The IUCN/SSC Shark Specialist Group
Shark News 6: March 1996
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Localised stock depletion; does it occur for sharks?
Terry Walker, Victorian Fisheries Research Institute
Introduction
For the purpose of this article localised stock depletion' refers to a
situation where a species occupies a range of separate regions and
where the density of animals in one or more of these regions is
reduced more than in the other regions by fishing or habitat
modification. Localised stock depletion is expected for sessile and
relatively slow moving animals such a scallops, abalone and lobsters
which are harvested more intensively in some regions than in others,
but is less expected for free-swimming animals such as sharks which
can readily move into previously occupied areas. In the following I
will briefly outline how localised stockdepletionhasbecomeapparent
in shark culling programs designed to protect bathers at beaches from
shark attack and how it might occur in artisanal, recreational and
industrial (i.e. modern large-scale commercial) fisheries.
Evidence for localised stock depletion
The concept of localised stock depletion for sharks first arose when
Holden (1977) drew attention to the catch per unit effort (CPUE)
trends for beach netting programs at two Natal locations - Durban
during 1952-1972 and Brighton Beach during 1961-1972. He
describes the trends as both having an initial steep decline followed
by a steady catch rate, a pattern expected during the early phase of
harvesting of any previously unfished stock. Because the initial catch
rates at Brighton in 1961 were as high as the initial catches at Durban
nine years earlier, and because the netted beaches are only about 10
km apart, Holden concluded that the populations were isolated and
that the sharks were territorial.
A school shark, Caleorhinus galeus, caught by demersal gillnet off southern Australia in
the world's longest-running industrial fishery targeting snarks. Photo: Terry Walker.
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Several other authors found the trend of initial CPUE decline
followed by stability foreach of a numberof,butnotall,sharkspecies.
Dudley and Cliff (1993a, b) present additional data for the Natal
beach meshing program, which by 1990 involved setting a total of 42 km
of netting at 43 beaches on the 560 km Natal coast, and Simpfendorfer
(1993) presents data for the Queensland beach protection program.
While most species captured by gillnets in these programs followed
this trend, Simpfendorfer found no trend for tiger shark Galeocerdo
cuvier and found constant or rising CPUE trends for several shark
species captured by drumlines. He suggests that rates of
inshore-offshore movements, seasonal and along-shore migration
patterns and amounts of time certain species spend inshore affect the
trends. Dudley and Cliff (1993a, b) also suggest that the trends for
some species depend on migration patterns and the 'degree of
residency'. In addition, they postulate that changes in predator-prey
interactions between shark species and between sharks and other
vertebrate species might contribute to the observed CPUE trends.
Although not well documented, localised stock depletion is also
likely to beexhibited in many of the world's unregulated artisanal and
recreational fisheries where sharks are either targeted or taken as part
of multispecies fisheries. Many of these fisheries have large numbers
of small fishing boats collectively applying high levels of fishing effort
in coastal waters. However, because these boats are restricted to a
range of only a few miles from shore and because most of the species
harvested are distributed widely inshore and offshore, the ranges of
these fisheries are small compared with the distribution ranges of the
shark species. Provided nursery grounds or major aggregations of
breeding sharks do not fall within the ranges of these fisheries and
there are not well-developed offshore industrial fisheries harvesting
the same species, inshore localised stock depletion of sharks with
associated falling CPUE trends can give the appearance of a fishery
in decline while the overall stock is only marginally depleted.
Examples of wider stock depletion
Industrial fisheries either targeting sharks or taking sharks as bycatch
operating over wide areas on the high seas and continental shelves of
the world have had a greater impact on stocks than inshore
localised fishing. For example, falling bycatches from the tuna
longlining fleets are indicative of a broad-scale stock reduction of
pelagic sharks (Taniuchi 1990) and the unregulated targeting of
the soupfin shark Galeorhinus galeus on the continental shelf off
California led to a complete fishery collapse during the 1940s. Fishers
in the industrial shark fishery off southern Australia targeting school
shark G.galeus and gummy shark Mustelus antarcticus believe that
the presence of sharks captured in bottom-set gillnets repels free-
swimming sharks from an area. Many express the view that habitat
disturbance and/or noise from trawl fishing also have the effect of
repelling sharks from an area. Hence to maintain their catch rates the
fishers tend to shift position after hauling the gear and for several
weeks will avoid grounds known to have been previously fished. The
effect of catching part of a population in an area and repelling other
sharks by the use of fishing gear can be viewed as temporary localised
stock reduction whereas permanently repelling sharks from an area
by habitat modification can be viewed as more permanent localised
stock depletion.
Stock depletion in the Port Phillip Bay nursery
An example of more permanent localised stock depletion of juvenile
school sharks in the Australian fishery is that described by Olsen
(1959) for Port Phillip Bay in Victoria. In response to intensive fishing
of juveniles, the catch from the Bay increased threefold from 1942 to
1944 and then fell rapidly until the early 1950s when they became
protected by the introduction of a legal minimum length. Olsen
(1954) identified the Geelong Arm in Port Phillip Bay as a nursery
area, where on several occasions during 1947-1951 he captured for
tagging more than 200 sharks per day on a handline. Since then
inshore fishers have caught only small numbers of school sharks from
anywhere in the Bay and monthly sampling over December-March
during 1993-1996 by the Victorian Fisheries Research Institute with
400 baited hooks attached to longlines and 150 m of gillnetting
(2-4 inch mesh-sizes) produced catches of only 0-10 juvenile sharks
per day.. This localised stock depletion of juvenile sharks in the Bay
is much more severe and occurred much earlier than the overall
reduction of stock biomass which current assessments indicate have
been reduced to below 25% of the biomass levels occurring before
the fishery began in the 1920s.
The lack of any stock recovery in Port Phillip Bay since the 1950s
and the high movement rates by adult sharks aredifficuitto reconcile.
On one hand, the wide dispersion of tagged school sharks, the long
migrations associated with parturition, and the complex distribution
patterns of various age-classes throughout southern Australia described
by Olsen (1954) are all consistent with the hypothesis of a single
panmictic population with sections of the stock at different life history
stagesoccupyingdifferentlocalitieswithintherangeofthedistribution.
On the other hand, the lack of recovery of juvenile sharks in Port
Phitlip Bay is more consistent with the hypothesis of discrete
subpopulations with limited interchange. The discrete breeding
subpopulations using different nursery areas would have to mix at
other life history stages to be consistent with Olsen's description.
Another hypothesis, which accounts for the lack of diffusion of school
sharks into the Bay and its diminished use as a major pupping ground
since the 1940s, is that the habitat of the Geelong Arm has become
less suitable for G. galeus.
References
Dudley, S.F.J., and Cliff, G. 1993a. Trends in catch rates of lar)
sharks in the Natal meshing program. In: Shark Conservatio
Proceedings of an International Workshop on the Conservation
Elasmobranchs held at Taronga Zoo, Sydney, Australia, 24 February
1991. (Eds J.G. Pepperell, J. West, and P.M.N. Woon). pp. 59-70.
Taronga Zoo: Sydney.
Dudley, S.F.J., and Cliff, G. 1993b. Some effects of shark nets in the
Natal nearshore environment. Environmental Biology of Fishes 36:243-255.
Holden, M.J. 1977. Elasmobranchs. In: Fish population dynamics.
(Ed. J. A. Gulland). pp. 187-216. John Wiley and Sons: London.
Olsen, A.M. 1954. The biology, migration and growth rate of the
school shark, Caleorhinus australis (Macleay) (Carcharhanidae)
in south-eastern Australian waters. Australian journal of Marine
and Freshwater Research 5:353-410.
Olsen, A.M. 1959. The status of the school shark fishery in south-
eastern Australian waters. Australian Journal of Marine and
Freshwater Research 10: 150-176.
Simpfendorfer, C. 1993. The Queensland Shark Meshing Program:
analysis of results from Townsville, North Queensland, In: Shark
Conservation: Proceedings of an International Workshop on the
Conservation of Elasmobranchs held at Taronga Zoo, Sydney,
Australia, 24 February 1991. (Eds J.G. Pepperell, J. West, and
P.M.N. Woon). pp. 71-85. Taronga Zoo: Sydney.
Taniuchi, T. 1990. The role of elasmobranchs in Japanese fisheries.
In: Elasmobranchs as Living Resources: Advances in the Biology,
Ecology, Systematics, and the Status of the Fisheries. (Eds H.L.
Pratt Jr., S.H. Gruber, and T. Taniuchi.) pp. 415-426. US
Department of Commerce, NOAA Technical Report NMFS 90.
Terry Walker
Principal Marine Scientist, Victorian Fisheries Research Institute,
PO Box 114, Queenscliff, Victoria, Australia 3225
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