Genetic diversity of marine fisheries resources - Possible impacts of fishing
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This report reviews the evidence for the genetic impact of fishing on marine fisheries resources. The most widely used method for measuring genetic diversity in natural populations has been protein electrophoresis; marine teleosts have levels of genetic diversity ranging from 0.0 to 18% and marine invertebrates from 0.4 to 32%. Genetic studies have shown that populations of marine species are less differentiated than freshwater species, experience temporal genetic changes, can be changed locally by pollution, and contain cryptic species. Genetic changes in populations occur through selection or drift. In natural populations fishing is a major source of mortality and is non random with respect to age and size of individuals. A common observation in heavily exploited teleost fisheries has been a decline in the age and/or size at sexual maturity. Size selective fishing would favour early maturity. However growth rate in some fishes is density dependent and increases when the stock is r educed; faster growth rates lead to a reduction in the age or size at onset of sexual maturity. Thus it is not possible to determine if the observed changes are genetic or compensatory in response to reduced stock density. Genetic drift is unlikely to be a major factor influencing levels of genetic diversity in many marine fisheries, except for some populations, e.g. giant clams, which have been reduced to near extinction levels. Some rare and endangered freshwater fishes show low levels of g enetic diversity. There is no evidence for loss of genetic diversity in collapsed stocks of pelagic species. While the stocks have collapsed from a commercial perspective most have maintained large population sizes at their lowest state. The use of hatcheries to produce seed for aquaculture and enhancement could lead to loss of genetic diversity in natural populations through escape of farm stock or inappropriate choice of broodstock. Experimental studies are required to determine the heri tability and the response to selection of life history characters of exploited species, and to determine if relaxation of fishing pressure allows the recovery of fast growing and late maturing genes or gene complexes in populations. Also it would be desirable to monitor levels of genetic diversity in recently exploited or highly exploited species. A combination of experimental and field studies would permit a more rigorous testing of genetic changes in exploited populations. If genetic change s are demonstrated in exploited species then changes to management would be needed to conserve natural levels of diversity.