Suid-AfrikaanseTydskrifvir Weleflskap Vol. 90 Maar11994 RESEARCH ON AGULHAS BANK 135
Predators of the Agulhas Bank
M.J. Smale, N.T. Klages, J.H.M. David
land V.G. Cockcroft
Port Elizabeth Musewn, P.O. Box 13147, Humewood, 6013 South Africa; and lSea Fisheries Research Institute, Private Bag X2, Rogge Bay, 8012.
The current knowledge of the population status and trophic inJeractions of four groups of predators, fish, seabirds, seals and cetaceans, on the Agulhas Bank is reviewed. Predatory fish are divided according to the communities they influence; pelagic, rocky reef and soft substrata. Although the biomass of fish on the Bank and their consumption are poorly known, the fish fauna is diverse. Judging by the catches made by demersal and line fisheries, the large stocks on the Bank must have a signijicanJ impact on each of the communities they prey on. The non-commercial species are particularly poorly understood. Resident seabird numbers (14 species) are relatively well known, their diets determined and con-sumption levels calculated. In contrast, these factors are poorly understood for migrant seabirds (77 species). The only pinniped, the South African fur seal, is a conspicu-ous component of the Bank, feeding throughout its area. Seal numbers, feeding patterns and consumption rates are well documented for the western Bank, but know-ledge of the easterly populations is scant. Some 32 ceta-cean species are known to occur in this region. This fauna is poorly studied and liule is known of the various populations or their dietary preferences. It is concluded thai this large marine ecosystem is poorly understood and this provides a major chaUenge for marine scientists.
The Agulhas Bank is a roughly triangular protrusion of the continental shelf bounded in the west by the cold and produc-tive Benguela Current and in the east by the warm Agulhas Current.l The benthic topography is varied and includes both soft sediments and large areas of rocky reef.2.3 The variety of habitats and oceanographic conditionsl results in high produc-tivity: which, in turn, supports a rich and diverse fauna.
In this article we review current knowledge of the predators of the Agulhas Bank. These are treated as four major groups; fishes, birds, seals and cetaceans.
Predatory rlSh
The high diversity of the fish fauna of the Agulhas Bank has been noted by Smale,4 Japp el
al.J
and Smale et al. (in prep.). Given this diversity of species and the limited amount of biological work that has been done on the Agulhas Bank, observations on this component of the predators must be seen as preliminary, particularly because of the difficulty in obtain-ing quantitative information on fish populations.The total biomass of fishes on the Agulhas Bank has been estimated to be 2.3 million tonnes, although this is believed to be an underestimate? Attempts to quantify the biomass of different trophic components of the fish fauna have not been published because of the limited amount of reliable estimates of non-<:ommercial species and the consequent need for dubious extrapolations. No attempts have been made in this article to quantify the biomass of predators.
Smale4 reviewed the fish predators of the Cape west coast and the Agulhas Bank and defined three broad groups: the pelagic community, which lives and generally feeds in the water column well above the substrata; the reef community, which inhabits rocky areas with the associated biota; and the
soft substrata community, which comprises benthic and epi-benthic biota.
Pelagic predators
Small pelagic c1upeoids and horse mackerel Trachuru trach-urus capensis are particularly important prey of this group.4 Predators include the highly migratory tuna, for ex~le Thun-nus albacores, T. obesus and KatsuwoThun-nus pelamis,s. geelbek Atractoscion aequidens, snoek Thyrsites atun and yellowtail Seriola lalandi,f>-l0 Other pelagic predatory teleosts include elf Pomatomus saltatrix and leervis Lichia amia, both of which are generally inshore species.6
Sharks which feed on pelagic fish and cephalopods include bronze whalers Carcharhinus brachyurus, dusky sharks C. ob-scurus and smooth hammerheads Sphyrna zygaena.lI Nwner-ous other fish and sharks that prey on the pelagic community have been little studied.4
Whereas clupeiform fishes often dominate the diet of pelagic predators, the squid Loligo vulgaris reynaudii is abundant on the Agulhas Bank 12 and is an important prey item of several species of fish.6.1I This is most marked on the inshore spawn-ing groundsY
The dynamics of predator-prey relationships within the pelagic system vary in relation to time, locality and size of predator. Furthermore, the availability of suitable prey has a marked influence on predation patterns and many of the preda-tors are migratory.
Rocky reef communities
Inshore
«
40 m) fish communities are species-rich and have complex trophic relationships.4.l4.1S Most of the work on feed-ing has been done in the eastern Bank region, although Nepgen7•16 has studied part of this fauna on the western Bank.
In shallow water
«
30 m), endemic sparids dominate the larger reef teleosts and fill a wide range of trophic niches from herbivores to piscivores?·1f>-21 Some species, however, such as seventyfour Polysteganus undulosus and red steenbras Petrus rupestris, are found on reefs at the edge of the continental shelf, at least as adults.22.23 Large red steenbras also occur inshore on the Agulhas Bank on some reefs where they have not yet been heavily exploited.23On the western part of the Agulhas Bank, engraulids, other fish, crab megalopae and juvenile inshore squid are important components of the diet of the carpenter, Argyrozona argyro-zona,I6 a piscivorous schooling sparid which is most conunon-Iy found on deeper reefs (> 30 m). On the eastern part of the Bank its prey includes pilchard and anchovy.8
The community composition and trophic interactions of deeper reef fish fauna is poorly known. With the exception of panga Pterogymnus laniarius,24-27 estimates of abundance of reef fishes are generally lacking, so it is not possible to assess their predatory impact. Of the known fauna, two species of horsefish, Congiopodus torvus and C. spinifer, the fingerfm Chirodactylus grandis and the jacopever Helicolenus dactylop-terus are all found in deeper communities. These species feed mainly on invertebrates, with the possible exception of H. dac tylopte rus.2B
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136 RESEARCH ON AGULHAS BANK Soft substrata
There have been several investigations of the fishes of soft
substrata on the eastem Bank, including subtidal
stud-ies.30 Many of the surf-zone are considered
opportun-istic.30•31
Nearshore littoral areas are used numerous teleost tors,32,19 including juveniles of top predatory teleosts such as kob Argyrosomus Jwlolepidotus~3 Inshore areas are important
as nurseries for juveniles of predatory as well as
some small species which feed mainly on small crustaceans
such as 19.32,34 Mysids are partiCUlarly dietary
items 10 many of the smaller members of the nearshore com-munityI9.30,32 and are patchily distributed spatially and
tempor-35 Benthic are numerous and
include the sandshark RhiIWbatos annulalus,36 stingrays37 and skates,l8 which feed on both invertebrates and fish. The
feed-ing of the larger elasmobranchs, such as the Zambezi
shark, Carcharhinus leucas, is poorly known.
In deeper waters
70
m) the commercial trawling fleetconcentrates mainly on six teleosts28 and, of these, hakes
Mer-luccius spp. are the most dominant piscivores.S,28,32 The
com-munity is much more complex, and is made up of at
least 46 of chondrichthyans,
123
teleosls and22
cepha-lopods from the Agulhas Bank (Smale et al.SO
) , and biomass
estimates have been made for at least part of this fauna? Of 19 teleosts taken in demersal trawls off the Benguela and ... ,..., Bank, seven, including buttersnoek Lepjdopus
cauda-IUS, and the dory Zeus capensis, preyed mainly on pelagic
39 Twelve which included the gumards
CheU-donichthys and C. queketti, exploited the
benthic/epi-benthic 29
Future research
Knowledge of fish of the Agulhas Bank is best for
commercial whilst non-commercial species and the
inhabitants of all reefs have been ignored. Despite
the ecological and economic importance of areas east of Cape
Agulhas. have generally received little attemion compared
10 the ecosystem. Consequently, biomass estimates
of each species are either imprecise or non-existent except for a few commercially exploited trawl fish?·27 This clearly needs
10 be addressed in atlempt to understand fisheries and their
in that the complexity and open nature of
the Agulhas Bank system, it is difficult to foresee when adequate quantification of the fish fauna would be made, given the present level of work in the area. The interactions are complex and affected by the size structure of the different
movement of predators and prey and the
variation of the ecosystem. Although of nrp'rI"'·" ... into guilds would be a first step in the exercise, basic
informa-tion on biomasses of the component is Sadly,
there is no research attempting to resolve these
problems.
The development of methods to sample reef at depth
also needs attention. studies would advance
consider-ably if a technique to avoid eversion of stomachs of deep-living teleosts could be developed, In addition to collecting data on commercial landings, information on discarded from commercial operations should be documented to help
develop an understanding of the impact of each Studies
on selectivity, rates and rates of digestion of
more local species are needed. as are studies of
communities. With such studies. it is likely that fisheries impacts on communities would be better understood. This would be of particular importance on the Agulhas Bank, where
there is a more diverse mix of and consequently
inter-SouJh African Journal Vol. 90 March 1994
actions are likely to be more complex than those found in the
oenguCJlil ecosystem.
Seabirds
Seabirds, birds that can or must obtain their food from the
sea, are a numerous and component of marine life
off South Africa and Namibia. Some 77 migrant species have been observed to date,40 in addition to the 14 species of seabird that breed in the area, half of which are endemic,"1 The aggre-gate of the best counts and estimates currently available amounts to a total of 4.6 million individuals off southern
Afri-ca.42 The of all resident seabird species bas been
slUd-ied at the nest and to some extent also at sea, but our know-ledge of the ecology of the non-breeding seasonal migrants is comparatively meagre.43 Because seabirds are putative compet-itors with fisheries, their role as cortsumers has come under investigation several times in recent
Some 40 of the 91 seabird recorded in southem
African waters occur regularly on the Agulhas Bank (Table 1), the rest vagrant or very rare.43 Twelve of the 14 resident species breed colonially in the region, either on islands or at secluded mainland sites for protection from predators. The
most important colonies 10 the Bank are
Table I. List of seabirds occurring regularly 00 the Agulhas Bulk . Resident spedes African penguin Cape gannet Cape cormOfllllt Whilebreasled cormorant Bank cormorant Crowned cormorant Kelp gull Hartlaub's gull Swifltem Roseale tern Damara lem lem Migrant species Wandering alblltross Shy albatross Blackbrowed albatross Yellownosed albatross Nonhem Gillnt petrel Southern Giani petrel Pintado petrel Greatwinged Softplumaged Broadbilled prioo Salvin's Antarctic prion Whilechinned petrel shearwaler Great shearwaler SOOI y shearwalel' European storm Wilsoo's storm
Leach's storm petrel Subantarctic skUll Arctic skua LOflgtallOO skua Sabine's gull Sandwich lem Common lem Arctic tern Antarctic lem Litlle lem Spheniscus de_rsus Morus capensis PhalaCTccorQJC capensis PhalacrccorQJC carbo PhalacrOCf)rQJC IUglectus PhalacrccorQJC corlJflOlus Larus dominicanus Larus hartlaubij Sterna bergii Sterna dougaliii Sterna balaellQl'JUfI HydroproglU caspia D icmedea exlllans Dicmedea cauta Dicmedea melanophris Dicmedea ch/(Norhyneluu MaCToMcles halli MaCTolUclu gigiallleus Daplion capense Plercd:romtl mtlCroplera Plercd:romtl mol/is P achyptw villala Pachyplw sa/vini P achyplw deso/aJa Proceilaria lUquiJwctialis Ca/olUctris diomedea Puff- gravis Puff- grise", HydrobaJes pelagicus Oceaniles oceanicus Oceancd:roma leucorhN CllIltaraCla OIII4raica Stercol'luius DOI'uiticlU Slercorarius longicaudus Larus sabUri Sterna UlJldvicellSis Sterna hirlUldo Sterna paradisea Sterna lIi/lala Sterna a/bifrOM
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Suid-Afrilcao.nse Tydskrif vir Wetenskap Vol. 90 MaartJ994 situated in False Bay, on Dyer Island, in Mossel Bay and in Algoa Bay. Colonies on islands off the western Cape are the source of endemic species which frequent the Bank during their non-breeding season. Most migrants derive from either the southern (e.g. the Procellariiformes) or the northern cold regions (e.g. the Palaearctic terns), and only a few tropical seabirds are regular visitors.
Seabird numbers in colonies and roosts are well documented (see Crawford et al.42) but there are few published estimates of seabird densities at sea off southern Africa. There is a particu-lar dearth of data for the Agulhas Bank, with only two ship-board surveys performed to date,45.46 the latter restricted to the western part during winter. Using this information as well as counts at breeding colonies and roosts, Crawford et aI.42 esti-mated that approximately 1.6 million seabirds can be assigned to southern South Africa, between Cape Point (34°30'S, 18°30'E) and East London (33°00'S, 28°00'E), from the coast out to the SOO-m bottom contour. The Cape cormorant, African penguin and Cape gannet were the most numerous (97%) of the resident species. Prions, shearwaters, white-chinned petrel, common/Arctic terns and stormpetrels domina-ted the migrants in terms of numbers (87%).
The structure of the seabird assemblage on the Agulhas Bank fluctuates substantially, depending on the season, because of migration and dispersal. For instance, Cape cormor-ants and Cape gannets take part in an extensive longshore migration each year in winter when following pilchard shoals moving up the coast to Natal in the 'sardine run'.41 It is gener-ally believed that the assemblage is richer in species and more numerous during winter because Southern Ocean birds remain near their breeding sites during the austral summer. There are, however, few published data (e.g. ref. 48) to prove that this is so on the Agulhas Bank. Indeed, seabird biomass estimates derived from shipboard transects (ref. 45, total swvey area 69 S28 kIn2), revealed higher values in summer (l 424 t) than in winter (l 298 t). Visitors from the nonhern hemisphere and from the Southern Ocean contributed more to standing bird biomass in summer (73.9%) than in winter (SI.S%), the balance being made up by resident species. Contrary to these findings are the estimates of Crawford et al.42 who calculated, for southern South Africa between Cape Point and East London (total area 130 000 Jan2), that the majority of migrants, 1.473 million, were present in winter versus 0.812 million individuals in summer. An extension of the Southern African Bird Atlasing Project49 to include the continental shelf would provide much-needed facts on seasonal variation in the seabird assemblage.
Information on seabird diets is best for those species that are easily caught at colonies, such as Cape gannets and African penguins, whereas dietary data for migrant species are usually scanty due to difficulties in sampling. Berruti et al.43 reviewed what is Imown about diet and feeding ecology, categorizing seabirds by foraging zone, feeding techniques, geographical area and seasonal occurrence. Three broad foraging guilds are recognizable, based on where seabirds encounter their prey: inshore and benthic, epipelagic, and scavenged, including trawler offal. Members of the first guild are fewest in numbers and overall impact. Most seabirds of the Agulhas Bank feed in the top metres of the water column, largely on epipelagic fish.
Anchovy and pilchard are the most important prey species, while horse mackerel, saury Scomberesox saurus scomberoides and redeye herring Etrumeus whiteheadi play a lesser role. Many migrants, but also Cape gannets, rely on offal provided by fishing boats. Zooplankton and micronekton are important prey constituents for the smaller species, such as prions, storm petrels and terns.
RESEARCH ON AGULHAS BANK 137
Crawford et al.42 have estimated that seabirds consume approximately 14000 t of carbon (or 140 000 t of wet food) per year on the shelf (out to the 500-m bottom contour) between Cape Point and East London, on the basis of bioener-getic equations and abundance data collected at colonies and
roosts, augmented with density figures from surveys. More than half of this (56%) was consumed by resident seabirds and
consumption was greater (57%) during the winter than in the sununer half of the year. Seven species (Cape connorants, whitechinned petrels, Cape gannets, sooty shearwaters, African penguins and blackbrowed and shy albatrosses) took 8S% of the total food eaten by seabirds.42 Duffy et aI.45 calculated an annual total food consumption of 72 868 t in an area from the shore to the 200-m depth contour between Cape Point and
Knysna (34 ° OS'S, 23 ° 2S'E). The estimates by Duffy et aI. and Crawford et al. are very close to one another when stand-ardized - 1.08 versus LOS t Jan-2 yr-! - despite differences in methodology between the two studies. Because the consumption of all piscivorous marine taxa is unknown, it is difficult to assess the relative impact of seabirds, although Duffy and Siegfricdso suspect that seabirds are not major consumers of marine fish resources.
There is strong evidence that the numerical composition of the southern African seabird community has changed quite substantially during the last century. Adams et a1.5! have
argued that, above all else, these large-scale changes were responses to anthropogenic influences and were not directlr related to natural changes in food availability. Crawford5 examined the population trends of the Cape cormorant, African penguin and Cape gannet, in the face of changes in the abun-dance of pilchard. There has been an increase in the numbers of Cape cormorants,S3 whereas both African penguins and
Cape gannets have shown a marked decline off the western coast of southern Africa since the 1950s, with the result that the majori~ of penguins and gannets now reside on the Agul-has Bank. .55 Similar data on changes in the numbers of migrants visiting the Agulhas Bank each year are not available. However, the larger Procellariifonnes. especially albatrosses. are likely to be less abundant now than before the advent of the long-line fishery in the Southern Ocean, which is responsible for large-scale killing of these birds.56
Future work
Resident seabirds are ideal subjects for long-term studies. Ongoing work should be continued at colonies such as· Bird Island. Algoa Bay, to study the influence of inter-annual environmental fluctuations and long-term climate change on
resident seabirds. These birds also provide indirect evidence of changes in epipelagic prey resources and may be useful for monitoring changes in exploited species independent of fishery data.
Competition for space between species needs to receive attention and particularly the effect of habitat modification. for example by guano scraping. When this is better understood in the eastern Cape, management of the species should improve. Seals
Seals have long been regarded as major predators of marine resources in the Benguela and other ecosystems.51 The South African (Cape) fur seal, ArctocephaJus pusillus pusillus is the only indigenous breeding pinnipcd in southern Africa and is by far the most abundant local marine mammal. Its potential impact on commercial and other fish and cephalopod resources is considerable and the conflict with commercial fishing inter-ests is well documented.s~
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138 RESEARCH ON AGULHAS BANK
Distribution
The South African fur seal is distributed around the south and west coasts over about 3 000 kIn of coastline and breeds at 25 colonies from Algoa Bay in the south-east 10 Cape Cross (Namibia) in the north-west. The bulk of the population is distributed along the west coast, only about 100/0 occurring on
the south coast between False and Algoa at five island
colonies. Three of these False Bay, Geyser Rock
and . Quoin Rock) occur in the westerly Agulhas! Atlantic
mixing and two (Seal Island, Mossel Bay and Black
Rocks, Algoa in the east, inshore of the warm (20-25°C)
Agulhas Current.
Population size
The seal population is censused primarily by aerial photo-graphy of black pups at breeding colonies. The total population is difficult to estimate because many animals are at sea at any
one time, but is considered to be the number of multiplied
by 4.5 (ref. 62) at under a of harvesting.
Using more recent data on mortality and pregnancy data, Wickens and Sheiton6S suggest that this factor is too low and
should be around 5.0 under a harvesting and even
high-er if thhigh-ere is no harvesting.
All available aerial estimates of pup numbers for south-coasl
colonies (Table 2) show a decline in pup from 1971
to 1982, in contrast to the west-coast population, which expanded at a rate of over 3.7% per annum during this
66 Since however, the population has resumed
growth (at a rate of about 4.5% p.a. up to 1989). The popula-tion growth since 1971 has apparently been entirely due to
growth at Seal Island, False Bay and Rock. Accurate
estimates for Mossel and Algoa Bay are because
of the extra expense of flying to these two colonies. Because the five colonies on the Agulhas Bank are all on small islands, space for breeding seals must be a limiting factor and will place an upper bound on the number of pups that can be reared. Consequently, unless new breeding colonies are founded, the total south-coast population may be expected to fluctuate within relatively narrow limits. In this to note that Robberg and Beacon Island
in Bay and the Islands of Jahleel, SI Croix and
Brenton in Algoa are all former seal-breeding colonies,
none of which has been recolonized. the current seal
Table 2. Summary of aerial surveys of SOUlh coaSI colonies: coonlS of
black pups oblllined from aerial photographs; laken between 16 lind 22 December each year.
1971 1976 1979 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991
Seal Isllll'ld Geyser False Bay Rock
14449 12312 8 188 8574 2679 6873 4099
Quoin Seal Island Blacic Rocks Rock Mossel Bay Algoa Bay 3744 1092 630 3234 I 177 380 ! 702 112 442 561 10017 9 151 I 074 899 II 010 12 116 5218 14105 !3 503 15484 13 898 17522 6954 8345 9584 8643 10187 12793 10 749 9651 II 522 I 644 1496 1 170 808 1 756 2041 800 1676 1 238 2367 TOIllI 25808 21566 13739 (21 702)* 17335 (30375)· (29947)*
·In cases where II survey was incomplete, the mOSI recent estimate for the miSSing colony has been used.
Journal. Vol.9O MQJ'ch /994
population is probably smaller than it was historically. The approximate total population size for the Bank area is 134 760, obtained by multiplying the most recent (1989) estimate of pup numbers (29 947, Table 2) by 4.5.
Seal harvesting
Harvesting may reduce seal population growth, but on the south coast harvesting of pups last took place in 1982, and the last bull was harvest in 1984. Between 1971 and 1984 a total of 47 002 pups and 5 095 bulls was harvested at all south-coast colonies.66•67
Seal flWvemenis
Studies of seal movements, determined Jrom the recapture of animals tagged as pups, show clearly that the predomirtant
movement by seals from the three colonies is
west-ward and northwest-ward.611 pups from these colonies were
harvested as far away as Cross in Namibia - a journey
of over 1 600 km. Others, recovered at sea as older animals,
were found on the grounds between Cape Point and
Lambert's and four were found mid-way between Cape
Cross and the River, illustrating how far these animals
move in their quest for food. Little is known about movements
from the two easterly particularly Black and
there have been only three recoveries of seals tagged at Mossel 611
Diet
Work on the diet of A. p. pusillus has been published by
Rand,57 David,60 Lipinski and David69 and CasUeyel al.70 Seal
diet is calculated from examination of the stomach contents of animals collected at sea on the feeding grounds. Most collect-ing cruises have been conducted on the west coast because the bulk of the seal population is found there, resulting in a relative scarcity of data for the south coast, where only three research cruises have been conducted since 1974. The results (Table 3) show thaI diet consists of about 85% teleost fish and
about 12% cephalopods, with a absence of rock
lob-ster and other crustacea. The reason for the difference from earlier estimales60 is that raw data have been re-analysed on a 'modified volume' basis using the actual weights of undigested
remains of different prey rather than the earlier
modified frequency-of-occurrence basis,71 which tends to over-estimate prey items such as squid and crustaceans.
The total consumption of each prey species over a year (Table 4) can be obtained by mUltiplying the number of
foraging animals (pups under one old do not but
are dependent on their mothers), the ration, by 365
days, by the proportion of each species in the diet.
1) Number of foraging animals
==
number of pups X 3.5.Hence, for 1969 2),29947 X 3.5 == 104 815 foraging
seals.
2) Daily ration [obtained from Equation 23 (growing marine
mammals) of Innes et al.,n] calculates a daily food
requirement according to the formula 18 O.123M X 0.8.
where 18 is ingested biomass day-I) and M is mean body
Table 3. Diet of South African fur seals on the southern COlISt of !.be Province sampled during cruises conducted in 1974. 1975 and 1985.
N Teleost Crst R. L. Elasm Other TOI.lIl Prey (g) 115 94907 13 453 5 6 3184 9 III S64
% of diet 85.1 12.1 0 0 2.9 0
Cpoda, Cephalopoda; Crst. Crustacea; R. L., rock lobster; Eas, Elasmo-branch; Prey (g). mass of undigested remains.
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Su,jd-Afrikao.nse Tydskrifvir Wetenskap Vol. 90 Maart1994
Table 4. Estimated annual consumption of seleaed prey by seals OIl the
south coast.
% of iden- % of lola! Annual
0011-Species tified fish 1 diet2 sumption
(1y
Anchovy 25.4 21.6 31 100 Horse mackerel 21.3 18.1 26060 Pilchard 17.7 15.1 21750 Round herring 0.8 0.7 1000 Chub mackerel 3.4 2.9 4200 Snoek 23 2.0 2880 Hakes 20.3 17.3 24900 Sole 1.8 1.5 2160 3.4 29 4200 12.1 17400 Tcul 94.2 135650
lFI'OOI Table VI of David (1987); 2from Table 3; 'see !.eX! for explanation
of calculation of annua1 consumption (= number of foraging animals X daily ration x proportion of each prey species).
mass (kg). the mean body mass of a seal 10 be 72 kg
(ref. 62), then the daily ration can be calculated as 3.76 kg day-I.
3) Total annual food consumption is 104 815 X 3.76 X 365 144 000 tormes. This consumption is divided among the prey species according to the proportions in Table 4. It can be seen that the most important prey are anchovy at about
31
000 t, followed by horse mackerel at 26 000 t, hake at 25 000 I,pilchard at 22 000 t and cephalopods 17 000 1. Informalion collected in 1985 the Sea Fisheries Research InstilUte on the eastern Agulhas Bank and from beached animals 10 showed that squid were more important there than in western areas.
FUlure research
Additional information is needed on the diet of fur seals on the Agulhas Bank, with emphasis on the easterly area. Studies on interactions with south-coast fisheries are in progress and should be expanded.
Several opportunities for research into predator
interactions present themselves. Seals historically inhabited several islands in Bay, but are now restricted to only Black Rocks. In view of the competition for habitat between seals and seabirds, especially penguins,Sl it may be opportune to monitor any expansion of the seal population in Algoa Bay to determine the effects on penguin nesting space.
Cetaceans
At least 32 cetacean species are known to occur in the Agul-has Bank area, based on slrandings, sighlings at sea and whaling statistics (Table 13--80 These can be divided into
three distinct groups:
Resident or frequently ,,(','ur'..,Il·" cetaceans within the Agulhas Bank area
Four delphined and one baleen whale species can be consid-ered as 'resident' and probably occur year-round within the defined area (Table 5). Dusky dolphins are seldom encountered east of Point and probably have a negligible affect on the resources of the Agulhas Bank.
Based on ship-board rales, Besl et al.i.! estimated
that 466 Bryde's whales inhabited South African waters and
that their annual consumption of fish was
between 19000 and 65000 t. Although some 80% of sightings
occurred east of Point, no estimates of Bryde's whale
numbers are available the Bank area.
Cockcroft and Peddemors81 estimated that the numbers of
RESEARCH ON AGULHAS BANK 139
common dolphins inhabiting the south and east coaslS may number between 15000 and 20000, although this estimate was based only on surveys between Port Elizabeth and Natal. There is little information on the slruClure of this population other than from animals captured in the Natal shark nelS, where 15% were unweaned calves and mature females outnumbered mature males.82 Consumption figures based on the estimated populalion size, an average non-calf weight of 100 kg and a food consumption of 4% of body weight
per
day, indicate that common dolphins in the Bank area could consume between 19 000 and 25 000 tomes of fish (66% by weight) and squid (33% by weight) annually.99Bottlenose and humpback dolphins are relatively
abundant in the inshore and deeper neritic waters of this region, although no population estimates for these dolphins south of Natal are available. However,
it
seems reasonable to assume thaI total numbers could approach several thousands. These dolphins have an average daily food consumption ofbe-tween 4% and 8% of body that total
re-source use,
both
of fish and cephalopods,S3 wood be""""w ...
It is important to note that the known occurrence of these cetaceans in the neritic zone does not preclude their presence further offshore. Other than the dusky and Indo-pacific hump-back dolphin, the 'resident' cetaceans listed in this seclion are also known to occur in walers beyond the continental shelf.Table 5. Common and scientific names of cetaceans frequmting !be
Agulhas Bank area between Point and EaS! London. Cetaceans resident In the Agulhas Bank area:
Boulenose dolphin Tursiops truncal us
Indo-Pacific humpback dolphin Sousa chimmsis
Common dolphin Delphinus delphis
Killer whale Orcmus ()Tca
1 Dusky dolphin Laglllwrhynchus ObsCUTUS
Bryde's whale BakJelWplera edeni
Cetaceans rrequenting the Agulhas Bank seasonally: Southern right whale EubaJtuna australis
Humpback whale Megaplera IWvtuQIIgliae
Cetaceans known to Inhabit the pelagic waten orr the Agulhas Bank. but may use the Bank's resources occasionally:
Risso's dolphin gFiseus
dolphin SleMlla coerweoalba
Spotted dolphin SleMlla altenuata
Fraser's dolphin Lage1Wdelphis hose;
False killer whale Pseudorca crassUJeflS
killer whale 1.AlI'~-'''U'''''' pilo! whale 2Shon-firmed whale Melonheaded whale
whale 2Pygmy sperm Whale 20warf sperm whale 2Southem boUIenose whale 2Cuvier's beaked whale 2Strap-tOOlhed whale Blainville'$ beaked whale 2Gray's beaked whale
beaked whale 2Hector's beaked whale Minke whale
Pygmy right whale Blue whale Sei whale Fin whale Peresa altelUUUa Globicephaia melas Globicephaia macrorhynchus Peporwcephaia electra P hyseler macrocephalus Kogia bre\liceps Kogia sinws Hyperoodon pianifroflS Ziphius cavil-osiris M esopJodQII ia)IQTdii Mesoplodoll cUIISirostris Mesoplodo1l grayi Mesoplodoll mirus Mesoplodoll Mclori BakJelWplera acu,Zoroslrala Caperea marginata BakJelWplera I'IUISCWUS BalaelWptera borealis BakJelWplera physalus
. , ' , _ .. ,~..A to the very west of the defined Agulhas Bank area; 2
predomin-SQUld-coaltfl2 cetaceans.
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140 RESEARCH ON AGULHAS BANK
Whether these are separate stocks and whether these stocks use continental shelf resources is unknown.
Cetaceans seasonally occurring on the Agulhas Bank area
Southern right and humpback whales are both seasonal visit-ors to the Agulhas Bank area, 'wintering' in warmer waters during the austral winter and spring. The major ponion of the former visiting this region remain in Agulhas Bank waters for several months, whereas the latter migrate through the area on the way to and from their wintering grounds. Neither is consid-ered to feed regularly while in these warmer waters, though both feed on copepods in Antarctic waters. Consequently, although good estimates of the numbers of both species are available, it is unlikely that either makes significant use of the area's living resources. Nevenheless, it is notewonhy that since the cessation of whaling in the 1930s, southern African stocks of both these species appear to be increasing?6
Pelagic cetaceans possibly using Agulhas Bank resources
Little is known of the cetacean species which are occasion-ally sighted on the Bank or found stranded on southern and eastern Cape beaches. No population estimates are available for any of these species. Although some may frequent inshore areas, all are generally pelagic, inhabiting either the continental slope or deeper waters.
Several baleen whales are known to occur in this region. Statistics for the Plettenberg Bay Whaling Station show that fin, blue and sei whales were taken from the Agulhas Bank area, suggesting that these whales may well use the Bank's zooplankton resources (Best, in press). Minke and pygmy right whales strand on the southern Cape coast and possibly make use of zoorlankton (euphausids and copepods, respectively) resources.83.
Killer whales are known to occur both inshore and offshore in this area and their food probably includes larger fish and squid and marine manunals. The remaining cetacean s~ecies all consume pelagic schooling fish and or squid (Table 1). 4.85--117
Future research
Little is known of the occurrence and distribution of most cetacean species in South African waters. Consequently, the establishment of an observer base to monitor the occurrence, identity and distribution of cetaceans, especially in the Bank area, may be profitable. This could be done through the use of existing ships' cruises, with either the training of participating individuals interested in cetacean sighting, or the training of personnel dedicated to cetacean sighting.
Discussion
Although the Agulhas Bank is recognized as pan of the Agulhas Current Large Marine Ecosystem,88 knowledge of the component species is poor, with the exception of resident sea-birds, seals and some conunercially exploited fish. Knowledge of other groups, such as non-conunercial teleosts, chondrich-thyans, cetaceans and migrant seabirds, is sketchy at best, as has been shown in this review. It is not surprising that popula-tion and biomass estimates are best for species which roost or return to land for reproductive purposes, because this makes estimates of numbers and sampling easier. By contrast, know-ledge of the size of most fish and all dolphin populations, which are difficult or expensive to sample adequately, are rudimentary.
Several seabirds, dolphins and many of the pelagic schooling fish eat mainly planktivorous fish, such as anchovy and pilchard. It is notewonhy that two pelagic predators of fish, penguins and gannets, have larger and more stable populations
SOUlh African Journal of Science Vol. 90 March 1994 on the Agulhas Bank than off the western coast of southem Africa, despite the established productivity and richness of the Benguela upwelling region.'9 Although this difference in popu-lation trends may be environmentally related, it must be recog-nized that the Agulhas Bank is productive even though it is thought to be less productive than the Benguela.90.9I Brown90 indicates, however, that there are considerable differences in productivity on the Cape south coast seasonally, and between inshore and offshore areas. The implications of this on pelagic fish production have yet to be fully understood, but it may affect their distribution and availability to predators. The imponant interrelationship of the Agulhas Bank and the Benguela has been illustrated for many species, including pelagic fishes and birds4.92 The apparent wealth of fish stocks of the Agulhas Bank3 makes the area valuable for expanding fishery interests, particularly because of continued growth in demand for fish to feed the rapidly expanding human popula-tion. Increased catches of pelagic and demersal fishes will almost cenainly impact on other components of the system, but as yet we have a very poor understanding of the standing stocks of most species or the interspecific interactions of most species, and consequently we are in no position to predict accurately the ripple effects through the marine ecosystem of increased harvesting.
Although the management of marine ecosystems is generally aimed at optimizing exploitation, ideally, ecosystems should be managed to incorporate other benefits, such as ecotourism. The economic benefits of non-exploitative use of resources (such as whale, seabird and shark watching and SCUBA diving) are great and deserve more attention when considering manage-ment needs.
Phase III of the Benguela Ecology Programme study is geared towards understanding critical aspects of the life history of pilchards, anchovy and squid. It should be recognized that in addition to these species, the Agulhas Bank is more complex than the Benguela, supponing a more diverse assemblage of species. At prcsent, research on the upper trophic system is poorly supponed. Indeed, although studies of predators and
their prey have provided insights into the occurrence and distri-bution of prey species on the Agulhas Bank, it is notable that this work is often ignored by fisheries biologists.93,94 Such work may nevenheless be a useful form of environmental and ecosystcm monitoring where predators may be relatively cheaply monitored.
Understanding the species-rich Agulhas Bank ecosystem seems a fonnidable challenge and, without a major research thrust, the status of our knowledge will remain severely restricted. If we wish to understand and manage man's influ-ence on the marine ecosystem, more intense and broader biological and physical oceanographic research is needed on the Agulhas Bank. Without it, we will be in a weak position to
understand, much less manage and predict, fisheries. Physical oceanographic studies are a vital component of this thrust because knowledge of driving mechanisms underpins our understanding of distribution of species and biological inter-actions.95,96 The imponance of studying the multi-species nature of large marine ecosystems is increasingly being recog-nized,4.78,88.97 and it follows that this is panicularly imponant in species-rich environments such as the Agulhas Bank.
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