Norms of Reaction in Fishes

Abstract

The norm of reaction is defined as the ‘phenotypic expression of a genotype in different environments’ (Schmaulhausen 1949). This concept is of importance in fisheries management because it describes how morphology and life history can change in response to changes caused by fishing. Specifically, high fishing pressure often reduces population densities and increases resource availability to each individual that survives. I summarise experiments on guppies that show that increased resource availability causes an increase in the size at maturity, a decrease in the age at maturity, increased fecundity, increased frequency of reproduction and an increase in population growth rate. An independent experiment shows that increased mortality, a more direct consequence of fishing, selects for earlier maturity at a smaller size and higher fecundity early in life. Both of these changes can be expected in response to high fishing pressure, but the plastic response to food availability is much larger than the evolved response to high mortality rates. This environmental effect can thus overshadow the direction of life history evolution, even though the latter still occurs. If this were true of a natural fishery, then there would be active selection for smaller body size, an undesirable result, that would be masked by the environmental response to high resource availability. I also evaluate the generality of these results with respect to the published literature and discuss the possible value of investigations on such model systems for the management of economically more important, but experimentally less tractable species.