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Marine Ecology Progress Series

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MEPS 555:249-260 (2016)  -  DOI:

Net loss of endangered humpback dolphins: integrating residency, site fidelity, and bycatch in shark nets

Shanan Atkins1,2,*, Maurício Cantor3, Neville Pillay2, Geremy Cliff4, Mark Keith5, Guido J. Parra6

1Endangered Wildlife Trust, Private Bag X11, Modderfontein, Johannesburg 1609, South Africa
2School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg 2050, South Africa
3Department of Biology, Dalhousie University, Halifax, NS B3H 4J1, Canada
4KwaZulu-Natal Sharks Board, Umhlanga 4320, South Africa
5Centre for Wildlife Management, University of Pretoria, Pretoria 0028, South Africa
6Cetacean Ecology, Behaviour and Evolution Lab (CEBEL), School of Biological Sciences, Flinders University, Adelaide, South Australia 5001, Australia
*Corresponding author:

ABSTRACT: Fisheries bycatch—the incidental catch of non-target species during fishing—is problematic for large marine vertebrates. Bather protection programmes that use gillnets to kill sharks cause the incidental mortality of humpback dolphins Sousa spp., potentially impacting the long-term survival of these threatened species. Understanding dolphins’ spatial and temporal use of gillnetted areas is critical for designing effective mitigation strategies. We photo-identified dolphins over 8 yr in a high-bycatch area (Richards Bay, South Africa) to assess the residency, site fidelity, and movement patterns of Indian Ocean humpback dolphins S. plumbea and evaluate how emigration, immigration, and mortality rates influence the use of Richards Bay at various temporal scales. Overall, residency was low but site fidelity was high, leading to high population turnover in the short term but low turnover over 6 mo and longer. There was clear individual variation in visitation but no evidence of seasonality. By considering such movements, the net loss of dolphins from the area became evident. While dolphins naturally emigrate from the area, the recognition of several catalogued individuals among the bycaught dolphins indicated that mortality in the shark nets contributes to the permanent loss of both residents and transients. Richards Bay may represent an ecological trap: high site fidelity indicates dolphins perceived the area as ecologically attractive, but high mortality due to shark nets makes it risky. We examined these results relative to gillnet bycatch mitigation methods and recommend that stakeholders collaborate as a mitigation team to prioritise management actions to reduce bycatch without compromising bather safety.

KEY WORDS: Bycatch mitigation · Incidental catch · Gillnets · Residency · Site fidelity · Cetaceans · Bather protection · South Africa

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Cite this article as: Atkins S, Cantor M, Pillay N, Cliff G, Keith M, Parra GJ (2016) Net loss of endangered humpback dolphins: integrating residency, site fidelity, and bycatch in shark nets. Mar Ecol Prog Ser 555:249-260.

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