Download Oceanographic Variability

Oceanographic Variability
Oceanography (e.g. water temperatures, currents, productivity, etc) and climate dynamics
have major influences on fish population dynamics and fisheries (Lehodey et al., 2006) in
addition to any fishery impacts. Ocean-climate systems have been shown to strongly
influence tuna fisheries in the Western and Central Pacific Ocean (WCPO) at various spatiotemporal scales and in different ways (Bour et al., 1981, Lehodey et al. 2003, Lehodey et al.,
2006). Changes in oceanography may influence vertical and horizontal movements of tunas
and other species, as well as eggs and larval survival. Individual tuna species display different
preferences (e.g. preferred temperature) and thus will respond differently to changes in
oceanography and climate (Fromentin and Fonteneau, 2001). While beyond the control of
fishery managers, it is important to take into account the influence of oceanography and
climate in order to better manage fisheries. Globally, tuna catches are highest in the western
equatorial Pacific warm pool, a region characterized by low primary productivity and the
warmest surface waters of the world’s oceans. However, the WCPO displays remarkable
dynamics in oceanography mostly linked to climatic changes (such as El Niño Southern
Oscillation, ENSO). In response, variations in tuna catches are reported at both regional and
domestic scales both seasonally and inter-annually. The major oceanographic processes that
impact on tuna distribution and abundance include in the WCPO are briefly discussed below.
1) Ocean processes that induce movement of water masses play on tuna distribution. Areas of
divergence or convergence of currents are of major importance as they induce physical
phenomena (upwellings, thermal fronts, eddies) that enhance local productivity and create
zones of forage availability. These, in turn, attract and concentrate tuna. A better knowledge
of these processes and their spatio-temporal variability remains a key issue for fishery
management.
2) Tuna movement is linked to horizontal displacement of surface isotherms and vertical
change in mixed layer depth that determine their surface habitat. In particular, east-west
migration of the warm pool-cold tongue pelagic ecosystem, should be considered carefully by
fisheries managers of the different countries. Analyses have demonstrated that inter annual
variability in the environmental conditions linked to climatic oscillation (ENSO) is evident in
the operation of the main tuna fisheries and the population dynamics of the tuna species.
ENSO affects tuna fisheries through environmental changes in the warm pool-cold tongue
system. During El Niño conditions, the distribution of purse-seine catch in the western and
central Pacific is generally displaced eastwards, indicating a spatial shift in the distribution of
tuna. There is an obvious impact for the Pacific Island Nations through variations in the level
of tuna catches in their Exclusive Economics Zones and consequently on their economical
revenue, according to the ENSO situation.
3) Contraction or extension of the warm pool also has a major impact on tuna recruitment
which varies among species. For tropical species, such as skipjack and yellowfin, El Niño
events would favour recruitment through extension of warm water spawning habitat, whereas
La Niña events would restricted the favourable spawning areas and reduce the recruitment.
For subequatorial species, such as South Pacific albacore, the opposite trends are found.
These phenomenons may be taken into account to provide some indication of likely future
catches within individual EEZs.
4) Global warming is also likely to affect regional tuna fisheries by raising average sea
surface temperature to levels currently experienced during El Niño and by increasing year to
year variability. Possible change in primary productivity in the tropical Pacific is also
hypothesized. These factors would affect distribution, abundance and catchability of tuna
fisheries, but further investigations are required to validate these assumptions.
El iño Southern Oscillation
El iño-Southern Oscillation (ESO; commonly referred to as simply El iño) is an
irregular climatic oscillation of 3–7 years that evolves under the influence of the dynamic
interaction between atmosphere and ocean (Philander 1990). “El Niño” usually refers to a
periodic warming in sea-surface temperatures across the central and east-central equatorial
Pacific (between approximately the date line and 120oW). In contrast, La Niña is
characterized by anomalously cool water in the central and east central equatorial Pacific.
ENSO also results in changes in the intensity and distribution of rainfall in the Tropics and in
changes in the patterns of sea level pressure and atmospheric circulation that affect many
areas worldwide.
Every El Niño is somewhat different in magnitude and in duration and this magnitude is
typically measured as the difference in air pressure between eastern and western Pacific (the
Southern Oscillation Index or SOI) (Picaut et al. 1996.). Prolonged periods (usually more
than 3 months) of increasingly negative SOI values (below-normal air pressure at Tahiti and
above-normal air pressure at Darwin) coincide with El Niño episodes whereas prolonged
periods of positive SOI values coincide with La Niña episodes.The SOI has been recorded
since the late 1800s, and Figure 1 provides a record of the SOI since 1950.
Figure 1 Southern Oscillation Index (SOI) based on the differences in air pressure anomaly
between Tahiti and Darwin, Australia. Dashed lines correspond to +/- standard deviation over
January 1951 to march 2009 period (Source: http://www.cpc.noaa.gov/data/indices/).
Ocean processes, fish distribution and movement
The ocean is comprised of horizontally and vertically moving water masses, each of which
can differ in their temperature, salinity, water pressure and other factors. These different
properties create different conditions or habitats for fish in different parts of the ocean. Each
fish species has evolved particular physiological and behavioral characteristics which allow
them to exploit specific conditions/habitats, rather than all conditions. Typically, if they move
outside these “habitats” then growth and survival can decline, hence fish tend to aggregate to
and move with their preferred habitats. For example, skipjack tuna are physiologically
adapted to warmer surface waters while bigeye tuna have physiological features that allow
them to exploit deeper colder waters (e.g. Brill et al., 2005).
Outside the influence of physiological restrictions, tuna and other fish will typically aggregate
to areas of high food (prey) density, whose occurrence are also determined by specific
oceanographic processes, which in turn are coupled to climatic processes. Areas where
currents meet and converge or diverge tend to have concentrations of food (or prey of large
pelagic fishes). Divergence of currents brings nutrient-enriched water from underlying cold
layer to surface to compensate the deficit induced by water displacement. The mixing of
warm and cool nutrient rich waters under the influence of sunlight leads to a process of
enrichment that result in the creation of local areas of high productivity (planktonic
“blooms”). The intense concentration of planktonic communities attract plankton feeding fish
which in turn attract the pelagic predators (tuna, billfish and sharks) that feed upon them.
Under the influence of convergent currents, the organisms from the divergence zone remain
aggregated in a large zonal band usually associated with a thermal front (Figure 2). These
features create zones of high food availability in otherwise barren tropical oceans
(Grandperrin, 1978). Therefore, seasonal and annual variations in current patterns and the
associated variation in the location and timing occurrence of upwellings, convergences and
divergences, have a major effect on tuna distribution. Oceanographic conditions also
influence the survival of eggs and larvae and therefore impact on recruitment to fish
populations (e.g. Bakun, 1996; Lasker et al. 1981). Results from statistical population
dynamics modeling (Lehodey et al., 2004) and related analyses (e.g. Langley et al., 2006)
point to a link between tuna recruitment, climatic fluctuations and oceanography.
Warm water
Cold salt water
t
Figure 2 Dynamics of currents and productivity (modified from Grandperrin, 1978)
The warm pool and cold tongue system
Tropical
Convergence zone
Warm pool
Cold tongue
Figure 3 Sea surface temperature (AVHRR Pathfinder) and chlorophyll a concentration
(Seawifs) in Western and Central Pacific Ocean detected by satellite in January 2000. Data
source: http://oceanwatch.pifsc.noaa.gov/las/servlets/dataset
The western equatorial Pacific is characterized by low primary production and warmer
surface waters (>28˚C) that typically show little seasonal variability (<1˚C). This water mass
is referred to as the “warm pool”. In the eastern and central Pacific Ocean, an upwelling
extends westward along the equator from the coast of South America. The water mass is
characterised by cold nutrient-enriched waters rising to the surface. This results in the
formation of a large zonal band in which there is high chlorophyll concentration that
correspond to the primary production. This band is commonly referred to as the “cold
tongue”. The warmer, low nutrient, low salinity surface saline waters from the warm pool
move seasonally eastwards under the influence of westerly wind events, and the area on the
eastern edge of the warm pool which encounters the westward advection of cooler waters
(cold tongue) is referred to as the tropical convergence zone. This is identified by a salinity
front or more approximately by the 29°C isotherm (see Figure 3). The location and extent of
the convergence zone is determined by the relative movements of the warm pool and cold
tongue waters, which are driven by equatorial winds, which in turn vary in direction and
strength under different climate conditions.
Impact of El ino phenomena on the warm pool-cold tongue system and related fisheries
Sea Surface Temperature
January 1999
January 1998
Chlorophyll a
January 1999
January 1998
Figure 4 Thermal structure and productivity of the equatorial Pacific during example El Niño
(January 1998) and La Niña conditions (January 1999), including representation of mean sea
surface temperatures (Source: http://poet.jpl.nasa.gov/) and chlorophyll a ( Source:
http://www.giovanni.gsfc.nasa.gov)
In an average situation, the convergence zone on the eastern edge of the warm pool oscillates
around longitude 180°, but very large zonal displacements (i.e. horizontal or longitudinal
shifts) occur in correlation with changes in the ENSO signal, sometimes over 50° of
longitude.
During El iño conditions – these are characterized by the expansion of the warm pool
eastwards (Figure 4), resulting in warmer than average waters in the central and eastern
Pacific, higher rainfall in that region, and cooler than average waters in the western Pacific.
During La iña conditions – these are characterized by stronger Pacific Trade winds, the
contraction of the warm pool into the equatorial western Pacific, higher rainfall in the western
Pacific and lower rainfall in the eastern Pacific (Figure 4).
The equatorial divergence that occurs within the SEC induces a shallow thermocline (~ 50 m)
in the eastern Pacific that deepens progressively westward (~150 m in the warm pool).
However, during the eastward displacement of the warm water masses accompanying an El
Niño, the thermocline deepens in the central and eastern Pacific while it rises abnormally in
the western Pacific (Figure 4). Conversely, during La Niña the warm pool is confined to the
extreme west of the equatorial Pacific (Picaut et al., 1996), the thermoclime deepens in the
western part while it rises in the eastern part.
The largest proportion of the tuna catch (mainly skipjack) in the Pacific Ocean is taken within
the warm pool area. Surface tuna fisheries, particularly purse-seine fisheries targeting
skipjack, appear to respond to the movement of the warm pool (Figure 5). Large scale
migration of tropical tuna in the western central equatorial Pacific have been correlated with
the position of the oceanic convergence zone, produced where the warm pool meets the cold
tongue. This nutrient-rich zone supports high concentrations of secondary productivity (small
fishes) in a band several hundred kilometres wide along the eastern edge of the warm-water
pool. Tuna are likely to seasonally follow this convergence zone to remain in waters with
relatively high concentrations of prey species in conditions suitable for reproduction.
Variations in tuna catches are reported at regional and domestic scales both seasonally and
inter-annually and are related to east-west migration of the warm pool-cold tongue pelagic
ecosystem. For longline fisheries, the vertical change in the thermal structure during El Niño
(La Niña) events results in the rising (deepening) and vertical extension (contraction) of the
temperature habitats of yellowfin and bigeye, thereby affecting the catchability of the species.
Figure 5 Examples of purse seine catches distribution in the equatorial Pacific during an El
Nino episode (top) and La Niña episode (bottom). Blue-unassociated sets, red-drifting FADs,
yellow-logs, green-anchored FADs.
Climate change and risks for fisheries
Climate change is currently one of the most scientifically debated and controversial issues.
While understanding climate change will be important for fisheries, it is clearly an even larger
issue for many Pacific Island states. In this section we will briefly discuss two of the more
widely accepted outcomes of climate change for oceanography and tuna fisheries. Clearly this
is an area of importance and a considerably active research so we expect to be able to greatly
expand upon the potential impacts in the coming years.
In considering the relationship between tuna, tuna fisheries and the climate-ocean system, we
must also consider the possibility that this relationship will change (or is changing) as a result
of climate change. The key question currently is – “How will it change?”. While there is
currently significant uncertainty about the implications of climate change for tuna fisheries,
considerable research is underway, and a number of observations and hypotheses have been
proposed.
Firstly, increases in global average sea temperatures have been observed. Sea surface
temperatures affect the patterns in atmospheric pressure, which in turn are responsible for
wind generation. It has been hypothesized that changes in wind generated surface currents
would not only modify the weather conditions but also alter the timing, location, and extent of
the upwelling processes upon which much oceanic primary productivity is reliant. Some early
studies suggest that primary productivity in tropical oceans would decline due to increased
stratification between warmer surface waters and colder deeper water (and consequent
reduction in upwelling) (Bopp et al., 2001). The implication of this is that a decline in the
upwelling system of the central and eastern equatorial Pacific may lead to reduced
productivity in that region which currently moves with currents to the western equatorial
Pacific and is a critical feature upon which our tuna stocks depend. This decreasing
production could lead to a decline in tuna abundance, particularly in the bigeye and adult
yellowfin population (the species targeted by the longline fleet).
A second observation is that El Niño events appear to have become more frequent in recent
decades (possibly linked to climate change) (Timmermann et al., 1999; Timmermann et al.,
2004) and are associated with an eastward shift of the major tuna resource (e.g. skipjack) in
the WCPO (Lehodey et al. 1997). Currently total catch does not appear effected by ENSO
related shifts in the stocks. It has been suggested that climate change may, however, imply
more permanent El Niños, which are likely to increase the annual fluctuations of the spatial
distribution and abundance of tuna. First simulations of global warming on skipjack and
bigeye tuna suggest a strong shift in the populations towards the eastern Pacific (see Figure
5). This is related to the weakening of the equatorial upwelling and equatorial current systems
predicted by the IPCC models. There are implications of this scenario for purse seine fleets.
Distant water fishing fleets should be able to adapt to changes in the spatial distribution and
abundance in tuna stocks. But domestic fleets would be vulnerable to fluctuations of tuna
fisheries in their Exclusive Economic Zones. More research is required to confirm current
theories regarding the implications of climate change for tuna fisheries.
Figure 6 Global warming and predicted changes in spatial distribution of bigeye and skipjack
adult biomass under the IPCC (SRES A2) scenario. (From Lehodey et al. 2008.).
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