Evolutionary effect of fishing on maturity in cod

Fisheries and other harvesting
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Age at first reproduction has declined markedly for Barents Sea cod over recent decades. In the 1940s, a cod typically reproduced for the first time when it was between 9 or 10 years old. In the 1990s, average age at first reproduction had declined to between 6 and 7 years. Reduced age at maturity may affect the reproductive capacity of the cod stock, and the cod’s role as an important top predator in the ecosystem. The possible explanation for the declining age at

maturation in Northeast Arctic cod is an adaptive response to high fishing pressure through many years and thus involves genetic changes in the population (Law and Grey, 1989; Jørgensen et al., 2009). Because the number of offspring that a cod can produce increases considerably with body size, older fish generally produce more offspring than young fish. Moreover, the eggs spawned by older cod are more viable than those from younger cod, thus the reproduction potential of the stock has been negatively affected by the development (see Sundby, 2000) for references). This may have significant consequences for cod recruitment and the role of cod as a top predator in the ecosystem. In addition, the decline in average age at maturity has caused the spawning stock to be made up of fewer age groups. This has made recruitment more dependent on environmental factors in recent decades compared to previous times when more age groups of older fish participated in the spawning (Ottersen et al., 2006). It should be noted that fisheries targeting larger more marketable sized individuals also reduces the age structure of the population, but additional evolutionary effects may exacerbate the causes of poor stock condition making it more difficult, and requiring longer time periods, for the stock rebuild if fishing pressure is reduced.

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