Environmental change effects on life‐history traits and population dynamics of anadromous fishes

1. Migration, the recurring movement of individuals between a breeding and a non‐breeding habitat, is a widespread phenomenon in the animal kingdom. Since the life cycle of migratory species involves two habitats, they are particularly vulnerable to environmental change, which may affect either of these habitats as well as the travel between them. 2. In this study, we aim to reveal the consequences of environmental change affecting older life‐history stages for the population dynamics and the individual life history of a migratory population. 3. We formulate a population model based on the individual energetics and life history to study how increased energetic cost of the breeding travel and reduced survival and food availability in the non‐breeding habitat affect an anadromous fish population. 4. These unfavourable conditions have impacts at the individual and the population level. First, when conditions deteriorate individuals in the breeding habitat have a higher body growth rate as a consequence of reductions in spawning that reduce competition. Second, population abundance decreases, and its dynamics change from a regular annual cycle to oscillations with a period of four years. The oscillations are caused by the density‐dependent feedback between individuals within a cohort through the food abundance in the breeding habitat, which results in alternation of a strong and a weak cohort. 5. Our results explain how environmental change, by affecting older life‐history stages, has multiple consequences for other life stages and for the entire population. We discuss these results in the context of empirical data and highlight the need for mechanistic understanding of the interactions between life‐history and population dynamics in response to environmental change.