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The much needed protection of European commercial fish, and their fisheries, cannot in general be achieved through technical measures, such as closed areas and mesh sizes, alone. Their conservation requires permanent reductions in fishing mortality, which means that less fish must be killed. Models have been developed that allow the quantitative evaluation of the long-term effects of closing areas to fishing on mature biomass and yields of exploited fish populations. The models need information on the demographic biology of the fish, the behaviour of the fishermen and the management system. They assume a quality of data that are available from European marine fisheries. A 'rule of thumb' is given to indicate the relative effects on mature biomass of reductions in juvenile and adult fishing mortality. For most exploited fish populations the mature stock would benefit more from proportional reductions in juvenile mortality. Permanent closures of nursery areas, in which there are high discards of juvenile fish, will assist in increasing the size of the mature stock, and will often increase yields to the fishery. In contrast, the closures of spawning grounds can be counterproductive. For fisheries managed by quotas, a displacement of the fleet from one locality to another, where the same population is fished, will generally have little effect if the same quota is taken. Closed areas may also be beneficial to unregulated fisheries, or those managed by effort, if fishing is displaced from areas of high catch rates causing less fish to be killed. Benefits are also likely where closed areas can supply a significant recruitment to adjacent areas, or where they can give protection to vulnerable species or habitats. More generally, it is possible to evaluate the short- and long-term effects on fish yield and mature biomass of closed areas. The results will be sensitive to the biology of the fish, particularly its population identity, distribution, migration and recruit[neat, the intensity of fishing, the behaviour of the fisherman, and the other management systems in place. Specific analyses are needed in most cases. Many commercial and non-commercial fish spawn on the Trevose fishing grounds off north Cornwall. New material shows the time and location of spawning. Closure of the commercial fishery over spring will not benefit the species regulated by quota if the fish are caught elsewhere. However, a closure is likely to benefit the many other non-commercial and unregulated species that are caught in the area. |
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