Genetic considerations in the introduction of aquacultured fish to natural ecosystems
By A. Magoulas, Department of Genetics and Molecular Biotechnology ,Institute of Marine Biology of Crete, P.O.Box 2214, Heraklion 71003, Crete, Greece.

Abstract

Introduction in the natural aquatic ecosystems of fish grown in captivity is a common phenomenon, which usually results either from planned stocking activities or from accidental escapes from farms. Although the introduction of genetic material in wild populations may be in some instances advisable, for example when a natural stock has been depleted for a long time, in most of the cases this practice is loaded with several threats for the genetic integrity of receiving populations. The inundation of a broad area by introduced genetic material may result in the loss of the genetic structure and variability and thus in the genetic homogenization of the populations in this area. Valuable gene pools, such as genes or co-adapted gene complexes could be lost through their replacement by exogenous genes, a phenomenon known as genetic introgression. This happens when the released fish interbreed with the wild ones and the genetic characteristics of the former differ from those of the latter.

Since the existence of natural population subdivisions may imply adaptation to local conditions, genetic assessments of the degree of population structuring and gene flow are necessary not only to preserve the existing biodiversity, but also to keep valuable adaptive resources. The assessment of the degree of genetic differentiation between cultivated and wild populations, as well as monitoring of the changes in genetic composition of the receiving populations after release, should constitute an integral part of any translocation or restocking program. Several types of genetic markers can be used for the analysis of genetic variation in populations, most of which are considered as evolutionary neutral. Between them, microsatellites are currently the most extensively used, since their high mutational rate and polymorphism offer high-resolution power. The inclusion in the studies of markers under selection could help in the understanding of the relationship between genetic divergence and fitness differences implying local adaptation in the populations.

A common practice of stocking is supportive breeding, whereby a certain number of wild fish are caught and reproduced in captivity and the progeny is released into the environment. This practice may be harmful for the native populations, even if no exogenous genetic material is introduced and the released fish are not adapted to the artificial rearing conditions, because it may lead in a reduction of the effective population size and hence of the genetic diversity of the receiving stocks.

To avoid problems caused by the accidental escapes of reared fish, specific measures such as farming of local stocks, manipulation of sex and ploidy (e.g. production of mono-female or sterile triploid fish) etc, should be employed.