Download likely effects of global climate change on the purse seine fishery for

LIKELY EFFECTS OF GLOBAL CLIMATE CHANGE ON THE PURSE SEINE FISHERY FOR CAPE ANCHOVY
ENGRAULIS CAPENSIS OFF THE WEST COAST OF SOUTHERN AFRICA (SE ATLANTIC)
B.M. Clark1, S. Young2, A. Richardson3
1. Zoology Department, University of Cape Town, Rondebosch 7701, South Africa
2. Environmental Evaluation Unit, University of Cape Town, Rondebosch 7701, South Africa
3.
Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth, PL1 2PB, United Kingdom
Abstract
This paper explores the possible effect that changes in global climate under a 2x atmospheric CO2 concentration may
have on the fishery for Cape anchovy. Anchovy is one of the dominant small pelagic fish species in the Benguela
upwelling system, and form the target of an important pelagic purse seine fishery in the region. Anchovy dominated
catches in the region from the mid 1960s until fairly recently, generating revenues in the region of $50 million per
annum for the South African fishing industry. The anchovy has a well defined life history pattern with distinct spawning,
transport and nursery regions. Recruitment varies widely from year-to-year, controlled by both spawner biomass and
environmental factors prevailing during the spawning and post-spawning period. Wind appears to be the main driving
force, mediated through an “optimal environmental window”. Winds that are either too strong or too weak lead to weak
upwelling and little nutrient enrichment of the euphotic layer (the former) or excessive turbulence and offshore transport
(the latter) both of which compromise survival of anchovy eggs and larvae and hence recruitment to adult populations.
Short periods of calm (known as Lasker events) promote stabilisation of the water column and can dramatically
enhance conditions for first feeding larvae.
National Centre for Atmospheric Research’s (NCAR) Climate System Model (CSM) was used to assess the likely
effects of changes in wind fields over the southeast Atlantic on the Cape anchovy Engraulis encrasicolus. Some
changes in sea level pressure and wind patterns were evident between the current and double CO 2 scenarios as
projected by this model. Of greatest significance was a projected small increase in easterly and south-easterly wind
vectors during the spawning period (early and late summer). The effects of this on upwelling intensity is projected to be
small (<1% change), but changes in mean turbulence are more substantial, in the order of 10% decrease on the
spawning ground and 3% increase on the main feeding ground. The frequency of extreme wind and turbulence events
is also expected to decrease quite substantially (by 50-60%). Given that turbulence values in the southern Benguela
are extremely high relative to other upwelling systems around the world, it is hypothesised that this change is likely to
favour anchovy recruitment by enabling more effective larval feeding. On the other hand, the incidence of Lasker
events over the western Agulhas Bank, where most of the successful recruits originate at present, is expected to
decrease in the future (by 20-27% reduction) and is likely to be detrimental for anchovy spawning in the future. This
may be compensated for by the increase in frequency of such events (up by 16-60%) on the eastern Agulhas Bank (a
marginal spawning ground at present), but there is no way of knowing whether this will be the case or not. Changes in
other variables, such as water temperature or an increase in the intensity of the frontal jet could also play a further
unquantified role.
Introduction
The Benguela upwelling system, situated along the west coast of southern Africa is considered one of the most
productive upwelling systems in the world (Bakun 1996, Crawford et al. 1997). As is typical in such systems,
prevailing equatorward winds drive surface waters offshore through to a process of Ekman drift. Cool, nutrientrich water upwells into the photic zone to replace the surface waters thereby promoting growth of phytoplankton.
The abundance of phytoplankton in the Benguela upwelling region supports large standing stocks of zooplankton
and small pelagic fish, which in turn supports an important commercial purse seine fishery. Two species of small
pelagic fish, anchovy Engraulis capensis and sardine Sardinops sagax dominate in the Benguela region.
Together they generate U$50-60 million per annum for the South African fishing industry (Booth & Hecht 2000).
These two species show alternating periods of dominance in the Benguela, since the fishery began in the early
1950s. Landings by the purse seine fishery were initially dominated by sardines, but were replaced by anchovy
as the mainstay of the fishery in the mid-1960s (Crawford et al. 1987). Anchovy continued to dominate catches
until 1995 (Schwartzlose et al. 1999); since then sardine have been resurging although no clear patterns of
dominance have yet emerged.
Anchovy are highly susceptible to perturbations in the environment as they have r-selected traits, they feed on a
short plankton-based food chain, and their recruitment is controlled to a large extent by egg and larval survival,
which in turn is dependent on the prevalence of suitable oceanographic and atmospheric conditions (Hunter &
Macewicz 1985, Cury & Roy 1989, Bernal 1991, Bakun 1996). Inappropriate water temperatures, for example,
may cause gonad atresia, while larval retention and transport to nursery areas relies on prevailing water currents
and winds, and larval survival depends on the presence of elevated food concentrations in their immediate
surroundings (Lasker 1975, Bakun & Parish 1980, Hunter & Macewicz 1985, Bakun 1990, Bakun 1996).
Changes in the global or regional climate are likely to impact pelagic fish stocks and hence their fisheries. This
paper explores the possible effect that changes in global climate under a 2x atmospheric CO2 concentration may
have on the fishery for Cape anchovy. The time taken for CO2 concentrations to reach this level varies widely
(IPCC 2001) depending on what emission scenario is used, but for the purposes of this study are not important as
it simply serves to demonstrate the potential effects such a change may have on the fishery. The study from
which this information is drawn formed part of the South African Country Study on Climate Change, reported on in
more detail by Clark et al. (2000) and Young (2001). The anchovy was selected as the focus of this paper
because of its considerable economic importance and as its biology is comparatively well understood.
Life history of the Cape anchovy Engraulis capensis and relationships with environmental
variables
The life history of anchovy has been well studied in the southern Benguela and is considered to be well defined,
with distinct spawning, transport and nursery regions (Hutchings et al. 1998). A simplified depiction of the life
history is included in Figure 1. Spawning is centred on the western Agulhas Bank south of the subcontinent,
peaking in November each year. Eggs and larvae are carried from here to the nursery grounds on the west coast
by frontal jet currents, which lie on the boundaries of the upwelling centres. Young fish start recruiting to the
fishery after about five months (April each year) but recruitment peaks in the period May to August when fish
have reached a size of 8-10 cm caudal length. The fishery relies mostly on these recruits for their catches, with a
smaller proportion (30%) being made up of adults from the previous year. Acoustic surveys carried out in
May/June each year indicate that recruitment strength varies widely from year-to-year, primarily as a function of
spawner biomass, although it is also strongly influenced by environmental factors (Hutchings et al. 1998). In any
particular year, different factors appear to dominate, but wind seems to be the main driving force.
S
28 o
Namibia
South Africa
30 o
Nursery
32 o
34o
Upwelling
Spawning
Transport
C
Recruitment
Losses
16 o
18 o
in
Po
ape
t
pe
Ca lhas
u
Ag
20 o E
Fig. 1. Principle spawning and recruitment areas for anchovy in the southern Benguela
system in relation to the major upwelling centres (redrawn from Hutchings et al. 1998)
Cury and Roy (1989) analysed the relationship between annual pelagic fish recruitment indices and wind intensity
in various upwelling regions around the world, and found a dome-shaped relationship with maximum recruitment
at intermediate wind speeds (Fig. 2). On the left hand side of the curve, two factors limit recruitment. Firstly, low
wind speeds result in weak upwelling and little nutrient enrichment of the euphotic layer. This results in low
phytoplankton abundance, the major source of food for fish larvae (Blaxter and Hunter 1982). Thus, weak
upwelling can result in larval mortality. Secondly, low wind speeds result in low turbulence in the water column.
This lack of mixing reduces the encounter rate between larvae and food particles (MacKenzie et al. 1994).
Conversely, strong winds can also limit larval survival and hence recruitment. As the dominant wind in the
Benguela upwelling system is equatorward, increased wind speeds lead to stronger offshore movement of the
surface waters causing the loss of eggs and larvae from their preferred food-rich coastal habitat (Bakun and
Parrish 1980, Sinclair 1988). Strong winds also enhance turbulent mixing in the surface layer (Kullenberg 1971,
Serra et al. 1998), which causes patches of larval prey organisms to disperse (Lasker 1978, Theriault & Platt
1981) and makes it difficult for larvae to obtain sufficient food and hence increases mortality through starvation.
High wind speeds and turbulence can also compromise the ability of larvae to capture food particles (MacKenzie
et al. 1994), and can affect primary production (Huntsman & Barber 1977) by mixing phytoplankton out of the
euphotic zone (Steele 1974). The optimal wind speed for larval survival in most upwelling systems is 5-6 m.s-1
(Cury & Roy 1989) but peak spawning of anchovy in the Benguela upwelling system occurs when wind speeds
are in the range of 5-8 m.s-1 (Shin et al. 1998). Frequency of winds is also known to impact on survival of pelagic
fish larvae (Lasker 1975). Frequent winds produce turbulent conditions, which deepens the upper mixed layer
and disperses food to below threshold concentrations (Lasker 1975, Shelton & Hutchings 1990). Quiescent
events on the other hand, allow the water column to stabilise and phytoplankton to form concentrated patches.
This enables enhanced feeding for first feeding larvae, which are particularly susceptible to starvation (Lasker et
al. 1970, Wroblewski & Rickman 1987), and hence dramatically increases chances of survival. Continuous
periods during which wind speeds remains weak enough to ensure low wind mixing are termed ‘Lasker events’.
Optimal periods between wind events are considered to be in the range of 4-6 days, as shorter periods than this
can result in excessive mixing and longer periods can result in a decrease in productivity (Mendelssohn & Mendo
1987, Wroblewski & Rickman 1987).
LIMITING
FACTOR
OPTIMAL
ENVIORNMENTAL
RECRUITMENT
WINDOW
UPWELLING
INTENSITY
WEAK
TURBULENCE
MODERATE
UPWELLING INTENSITY
STRONG
TURBULENCE
LOW
HIGH
Fig. 2. The relationship between recruitment and both turbulence and upwelling intensity (after Cury and Roy
1989).
Global climate change scenarios used to assess likely impacts on anchovy stocks
Numerical models generally referred to as General Circulation Models (GCMs), provide the only quantitative
estimates of future climate change. These models are based on physical laws represented by mathematical
equations that are solved using a three-dimensional grid over the globe. In this study, the National Centre for
Atmospheric Research’s (NCAR) Climate System Model was used to assess the likely effects of changes in wind
fields over the southeast Atlantic on the Cape anchovy. The area and period of particular interest for this study
extended from 15° to 42.5°S and from 0° to 40°E and covered the period September to February, the time of
maximum spawning and larval first feedingperiod for anchovy. For each month, wind field maps were generated
for the present day, double CO2, and anomaly situations. Specific data on u and v components of wind were also
obtained from five grid cells within the larger area (Port Nolloth, Lamberts Bay, Cape Town, Western Bank and
Eastern Bank Grid Cells - Figure 3). Wind speed and turbulence were derived from these data – wind speed from
the square root of the sum of the u and v components squared, and turbulence by cubing the wind speed value.
Alongshore wind stress was also calculated for each of the grid cells as a proxy for upwelling, using the equation
 = CDa |Ũ|Ũ
where:
 is the vector wind stress at the sea surface
CD is the drag coefficient (here assumed to be unity)
a is the density of air (here assumed to be unity)
|Ũ| is the magnitude of the wind velocity
Ũ is the wind velocity along the coast
Numbers of days showing extreme wind speeds (i.e. wind speeds >15 m.s -1), extreme turbulence (turbulence
values >5000 m3.s-3) and the number of type A and type B Lasker events (5 and 6 consecutive days with wind
speeds less than 6 m.s-1 respectively) were also calculated and compared between the current and future
scenarios.
Likely changes in climatic conditions over the Benguela upwelling region
Fig. 3. Locations of the five grid cells investigated in the southern Benguela upwelling region
Certain marked changes in sea level pressure and hence wind patterns are evident between the current and
double CO2 scenarios as projected by the CSM global climate model. In the present simulation, the South
Atlantic High Pressure System (SAHP) and the continental trough, which dominate weather patterns in the
region, change in intensity and/or position from early to late summer (Fig. 4). In the future projected scenario
(double CO2), the SAHP is projected to intensify and move south and east of the subcontinent and that the
continental trough is likely to shift northwards, both of these effects being manifest during late summer. The
effects of this can be seen in the anomaly patterns (2x CO2 – 1x CO2), which display a general SW-NE pressure
gradient extending across the region (Fig. 4). The net result of these changes in pressure gradients is a
projected increase in easterly and south-easterly wind vectors during early and late summer (September and
January- February), but little change in mid summer (Fig. 5). Effects of this on ocean dynamics, exemplified by
changes in mean wind speed, numbers of extreme wind events (days with wind speeds >15 m.s -1), mean
turbulence, extreme turbulence events (days with turbulence values > 5000 m3.s-3), and frequency of Type A and
B Lasker events in the five grid cells studied, are summarised in Table 1. Mean wind speeds are projected to
decrease marginally in the southern areas (on the spawning grounds) but to increase further north (feeding
grounds). The number of extreme wind events and mean turbulence values are projected to decrease on the
spawning grounds but to increase further north, while extreme turbulence values are projected to decrease
throughout the region (except Lamberts Bay where these are already zero). Type A and B Lasker events are
projected to increase across all areas except the main spawning grounds on the western Agulhas Bank.
Table 1. Projected effects of a doubling of atmospheric CO2 on mean wind speed, numbers of extreme wind
events (days with wind speeds >15 m.s-1), mean turbulence, extreme turbulence events (days with turbulence
values >5000 m3.s-3), and frequency of Type A and B Lasker events in the five grid cells in the southern Benguela
upwelling region.
Eastern Bank
m.s-1
Western Bank
Present: 7.4
m.s-1
Cape Town
Present: 7.0
speed
Future: 6.9 m.s-1
Future: 7.3 m.s-1
Future: 7.6 m.s-1
Future: 8.1 m.s-1
Future: 10.0 m.s-1
Extreme wind
Present: 13 days
Present: 23 days
Present: 17 days
Present: 2 days
Present: 17days
events
Future: 12 days
Future: 16 days
Future: 17 days
Future: 3 days
Future: 11 days
Mean
Present: 586.5
Present:
Present:
Present:
Present:
turbulence
m3.s-3
658.0
values
Future:
Future:
Future: 670.2
Future: 712.8
Future:
528.1 m3.s-3
596.1 m3.s-3
m3.s-3
m3.s-3
1268.6 m3.s-3
Extreme
Present: 12 days
Present: 15 days
Present: 17 days
Present: 0 days
Present: 6 days
turbulence
Future: 6 days
Future: 7 days
Future: 7 days
Future: 0 days
Future: 3 days
Type A
Present: 25
Present: 22
Present: 12
Present: 8
Present: 3
Lasker
Future: 29
Future: 16
Future: 13
Future: 15
Future: 5
Type B
Present: 10
Present: 10
Present: 6
Present: 3
Present: 2
Lasker
Future: 16
Future: 8
Future: 8
Future: 8
Future: 2
738.5
m3.s-3
Present: 8.0
694.4
m.s-1
Port Nolloth
Mean wind
m3.s-3
Present: 7.7
Lamberts Bay
m.s-1
m3.s-3
Present: 9.9 m.s-1
1234.7 m3.s-3
values
events
events
Likely effects of climate on anchovy in the southern Benguela upwelling system
Projected changes in wind speed across the whole of the southern Benguela under a double CO2 scenario using
the CSM model are minimal (<1% change, Table 1) but do show some month-to-month variability (Fig. 5). The
effects of this on upwelling intensity and hence on anchovy recruitment and growth are thus projected to be small.
However, changes in mean turbulence are more substantial, with an ~10% decrease on the spawning grounds
and 3 % increase on the main feeding grounds. MacKenzie et al. (1994) notes that low turbulence can be
detrimental to recruitment by reducing the encounter rate between larvae and food particles. However, given that
turbulence values in the southern Benguela are extremely high relative to other upwelling systems around the
world, such as off the California and Peru coastlines where anchovy spawn successfully in turbulence conditions
<250 m3.s-3, this change is likely to favour anchovy recruitment by enabling more effective larval feeding. The
frequency of extreme wind and turbulence events are projected to decrease dramatically by 50-60% across the
entire southern Benguela . Little has been written on the effects of extreme events on anchovy recruitment and
survival, but it is conceivable that extreme events could severely impact anchovy larval survival, particularly in
years when they are already stressed due to unusually high turbulence or low food availability. The projected
reduction in the frequency of extreme wind and turbulence events on the fishery for anchovy is projected to be
positive.
Projected changes in the frequency of Lasker events vary considerably across the southern Benguela. The
number and frequency of these events are considered critical for anchovy larval survival (Lasker 1975) and are
hence of considerable interest here. The western Agulhas Bank, which is located over the principal spawning
ground, is projected to have a decrease in Type A and B Lasker events in the future (20-27% reduction) which is
likely to be detrimental for anchovy spawning in the future. This may be compensated for by the increase in
frequency of such events on the eastern Agulhas Bank (up by 16-60%). However, the eastern banks are
considered to provide fewer recruits to the fishery at present, due to higher predation losses from cannibalism as
the larvae move through the concentrations of spawning adults on the western banks on their way to the feeding
grounds up the west coast (Hutchings et al. 1998). There are also several other factors that may affect spawning,
larval survival and recruitment and may have a bearing on the fishery in the future. These include, for example,
the effects of changes in water temperature, and changes in the frontal jet current that anchovy are hypothesised
to rely on to bring eggs and larvae around the Cape Peninsula to the west coast in order to feed. Anchovy have
strict temperature requirements for spawning, optimal temperatures being in the range of 16-19°C (Anders 1965,
King 1977). Sea surface temperatures are influenced by turbulence and upwelling, and may rise above thi
preferred level for anchovy under conditions of reduced turbulence. Effects of any changes in the frontal jet are
also hard to predict, although a decrease in wind speeds would in all likelihood result in a decrease in the
strength of the frontal jet. Such a decrease could cause eggs and larvae to take longer to reach the feeding
grounds, thus increasing losses due to predation and cannibalism (Valdés et al. 1987). Conversely, an increase
in southerly or easterly winds could result in increased offshore flow, thereby promoting the loss of eggs and
larvae offshore. An increase in current strength may also have a similar effect to increased turbulence in that it
may prevent larvae from feeding effectively. These are all speculative comments and simply serve to highlight
the complexity of the situation and the difficulties in accurately forecasting the likely effects of global climate
change on the anchovy fishery.
Rather than serving to provide a definitive projection of the likely effects of climate change on anchovy population
and the associated fishery, this study serves rather to highlight our current incomplete understanding of the
complex factors that play a role in structuring populations of marine organisms, and the difficulties involved with
projecting the likely consequences of climate change on these species.
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