Bottlenose dolphins are being caught and killed in trawl nets in Western Australia’s north at unsustainable levels, a study warns.
The finding is based on analysis of the Pilbara trawl which supplies fish to the Perth market, targeting emperor, snapper, trevally, cod and grouper.
A report last year by the federal environment department indicated between 11 and 17 bottlenose dolphins were killed every year in the trawl’s bycatch.
Independent observers have previously put the rate as high as 50 a year in peer-reviewed research.
In a new international study published in the Conservation Biology journal, researchers used a modelling tool incorporating chance events to assess population declines among bottlenose dolphins in the Pilbara.
Dr Simon Allen, an adjunct research fellow at the University of Western Australia, said the study found capture rates remained unsustainable even with mitigation efforts.
“Bycatch reduction devices were placed in the trawl nets in 2006 and there has been some monitoring since, but no quantitative assessment of the impact of fishery-related dolphin mortality was ever carried out,” Allen said.
“We set out to model different levels of dolphin capture, including those reported in fishers’ logbooks and those reported by independent observers.
“Unfortunately, our results show clearly that even the lowest reported annual dolphin capture rates are not sustainable.”
Previous models had focused simply on the maximum number of marine animals that could be killed without affecting the sustainability of the population.
The new study takes into account environmental and demographic factors, including the dependency of offspring on their mothers and chance events such as heatwaves.
It found the “acceptable” number of bottlenose dolphin bycatch deaths was between two and eight a year, compared to 16 under the less-sophisticated model.
“These results suggest that reported bycatch rates are unsustainable in the long term, unless reproductive rates are consistently higher than average,” the authors found.
