Bottlenose dolphins leaping alongside trawler. Photo: Simon Allen

New research from Murdoch University scientists has revealed that around 500 dolphins have been caught in the Pilbara trawl fishery in the past decade.

The research, by PhD candidate Simon Allen and colleagues from the Murdoch University Cetacean Research Unit, is based on independent observer data.

"The data shows us that the methods and devices in place to reduce dolphin bycatch simply aren't working, despite the industry's best efforts," Mr Allen said.

"Independent observers reported catch rates of about 50 per year, which is double the number reported by the skippers of these vessels."

Mr Allen said under reporting is not unusual around the world, especially where marine mammal capture is illegal.

"This doesn't necessarily mean that skippers are deliberately under reporting. Fishers are concentrating on the job at hand and may not see a dead dolphin fall out of the net on winch up," he said.

A recent report by Western Australia's Department of Fisheries detailed similar findings, but instead suggested that the self-reporting mechanisms in place were accurate and that the impact poses 'negligible risk'.

Co-investigator Professor Neil Loneragan said the paper makes a number of recommendations to better monitor and reduce bycatch.

"The bycatch reduction devices had bottom opening escape hatches, with an unknown quantity of bycatch falling out and therefore not being reported," he said.

"Modified bycatch reduction devices with top-opening escape hatches may be more effective."

The scientists have also called for a reinstatement of independent observers, as well as in-net video collection so that bycatch can be accurately measured.

"The next step is to calculate the acceptable levels of human-caused dolphin mortality, which requires an estimate of the dolphin population size in the region," Mr Allen said.

"If the ongoing bycatch exceeds that threshold, switching to alternative, less destructive fishing methods, like trap or line fishing, should be considered." The paper has been published in open access journal PLOS ONE.

More information: Allen SJ, Tyne JA, Kobryn HT, Bejder L, Pollock KH, et al. (2014) "Patterns of Dolphin Bycatch in a North-Western Australian Trawl Fishery." PLoS ONE 9(4): e93178. DOI: 10.1371/journal.pone.0093178

Journal information: PLoS ONE

Provided by Murdoch University