Humpback dolphin bycatch in the shark nets in KwaZulu-Natal, South Africa

Abstract

Humpback dolphins (Sousa plumbea) in South Africa are classified as Vulnerable and one quantifiable threat is accidental mortality in the shark nets in KwaZulu-Natal. We investigated the spatial, temporal and life history patterns of this bycatch to guide mitigation strategies to decrease humpback dolphin capture. A total of 203 individuals were caught between 1980 and 2009. We analysed patterns of captures in relation to the area of capture (location) and year and month of retrieval from the nets. We also analysed the distribution of the sex and size (body length) of humpback dolphins. Most catches (61%) occurred at Richards Bay in the northern part of their distribution in South Africa. Annual catch rate fluctuated considerably and there was little seasonality. The sex ratio was male-biased (1.55:1) and in particular skewed towards adolescents (56%) which constituted the majority of the catch. We suggest that mitigation strategies be focused at Richards Bay, throughout the year. Of the existing shark net mitigation strategies, changing fishing gear from nets to baited hooks (drumlines) could be useful to decrease humpback dolphin capture rates.