Published online 7 March 2008 | Nature | doi:10.1038/news.2008.658

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Trawling shallower depths can affect fish in the deep sea.

Trawlers: they reach deeper than thought. Punchstock

The huge cables used by commercial deep-sea fishing trawlers have a much greater reach than anyone realized. That's the conclusion of a new analysis of fish surveys that reveals dramatic declines in the abundance of fishes in deep waters off Ireland.

David Bailey of the University of Glasgow, UK, took a fresh look at a unique record of seasonal scientific fish trawls begun in 1979 by collaborator John Gordon of the Scottish Association of Marine Science in Oban. He presented the findings on 5 March at the Ocean Sciences Meeting in Orlando, Florida.

The trawls were run at the Porcupine Seabight, southwest of Ireland, at depths of a few hundred to 4,000 metres. "What's truly amazing about this is that they go all the way from the top of the [continental] slope out onto the abyssal plain," says Jeffrey Drazen, a deep-sea fish specialist at the University of Hawaii at Manoa, who was not involved in the study. "It's the only data set like it in the world," he says.

Bailey and his co-authors compared the abundance of fish in the trawls during 1979–89 against results from trawls in the same area during 1997–2002. Commercial trawling, which typically drags nets across the ocean bottom, increased during this time period and went to greater depths. The study found dramatic declines of about 50% in overall fish abundance over the years.

This is no surprise for depths down to 1,500 metres, where commercial fishermen operate, targeting species such as the orange roughy and grenadiers. But the declines were also dramatic at greater depths, as deep as 3,000 metres in some cases. Results were similarly dismal for all of the more than 20 fish species examined, which included many, such as eels, that are not commercially harvested.

Deep damage

The likely cause for the declines at lower depths, says Drazen, is that many deep-sea fish species are ‘ontogenetic migrators’. This means that they spend their younger years in shallow waters, then make their way to deeper grounds as they get older. So, trawling younger fish at one depth can ultimately reduce the number of fish that live long enough to move deeper. This is true even for species of non-commercial interest, because a ride in a trawl net is usually deadly to a deep-sea fish, even if it is eventually discarded as bycatch.

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"No one had really looked at this issue of ontogenetic migration and how it might affect populations below the fishing depths," says Drazen, "That's the big contribution that [Bailey] and his co-authors have made."

Based on the study, the group concludes that the impact of commercial trawling on fish abundance extends to roughly double the depth, and so double the area, where fishing actually takes place — an increase from 11% to 23% of the sea floor. "The impacts of fishing in the world's oceans are greater than we previously thought," says Drazen. "Regions of the ocean that we thought were pristine or relatively untouched by fishing are not."

Other negative effects from deep-sea trawling have previously led to calls for bans of the practice.