Deep nets could save lives

Researchers say 626 common dolphins were caught in commercial mackerel trawl nets in New Zealand waters between 1995 and 2011. Photo / Alan Gibson

A new study suggests the number of dolphins being captured and killed in commercial trawling nets could dive if vessels changed their fishing methods.

The research, led by Dr Finlay Thompson of Wellington-based Dragonfly Science and published in scientific journal PLOS One, estimates 626 common dolphins were caught in commercial mackerel trawl nets in New Zealand waters between 1995 and 2011.

There were 119 common dolphin capture events across 4299 observed tows in that time, and about 60 per cent occurred during trawls where the top of the net, or headline, was 40m below the surface.

The study concluded the probability of dolphin captures could be halved if the headline depth was increased by 21m.

"I think it is a challenge for [the industry] that they need to confront," Dr Thompson said yesterday. "It's not my place to say what they should or shouldn't do, but the impact on the fishery from catching dolphins is increasing in importance by the fact that these kinds of studies are coming out and forcing them to respond."

He acknowledged capture rates had fallen since the end of the study period.

Because there were as yet no scientific estimates of common dolphin numbers off our coasts, it was difficult to gauge the precise impact on populations, he said.

"There's a fairly large population of common dolphins and they catch quite a few of them, and until we were really reporting on this, I don't think anybody was reporting on the scale of that interaction."

Victoria University Professor of Fisheries Matthew Dunn said that although the study gave an insight into the problem, estimated numbers should not be fixated on because uncertainties around how many were being caught and why remained large.

"Nevertheless, this study is our best understanding at the moment, making good use of the available information, and the conclusions are sensible."

The chances of catching a dolphin increased if a trawler fished near the surface, at night, for a long time, and off the west coast of the North Island, he said.

Whether the nets could really be fished deeper while still catching economical quantities of fish was now a question for the fishers, he said.

Don Carson, of industry group Seafood New Zealand, said commercial fishers had made increased efforts to prevent dolphin bycatches over recent years.

These included not fishing between 2.30am and 4.30am, halting operations if dolphins were spotted near vessels, removing nets from the surface if gear failed and attaching acoustic pinger devices to warn dolphins off.

By the numbers

626
The estimate of common dolphins accidentally netted by mackerel trawls in New Zealand waters between 1995 and 2011

119
Common dolphin capture events, many involving more than one dolphin, recorded over the period

141
The peak in capture events recorded in 2002-03 due to expansion in the mackerel fishing industry

21m
The suggested increase in trawler net headline depth recommended by the study authors, who believe this could halve the number of dolphins accidentally caught.