Artificial Selection on Growth and Correlated Selection on Competitive Behaviour in Fish

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Abstract
There is widespread concern that artificial selection for rapid growth will indirectly select for aggressive fish, thus nullifying genetic stock improvement programs based on selection. We show that this unfortunate result is highly unlikely. The argument is based on a game-theoretic analysis of two general features of fish competition: (1) the growth of some individuals is often suppressed even when food is abundant and (2) the behaviour of a fish towards other members of a population depends on its relative size. The commonsense view that growth rate selection favours aggressive fish is shown to be correct only when aggressive behaviour is independent of relative size and resources are severely limited. In experimental and aquaculture environments, growth rate selection is shown to favour a decrease in the value of both aggressive and submissive behaviour, i.e. fish which ignore each other. Artificial selection for rapid growth will therefore indirectly select for tameness, not aggression.