MINIMIZING ADVERSE-EFFECTS OF FISH CULTURE - UNDERSTANDING THE GENETICS OF POPULATIONS WITH OVERLAPPING GENERATIONS

Although an increasing number of natural fish populations are being co ntaminated by exogenous immigrants, knowledge is poor regarding the ge netic changes expected to occur in a wild stock once an introgression has taken place. One reason for this lack of knowledge appears to be t hat the theory for the genetic dynamics is poorly developed and compli cated for age-structured populations with overlapping generations. Usi ng newly developed theory and results from computer simulations, the g enetic aspects of age-structured populations with overlapping generati ons are discussed, especially the detection of contamination and the g enetic dynamics following hybridization. When generations overlap, the amount of temporal allele frequency shift is generally larger than fo r a population of equal genetically effective size with discrete gener ations. This is even more pronounced for the separate cohorts than for the population as a whole. Therefore, when testing for temporal genet ic heterogeneity, a higher frequency of statistically significant resu lts 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 m assive influx of exogenous genes may be very difficult to detect, part icularly in the absence of genetic and demographic monitoring data fro m 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 ap parent for many salmonid species where only a subset of the existing a ge classes may be available for sampling. (C) 1997 International Counc il for the Exploration of the Sea.