Predicting population responses to restoration of breeding habitat in Atlantic salmon

Abstract

SummaryHabitat degradation is seriously threatening stream biodiversity and productivity world‐wide. For salmonid fish, restoration projects in disturbed systems commonly aim at increasing the availability of suitable breeding habitats, but expectations of effects on population abundance based on explicit models are lacking.We used a spatial population model to predict how breeder dispersion may influence population dynamics of salmonids. Simulations involved changing the relative abundance of habitat suitable for two juvenile size classes (small fry and larger parr), the type of density‐dependent regulation (effects on survival only vs. effects on survival and growth) and intercohort competitive mechanisms.Increased breeding dispersion could alter patterns of density‐dependent mortality and increase equilibrium adult abundance and maximum sustainable yield (MSY). However, there was a strong interaction between stage‐specific habitat abundance and breeding dispersion. The most pronounced effects of breeding dispersion were observed under intermediate levels of fry habitat abundance.When fry habitat was abundant, density‐dependent mortality was most intense during the parr stage, and increased breeding dispersion did not increase adult abundance or MSY. In fact, when populations were regulated in the parr stage, increased breeding dispersion could cause decreased adult abundance because of the effects of intercohort competition. This negative effect only occurred, however, when competition among juveniles was symmetric (no age or size advantage).Synthesis and applications.The population effects of restoring breeding habitat can differ among environments, and we have demonstrated how these differences can be understood in the light of stage‐specific density dependence. Increasing spatial dispersion of breeders may often be an efficient measure for conserving threatened populations and increasing yields in fished populations. However, if direct evidence or habitat considerations suggest that fish populations experience density‐dependent mortality during the parr stage, attempts to increase abundance through increased breeding distribution or artificial stocking should be avoided, as these are likely to be ineffective or even detrimental. The difference between fishing rates providing maximum sustainable yield and extinction is particularly small for such populations, suggesting that fish managers should adopt a conservative and flexible regulation regime.