Northern cod, comprising populations of Atlantic cod (Gadus morhua) off southern Labrador and eastern Newfoundland, supported major fisheries for hundreds of years1. But in the late 1980s and early 1990s, northern cod underwent one of the worst collapses in the history of fisheries2,3,4. The Canadian government closed the directed fishing for northern cod in July 1992, but even after a decade-long offshore moratorium, population sizes remain historically low4. Here we show that, up until the moratorium, the life history of northern cod continually shifted towards maturation at earlier ages and smaller sizes. Because confounding effects of mortality changes and growth-mediated phenotypic plasticity are accounted for in our analyses, this finding strongly suggests fisheries-induced evolution of maturation patterns in the direction predicted by theory5,6. We propose that fisheries managers could use the method described here as a tool to provide warning signals about changes in life history before more overt evidence of population decline becomes manifest.
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We are grateful to the many fisheries biologists and technicians who participated in the data collection underlying this study. We also thank S. Barot, O. R. Godø, T. O. Haugen, N. C. Stenseth and L. A. Vøllestad for discussions. S. Barot, W. B. Brodie, O. R. Godø and S. J. Walsh are thanked for their help in initiating the interaction that led to this study. U.D. gratefully acknowledges financial support from the Austrian Science Fund and the Austrian Federal Ministry of Education, Science, and Cultural Affairs. This research has been supported by the European Research Training Network ModLife (Modern Life-History Theory and its Application to the Management of Natural Resources), funded through the Human Potential Programme of the European Commission.
The authors declare that they have no competing financial interests.
The probabilistic maturation reaction norm method. (DOC 38 kb)
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