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. Clark^1^, S. Young^2^, A. Richardson^3^

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 CO~2~ 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, nutrient-rich 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
CO~2~ concentration may have on the fishery for Cape anchovy. The time
taken for CO~2~ 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.

![](media/image1.wmf){width="3.240972222222222in"
height="4.661111111111111in"}

> 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).

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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 CO~2~, 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

 = C~D~ρ~a~ \|Ũ\|Ũ

where:  is the vector wind stress at the sea surface

C~D~ is the drag coefficient (here assumed to be unity)

ρ~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 m^3^.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.

![](media/image2.png){width="5.25in" height="4.31875in"}

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# 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 CO~2~ 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 CO~2~), 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
CO~2~ -- 1x CO~2~), 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 m^3^.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 CO~2~ 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 m^3^.s^-3^), and frequency of Type A and B
Lasker events in the five grid cells in the southern Benguela upwelling
region.

+--------+-----------+-----------+-----------+-----------+-----------+
|        | Eastern   | Western   | Cape Town | Lamberts  | Port      |
|        | Bank      | Bank      |           | Bay       | Nolloth   |
+--------+-----------+-----------+-----------+-----------+-----------+
| Mean   | Present:  | Present:  | Present:  | Present:  | Present:  |
| wind   | 7.0       | 7.4       | 7.7       | 8.0       | 9.9       |
| speed  | m.s^-1^   | m.s^-1^   | m.s^-1^   | m.s^-1^   | m.s^-1^   |
|        |           |           |           |           |           |
|        | Future:   | Future:   | Future:   | Future:   | Future:   |
|        | 6.9       | 7.3       | 7.6       | 8.1       | 10.0      |
|        | m.s^-1^   | m.s^-1^   | m.s^-1^   | m.s^-1^   | m.s^-1^   |
+--------+-----------+-----------+-----------+-----------+-----------+
| E      | Present:  | Present:  | Present:  | Present:  | Present:  |
| xtreme | 13 days   | 23 days   | 17 days   | 2 days    | 17days    |
| wind   |           |           |           |           |           |
| events | Future:   | Future:   | Future:   | Future: 3 | Future:   |
|        | 12 days   | 16 days   | 17 days   | days      | 11 days   |
+--------+-----------+-----------+-----------+-----------+-----------+
| Mean   | Present:  | Present:  | Present:  | Present:  | Present:  |
| turb   | 586.5     |           |           |           |           |
| ulence | m         | 658.0     | 738.5     | 694.4     | 1234.7    |
| values | ^3^.s-^3^ | m         | m         | m         | m         |
|        |           | ^3^.s-^3^ | ^3^.s-^3^ | ^3^.s-^3^ | ^3^.s-^3^ |
|        | Future:   |           |           |           |           |
|        |           | Future:   | Future:   | Future:   | Future:   |
|        | 528.1     |           | 670.2     | 712.8     |           |
|        | m         | 596.1     | m         | m         | 1268.6    |
|        | ^3^.s-^3^ | m         | ^3^.s-^3^ | ^3^.s-^3^ | m         |
|        |           | ^3^.s-^3^ |           |           | ^3^.s-^3^ |
+--------+-----------+-----------+-----------+-----------+-----------+
| E      | Present:  | Present:  | Present:  | Present:  | Present:  |
| xtreme | 12 days   | 15 days   | 17 days   | 0 days    | 6 days    |
| turb   |           |           |           |           |           |
| ulence | Future: 6 | Future: 7 | Future: 7 | Future: 0 | Future: 3 |
| values | days      | days      | days      | days      | days      |
+--------+-----------+-----------+-----------+-----------+-----------+
| Type A | Present:  | Present:  | Present:  | Present:  | Present:  |
| Lasker | 25        | 22        | 12        | 8         | 3         |
| events |           |           |           |           |           |
|        | Future:   | Future:   | Future:   | Future:   | Future: 5 |
|        | 29        | 16        | 13        | 15        |           |
+--------+-----------+-----------+-----------+-----------+-----------+
| Type B | Present:  | Present:  | Present:  | Present:  | Present:  |
| Lasker | 10        | 10        | 6         | 3         | 2         |
| events |           |           |           |           |           |
|        | Future:   | Future: 8 | Future: 8 | Future: 8 | Future: 2 |
|        | 16        |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+

# 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 CO~2~ 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 m^3^.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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