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

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.