Minimizing adverse effects of fish culture: understanding the genetics of populations with overlapping generations

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Although an increasing number of natural fish populations are being contaminated by exogenous immigrants, knowledge is poor regarding the genetic changes expected to occur in a wild stock once an introgression has taken place. One reason for this lack of knowledge appears to be that the theory for the genetic dynamics is poorly developed and complicated for age-structured populations with overlapping generations. Using newly developed theory and results from computer simulations, the genetic aspects of age-structured populations with overlapping generations are discussed, especially the detection of contamination and the genetic dynamics following hybridization. When generations overlap, the amount of temporal allele frequency shift is generally larger than for a population of equal genetically effective size with discrete generations. This is even more pronounced for the separate cohorts than for the population as a whole. Therefore, when testing for temporal genetic heterogeneity, a higher frequency of statistically significant results may be expected than can be explained by genetic drift caused by a restricted effective population size. During introgression, a sudden infusion of new genes initiates marked allele frequency fluctuations, and in salmonids this “genetic instability” may persist for several decades. In spite of these potentially dramatic fluctuations, even a massive influx of exogenous genes may be very difficult to detect, particularly in the absence of genetic and demographic monitoring data from the natural population prior to immigration. A conservative attitude is recommended when interpreting allele frequency differences within populations where the history and the demographic characteristics are poorly known. The risk of incorrect interpretations is particularly apparent for many salmonid species where only a subset of the existing age classes may be available for sampling.