Allen / Loneragan

Individual dolphins have been captured on film repeatedly entering fishing nets to plunder the catch for their dinner.

Dolphins are regularly, if infrequently, hauled out of the sea in trawler nets. With many marine mammals threatened, such bycatch is a major concern. This new work highlights that dolphins seek out and actually venture inside huge nets pulled through the oceans by modern fishing vessels, and could eventually lead to improved methods for preventing such deaths.

Dolphins are often spotted in the vicinity of fishing boats, but the extent to which they actually enter nets is less well known, say Neil Loneragan, Simon Allen, and Vanessa Jaiteh, marine mammal researchers at the Murdoch University in Australia. To shed some light on this, their team placed video cameras inside nets for the entire duration of fishing trawls off the northwest Australian coast.

They were able to identify individual animals accompanying several trawls. Of the 29 dolphins they could identify inside nets, seven returned repeatedly, they report in Marine Mammal Science.

“Once a trawler moves to a new area, dolphins quickly gather around the stern of the vessel and are associated with it for much of the time they are in the area. Hence, it is most likely that all individuals incidentally caught inside trawl nets in this fishery have deliberately entered the nets for the foraging opportunities this presents,” said the researchers in an email to Nature.

Previous video work has spotted dolphins in nets, but did not identify individuals or cover entire trawls. “The identification of individuals was significant as a number of them were repeatedly recorded in the nets, which strongly suggests that it is only a subset of the dolphin population in the area that enters trawl nets,” they say.

Simon Northridge, a marine mammal research at the University of St Andrews in Scotland who was not involved in the research, says that the popular idea of dolphins being swept up in trawls is erroneous. These nets are generally pulled through the water quite slowly.

“There’s no way a dolphin would not know a trawl was there,” he says. “The fact animals get caught in trawls is suggestive they’re deliberately entering them.”

Given that the animals are actually caught only infrequently, the key questions are exactly why animals do get trapped and killed. By monitoring entire trawls Loneragan, Allen and Jaiteh say they will be better able to precisely estimate the rate at which the animals are captured and to assess the effectiveness of efforts to minimise it. As the animals are seeking out such nets and are likely to often find themselves inside, ‘escape hatches’ – basically a hole in the top of the net – may present the best solution.