SUMMARY AND RECOMMENDATIONS

Genetic changes due to selection

Experimental studies on aquatic organisms have demonstrated that selection can change the mean life history characters of populations within a few generations. In natural populations fishing is a major source of mortality and is non random with respect to age and size of individuals. Frequently the observed changes in life history characters in exploited populations are in the direction predicted from selection experiments and theory. In teleosts one of the most common observations has been a decline in the age and/or size at sexual maturity (and the examples cited are not exhaustive). Because the onset of sexual maturity is under both genetic and environmental control the genetic impact of fishing has been difficult to evaluate. Genetic concepts were not considered in the early fisheries literature reporting changes in life history characters, and the equilibrium models of fisheries management, which are ecologically based and dominated by compensatory effects, have overlooked a genetic component to fishing. It was not until Rickers (1980, 1981, 1982) and Beachams (1983a, 1983b, 1983c) detailed and thoughtful accounts were published in the early 1980s that genetic selection was considered to be a potential problem of overexploitation. All of the examples of selection have been in stocks which have been over exploited. Tighter management controls on stocks may help to reduce the likely genetic impact of fishing in the future.

If there is uncertainty about selection for reduced age/size at maturity in some species then stronger evidence for selection comes from size selection in coho salmon (Gross 1985, 1991) and from the long-term size changes in the semelparous Pacific salmon (Ricker 1980, 1981). Reduction in genetic diversity in orange roughy has occurred in virgin stocks that have been fished beyond the maximum sustainable yield.

In none of the fisheries exhibiting a genetic response to exploitation has there been a total curtailment of fishing to test if the original life history traits can be returned to their former state. Only in the anchovy Engraulis capensis has there been an increase in the length at 50% maturity following an increase in biomass due to recruitment, presumably as a compensatory regulation (Shelton and Armstrong 1983).

Genetic changes due to drift

Genetic drift affects small populations and some rare and endangered freshwater species show low levels of genetic diversity. There is no evidence for loss of diversity due to drift in marine populations, but most populations have not been reduced to near extinction levels. Populations of marine bivalves that have been severely reduced in numbers have not been tested for genetic diversity. There is no evidence that collapsed stocks have suffered a loss of genetic diversity, but evidence is restricted to a comparison of electrophoretic data collected after the collapse. Although the stocks have collapsed from a commercial perspective most have maintained large population sizes even at their lowest state and thus may not be expected to loose genetic variation due to drift.

The increasing use of enhancement techniques to rebuild depleted stocks could have a genetic impact on coastal resources. Inappropriate choice of broodstock and loss of diversity in the hatchery seed could change the genetic composition of the wild population.

Recommendations

Recommendations that follow from this report fall into three areas:

1. Experimental studies are required to determine the heritability and 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. While it is unlikely that an open marine fishery would be allowed to recover to some early exploitation level, it would be desirable to use an experimental population (lake or laboratory) to test for genetic changes under heavy and relaxed levels of exploitation. Experiments have been designed for the pink salmon Oncorhynchus gorbuscha fishery to test the heritability of growth rate and the effects of size selective fishing on a natural population, and it has been shown that there would be no economic disadvantage to conducting such experiments (McAllister and Peterman 1992).
2. It would be desirable to collect and record data on levels of genetic diversity levels in exploited species, especially recently exploited or lightly exploited species. To date most genetic studies have focused on stock identification. The skills and resources developed for these studies could be refocused on intrapopulation studies and use both life history characters and molecular markers to examine genetics of recruitment and genetic changes in stocks. A combination of experimental and field studies would permit a more rigorous testing of genetic changes in exploited populations.
3. Changes to management controls should be considered to increase the minimum size in heavily exploited stocks. Raising the size limit, provided that it is not achieved by higher discard rates of undersize fish, would reduce the selection pressure for early maturity. Controls may take the form of gear restrictions or modifications to avoid the capture of juveniles and/or extensive closed areas to protect juveniles from fishing. To date most marine reserves are small and concentrate on the immediate sub littoral zone with a focus on recreational interests rather than scientific principles of conservation. For enhancement programmes it is recommended that the released seed should be produced from local broodstock and from a minimum effective number of parents (Ne = 50).