Individual growth and its implications for the recruitment dynamics of stream-dwelling marble trout (Salmo marmoratus)

The role of density-dependent individual growth in the recruitment dynamics of fish populations has rarely been analysed in a quantitative framework. Variations of mean size of juveniles in response to changes in population densities have frequently been observed in salmonids and it has been shown that body-size differences at the juvenile stage can persist through the sub-adult and adult stage. As fecundity and sexual maturation are often a function of body size, inter-cohort variations in individual growth may ultimately affect the reproductive output and, consequently, the amount of yearly recruitment. Here, we present a simulation analysis investigating the effects of density-dependent growth on the reproductive output and, ultimately, on the population dynamics of stream-dwelling salmonids. The demographic model used for the numerical analysis was parameterised using marble trout Salmo marmoratus as a reference species and explicitly accounted for the occurrence of major floods events causing the population to collapse to very low densities. Our simulations showed that density-dependent individual growth is a powerful mechanism to foster population resilience through compensatory recruitment. In addition, we argue that density-dependent growth also helps regulate recruitment at high population densities. We show that even slight variations in the growth trajectories of fish substantially affect the size and the dynamics of the population.