The Effects of Fishing on the Timing of Maturity in North Sea Cod (Gadus Morhua L.)
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- Rowell C.A. (1993) The Effects of Fishing on the Timing of Maturity in North Sea Cod (Gadus Morhua L.). In: Stokes K., McGlade J.M., Law R. (eds) The Exploitation of Evolving Resources. Lecture Notes in Biomathematics, vol 99. Springer, Berlin, Heidelberg
Although there is a body of theoretical work on evolutionary responses to harvesting there have been relatively few attempts to apply this to commercially harvested wild populations. In this paper the long term data on North Sea cod are examined for trends in size at maturity. The preliminary work shows that there are differences between areas of the North Sea in the recent data, and so the long term analysis is confirmed to the one area (Round Fish Area 4) for which there was sufficient data. A shift towards maturation at smaller size was found. As there is no evidence for a corresponding change in growth rates it is likely this reflects a change towards maturation at younger ages.
A model of the effects of increased adult mortality on the optimal timing of maturation was also constructed. The model suggested an increasingly large advantage to fish maturing early as the adult mortality increased: a change in the same direction as was found in the data. No record of an environmental trend that could be expected to cause a shift towards maturation at lower ages, and a smaller size was found. It is suggested, therefore, that the observed change may be the result of an evolutionary change resulting from selection imposed by harvesting.
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