Low impact of first-time spawners on population growth in a brown trout population

Abstract

For species with individual variation in reproductive success, experience in breeding and the distribution of different breeders is important for population productivity and viability. Human impacts, such as climate change and harvesting, can alter this distribution and thus population dynamics. Here, we investigated the effect of spawning experience on population growth in a population of migratory brown trout Salmo trutta subject to stressors including migration barriers, harvesting, and climate change. We described the population dynamics with a structured integral projection model that differentiates between first-time and repeat spawners. We then took a scenario-based approach to test to which extent spawning experience has a positive effect on the population growth of brown trout by running 3 different model simulations: a baseline scenario with no changes to the reproductive output of the population, a non-selective scenario in which the reproductive output of all spawners was reduced, and a selective scenario where the reproductive output of only first-time spawners was reduced. We found that the reproductive output of repeat spawners is more important than that of first-time spawners for population growth, in line with other studies. Moreover, the contribution of first-time spawners to the population growth through their own survival is more important than their contribution to growth through reproduction. To ensure the continued existence of the study population, survival of first-time spawners and reproductive success of repeat spawners should be prioritised. More generally, including breeding experience adds more mechanistic detail, which ultimately can aid management and conservation efforts.