The Third International Stock Enhancement & Searanching

Theme 5: Abstract

Genetic Risk of Stocking on Fishes Having Environmentally Influenced Sex-determination


Minoru Kanaiwa
and Yasushi Harada
Tokyo University of Agriculture, Dept. of Aqua-Bioscience and Industry
196 Yasaka, Abashiri, Hokkai 099-2493 Japan
m3kanaiw@bioindustry.nodai.ac.jp

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Partial Abstract:
Sex determination of many fish species is influenced by environmental variables. For example, Japanese flounder has male heterogametic sex determination (XX female - XY male) but XX juveniles can develop to become functional males depending upon environmental conditions. Environmental variables such as temperature, salinity and/or pH reverse a genetically determined sex against genetic sex as sex-reversal. If such a species is involved in a stocking operation, the sex-reversal ratio between wild and hatchery reared juveniles may be different, and there is a risk that the gene for sex determination may be eliminated. We have evaluated conditions for the elimination of the sex determination gene for Japanese flounder and discussed how we can reduce such a risk (Kanaiwa & Harada 2002). In this paper, we extend the analysis to more general situations with both one locus-two allele and environmental sex determination.

We construct the one locus-two allele, A and B, population genetics model. We consider the scenario for which environmental variables make only a one way sex change from genetic sex: i.e., either genetic female or male alone changes to reversed sex by environmental variables. First, we evaluate the condition for the elimination of one of two genes for sex determination when there is no stocking operation. Then, we evaluate the condition for the elimination of one gene for sex determination in the presence of a stocking operation.

From these results, we can argue that there are some genetic risks to the sex-determining system by releasing sex-reversed individuals, considering that environmentally influenced sex determination may be rather common in fishes. We can also argue that such a risk increases in scenario 1 with a stocking operation. To reduce these risks, it is useful to use individuals born in the wild as parents for hatchery production. If we cannot employ scenario 3, we need to have a better understanding of the sex determination system for each target species of the stock enhancement program.