Impact of protective shark nets on sea turtles in KwaZulu-Natal, South Africa, 1981–2008

Abstract

Shark nets were established along the KwaZulu-Natal coast in 1952 to protect bathers against shark encounters. Sea turtle bycatch data for the period 1981–2008 in these shark nets indicated that loggerheads Caretta caretta were caught most frequently at 40.9 y^-1 or 1.11 km-net^-1 y^-1, followed by green turtles Chelonia mydas at 0.32 km-net^-1 y^-1 and leatherbacks Dermochelys coriacea at 0.14 km-net^-1 y^-1. Catch rate of loggerheads increased throughout the study period, and was positively correlated with the increase in the number of nesting females from the recovering population in Maputaland, South Africa (r = 0.52, p < 0.05, n = 28). This increase is ascribed to successful nest conservation. Leatherback bycatch remained stable, following their nesting trends, although they receive similar conservation efforts on nesting beaches as loggerheads. Bycatch of green turtles, although not a nesting species in South Africa, was stable over time. Mortality varied between species (from 53.2% to 70.6%) with loggerheads being the most hardy, followed by leatherbacks and green turtles. Few hawksbills Eretmochelys imbricata and olive ridleys Lepidochelys olivacea were caught in the shark nets. Fewer sea turtles are caught by shark nets than by longlines and because the nesting populations of loggerheads, green turtles and leatherbacks are either stable or increasing in the South-West Indian Ocean, shark net bycatch was considered negligible and sustainable.

Keywords: bycatch, CPUE, green turtle, leatherback, loggerhead, mortality, seasonality, sex ratio

African Journal of Marine Science 2012, 34(2): 249–257