Cold fins, murky waters and the moon: what affects shark catches in the bather-protection program of KwaZulu–Natal, South Africa?

Abstract

The influence of environmental variables on shark catch in the bather-protection program along the eastern coast of South African was investigated for 11 commonly caught species (Carcharhinus limbatus, C. obscurus, C. brachyurus, C. plumbeus, C. brevipinna, C. leucas, Carcharodon carcharias, Carcharias taurus, Sphyrna lewini, S. zygaena, Galeocerdo cuvier). Data for the period 1986–1994 were analysed using generalised additive models and generalised additive mixed models. The influence of temporal and spatial factors was respectively considered and removed within a standardisation procedure to investigate and predict the influence of lunar cycle, sea-surface temperature (SST) and water visibility on daily catch. The catches of C. taurus and C. brevipinna were significantly affected by all three factors. Catches of all other species were affected by at least one of the three factors, e.g. six were affected by lunar phase, six by SST and seven by water visibility. The results suggested that measurable, predictable relationships exist between environmental conditions and presence and, consequently, catch of shark species in this program. Understanding these relationships could be useful to mitigate against unwanted catch and to further reduce risk for bathers.