Dolphin bycatch in trawler nets still too high for sustainable populations, study finds

An international team of researchers has warned that the accidental capture of dolphins by boats using trawl fishing methods will have an unsustainable impact on dolphin populations.

The study looked at a trawl fishery in the Pilbara and the unwanted nets, or bycatch, of dolphins.

Although the trawls have been fitted with devices designed to reduce bycatch since 2006, the study concluded that even the lowest recorded dolphin catch rate was too high.

Professor Neil Loneragan of Murdoch University said estimates from captains and independent observers show that between 16 and 25 dolphins were caught in nets each year in the Pilbara fishery.

“Most people would think it won’t be significant, but you have to take into account that dolphins only produce one or two calves and they will have the calf for three years,” he said.

“They only produce young [at the most] every three years once they have reached maturity, so culling the animals can have a huge impact.”

The study revealed that protected species were invariably caught as bycatch by fishing trawlers.(Provided: Simon Allen)

Lecturer at the University of Bristol, Simon Allen, said the team had analyzed the impact on the population based on different bycatch rates.

“We set out to model different levels of dolphin capture, including those reported in fishermen’s logbooks and those reported by independent observers. Unfortunately, our results clearly show that even the lowest annual dolphin capture rates reported are not sustainable,” Dr Allen said.

The study’s lead author, Oliver Manlik of the United Arab Emirates University, said even the lowest estimates of dolphin bycatch were “unsustainable”.

“This not only raises concerns for the dolphin population, but highlights a problem with other assessments that do not account for random events, such as heat waves,” Dr Manlik said.

Dolphins aren’t the only unintended victims, according to Simon Allen, a lecturer at the University of Bristol.

“The Pilbara Fish Trawl interim managed fishery targets emperor, snapper, trevally, cod and grouper,” said Dr Allen.

“However, it also results in the capture of protected species including dolphins, turtles and sea snakes, as well as a variety of endangered sharks, rays and sawfish.”

Sawfish in a net.
It is not uncommon for species such as sawfish to be caught in the nets.(Provided: Simon Allen)

An ecological risk assessment of the Pilbara Fish Trawl Interim Managed Fishery, examining the impact on target and non-target species, is expected to be published later this year.

The team of researchers hoped that their new model for assessing sustainable wildlife mortality limits, which they used in their study, could be implemented more widely.

‘We hope this approach will be incorporated into the Department of Primary Industries and Regional Development’s assessment of fisheries in its ecological assessments,’ Prof Loneragan said.

“Currently, we don’t see a quantitative approach being used to assess dolphin bycatch, and this model [developed by the team of researchers] provides a mechanism for doing so, as well as the potential assessment of the impact on other species.”

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