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The effect of fishing on the evolution of North Sea cod

Bridson, Jessica (2001) The effect of fishing on the evolution of North Sea cod. PhD thesis, University Of Strathclyde.

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Abstract

With the recent collapses of many major fish stocks and North Sea cod seeming to be the next on a long list, it has become apparent that our actions have a great effect on fish populations. This thesis looks at how fishing can not only have an immediate effect such as causing declines, but can also affect the evolution of a stock. A population model is built which is first examined for stability properties. A comparison is also made of the model under fishing and when fishing is absent. A measure of population fitness in terms of ability to invade other populations is then established. This measure is used to examine the sensitivity of the model to parameter values. This is also done for models which use different functions to model life history in order to determine the importance of model choice. Components of fishing mortality are considered with respect to their impact on the stock, for both the main model and the alternate models. Finally, spatial and seasonal considerations are added in a simple way to check if a single region model can be trusted to model the whole of the North Sea. It is found that although the model is sensitive to the choice of growth function, generally growth has the most effect on population fitness. It is also shown that the level of fishing has more impact on the fitness and yield of the stock than the initial capture length. Thus, it is more important to reduce fishing effort, than change aspects of the fishery such as mesh size in nets. Furthermore the spatial model shows that the establishment of reservoirs, or no-fishing zones, should be done carefully in order not to favour a decrease in growth rate of the stock.