The Atlantic cod has, for many centuries, sustained major fisheries on both sides of the Atlantic. However, the North American fisheries have now largely collapsed. A new article from scientists at the University of Iceland and Marine Research Institute in Reykjavik provides insights into possible mechanisms of the collapse of fisheries, due to fisheries-induced evolution.

Cod fishing is of highest intensity in shallow water in Iceland and it selects against genotypes of cod adapted to shallow water. The new PLoS One article reports a significant difference in Darwinian fitness (relative survival rate) between shallow-water and deep-water adapted cod. The shallow-water fish have only 8% of the fitness of deep-water fish. This difference can lead to rapid elimination of shallow-water fish in only a few generations with drastic effects on the population and the fishery.

Using molecular population genetics, the authors reports steep changes in the frequency of genotypes at a single genetic locus with depth: a gradient of nearly one half percent drop in frequency per meter. The genotypes at the locus are directly related to behavioral types that select deep vs shallow water habitat by genotype.

"There is no direct targeting of specific genotypes. Instead the intense selection results from the interaction of fish that select their habitat by genotype and fishermen choosing to fish in the preferred habitat of the fish," said Einar Arnason, professor of population genetics and lead author.

In addition to the molecular results, the study also demonstrates that the length and age at which the fish become mature have decreased. So-called "probabilistic maturation reaction norms" show that the length at which there is a 50% probability of becoming mature, has, on average, decreased nearly one centimeter per year. The changes observed very likely are evolutionary genetic changes and not simply plastic phenotypic responses to the environment. They are comparable to changes that preceded the collapse of northern cod at Newfoundland.

This finding further supports the hypothesis of an imminent collapse of Icelandic cod due to the intense fisheries-induced selection. The cod fishery at Iceland is one of the world's few remaining cod fisheries. The study appears to have met all criteria for concern that this fishery is threatened.

"Can anything be done to avert collapse?" the authors ask. A strategy that would remove selection pressures against shallow-water adapted fish would seem to be the answer. The authors speculate that immediate establishment of large no-take reserves might be the right strategy by relieving selection pressures on all genotypes.

The findings provide general lessons for population and conservation genetics that anthropogenic changes in habitat can lead to intense selection even if the mortality is non-selective in the habitat in which it occurs. The study highlights the importance of applying Darwinian principles and evolutionary thinking to fisheries and conservation science.

The study was funded by grants from the Icelandic Research Fund and resources from the Marine Research Institute and the University of Iceland. Specimens were obtained during Marine Research Institute Surveys. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.