Reducing dolphin deaths in fishing trawls

July 14, 2010

Each year an estimated 20 to 50 dolphins die when they are accidently caught and asphyxiate in trawl fishing nets off Western Australia’s northwest, a Murdoch University research project has shown.

Simon Allen from Murdoch University’s Cetacean Research Unit started researching dolphin ‘bycatch’ (unintentional capture of marine life in fishing nets) in 2007 when he, Dr Lars Bejder and Professor Neil Loneragan won funding to study the interactions between dolphins and the fishing trawl industry.

“A short study by the Western Australian Department of Fisheries (DoF) in 2002 estimated that dolphin mortality rates were around 100 per year. Through working with the fishing industry to alter trawl net design, we have been able to reduce the number of dolphin deaths,” Mr Allen said.

Trawl fishing nets now feature bycatch reduction devices (BRDs) that are designed to prevent unwanted objects such as rocks, corals and sponges and bycatch from being caught in the trawl net.

“Bycatch reduction devices consist of an exclusion grid and an escape hatch in the bottom of the net. By moving these devices forward in the nets, bycatch was further reduced, but recent data from underwater videos mounted inside the nets suggests that further improvements can be made,” Mr Allen said.

“Our research, which is funded by the Fisheries Research and Development Corporation (FRDC), the WA DoF and the Pilbara Fish Trawl Interim Managed Fishery, showed that air breathing animals, like dolphins, swim upwards when under stress.

“Current BRDs have bottom-opening escape hatches only and, in some cases, dead dolphins fell through them before the nets were hauled up and were therefore not recorded. Only those dolphins landed on deck were recorded by skippers and observers, meaning dolphin bycatch figures could be much greater than 20 to 50 per year.

“Furthermore, trials of nets with top-opening escape hatches for air-breathing animals are vital. The fishing industry have been working with us to reduce the number of dolphin deaths – but the state and federal governments would like to see the numbers reduced even further.”

In their final report to FRDC, Mr Allen and Professor Loneragan recommend further research to determine the size of dolphin populations that are impacted by trawl fishing. They also recommend more independent observations of bycatch numbers as dolphin deaths are under-reported.

Mr Allen and his colleagues will now investigate the genetic structure and abundance of dolphin populations of northwestern Australia. This will allow researchers to assess whether the dolphin populations can survive the current levels of mortality caused by trawl fishing. The Australian Marine Mammal Centre funds this ongoing research.

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