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Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Potential Climate Change Impacts on Marine Resources
of the Northeastern United States
Michael Fogarty1, Lewis Incze2, Richard Wahle3 , David Mountain1, Allan Robinson4, Andrew
Pershing5, Katherine Hayhoe6, Anne Richards1, and James Manning1
1
National Oceanic and Atmospheric Administration, Woods Hole, MA 02543
University of Southern Maine, Portland, ME 04101
3
Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME 04575
4
Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138
5
University of Maine and Gulf of Maine Research Institute, Portland ME, 04101
6
Department of Geosciences, Texas Tech University, Lubbock, TX 79409
2
Executive Summary
Projected changes in the marine environment off the northeastern United States as a result of
climate forcing include changes in water temperatures, salinity, wind stress, local precipitation
patterns, and cloud cover, with potential ramifications for water column structure and circulation.
These, in turn, can affect many aspects of marine systems, beginning with the transport, production
and dynamics of planktonic communities and extending through various parts of the food web.
Energy flow has many possible pathways in marine systems, which makes it difficult to predict
system responses to the multiple types of forcing that can attend climate change, for example,
changes in the degree and timing of winter cooling as well as summer warming, and changes in
stratification, nutrient supplies, surface mixing, and photosynthetically active radiation. Because of
the dominant role of temperature on the physiology and ecology of marine organisms, the potential
effects of changing temperature regimes is a logical place to begin any assessment of potential
climate change impacts. Increased water temperatures predicted by climate change models will result
in a poleward shift in the distribution of many subtropical and temperate water species in the
northeast, including economically important fish and shellfish species; changes in productivity of
these species; and possibly increased uncertainty for the fishing industry and resource managers
during periods of ecological transition. Indeed, ecological transitions add to the already difficult job
of ocean prediction and managing integrated human impacts on all parts of the ecosystem. In this
report, we highlight potential changes in two exploited species that have strongly shaped the identity,
character and incomes of New England fishing communities: the American lobster, Homarus
americanus, and Atlantic cod, Gadus morhua. These are good selections because both species can
be predicted to respond to increasing temperature. We stress that numerous other concerns warrant
attention in the near future but are more complicated, such as the spread of marine invasive species,
occurrences of harmful algal species, and the conservation of biodiversity in general.
The ocean components of GCMs currently in use exhibit significant biases in simulations of
the western North Atlantic. This bias results from the inability of the models to simulate the
separation of the Gulf Stream from the eastern U.S. coast correctly, resulting in an eastward
displacement of the North Atlantic Current (the northern extension of the Gulf Stream). This results
in a reduction in Gulf Stream water flowing into the far North Atlantic and circulating into the
Labrador Sea, inducing a cold and fresh bias in that region. In recognition of these biases, we
calibrated GCM model outputs against observed surface and bottom temperature fields and used
predictive relationships to make projections of bottom water temperatures from surface temperature
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anomalies for three coupled ocean-atmosphere GCMs under high and low IPCC emission scenarios.
We developed projections for four major regions of our study area: Northern Mid-Atlantic Bight,
Georges Bank, Western Gulf of Maine, and Eastern Gulf of Maine.
Dramatic differences are evident for the low and high emission scenarios for projected
bottom temperature levels. For the low emission scenario in particular, substantial differences are
noted among models and over the time course of the simulations. Under the low emission scenario,
the average bottom temperature anomalies did not exceed 1.5oC in either season by the year 2085.
For the high emission scenario, the projected bottom temperature anomalies exceeded 3.5oC in some
areas/seasons by 2085.
We explored the feasibility of linking a coupled ocean-atmosphere general circulation model
[the Parallel Ocean Climate Model (PCM)] with the Harvard Ocean Prediction System (HOPS) in an
attempt to account for the effects of complex topographic features and finer-scale hydrographic
features in the Gulf of Maine. HOPS has been used for realistic simulations and real-time forecasts in
the Gulf of Maine in other applications. We employed a detailed Gulf of Maine Feature Model which
links together sub-basin scale circulation features (such as the Maine Coastal Current, Northeast
Channel inflow, and Georges Bank gyre) and allows realistic high-resolution initialization of HOPS
in the Gulf of Maine. Resources were not available at this time for a full set of model runs under
varying emission scenarios, but we provide initial results to demonstrate proof of concept in
anticipation of a more extensive effort to follow.
Increased water temperatures over the last decade have been associated with sharp declines in
lobster landings in southern New England and in Long Island Sound. A critical threshold temperature
of 20oC has been exceeded frequently during summer over the last decade, and there is a high
probability that this level will consistently be exceeded by mid-century, resulting in a possible loss of
the lobster fishery in these areas. The dockside value of the lobster fishery in southern New England
and the Mid-Atlantic states is small relative to that in northern New England. Nonetheless, the sharp
decline in the southern portion of the lobster fishery over the last decade has created local hardships
in the fishing industry and has elicited substantial concern farther to the north. New research
programs have been implemented to understand the factors underlying the decline and whether other
lobster-producing areas are at risk.
In contrast, increased temperatures in the Gulf of Maine within the ranges predicted under
alternative climate change scenarios hold the potential for increased lobster productivity. This would
result from possible increases in the amount of thermally suitable habitat for settlement (initial
recruitment to the bottom from the planktonic stages) as well as enhanced individual growth rates,
assuming that other climate-induced changes do not negatively affect basic productivity states of the
system. To the extent that disease and hypoxic conditions experienced in the southern areas are linked
to changing temperature patterns, lesser impacts are expected in the Gulf of Maine, where model
projections indicate more moderate temperature increase.
Atlantic cod landings in the Gulf of Maine and on Georges Bank have declined substantially
over the last decade. These declines have been clearly linked to intensive exploitation, but the role of
environmental conditions in concert with over-harvesting cannot be ignored. Results of a metaanalysis of North Atlantic cod stocks suggest that increases in mean annual water temperatures above
8oC will result in a decline in recruitment (the number of young surviving to enter the fishery).
Georges Bank, historically the major cod producing region in the northeastern United States, had a
mean annual temperature of 9oC during the period 1978-2002. This suggests declining recruitment
and reduced potential for recovery of cod populations over a substantial part of the northeastern shelf
if further increases in bottom temperatures are realized. The 8oC threshold is also currently exceeded
in the near shore southern New England area, which has historically supported small spawning
aggregations of cod. Consideration of cod distribution patterns throughout the North Atlantic
suggests that a 12oC mean annual bottom temperature describes the practical limit of cod
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distributions. If the bottom temperature projections under the high emission scenario are realized, it
is likely that this level will be surpassed in the near-shore southern New England region and Georges
Bank regions by the end of this century. The mean bottom water temperature in the Gulf of Maine as
a whole during the period 1978-2002 was 7.1oC, and if the bottom temperature anomaly projections
for this region hold, the 12oC threshold will not be exceeded. Unknown at this time is how other
climate-related changes in the system would affect cod, such as through changes in prey and predator
communities.
Comparison of the projected impacts on cod and lobster indicates that both are potentially
vulnerable to declines and loss of habitat in the southern portion of their range if projected bottom
temperature increases occur, particularly under the high emission scenario. Cod is at the extreme
southern extent of its range in the Northern Mid-Atlantic Bight. Lobster reaches the southern extent
of its range in the vicinity of Cape Hatteras, but is largely confined to deeper waters in the southern
Mid-Atlantic Bight. As noted, cod is potentially vulnerable to loss in thermal habitat on Georges
Bank under the high emission scenario. Highest densities of lobster on Georges Bank are
concentrated in the submarine canyons on the continental slope and are less vulnerable to temperature
change in this region. Within the Gulf of Maine, cod are expected to exhibit a neutral to slightly
negative change if projected bottom temperature anomalies are realized by the end of this century. In
contrast, bottom temperature anomalies within the range predicted for the Gulf of Maine are
potentially favorable for this species. These differences highlight the fact that, in general, the
potential impacts of climate change in the ocean will exert differential effects on different species and
both declines and increases in productivity are likely to occur for different components of marine
communities within the region.
An economic impact assessment of the effects of projected climate change on the target
species is beyond the scope of this report. The potential loss of thermal habitat with resulting loss of
productivity for both lobster and cod in the southern portion of the study area is substantial,
particularly under the high emission scenario. Although the loss of access to both these species would
be potentially economically significant in these areas, we can anticipate some adaptation on the part
of the fishing industry, principally through switching to alternative species. For example, an increase
in water temperatures in southern New England is potentially favorable for an increase in blue crab
(Callinectes sapidus, Fogarty and Lipcius, in press) and it is conceivable that some lobster fishers
could switch to the lucrative blue crab fishery if such an increase occurs. Similarly, possible increases
in abundance and productivity of sub-tropical finfish species in the Northern Mid-Atlantic Bight
(e.g., Sciaenids such as weakfish, spot, and drum) could potentially offset some losses of cod in the
southern part of its range. Overall, the relative contribution of the landings of both lobster and cod in
the southern portions of this study area are relatively low in relation to the total. Should large-scale
losses of cod on Georges Bank occur or should adverse impacts on lobster (not currently anticipated)
occur in the Gulf of Maine, however, the economic impacts would be substantially more serious.
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1. Introduction
Future climate predicted under increasing anthropogenic emissions of greenhouse gases will
potentially strongly alter the physical structure of the oceans, with direct implications for marine
ecosystems and human societies (IPCC 2001). These changes need to be considered in the context of
impacts resulting from other human activities including fishing, pollution, and habitat loss due to
coastal development. Climate change can interact with other human-induced changes to alter the
fundamental production characteristics of marine systems. Living marine resources have sustained
human cultures for millennia as an essential source of protein and as a cornerstone of maritime
commerce and trade. However, continuously increasing fishing pressure and demand related to the
burgeoning human population has resulted in significant declines in many previously abundant fish
and shellfish populations. It is now clear that human harvest of these fishes can exceed their
production potential in the world’s oceans (Worm et al. 2006). Climate change will potentially
exacerbate the stress on some living marine resources imposed by harvesting and other anthropogenic
activities in many regions, but will potentially enhance production for other populations in some
areas. Considerable emphasis is now being placed on understanding the causes and consequences of
climate change in temperate marine systems (e.g. Harvell et al., 2002; Helmuth et al., 2002; Barnett
et al., 2005; Drinkwater, 2005; Sutton & Hodson, 2005; Altieri & Witman, 2006) to prepare for
anticipated alterations in ecosystem structure and function.
Fishing has played a vital role in the cultural and economic fabric of the northeastern United
States. Indeed, the identity and current-day economy of many northeastern coastal communities is
deeply tied to fishing and to iconic species such as cod and lobster. Combined dockside revenues
from oceanic and estuarine commercial fisheries in the northeastern U.S. in 2004 exceeded $1.2
billion. In 2004, shellfish landed revenue was just over $900 million, dominated by lobster and sea
scallop (Placopecten magellanicus) returns. Dockside revenue from lobsters alone in 2004 was $366
million. Overall, lobster landings have undergone dramatic increases over the last three decades, but
landings in the southern part of the range have declined sharply from recent peaks in the mid-1990s.
The landings of cod in the northeast have declined substantially since 1995, and were worth an
estimated $21.7 million dollars in 2004. Hoagland et al. (2005) estimated the economic contribution
of the seafood industry to the overall economy in the Northeast as $10.4 billion in 2003 as a result of
the direct and indirect effects of commercial fishing activity and associated industries.
Approximately 76,530 jobs in the Northeast in 2004 were dependent on the production of
seafood by commercial harvesters, wholesalers, and processors (Hoagland et al. 2005). In that year
there were 2,508 vessels actively fishing under federal fishing permits in the region. Many smaller
vessels not requiring federal permits also operate in the region, typically fishing exclusively within
state territorial waters. In Maine, for example, there were more than 7,000 lobstermen and
approximately 4,000 boats involved in lobster fishing in 2004. Most of these fished within state
waters.
Here, we review aspects of the potential changes in marine ecosystems off the northeastern
United States in response to climate-induced forcing, with a focus on temperature effects on living
marine resources. We provide an overview of anticipated general alterations in marine ecosystems in
response to climate change, review likely impacts on the northeast continental shelf, and highlight
potential effects on the distribution and abundance of the American lobster and Atlantic cod to
illustrate the range of possible impacts throughout the region.
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2. Background
Climate Change Impacts on the Marine Environment of the Northeastern U.S.
The region covered by this paper includes the continental shelf of the northeastern United
States. Major sub-regions include the northern mid-Atlantic Bight and southern New England shelf
(south of Cape Cod), Georges Bank, and the Gulf of Maine (Fig. 1). In some areas the continental
shelf extends more than 200 km from shore. Over-all, the shelf is characterized by southward
(equatorward) flow of cold and comparatively fresh water from the sub-Arctic North Atlantic
(Labrador Sea and beyond: Belkin et al. 2004) and the Gulf of St. Lawrence, but the region,
especially the southern part, is substantially influenced by across-shelf exchanges with the
neighboring slope sea which is conditioned by the northward-flowing Gulf Stream (Figs. 2, 3; see
review by Townsend et al. 2006). At middle depths along the continental margin, slope water can be
of northern or southern origin, depending on latitude and variable forcing of the slope water systems.
The Gulf of Maine is particularly complex in this regard, because it contains several deep basins with
a single deep-water connection to the slope, the Northeast Channel (Fig. 1). The dominant slope
water at this latitude is strongly influenced by remote forcing, some of it linked to shifts in
atmospheric pressure distributions over the North Atlantic [for example, the North Atlantic
Oscillation (NAO) index: Rogers 1984, Pershing et al. 2001, Greene and Pershing 2003 ].
Temperature, salinity, nutrients and plankton all vary as a function of slope-water sources (Thomas et
al. 2003, Pershing et al. 2005).
Fig. 1. Map of the study area with selected geographical features labeled.
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Continental
shelf
waters
experience cooling and mixing during
winter months, when nutrients are
replenished in the surface layers, and
stratification during the warm months.
Over much of the continental shelf,
seasonal stratification isolates the
surface from lower depths, leading to
enhanced warming, plankton blooms
and eventual nutrient limitation of the
surface wind-mixed layer. A pool of
cold bottom water typifies mid-shelf
areas during summer
months.
Stratification decays in the fall as the
surface layer cools (thus becoming
less buoyant) and the frequency and
intensity of strong winds causes
increased mixing.
Late fall is
generally the warmest time of year on
the bottom. An exception to the
above scenario exists where tidal
mixing is particularly strong, such as
in the northern and eastern portions of
the Gulf of Maine and over shallow Fig. 2. Schematic of the dominant circulation features that affect
regions of Georges Bank and shelf and slope water systems in the northeastern U.S. (after
Nantucket Shoals. In the Gulf of Townsend et al. 2006).
Maine, comparatively cold, nutrientrich waters that are mixed upward by tidal mixing persist as a comparatively cold feature through
summer and are transported via the cyclonic (counter-clockwise) coastal current system. The
combination of tidal mixing, cold-water currents and stratified regions in the Gulf of Maine produces
exceptionally steep horizontal temperature gradients in this region during warm months of the year
(Fig. 3). Also see later modeling results, this paper).
Winter-time cooling and convection can vary substantially from year to year, affecting minimum winter
temperature and the depth of mixing. Impacts extend into the spring and summer in the form of temperature of
the cold bottom water pool and, in deep areas such as the Gulf of Maine, concentrations of nutrients introduced
to the surface. Throughout much of this region, however, bottom water characteristics are affected not only by
winter conditioning, but also by advection into the Gulf from the slope sea.
Vernal stratification and autumnal de-stratification can be shifted earlier or later by weather,
including cloud cover (because of its impact on radiative heating), wind, air temperature, and
freshwater input. Even winter conditions can vary by factors as subtle as cloud cover (which affects
the supply of light for photosynthesis) and wind; together, they affect late winter primary production
in shallow, near-shore waters. Thus, conditions on the northeast continental shelf are subject to
interannual variability in weather as well as longer-term changes in weather patterns and climate, and
local and remote forcing. Temperature, salinity, nutrients, light, the timing of seasonal transitions,
and advection of planktonic biota, including propagules, are all factors that can bring change to a
region. These changes, in turn, can affect higher trophic levels through temperature effects on
physiology, migrations, reproduction, alterations in predator and prey communities, and diseases.
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Fig. 3. Monthly climatologies (1974-2004) of satellite-derived sea surface temperature over the
northeastern U.S. continental shelf and adjacent ocean (cf. schematic circulation shown in Fig. 2).
Temperature History
Sea surface temperature records extend back over a hundred years at shore stations in Woods
Hole, Massachusetts (south side of Cape Cod, Nixon et al. 2004) and in Boothbay Harbor, Maine
(Fig. 1 for locations). Time series exist of shorter duration for numerous other locations, including
Steel Pier, New Jersey, Georges Basin in the central Gulf of Maine (bottom temperature, see
Townsend et al. 2006), Boston Harbor and Newport, Rhode Island (Nixon et al. 2004) and the
southwestern Bay of Fundy (Prince 5, Fig. 1). Most of these show a period of cold temperatures
during the mid-1960s which probably reflect enhanced southward advection of cold northwest
Atlantic water associated with a period of negative NAO anomalies (see later discussion). Data from
the two longest time-series in the Gulf of Maine and Woods Hole also show a period of very warm
temperatures during the 1950s.
The Boothbay time-series, based on daily observations with few significant interruptions
since 1905, can be used to look at the long-term temperature change (Fig. 4) and the time of year that
those changes occurred (Fig. 5). Compared to the “twentieth century” mean (1905-1999), positive
anomalies of 1-2°C occurred primarily during summer months prior to 1950. During the warm period
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Winter, Summer and Annual Temperature
18
of the early 1950s, positive
anomalies occurred throughout the
14
year, but were strongest (3-4°C) in
12
fall, winter and early spring.
10
Positive anomalies generally favored
8
non-summer months in the years that
6
followed until the 1990s, when they
again included most months of the
4
year. In recent years, the strongest
2
positive
anomalies (3-4.5°C) have
0
been
from
late summer into fall and
-2
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
early winter.
These recent
anomalies
coincided
with
a period of
Fig. 4. One hundred years (1905-2005) of sea surface
great increase in the abundance of
temperature in Boothbay Harbor, Maine . Data are annual
small copepods whose populations
values of winter (DJF, blue), summer (JJA, red) and 12-month
would benefit from the extended
(black) means.
season of warm/stratified conditions
(see later discussion).
Possible
explanations include greater radiative input from clearer skies, less vertical mixing from wind events,
decreased heat losses to the atmosphere, increased stability from salinity changes, and increased
coastal convergence/decreased divergence caused by changes in wind forcing. During winter,
decreased heat loss to the atmosphere probably dominates the trend and results in decreased depth of
winter mixing. The annual temperature means at Prince 5 show similar patterns as at Boothbay,
except that the temperature difference between the two gets larger as Boothbay temperatures become
warmer.
Satellite and buoy data for
1
offshore regions of the Gulf of
4
2
Maine, Georges Bank and
3
3
southern New England/Mid2
4
Atlantic Bight waters have shorter
5
1
histories and are useful for a
6
0
7
variety of purposes over shorter
-1
8
periods of time. There is an
9
-2
extensive
data
base
of
10
-3
temperature, salinity and density
11
-4
profiles and other water column
12
05
15
25
35
45
55
65
75
85
95
05
properties, but these also are best
Year: 1905 - 2005
suited to focused questions
concerning spatial or shorter
temporal patterns, and are not
Harbor, computed against the “20th century” (1905-1999) means.
Note the shifting patterns of positive temperature anomalies from
considered further in this paper.
16
Month
Temperature °C
summer to non-summer and year-round. White boxes are
missing values.
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Remote Forcing
The North Atlantic Oscillation (NAO) is the dominant climate mode in the North Atlantic
sector (Hurrell, 1995). NAO refers to a characteristic pattern of variability in atmospheric sea-level
pressure over the North Atlantic. The strength of the two main pressure centers, the Icelandic (subpolar) Low and the Azores (sub-tropical) High, are correlated: when the Icelandic Low deepens, the
Azores High typically becomes higher, and vice versa. This pattern can be summarized by a simple
index by differencing the atmospheric pressures at stations located within the two pressure centers
(typically Lisbon, the Azores or Gibralter for the High, and Reykjavik for the Low). The NAO Index
is correlated with a wide range of atmospheric and oceanographic phenomena in the North Atlantic
sector. When the NAO is positive, westerly winds shift northwards and intensify, bringing warmer
and wetter conditions to northern Europe. The area over the Labrador Sea tends to be colder, leading
to increased formation of Labrador Sea Deep Water (LSDW). Conversely, negative NAO conditions
lead to a southward shift in the westerlies and colder conditions in northern Europe. The cold
weather leads to increased formation of Norwegian Sea Deep Water, while LSDW production is
diminished.
Over the Gulf of Maine, weather conditions have little correlation with the NAO. Rather, the
NAO influences the NW Atlantic through its association with the regional oceanography. Changes in
the NAO index have been linked to shifts in the position of the Gulf Stream (northward following
positive NAO conditions) and to the volume transport in the Labrador Current (increased following
negative NAO conditions: Rossby 2000, Taylor 1998). Shifts in the Gulf Stream and the appearance
of colder Labrador Slope Water in the Gulf of Maine occur 1-2 years after a shift in the NAO
(MERCINA 2001; Greene and Pershing 2003; Drinkwater 2006). At this point, physical
oceanographers are debating the degree to which the Gulf Stream and Slope Water changes are
linked, and whether they are caused primarily by shifts in the wind field or hydrographic changes
originating in the Labrador Sea and beyond.
Changes in the physical oceanography of the NW Atlantic related to the NAO have been
related to changes in biology. Variations in the abundance of Calanus finmarchicus, the dominant
zooplankton (copepod) species during spring and summer, have been linked to NAO- associated
changes in the physical oceanography (Greene 2001; Conversi 2001; MERCINA 2001). Shifts in the
abundance of C. finmarchicus have, in turn, been linked to reproductive success in the endangered
northern right whale, Eubalaena glacialis (Greene 2003, 2004). It is likely that changes in C.
finmarchicus have other biological consequences as well, although these have not been quantified.
Popular reference to the NAO as the “Atlantic’s El Niño” is a misleading analogy. Unlike the
El Niño-Southern Oscillation (ENSO), which is a well-understood series of events in the ocean and
atmosphere, the NAO is a pattern, and the mechanisms that cause this pattern are not well understood.
Two examples illustrate this point. Cassou et al. (2004) reanalyzed surface pressure fields over the
North Atlantic using cluster analysis and found four characteristic states in the pressure distribution.
One resembled the positive NAO with enhanced pressure centers, another resembled the negative
NAO, and two others showed little resemblance to the NAO. In a separate study, Joyce (2002)
considered the correlation between the NAO index and weather conditions over North America. He
found that the association with the NAO waxed and waned: during some periods, the NAO was a
good predictor of weather, while at other times it provided no useful information.
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Despite its limitations, the NAO Index is a useful summary of physical conditions in the
North Atlantic. If we knew how climate change would affect the NAO, we could exploit established
correlations with the NAO to gauge the impact of climate change on the NW Atlantic. Interestingly,
many GCMs, including those used in the Northeast Climate Impact Assessment (this study), predict a
dominance or strong presence of positive NAOs over the next 50-100 years, especially the winter
index and especially in the higher carbon emissions scenarios. The basis for these predictions
requires further analysis, but these results are promising.
One of the key features of the response of the NW Atlantic to the NAO is that the forcing
comes from upstream, specifically, the Labrador Sea. In addition to the NAO changes, the Labrador
Sea seems to be a key area in other interannual changes in the Gulf of Maine. During the 1990s, the
NW Atlantic experienced widespread freshening (Smith 2001; Mountain 2003, 2004). This
freshwater was not associated with increased precipitation or river input; rather, the freshwater
originated beyond the Gulf of St. Lawrence (Frank 2003). During this period, the salinity in the
Labrador Sea, especially on the shelf, was much lower, and it seems likely that the two events are
related. Freshening in the Labrador Sea, and at higher latitudes in general, through a speed-up in the
hydrologic cycle (Curry, 2003, 2005) or increased ice melt is a hallmark of most climate change
scenarios. Thus, it seems likely that conditions in the Gulf of Maine during the 1990s could provide a
window into future physical and biological changes that could be linked to global climate change
models.
In addition to the freshening, biological conditions in the 1990s were much different than
previous decades. Pershing et al. (2005) documented an increase in most of the dominant copepod
taxa around 1990, one that persisted throughout the decade. The notable exception to this pattern was
a decrease in Calanus finmarchicus. Changes in the zooplankton community were the result of
greatly enhanced fall and winter populations of smaller zooplankton, and were associated with
increased fall phytoplankton abundance and warmer temperatures, presumably due to increased
stratification during this season (see above discussion of SST anomalies in the Boothbay Harbor
record).
3. Climate Models and Emission Scenarios
Our analysis is based on simulations from three atmosphere-ocean general circulation models
(AOGCMs): NOAA/GFDL CM2.1 (Delworth et al., 2005), UKMO HadCM3 (Pope et al., 2000), and
DOE/NCAR PCM (Washington et al., 2000). These are three of the latest generation of numerical
models that couple atmospheric, ocean, sea-ice, and land-surface components to represent historical
climate variability and estimate projected long-term increases in global temperatures due to human
emissions. As shown in Table 1, atmospheric processes are simulated at a horizontal resolution of 2.5
by 2 degrees (GFDL), 3.75 by 2.5 degrees (HadCM3) and T42 or ~2.8 by 2.8 degrees (PCM). Ocean
variables including salinity, potential temperature, and three-dimensional currents, simulated on a tripolar grid by GFDL and PCM, were interpolated onto a regular 1x1 degree grid using depthdependent areal weights. HadCM3 ocean simulations were performed on a regular 1.25x1.25 degree
grid so required no interpolation. Climate sensitivity, a metric that captures the magnitude of the
model-simulated increase in global temperature in response to a doubling of atmospheric CO2
concentration, is 1.5oC for GFDL, 1.3oC for PCM and 3.3oC for HadCM3, covering the low to
medium part of the IPCC range of 1.5-4.5oC.
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Table 1. AOGCM model resolution.
HadCM3
GFDL
PCM
Atmospheric resolution
2.5o x 3.75o
2o x 2.5o
T42 (~2.8o x 2.8o)
Horizontal resolution
1.125o x 1.125o
1o x 1o
(interpolated tri-polar)
1o x 1o
(interpolated tri-polar)
Vertical resolution
20 levels
50 levels
32 levels
Top 6 layers (depth, m)
5, 15, 25, 35, 48, 67
5, 15, 25, 35, 45, 55
13, 38, 64, 90, 118,
147
Emission Scenarios
Future simulations are forced by the IPCC Special Report on Emission Scenarios (SRES,
Nakićenović et al. 2000) higher (A1fi), mid-high (A2) and lower (B1) emissions scenarios. These
scenarios describe internally consistent pathways of future societal development and corresponding
greenhouse gas emissions, and cover a wide range of alternative futures based on projections of
economic growth, technology, energy intensity, and population. The SRES scenarios are not assigned
probabilities, but rather can be viewed as possible futures, with the actual path depending on
technology, economic development and political will. The A1fi and B1 scenarios used in this study
bracket the range of SRES scenarios, and can be thought of as lower and higher bounds that
encompass most, but not all, potential non-intervention emissions futures. At the higher end, rapid
introduction of new technologies, extensive economic globalization, and a fossil-intensive energy
path causes A1fi GHG emissions to grow steadily throughout the century. In the A1fi scenario, CO2
emissions climb throughout the century, reaching almost 30 Gt/yr or 6 times 1990 levels by 2100. A2
emissions also reach 30 GtC/yr by 2100, although cumulative emissions over the century are slightly
lower than A1fi. Emissions under the B1 scenario are even lower, based on a world that transitions
relatively rapidly to service and information economies. CO2 emissions in the B1 scenario peak at
just below 10 Gt/yr - around two times 1990 levels – at mid-century and decline slowly to below
current-day levels. Together, these scenarios represent the range of IPCC non-intervention emissions
futures with atmospheric CO2 concentrations reaching approximately double and triple pre-industrial
levels, at 550 ppm (B1) and 970 ppm (A1fi), by 2100. Projections for the A1fi and B1 scenarios were
used for the GFDL and PCM models, but as HadCM3 did not save any ocean output for its A1fi
simulation, we used A2 and B1 instead.
Bias in Oceanographic Projections
GCMs currently in use exhibit significant biases in simulations of the western North Atlantic
(e.g., Dai et al. 2005). This bias results from the inability of the ocean component of the model to
simulate the separation of the Gulf Stream from the eastern US coast correctly, resulting in an
eastward displacement of the North Atlantic Current (the northern extension of the Gulf Stream).
This results in a reduction in Gulf Stream water flowing into the far North Atlantic and circulating
NECIA • 11
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
into Labrador Sea, inducing a cold and fresh bias in that region. This has important implications for
simulating temperature and salinity regimes over the northeastern U.S. continental shelf because of
the importance of far-field forcing. We made a detailed examination of the biases for the Gulf of
Maine by comparing model predictions of temperature and salinity against direct observations for this
region (See Appendix 1). In all subsequent analyses for all areas, we used only projected anomalies
in an attempt to capture the range of potential changes in oceanographic variables while recognizing
the bias in absolute projections (see below).
Bottom Water Temperature Projections
Because of the overriding influence of bottom water temperature on the physiology of our
target species, we focused on developing projections of this variable for the major geographical
regions of interest. We developed predictive relationships between surface and bottom temperatures
for the Northern Mid-Atlantic Bight, Georges Bank, the Western Gulf of Maine and the Eastern Gulf
of Maine for spring and autumn using hydrographic observations during Northeast Fisheries Science
Center surveys throughout the region. Although observations are made throughout the year, the most
intensive sampling is in spring and autumn in conjunction with bottom trawl surveys. Predictive
relationships were developed by regressing surface temperature on bottom temperature in the season
of interest and then checking for improvement in fit by adding the surface temperature for the other
season.
In general, spring bottom temperature was closely related to spring surface temperature,
particularly for the MAB and GB regions (accounting for 63% to 89% of the variability) while fall
bottom temperature was best predicted by a combination of spring and fall surface temperature
(accounting for 33% to 60% of the variance). Correlation between bottom and surface temperatures
was much lower for GOM, with surface temperatures accounting for only ~20-50% of the year-toyear variance. Accordingly, projections for the Gulf of Maine in particular require care in
interpretation.
The seasonal and region-specific linear fits were applied to model-simulated spring and fall
surface temperature anomalies for each region (relative to 1970-2000) in order to produce bottom
temperature anomalies covering the full period of model simulations, from 1900 through 2099.
Projected water temperature patterns by region for spring and autumn are provided in Figures 6 and 7
under the low and high emission scenarios.
Dramatic differences are evident for the low and high emission scenarios for projected
bottom temperature anomalies (Figures 6 and 7). For the low emission scenario in particular,
substantial differences are noted among models and over the time course of the simulations.
Ensemble averages over the three models for projections of bottom water temperature anomalies by
region for the period 2080-2084 are provided in Table 2
Under the low emission scenario, the average spring bottom temperature anomalies (oC) range from
0.89 (northern Mid-Atlantic) to 1.33 (western Gulf of Maine) and, in autumn, from 1.17 (Eastern
Gulf of Maine) to 1.48 (Western Gulf of Maine). For the high emission scenario, the projected spring
bottom temperature anomalies range from 2.044 (eastern Gulf of Maine) to 3.64 (northern MidAtlantic) and, during autumn, from 1.96 (eastern Gulf of Maine to 3.48 (Georges Bank).
For the low emission scenario therefore, the spring and autumn projected temperature
anomalies are less than 1.5 oC and are relatively consistent among areas in comparison with the high
emission scenario. For the latter, wider regional differences are evident with substantially higher
projected bottom temperature increases in the southern areas.
NECIA • 12
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Spring
N. MidAtlantic
2
2
1
1
0
0
-1
Autumn
-1
3
2
3
Georges Bank
2
Bottom Temperature Anomaly
1
1
0
0
-1
-1
-2
-2
2
W. Gulf of Maine
2
1
1
0
2
0
E. Gulf of Maine
1
1
0
0
2020
2040
2060
GFDL
HAD-1
2
2080
2020
2040
HAD-2
PCM
2060
2080
Time Period
Figure 6. Projected average bottom water temperature anomalies (oC) for four
time periods (2020-2024, 2040-2044, 2060-2064, and 2080-2084) in spring
and autumn, by region, under IPCC low emission scenarios for the GFDL,
Hadley (HAD, 2 scenarios) and Parallel Climate (PCM) models.
NECIA • 13
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
5
4
5
Spring
N. MidAtlantic
Autumn
4
3
3
2
2
1
1
0
0
5
Bottom Temperature Anomaly
4
-1
5
Georges Bank
4
3
3
2
2
1
1
0
3
0
3
W. Gulf of Maine
2
2
1
1
0
0
2
E. Gulf of Maine
1
1
0
0
2020
2040
2060
GFDL
HAD
PCM
2
2080
2020
2040
2060
2080
Time Period
Figure 7. Projected average bottom water temperature anomalies (oC) for four
time periods (2020-2024, 2040-2044, 2060-2064, and 2080-2084) in spring and
autumn by region under IPCC high emission scenarios for the GFDL, Hadley
(HAD, 2 scenarios combined) and Parallel Climate (PCM) models.
NECIA • 14
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Table 2. Ensemble averages over all models for projections of bottom water temperature anomalies
by region and season for Low and High emission forecasts for the period 2080-2084.
Spring Low
N. Mid-Atlantic
0.890
Georges Bank
1.241
W. Gulf of Maine E. Gulf of Maine
1.330
1.052
Spring High
3.640
3.367
2.137
2.044
Autumn Low
1.249
1.254
1.481
1.172
Autumn High
2.770
3.482
2.160
1.958
Linking AOGCMs with Finer-Scale Hydrodynamic Models
To more fully evaluate options for developing projections of water mass characteristics, we
explored the feasibility of linking a coupled ocean-atmosphere general circulation model [the Parallel
Ocean Climate model (PCM)] at 1o resolution with the Harvard Ocean Prediction System (HOPS),
which has been used for realistic simulations and real-time forecasts in the Gulf of Maine in previous
applications. We employed a detailed Gulf of Maine Feature Model (5 km resolution) that links
together sub-basin scale circulation features (e.g. the Maine Coastal Current, Northeast Channel
inflow, and the Georges Bank gyre: Gangopadhyay et al. 2002) and allows realistic high-resolution
initialization of HOPS in the Gulf of Maine. To address the bias in mean conditions predicted by the
GCM, we employed a perturbation approach in which the model-based
projected future changes in temperature and salinity were superimposed onto observed climatological
fields. A full description of the methods employed in linking the PCM and HOPS models is provided
at: http://oceans.deas.harvard.edu/UCS/.
Resources were not available at this time to conduct a full set of model runs under varying
emission scenarios. Accordingly, we provide initial results to demonstrate proof of concept in
anticipation of a more extensive effort to follow. Representations of the dynamically adjusted surface
temperature and salinity fields for the reference year (2000) and in 2085 are provided in Figure 8.
4. Lobster Case Study
The American lobster (Homarus americanus) is one of the most highly prized fishery
resources of the Atlantic coast. The state of Maine produces more than half of the annual U.S.
landings, and the lobster industry is a mainstay of the state’s economy. In addition to the monetary
value of landings, lobstering supports a unique way of life in fishing villages all along the New
England coast. The lobster industry and related support facilities and activities help define the
heritage and character of these communities and provide a focal point for tourism, another major
economic activity.
NECIA • 15
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Dynamically adjusted fields for September 2000
Dynamically adjusted fields for September 2085
Temperature
Salinity
Figure 8. Projected changes in surface temperature (oC) and salinity (PSU) in the Gulf of Maine
between September 2000 and September 2085.
NECIA • 16
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
The geographic range of the American lobster encompasses one of the steepest latitudinal
gradients in sea surface temperature in the North Atlantic. Over the distance of a few hundred
kilometers, summer maximum temperatures have historically ranged from the mid-20’s ºC in
southern New England to only about 12ºC at the mouth of the Bay of Fundy. The southern limit of
the coastal distribution of the lobster is Long Island and northern New Jersey. South of that boundary,
lobsters become increasingly restricted to deeper cooler water at the edge of the continental shelf off
Virginia and North Carolina. At the northern limit of its range in the northern Gulf of St. Lawrence,
lobsters are largely restricted to waters less than 50m deep that warm during the summer.
Lobster landings in the United States nearly tripled over the past two decades. Explanations
for the rapid increase in landings have focused on a combination of sharp increases in fishing effort
and in the total area fished, enhancements in fishing technology, favorable water temperatures
(particularly in the Gulf of Maine), and decreases in predatory fish populations (including cod) as a
result of over-exploitation (Fogarty 1995). In the state of Maine, landings exhibited an increase
during the warm period 1945-1955 with a time delay consistent with the time required to reach
recruitment to the fishery (Fogarty 1988). The recent increase in landings also has occurred during a
period of rapidly increasing temperatures, but with temperature slightly lagging landings (Figure 9).
Temperature alone, therefore, does not explain the increase in landings, but may impact the largerscale distribution patterns. Figure 10 shows results from standardized research vessel surveys during
the relatively cold period 1965-69 and the recent warm period during 2000-2004. The surveys do not
include rocky substrate areas, a preferred lobster habitat. However, there are striking increases in the
abundance of lobsters in soft substrate habitats, and an apparent shift in the center of density toward
higher latitudes within the substrate types sampled by the survey gear.
Incze et al. (2006) show that the planktonic and first settled stages of lobsters in the Gulf of
Maine show large interannual variations that appear to be due to environmental forcing (planktonic
survival and transport). Series of good and bad years during the earliest life history stages then set the
potential for subsequent population size and fishery yields (Wahle et al. 2004).
30
12
Temp
25
11
20
10
15
9
10
8
5
7
0
6
1905
1915
1925
1935
1945
1955
1965
1975
1985
1995
Year
Figure 9. Landings of American lobster in Maine over the
last century and mean annual sea water temperature at
Boothbay Harbor.
NECIA • 17
Temperature (oC)
Landings ('000 mt)
Landings
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Figure 10. Abundance of American lobster in research vessel surveys (kg/standard tow)
conducted by the National Marine Fisheries Service during 1965-69 (left panel) and
2000-2004 (right panel).
Temperature Effects on Lobster Biology
Physiological Responses: While the American lobster is generally described as living in
environments where water temperature ranges from 5-20ºC (Aiken & Waddy, 1986), the reality is
more complex. Early work on temperature tolerance of the American lobster was motivated by the
need to maximize survival during storage and shipping of the commercial catch. It therefore focused
on lobsters of marketable size, most of which are adults. The maximum and minimum lethal
temperature limits for lobsters varies somewhat with the oxygen content and salinity of the seawater.
McLeese’s (1956) classic laboratory experiments illustrate the interactive effects of temperature,
dissolved oxygen, and salinity on the survivability of lobsters (Figure 11). Life at high temperatures
is made precarious by the fact that the biological oxygen demand of ectothermic organisms increases
with temperature just as oxygen solubility in water diminishes. In general, in sea water at
temperatures below about 20ºC, lobsters are not stressed as long as oxygen concentrations are above
2 mg O2 l-1. Recent work on Long Island Sound lobsters confirmed that as water temperature rises
above 20.5ºC, the respiration rate of lobsters increases significantly and the animals come under
stress (Chang 2004, Powers et al. 2004, Dove et al. 2005). As a result, 20ºC has been used by the
Connecticut Department of Environmental Protection as a physiological stress threshold for lobsters
in Long Island Sound.
Prior acclimation can enhance survival under extreme conditions. McLeese found the lower
lethal limit of lobster to be 1.8ºC for lobsters acclimated at 17ºC, but 5ºC for lobsters acclimated at
27.5ºC. A lobster’s ability to live in diluted seawater also is temperature-dependent. At temperatures
below 20ºC they can survive quite well in half-strength seawater (approximately 15 PSU), but
survival at this salinity decreases rapidly above 20ºC (McLeese 1956).
NECIA • 18
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
O
Toler
an
Zone ce
T
24
16
8
30
Leth
al Zo
ne
20
S
2
Salinity
Finally, as molting (part
of the growth cycle) occurs
during the warmer months,
resistance to physiologically
stressful conditions may rise and
fall with the molt cycle. Molting
lobsters have been found to be
less resistant to high temperature
and low dissolved oxygen and
salinity than lobsters during
intermolt periods (Waddy et al.,
1995).
4
Ox
6
yg
en
Movements and migrations:
Temp
Thermal
tolerances
strongly
eratu
10
influence lobster movements and
re
seasonal migrations. Lobster
movements in estuaries and
Figure 11. Diagram of the boundary of lethal conditions for
embayments are known to be
lobsters in various combinations of temperature (oC), salinity
linked to seasonal changes in
(ppt) and oxygen (mg/l). T – region in which temperature
temperature.
During
spring,
alone acts as a lethal factor, S- region in which salinity alone
lobsters become more active as
acts as a lethal factor, O- region in which oxygen alone acts
waters warm, and they move into
as a lethal factor. (Figure and caption after McLeese 1956).
the warming shallows of bays and
estuaries to the extent that they
can tolerate reduced salinities (Jury 1999). In southern New England, fishermen commonly observe a
mid- to late summer movement into deeper channels and out of estuaries that reach temperatures
greater than about 20ºC, followed by a return in the fall as temperatures cool for a time before the
winter emigration. Laboratory experiments indicate that lobster activity does not vary linearly with
temperature; rather, there is a threshold at ~10ºC above which lobsters become quite active and below
which they are more quiescent (Jury 1999). In experiments conducted in laboratory tanks providing a
horizontal gradient from 12 to 22ºC over a 1.8 m distance, adult lobsters that were acclimated at a
summer ambient temperature of 15.5ºC in the Gulf of Maine, avoided temperatures warmer than 19ºC
and colder than 13ºC, and spent most of their time at 16.5ºC, slightly warmer than the ambient
conditions from which they came.
Growth and maturity: Regional differences in the growth rate of juvenile and adult benthic stages are
readily attributable to temperature. Below 5oC metabolism is slowed to a point where molting does
not occur. Under stressfully warm conditions lobsters may not molt or may die in the process
(Waddy et al. 1995). The southern Gulf of St. Lawrence and southern New England, where summer
temperatures rise near or above 20oC, have some of the fastest growing lobsters, whereas in the colder
waters of the northern Gulf of St. Lawrence and the Bay of Fundy, lobsters grow more slowly. Under
constant 20oC, H. americanus in captivity has been reared to marketable size (83 mm carapace length,
CL) in two years (Hughes, 1972), but lobsters are not likely ever to experience that level of growth in
the wild, where it is estimated to take 5-10 years to become harvestable, depending on ambient
temperatures. Year-to-year differences in temperature substantially alter the timing and frequency of
molting (Templeman, 1936; Munro & Therriault, 1983; Comeau & Savoie, 2001, 2002).
NECIA • 19
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Laboratory manipulation of the seasonal temperature cycle alters the molt cycle. If lobsters
are held at 10oC or higher in the spring they quickly enter pre-molt, but if they are held at the same
temperature into autumn, the molt is inhibited even the next summer (Waddy et al. 1995). Moreover,
artificially warm temperatures of 15-20oC experienced during the winter can over-ride seasonal molt
inhibition so that the molt may occur at any time of year (Waddy et al. 1995). Still, lobsters reared
under constant temperature retain some seasonality in their growth cycle. American lobsters reared
from hatching at a constant 20oC in the absence of a seasonal temperature cycle nonetheless exhibit a
seasonal pattern of molting with the greatest frequency in spring and autumn and lowest in midsummer and winter. This indicates that other factors, such as photoperiod or an endogenous rhythm,
also affect seasonal patterns of growth (Aiken & Waddy, 1989).
Warm temperatures hasten the onset of maturity. Lobsters in warmer regions not only grow
faster, but they mature at a smaller size than conspecifics living in cooler water. Females mature
between 68 and 76mm carapace length (CL) in the warmer parts of their range, but between 87 and
97 mm CL in the cooler parts (Estrella & McKiernan, 1989; Comeau & Savoie 2001).
Finally, seasonality in the temperature cycle influences the timing of the hatch. In hatchery
settings, hatching has been observed when temperatures rise to between 12.2 and 15ºC. In the field,
the temperature at which newly hatched stage I larvae first appear in the water column is generally
consistent with the hatchery findings: 11.0-13.6ºC in southern New England, 9.0-12.7ºC in Cape Cod
Bay, 7.9-13.9ºC in the Gulf of Maine, 4.2-10.6ºC in Northumberland Strait, and 10.0-13.8ºC in
Newfoundland (Ennis 1995). Thus, the hatch generally occurs in southern New England in early
summer (late-June to early July) and only begins in August in the northern Gulf of Maine.
Larval and Postlarval Responses to Temperature: Development time from hatching to the postlarva
(last planktonic stage) is strongly influenced by temperature (MacKenzie 1988). The absolute times
projected by MacKenzie’s equations appear to overestimate the time required in the Gulf of Maine
(Incze et al. 2006), probably because of a strong seasonal (endogenous) component to the
temperature-dependent rates (Waddy et al. 1995). Temperatures above 24ºC are lethal to larvae and
postlarvae, and development is severely inhibited at temperatures below 12ºC (MacKenzie 1988).
Postlarval vertical movements and settlement behavior are temperature dependent as well.
This could have important implications for whether postlarvae ultimately settle in suitable benthic
habitat. Laboratory experiments have demonstrated that postlarval lobsters are deterred by steep
temperature gradients, and will terminate the dive and return to the surface if they encounter a strong
thermocline (Boudreau et al. 1992). In the field, Annis (2005) observed individual postlarvae to
consistently remain above the 12ºC isotherm during the stratified summer season. Avoidance of
temperatures <12ºC is consistent with the temperature requirements to complete larval development
(McKenzie 1988). The limited data available on the vertical distribution of newly settled lobsters
substantiates this pattern in that few if any newly settled lobsters have been found deeper than 20-30
m in the western coastal Gulf of Maine (Wilson 1999, Incze et al. 2006). The implication of this
behavior is that if temperatures warmed sufficiently above 12ºC in the decades to come, new nursery
grounds may become available that have historically been too cold for settlement.
Implication of Model Results for Future Lobster Distributions
Deteriorating conditions in the south: The observed time series of bottom temperatures at Millstone
Power Station in eastern Long Island Sound from 1979-2004 indicates that August temperatures have
been increasing on average over the period, and as a result have been rising more frequently above
the 20ºC stress threshold (Figure 12).
NECIA • 20
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Bottom Temperature (oC)
24
22
20 18
16
1975
1980
1985
1990
1995
2000
2005
Year
Figure 12. August bottom water temperatures (oC) at Millstone Power
Station in eastern Long Island Sound . Twenty-degree threshold indicated
as horizontal line.
Increases in air temperatures are consistently predicted over the next century for each of the
GCMs under both high and low emission scenarios. The frequency of extreme summer temperature
events is also predicted to increase. Although a direct translation to projected water temperatures is
not possible for reasons described above, it is possible to infer general trends and qualitative
outcomes. Given the high frequency of events in which the 20ºC stress threshold has been exceeded
in the last decade, it can be anticipated that this level will be consistently exceeded by mid-century in
Long Island Sound and likely in other near shore waters of the northern mid-Atlantic Bight. In short,
for eastern Long Island Sound the average bottom conditions in August, which are currently
marginally stressful, are expected to become substantially more so by the second half of the century,
with more frequent episodes of temperatures in the very stressful range of 25ºC.
Anatomy of a die-off: It is instructive to go into some detail on the sequence of events during the
Long Island Sound mortality event of 1999 because of its relevance to the climate assessment for the
lobster fishery and other coastal marine resources. The following summary represents the scientific
consensus regarding the factors contributing to the die-off as presented by Pearce and Balcom (2005)
in their synopsis of research focused on the episode.
Historical records indicate that during the months of July and August 1999, basin-average
temperatures in Long Island Sound were at decade highs for that time of year (Wilson & Swanson,
2005). Bottom water temperatures in August were greater than 21ºC and in some locations exceeded
23.5ºC. Temperatures above 20ºC continued into October. During that time, lobsters in the western
Sound were found to be infected with the parasitic Neoparamoeba pemaquidensis (Mullen et al.
2005). Lobster abundance in the Sound was estimated to be near historic highs as it was throughout
the species range. As lobsters moved to deeper, cooler water during the summer to escape the warm
hypoxic zones in the shallows, they became increasingly crowded and subject to further oxygen stress
and disease exposure. In late August, winds from a cold front moving through the region mixed the
NECIA • 21
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
water column, bringing warm surface water to the bottom and raising bottom water temperature
several degrees. Although summer hypoxic conditions were beginning to dissipate in early
September, the warmer conditions at the sediment-water interface facilitated the release of toxic
sulfides and ammonium from the sediments, further weakening or killing lobsters. In early
September, the remnants of Tropical Storm Floyd passed over the region with more winds and
substantial rainfall that mixed warm water to the bottom and lowered the salinity. Many lobsters
were afflicted with paramoebiasis by this time and were unable to recover from an already weakened
state. There is debate as to whether exposure to pesticides entering the Sound by way of run-off
could exacerbate the problem.
Other afflictions of lobster at the southern end of their geographic range appear to be
correlated with chronic exposure to warm temperatures. Dove (2005) described a condition called
calcinosis in which lobsters living in stressful warm-water conditions have severe changes in blood
ion balance, pH, protein levels, and blood cell counts. Epizootic shell disease is another pathological
condition that can be lethal and appears to be temperature related, although it has not been found to be
associated with the die-off in western Long Island Sound. It has occurred with highest prevalence
from eastern Long Island Sound to Buzzards Bay, MA and has been recorded in Cape Cod and north
of Boston more recently (Castro et al. 2006, Glenn and Pugh 2006). Smolowitz et al. (2005)
proposed that elevated temperature could promote shell disease in two ways: (1) by compromising
lobsters’ ability to remove bacteria because of physiological stress and compromised cleaning
behavior; and (2) increasing the growth rate of pathogens.
Beneficial effects in cooler regions: Warming in the cooler northern reaches of the species range may
have positive impacts on lobster populations. Lobster landings are correlated with temperature (over
the acceptable temperature range), reflecting immediate effects on vulnerability to capture and
delayed effects from events in early life history (Fogarty 1995). Possible positive effects of climate
warming are a prolonged growing season, more rapid growth, an earlier hatching season, faster
planktonic development (which may increase survival and settlement), and a smaller size at sexual
maturity.
Warming may also make new areas of sea bed suitable for larval settlement. While shallow
estuaries and embayments at the southern extreme of the species’ range may become less hospitable
to lobsters as the climate warms, areas previously too cold for lobsters to thrive may become more
habitable. This includes deeper waters below the thermocline as well as regions that do not become
thermally stratified during summer months. An example of the latter in the northeastern U.S. is the
eastern coast of Maine. This segment of the Gulf of Maine coast is strongly subject to the influence
of the cold Eastern Maine Coastal Current which carries tidally mixed water from the Bay of Fundy
southward along the Maine coast before it turns offshore near Penobscot Bay. Summer bottom water
temperatures in this region typically do not get much higher than 12ºC, except in protected shallow
bays and inlets. Lobster larval settlement, juvenile abundance, and harvests per unit of coastline along
this segment of coast have on average been substantially lower than in western regions (Wahle &
Steneck, 1991; Steneck & Wilson, 2001). Detectable densities of newly settled lobsters in this region
have only been reported in shallow bays and inlets (Wahle unpublished). While there is no direct
evidence to date that above-average temperatures have resulted in settlement in previously
unpopulated nursery habitat, it is hypothesized that, provided a larval supply, settlement will be more
likely where temperatures rise above 12ºC in the future.
5. Cod Case Study
Cod has been a mainstay of the commercial fishery in New England since the 17th century,
and a carved representation of the ‘sacred cod’ adorns the State House of Massachusetts.
NECIA • 22
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Populations of Atlantic cod off the northeastern United States are at the southern extent of the species
range in the Northwest Atlantic. Accordingly, climate change can be expected to affect their
distribution and abundance. For this reason, we focus on potential temperature effects on cod
dynamics, with supplemental information on the role of other environmental drivers.
Landings of cod have undergone large-scale and coherent fluctuations over the last century in
the Gulf of Maine and on Georges Bank and are currently at low levels in both regions (Figure 13).
Periods of low cod landings have corresponded with periods of high water temperature, although
many fishery-related factors affect landings in addition to environmental variables.
A comparison of the distribution and abundance of cod derived from Northeast Fisheries
Science Center research vessel surveys during the relatively cold period 1965-1969 and the recent
warm period during 2000-2004 suggests that cod were more broadly distributed in the earlier period
than at present, including the southern New England region (Figure 14).
Temperature Effects on Cod
55
20
Georges Bank
GoM Cod
45
15
40
35
30
10
25
20
15
5
10
5
0
1900
0
1920
1940
1960
1980
2000
Year
12
20
GoM Cod
Temp
18
11
16
10
14
12
9
10
8
8
6
7
Temperature (C)
Landings ('000 mt)
Georges Bank
Landings ('000 mt)
50
Temperature
(C)
Gulf of Maine
Throughout the North
Atlantic, cod populations inhabit a
broad spectrum of seasonal
temperature regimes, ranging
from less than -1oC to over 20oC,
with annual mean temperatures
from 2-12oC (Dutil and Brander
2003; Drinkwater 2005). The
optimal physiological temperature
for a population largely reflects
adaptive
responses
to
environmental conditions (Jobling
1988). Examining the response
of cod populations to historical
changes within this broad
spectrum of temperature regimes
can provide important insights
into the potential change in
distribution and abundance under
alternative climatic conditions.
In the northeastern U.S.,
cod spawning locations range
from the coastal Gulf of Maine
and Georges Bank to southern
New England (Lough 1984,
Lough and Bolz 1989, Serchuk et
al. 1994). Eastern and western
spawning contingents have been
reported on Georges Bank,
(Garrison et al. 2002, Lough et al.
2002). Spawning progresses from
south to north, with spawning
60
4
6
2
0
1900
5
1920
1940
1960
1980
2000
Year
Figure 13. Landings of Atlantic cod on Georges Bank and in the
Gulf of Maine (upper panel), and cod landings in the Gulf of
Maine in relation to annual mean water temperature at Boothbay
Harbor (lower panel).
NECIA • 23
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
commencing earlier in the south (Lough et al. 2002). Although the southern New England spawning
components are combined with the Georges Bank population for management purposes, the southern
groups may experience more immediate effects of increasing water temperatures because they inhabit
the southernmost spawning habitat areas of the Northwest Atlantic.
Figure 14. Distribution and abundance (kg/standard tow) of cod in Northeast Fisheries Science
Center surveys during 1965-1969 and 2000-2004.
Interannual variations in water temperature do not seem to affect the location of spawning,
but do result in variations in the timing of spawning. The relationship is complicated, however,
because temperature alone does not act as a trigger for the initiation of spawning (Hutchings and
Myers, 1994). Location and timing of spawning may be of considerable consequence for the
subsequent growth and survival of the early life stages.
In a meta-analysis of nine cod stocks throughout the North Atlantic, including Georges Bank,
Planque and Frédou (1999) examined the relationship between recruitment and temperature. Increases
in temperature were shown to have a positive effect on recruitment in areas with temperatures close to
the lower tolerance limit of cod, and negative effects in stocks inhabiting areas at the upper part of the
temperature range. Stocks inhabiting regions characterized by intermediate temperature regimes
showed no detectable effect of temperature. Recruitment of the Georges Bank stock was classified as
neutral with respect to temperature change within the range of temperatures included in the analysis
(see also Drinkwater 2005, Brodziak and O’Brien 2005). Planque and Frédou (1999) and Drinkwater
(2005) determined the rate of change of recruitment with respect to change in temperature across
stocks and suggested that further increases in temperature beyond a mean annual bottom temperature
of 8.5°C would result in decreases in recruitment on Georges Bank. Brander (2000) examined the
abundance of one-year-old fish in relation to average surface temperature (April-June) for five
Northeast Atlantic cod stocks and concluded that the optimal mean annual bottom temperature for
recruitment lies in the range 5-7°C for these populations.
NECIA • 24
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
Temperature has also been shown to exert important influences on cod growth (Brander 1995).
Brander (1995) showed a strong effect of temperature on the mean weight-at-age of cod for 17 stocks
throughout the North Atlantic. Similarly, Rätz and Lloret (2003) discovered a close relationship
between fish condition and average bottom temperature. These differences in growth have important
implications for reproductive potential of the stock and the overall resilience of the population.
Condition was highest for those stocks inhabiting warmer-water habitats within the range of
temperatures examined. A key question is whether further increases in temperature for cod stocks off
the northeastern United States would negatively affect growth and reproduction. McKenzie (1934,
1938) suggested that cod ceased feeding entirely at temperatures above 17°C. Rose (2005) reported
ranges for cod feeding and spawning of 1-10°C and -0.5 to 6°C, respectively.
Temperature also significantly affects the development rates of eggs and yolk-sac larvae
(Pepin et al. 1997). Egg mortality is also significantly correlated with temperature (Pepin et al. 1997),
where lower temperature leads to higher mortality rates. Highest egg survival occurred at 6oC in
laboratory studies (Pepin et al. 1997). On Georges Bank, bottom water temperature is positively
related to both egg and larval survival, acting in combination with age diversity of repeat spawners
and egg distribution patterns within the range of observed temperatures (L. O’Brien, NMFS, Woods
Hole, MA, pers. comm.).
Atlantic cod landings in the Gulf of Maine and on Georges Bank have declined substantially
over the last decade. These declines have been clearly linked to intensive exploitation, but the role of
environmental conditions in concert with over-harvesting cannot be ignored. Results of a metaanalysis of North Atlantic cod stocks suggest that increases in mean annual bottom water
temperatures above 8.5oC will result in a decline in recruitment (the number of young surviving to
enter the fishery). Georges Bank, historically the major cod producing region in the northeastern
United States, had a mean annual temperature of 9oC during the period 1978-2002. This suggests
declining recruitment and reduced potential for recovery of cod populations over a substantial part of
the northeastern shelf if further increases in bottom temperatures are realized. The 8.5oC threshold is
also currently exceeded in the near-shore southern New England area, which has historically
supported small spawning aggregations of cod. Consideration of cod distribution patterns throughout
the North Atlantic suggests that a 12oC mean annual bottom temperature describes the practical limit
of cod distributions. If the bottom temperature projections under the high emission scenario are
realized, it is likely that this level will be surpassed in the near-shore southern New England region
and Georges Bank regions by the end of this century. The mean bottom water temperature in the Gulf
of Maine as a whole during the period 1978-2002 was 7.1oC and if the bottom temperature anomaly
projections for this region hold, the 12oC threshold will not be exceeded. Unknown at this time is
how other climate-related changes in the system would affect cod, such as through changes in prey
and predator communities.
North Atlantic Oscillation
Brander and Mohn (2004) examined the relationship between NAO and recruitment for nine
Northeast Atlantic and four Northwest Atlantic cod stocks. Their results indicate that the influence of
NAO is subject to a pronounced geographical pattern. A positive NAO has a negative effect on cod
recruitment in the North Sea, Baltic Sea, Irish Sea, Georges Bank and Eastern Scotian (Nova Scotia)
Shelf, and a positive effect on the stocks of Iceland and Labrador/Newfoundland. The magnitude of
the NAO’s effect is also linked to geographical location, depending on the relationship between the
NAO index and the underlying physical factors, such as wind speed and direction, cloud cover, and
temperature. Brander (2005) re-examined the NAO’s influence on cod recruitment and showed that
high recruitment is observed more often when spawning stock biomass is medium or high, while low
spawning stock invariably leads to a lower frequency of high recruitment. In years with a low
NECIA • 25
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
spawning stock biomass, a high NAO index resulted in a much higher frequency of low recruitment
levels. The consequence of this relationship between NAO, recruitment and spawning stock biomass
is that stock recovery from low biomass levels will be highly dependent upon environmental
conditions. Presently, spawning stock biomasses of most cod stocks are at sharply reduced levels
(Brander 2005), and it is interesting that efforts to restore the severely reduced cod stocks in the
northeastern U.S. over the past decade of positive NAOs have met with marginal success.
Prey Availability
The availability of suitable food items at critical life history stages can exert considerable
influence on survival during the early life history of cod. Cod spawning is relatively constant between
years with respect to both location and timing. However, the spawning of Calanus finmarchicus, a
principal prey item of larval and pelagic juvenile cod, is highly temperature-dependent and may vary
by as much as six weeks between cold and warm years (Ellertsen et al. 1989). The result of this
temperature-dependent copepod production and advection is a mis-match in timing of peak larval
production in relation to prey production in years with warm and cold environmental temperatures.
Lynch et al. (2001) modeled larval cod growth in relation to larval fish trophodynamics,
zooplankton population dynamics and hydrodynamic turbulence. The smallest larvae are most
susceptible to variations in prey availability and their feeding rates are negatively influenced by
turbulence. In contrast, larger larvae benefit from turbulence-enhanced encounter rates and their
growth rates depend strongly on the distribution of both C. finmarchicus and copepods of the genus
Pseudocalanus.
Retention
Successful recruitment is dependent on retention of early life stages within productive areas
characterized by appropriate environmental and feeding regimes. On Georges Bank, the location and
depth of spawning together with wind-driven advection have important impacts on the distribution of
the larvae (Lough et al. 1994, Werner et al. 1993). Eggs and larvae from the spawning grounds on the
northeastern peak of Georges Bank are advected along the southern flank of Georges Bank in the
general clockwise circulation, concentrating between the shallow, permanently mixed area over the
bank and the seasonally stratified area over the shelf-slope (Lough 1984, Lough and Bolz 1989,
Lough et al. 1994, Mountain et al. 2003, Serchuk et al. 1994). Larvae remaining in the surface
Eckman layer (< 25 m) have a high risk of being advected off the Bank, particularly in years where
there is a strong along-shelf, east-northeastward wind stress. In contrast, larvae below the surface
layer appear to be advected along the southern flank of the Bank. Larvae shoal-ward of the 60-70 m
isobath have a higher probability of being retained on the Bank during spring than those further
offshore (Lough et al. 1994, Werner et al. 1993). The distribution and drift patterns within the water
column are essentially driven by convergence/divergence mechanisms in relation to stratification and
frontal activity (Lough and Bolz, 1989; Lough and Manning, 2001). Thus, climate-related changes in
wind and other factors affecting circulation on Georges Bank have the potential to affect cod
recruitment.
6. General Conclusions
Substantial uncertainty exists in projections of the effects of climate change in the marine
environment off the northeastern United States. Existing coupled Ocean-Atmosphere General
Circulation Models do not resolve circulation and other important dynamical physical processes on
NECIA • 26
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
continental shelves, and they exhibit known biases with respect to predicted water mass properties in
the northwestern Atlantic Ocean. We have attempted to reduce the effects of these biases by
calibrating model output against direct observation for the period of instrumental record, and then
considering only relative changes in predicted properties. That is, we used GCM model output from
present-day and future runs to estimate temperature change over time, and then added the predicted
difference to the current-day observations. The projections examined in this report should be viewed
as provisional estimates, and caution must be exercised in interpreting results. Previous attempts to
examine the effects of global climate change on marine resources in this region have utilized
projections of atmospheric temperatures (e.g., Drinkwater 2005) or sea surface temperature estimates
derived from GCMs (e.g., WWF 2005). We have attempted to refine these approaches by examining
potential changes in bottom water characteristics as a small step forward.
A full assessment of potential climate impacts on the marine environment of continental
shelves ultimately requires coupling of the results of Ocean General Circulation Models with finerresolution hydrodynamic models in a fully nested structure. Such coupling properly accounts for
responses of the shelf system to the combined effects of forcing at the boundaries with local forcing
and adjustments. In the northeastern U.S., such a nested approach is necessary to capture features
such as topography and vertical mixing which affect circulation and the distributions of temperature,
salinity, nutrients and plankton. This need is most pronounced in topographically complex areas such
as the Gulf of Maine. Our work using a perturbation approach to developing dynamically adjusted
fields for a fine-scale hydrodynamic model (HOPS) forced by anomalies derived from a GCM is an
initial step in this direction. Development of a fully nested model, however, will require much further
effort.
Regional models can incorporate other factors that we have not discussed here, such as
changes in precipitation patterns (both seasonal patterns and extreme events), cloud cover, wind
forcing, and advective exchanges with the neighboring ocean. To make further progress and
incorporate some of these other important variables, we recommend that NPZ (nutrientphytoplankton-zoolankton) or NPZD (add detritus) models should be coupled to the regional
circulation models. Such models would link climate change and remote forcing to the internal
plankton dynamics of shelf systems. More advanced models will be of immense value as we try to
predict, monitor and evaluate environmental change over the next century, and try to understand and
manage anthropogenic influences on the coupled natural and human ecosystem.
In examining cod and lobster as our examples, we neglected many other aspects of the marine
environment that we should mention briefly. Of practical and pressing concern is the question of how
climate forcing would affect marine invasive species that can substantially alter ecosystems. A
second, significant question relates to the temporal and geographical extent of harmful algal blooms
that can stop shellfish harvests (because the shellfish cannot be consumed) and affect human health
through non-consumptive means (for example, through aerosols). A third question is the effect on
endangered or threatened species and what, if any, action can be taken to mitigate the climate
impacts. Indeed, a warming climate might be expected to alter many parts of the marine biosphere,
with direct and indirect effects on the ecosystem and human interests, including commercial,
recreational, and aesthetic values. Effective conservation and management require that we
understand (and preferably anticipate) the factors driving system change.
Comparison of the projected impacts of climate change on cod and lobster indicates that both
are potentially vulnerable to population declines and losses of habitat in the southern portion of their
range if projected bottom temperature increases are realized, particularly under the high emission
scenario. Cod is at the extreme southern extent of its range in the northern Mid-Atlantic Bight.
Under the high emission scenario, cod is potentially vulnerable to a loss of productivity and thermal
habitat on Georges Bank, historically the dominant production region of the northeastern U.S. shelf.
Within the Gulf of Maine, cod are expected to exhibit a neutral to slightly negative change if the high
NECIA • 27
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
end of projected bottom temperature anomalies are realized by the end of this century. While lobster
reaches the southern extent of its range in the vicinity of Cape Hatteras, farther south than cod, it is
largely confined to deeper waters in the southern Mid-Atlantic Bight. In the northern Mid-Atlantic
Bight, temperature is already a problem in some areas, and continued warming will pose an
increasing threat to lobster populations over the southern New England shelf. On Georges Bank, the
highest densities of lobsters are concentrated in the submarine canyons on the continental slope,
where they are less vulnerable to temperature change in this region. In contrast, bottom temperature
anomalies within the range predicted for the Gulf of Maine, the dominant production region for
lobster, are potentially favorable for this species. These differences highlight the fact that, in general,
the potential impacts of climate change in the ocean will exert differential effects on different species,
and both declines and increases in productivity are likely to occur for different components of marine
communities within the region. Although the bottom water temperature predictions are tentative, we
note that differences between low and high emission scenarios are largest for the northern MidAtlantic Bight and Georges Bank.
An economic impact assessment of the effects of projected climate change on the target
species is beyond the scope of this report. The potential loss of thermal habitat with resulting loss of
productivity for both lobster and cod in the southern portion of the study area is substantial,
particularly under the high emission scenario. Although the loss of access to both these species would
be potentially economically significant in these areas, it is likely that some adaptationin the fishing
industry can be anticipated, principally though switching to alternative species. For example, an
increase in water temperatures in southern New England is potentially favorable for an increase in
blue crab (Fogarty and Lipcius, in press), and it is conceivable that some lobster fishers would switch
to the lucrative blue crab fishery if such an increase occurs. Similarly, possible increases in
abundance and productivity of sub-tropical finfish species in the northern Mid-Atlantic Bight (e.g.,
Sciaenids such as weakfish, spot, and drum) could potentially offset some losses of cod in the
southern extent of the range. Overall, the contribution of the landings of both lobster and cod in the
southern reaches of this study area are relatively low in relation to the total. Should large-scale losses
of cod on Georges Bank occur or should adverse impacts on lobster (not currently anticipated) occur
in the Gulf of Maine, the economic impacts would be substantially more serious.
Acknowledgments
We express our appreciation to Nick Wolff and Michelle Traver for data analysis and
Adrienne Adamek for GIS work. We are grateful for editorial reviews by James McCarthy and three
anonymous referees. The support and encouragement of Erika Spanger-Siegfried and the Union of
Concerned Scientists are also gratefully acknowledged.
NECIA • 28
Fogarty et al. 2007 • Climate Change and Marine Resources Impacts
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Appendix 1
Bias Assessment in GCM Model Output
To test for biases on the northeastern U.S. continental shelf, we compared model-predicted
temperatures with field data from NOAA Northeast Fisheries Science Center (NEFSC) survey cruises
from the Gulf of Maine. For each cruise in the NEFSC database an average temperature and salinity
was calculated for the water column and for 0-25 m and 104-133 m depth layers, to correspond with
model output.
Only cruises since 1991 were used, since that is when an electronic
conductivity/temperature/depth (CTD) instrument began to be routinely used for all hydrographic
data collection. The average date of the observations from each cruise also was determined. The
model values for the month containing the date of the observations were matched to the observed
temperature and salinity values for both layers.
For temperature in the surface layer over the ~15 year period, the model and observed series
had approximately the same annual means. However, model values were about 4°C too warm in
summer and about 4°C too cold in winter compared to the observations. In the deep layer over the
same time period, modeled temperatures were always unrealistically cold. In contrast to other
(broader geographical scale) analyses that suggest a fresh bias in prediction of salinity fields, model
surface and deep layers in the Gulf of Maine were about 1 PSU too high compared to observations
during the period of instrumental record.
The overly large seasonal cycle in the model surface layer temperature suggests that the
seasonal heating and cooling are not appropriately mixed into the deep parts of the water column.
This suggests that the model (or downscaling of the original model results to the interior of the Gulf)
does not properly represent the vertical physical processes in the Gulf. This could explain, at least in
part, why the model deep layer salinities are too high. The model values of 34-34.5 PSU are
appropriate for oceanic water outside the Gulf, but within the Gulf, vertical mixing with the lower
salinity upper layers reduces the deep salinity. The cause of the very low deep layer temperatures is
not obvious, as nowhere in the region, including outside the Gulf, are such cold temperatures
observed at depth. All other comparisons of model outputs for the surface layer with a hydrographic
data base revealed similar over-estimation of surface temperatures and a lack of resolution of the
horizontal temperature gradients that characterize this region. This was not surprising given the
importance of regional scale processes (tidal mixing and currents) in controlling water movements
and temperature distributions.
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