Revisiting acoustic deterrence devices: Long-term bycatch data from South Africa’s bather protection nets

The KwaZulu-Natal (KZN) Sharks Board in Durban, South Africa, has been managing bather protection nets off KZN’s most popular beaches since 1952 to mitigate shark-human interaction. The nets catch potentially dangerous sharks, but also take a bycatch of other marine megafauna, including the Indo-Pacific bottlenose dolphin Tursiops aduncus. A total of 1169 T. aduncus were caught between 1980 and 2015 (annual mean 32.5 ± 13.4). Most were young and immature. To mitigate cetacean bycatch, acoustic devices (pingers, whale alarms, and biosonar reflectors) were installed at selected beaches for some years between 1993 and 2015. On nets with pingers, most dolphins were caught close to active pingers, with 44 out of 72 animals (61%) caught within 50 m and 25 (35%) within 10 m. Bycatch varied inter-annually and between neighboring beaches both with and without pingers and there was no clear indication that pingers reduced bycatch. As this is an operational bather protection program, no controlled experiments on pinger efficacy could be conducted. Understanding the acoustic environment into which pingers are deployed is important in order to optimize pinger specifications and frequencies in terms of how far the pings propagate, how susceptible they are to being masked by ambient noise, and how detectable they are by target species.