Research Document 1998/156

Genetic impacts on wild Atlantic salmon (Salmo salar L.) stocks of escaped farm conspecifics: An assessment of risk

By E. Verspoor

Abstract

The potential exists for river stocks of wild Atlantic salmon (Salmo salar L.) to be genetically altered when escaped farm conspecifics enter rivers. Changes can occur indirectly, through ecological interactions, or directly, by interbreeding. Wild salmon stocks are composed of locally adapted populations whose gene pools are moulded by selection to increase survival and reproductive success i.e. fitness, and adaptive differences can be assume to exist between farmed and wild salmon. As a result, genetic changes have the potential to alter fiftness. The resulting fitness change is unlikely to be positive. Limitations in our knowledge, and in our ability to detect and monitor both changes and fitness, make it difficult to predict the extent of fitness depression which might occur. Variation in the genetic characteristics of farm and wild stocks, and the environmental circumstances associated with interactions, ensure each outcome will be more or less unique, and impacts could be short or long term. Where relative numbers of farm salmon or genetic differences with wild stocks are small, interbreeding is sporadic, and wild stocks are healthy, natural selection can be expected to reverse most adaptive genetic change. If relative numbers of farm salmon or genetic differences are large, interbreeding is persistent, and stocks are declining, stock viability can be expected to be further reduced. Long term, or even permanent, changes to stock character may result and contribute to the demise of wild stocks or their constituent populations. In the absence of a predictive capacity, current management must proceed using a precautionary approach. Where feasible, escapes of reproductively competent farm fish should be eliminated. Otherwise escapes shoud be reduced below natural straying rates and the sustainable abundance of wild stocks maximized. Research should focus on increasing empirical understanding by assessing specific interaction scenarios, and enhancing understanding of the nature and extent of local adaptation in the Atlantic salmon.

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