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Marine and Freshwater Research Marine and Freshwater Research Society
Advances in the aquatic sciences
RESEARCH ARTICLE

Population status of 14 shark species caught in the protective gillnets off KwaZulu–Natal beaches, South Africa, 1978–2003

Sheldon F. J. Dudley A C and Colin A. Simpfendorfer B
+ Author Affiliations
- Author Affiliations

A Natal Sharks Board, Private Bag 2, Umhlanga Rocks, 4320, South Africa.

B Center for Shark Research, Mote Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, FL 34236, USA.

C Corresponding author. Email: dudley@shark.co.za

Marine and Freshwater Research 57(2) 225-240 https://doi.org/10.1071/MF05156
Submitted: 22 August 2005  Accepted: 17 January 2006   Published: 10 March 2006

Abstract

Shark nets have been set off the beaches of KwaZulu–Natal, South Africa, since 1952 to reduce the risk of shark attack. The nets fish in fixed localities 400 m from shore and both directly affect local shark populations and act as fisheries-independent monitoring devices. Reliable catch information at the species level was available for the period 1978–2003. Trends in catch rate and size were used to assess the population status of 14 commonly caught shark species. In addition, a demographic modelling approach was used in conjunction with the catch information to assess the potential effect of the nets on populations. Catch rates of four species (Carcharhinus leucas, C. limbatus, Sphyrna lewini and S. mokarran) showed a significant decline, as did the mean or median length of three species (Carcharhinus amboinensis, C. limbatus and female Carcharodon carcharias). For three species that showed declining catch rates or length the potential effect of the shark nets was assessed to be low, suggesting that other sources of catch were responsible for the declining status. The potential effect of the shark nets was assessed to be high for two species (Carcharhinus obscurus and Carcharias taurus, neither of which showed declines in catch rate or length), because of very low intrinsic rates of population increase.

Extra keywords: catch, CPUE, demographic modelling, maturity, size.


Acknowledgments

The operations staff of the NSB maintain the KZN shark nets and record all catches. Catch, effort and dissection data are captured by staff in the research department of the NSB. Geremy Cliff and Sabine Wintner contributed unpublished life history information. We are grateful to Ken Goldman (Jackson State University, Jackson, MI) for providing von Bertalanffy growth data on Carcharias taurus. Mote Marine Laboratory hosted SD in 2004 and permitted CS to participate in this project.


References

Aitken A. P. (2003). The biology and tourism potential of the tiger shark Galeocerdo cuvier and the whale shark Rhincodon typus in KwaZulu–Natal, South Africa. M.Sc. Thesis, University of Cape Town, South Africa.

Allen, B. R. , and Cliff, G. (2000). Sharks caught in the protective gill nets off KwaZulu–Natal, South Africa. 9. The spinner shark Carcharhinus brevipinna (Müller and Henle). South African Journal of Marine Science 22, 199–215.
Bass A. J., D'Aubrey J. D., and Kistnasamy N. (1973). Sharks of the east coast of southern Africa. l. The genus Carcharhinus (Carcharhinidae). Oceanographic Research Institute Investigational Report 33, Durban, South Africa.

Bonfil, R. , Meÿer, M. , Scholl, M. C. , Johnson, R. , O'Brien, S. , Oosthuizen, H. , Swanson, S. , Kotze, D. , and Paterson, M. (2005). Transoceanic migration, spatial dynamics, and population linkages of white sharks. Science 310, 100–103.
Crossref | GoogleScholarGoogle Scholar | PubMed | Cliff G., van der Elst R. P., Govender A., Witthuhn T. K., and Bullen E. M. (1996). First estimates of mortality and population size of white sharks on the South African coast. In ‘Great White Sharks: The Biology of Carcharodon carcharias’. (Eds A. P. Klimley and D. G. Ainley.) pp. 393–400. (Academic Press: San Diego, CA.)

Cockcroft, V. G. (1990). Dolphin catches in the Natal shark nets, 1980–1988. South African Journal of Wildlife Research 20, 44–51.
Compagno L. J. V., Ebert D. A. and Smale M. J. (1989). ‘Guide to the Sharks and Rays of Southern Africa.’ (Struik: Cape Town.)

Cortés, E. (2002). Incorporating uncertainty into demographic modelling: application to shark populations and their conservation. Conservation Biology 16, 1048–1062.
Crossref | GoogleScholarGoogle Scholar | Davies D. H. (1963). Shark attack and its relationship to temperature, beach patronage and the seasonal abundance of dangerous sharks. Oceanographic Research Institute Investigational Report 6, Durban, South Africa.

Davies D. H. (1964). ‘About Sharks and Shark Attack.’ (Shuter and Shooter: Pietermaritzburg, South Africa.)

De Bruyn P. A. (2000). Age, growth, feeding, reproduction and exploitation of the scalloped hammerhead shark, Sphyrna lewini, in the coastal waters of KwaZulu–Natal. M.Sc. Thesis, University of Port Elizabeth, South Africa.

De Bruyn, P. A. , Dudley, S. F. J. , Cliff, G. , and Smale, M. J. (2005). Sharks caught in the protective gill nets off KwaZulu–Natal, South Africa. 11. The scalloped hammerhead shark Sphyrna lewini (Griffith and Smith). African Journal of Marine Science 27, 517–528.
FAO (Food and Agriculture Organization of the United Nations) (2004). Report of the FAO Ad Hoc Expert Advisory Panel for the Assessment of Proposals to Amend Appendices I and II of CITES Concerning Commercially-exploited Aquatic Species. FAO Fisheries Report 748, FAO, Rome. Available online at: ftp://ftp.fao.org/docrep/fao/007/y5523e/y5523e00.pdf [verified February 2006].

Fennessy, S. T. (1994). Incidental capture of elasmobranchs by commercial prawn trawlers on the Tugela Bank, Natal, South Africa. South African Journal of Marine Science 14, 287–296.
Francis M. P. (1996). Observations on a pregnant white shark with a review of reproductive biology. In ‘Great White Sharks: The Biology of Carcharodon carcharias’. (Eds A. P. Klimley and D. G. Ainley.) pp. 157–172. (Academic Press: San Diego, CA.)

Govender, A. , and Birnie, S. L. (1997). Mortality estimates for juvenile dusky sharks Carcharhinus obscurus in South Africa using mark-recapture data. South African Journal of Marine Science 18, 11–18.
Hilborn R., and Walters C. J. (1992). ‘Quantitative Fisheries Stock Assessment. Choice, Dynamics and Uncertainty.’ (Chapman and Hall: New York.)

Holden M. J. (1977). Elasmobranchs. In ‘Fish Population Dynamics’. (Ed. J. A. Gulland.) pp. 187–215. (John Wiley and Sons: New York.)

Hueter, R. E. , Heupel, M. R. , Heist, E. J. , and Keeney, D. B. (2005). Evidence of philopatry in sharks and implications for the management of shark fisheries. Journal of Northwest Atlantic Fishery Science 35, 239–247.
IOTC (Indian Ocean Tuna Commission) (2005). Information on shark finning fisheries. IOTC-2005–S9-08[EN]. IOTC, Victoria, Seychelles.

Joung, S. J. , Liao, Y. Y. , and Chen, C. T. (2004). Age and growth of sandbar shark, Carcharhinus plumbeus, in northeastern Taiwan waters. Fisheries Research 70, 83–96.
Crossref | GoogleScholarGoogle Scholar | Pepperell J. G., West J., and Woon P. (Eds) (1993). ‘Shark Conservation. Proceedings of an International Workshop on the Conservation of Elasmobranchs, Sydney, Australia, 24 February 1991.’ (Zoological Parks Board of NSW: Mosman.)

Peterson, I. , and Wroblewski, J. S. (1984). Mortality rate of fishes in the pelagic ecosystem. Canadian Journal of Fisheries and Aquatic Sciences 41, 1117–1120.
Simpfendorfer C. (1993). The Queensland Shark Meshing Program: analysis of the results from Townsville, north Queensland. In ‘Shark Conservation. Proceedings of an International Workshop on the Conservation of Elasmobranchs, Sydney, Australia, 24 February 1991’. (Eds J. G. Pepperell, J. West and P. Woon.) pp. 71–85. (Zoological Parks Board of NSW: Mosman.)

Simpfendorfer C. A. (2004). Demographic models: life tables, matrix models and rebound potential. In ‘Elasmobranch Fisheries Management Techniques’. (Eds J. E. Musick and R. Bonfil.) pp. 187–204. (Asia-Pacific Economic Cooperation Secretariat: Singapore.) (http://www.flmnh.ufl.edu/fish/organizations/ssg/EFMT/9.pdf)

Simpfendorfer, C. A. , and Unsworth, P. (1998). Gill-net mesh selectivity of dusky sharks (Carcharhinus obscurus) and whiskery sharks (Furgaleus macki) from south-western Australia. Marine and Freshwater Research 49, 713–718.
Crossref | GoogleScholarGoogle Scholar | Sousa M. I., Marshall N. T., and Smale M. J. (1997). The shark trade in Mozambique. In ‘The Trade in Sharks and Shark Products in the Western Indian and Southeast Atlantic Oceans’. (Eds N. T. Marshall and R. Barnett.) pp. 67–79. (TRAFFIC East/Southern Africa: Nairobi.)

Springer S., and Gilbert P. W. (1963). Anti-shark measures. In ‘Sharks and Survival’. (Ed. P. W. Gilbert.) pp. 465–476. (D. C. Heath and Company: Boston, MA.)

Van der Elst, R. P. (1979). A proliferation of small sharks in the shore-based Natal sport fishery. Environmental Biology of Fishes 4, 349–362.
Crossref | GoogleScholarGoogle Scholar | Wallett T. (1983). ‘Shark Attack in Southern African Waters and Treatment of Victims.’ (Struik: Cape Town.)

Walters C. J., and Martell S. J. D. (2004). ‘Fisheries Ecology and Management.’ (Princeton University Press: Princeton, NJ.)

West J. G. (1993). The Australian Shark Attack File with notes on preliminary analysis of data from Australian waters. In ‘Shark Conservation. Proceedings of an International Workshop on the Conservation of Elasmobranchs, Sydney, Australia, 24 February 1991’. (Eds J. G. Pepperell, J. West and P. Woon.) pp. 93–101. (Zoological Parks Board of NSW: Mosman.)

Wintner, S. P. , and Cliff, G. (1996). Age and growth determination of the blacktip shark, Carcharhinus limbatus, from the east coast of South Africa. Fishery Bulletin 94, 135–144.
[verified February 2006].