Icelandic Cod in Treacherous Waters

29 May 2009 (All day)

Einar Agustsson/Marine Research Institute

Knife-edge? Cod in shallow waters may be facing collapse.

The cod fishery around Iceland is one of the largest in the world, yielding roughly 200,000 metric tons a year. The stocks are in far better shape than the collapsed fisheries in the western Atlantic. Nonetheless, new research on cod genetics suggests that fishing is changing the population in ways that could lead to a partial collapse.

Icelandic cod are caught at a variety of depths: Most fishing goes on with lines and nets in relatively shallow coastal waters less than 100 meters deep. Bottom trawling can target cod down to 200 meters or so.

A few years ago, researchers using data loggers attached to fish discovered that the cod prefer distinct habitats, with some living exclusively in shallow water and others staying offshore, only coming near the coast in the spring to breed. The genetics of the two groups reflect the disparity: They have different versions of a gene called pantophysin I, whose function isn't known.

Einar Árnason, a geneticist at the University of Iceland, Reykjavík, wondered whether fishing might be altering the genetic composition of the cod stock. He and his colleagues genotyped more than 8000 fish to find out how the frequency of different versions of the gene, or alleles, changed in the two populations between 1994 and 2003. As they report this week in PLoS ONE, the shallow-water genotype has become less common over time. That makes sense, because the fishing is more intense there.

If fishing remains intense, Árnason predicts, "shallow-water fish will disappear" within 10 years. If the deep-water cod don't then spread into the shallows--and Árnason doubts they will, because their genetic difference suggests they are adapted to deep water--the size of the total population would shrink. Moreover, the industry would have to switch to expensive deep-water trawling. Árnason and his colleagues argue that to avert a collapse, large marine reserves where fishing is prohibited are required.

David Conover of Stony Brook University in New York state says the important question is whether the deep-sea cod will ever evolve a preference for shallow waters.

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