Individual energy dynamics reveal nonlinear interaction of stressors threatening migratory fish populations

1. Migratory fish populations, like salmon, have dramatically declined for decades. Because of their extensive and energetically costly breeding migration, anadromous fish are sensitive to a variety of environmental stressors, in particular infrastructure building in freshwater streams that increases the energetic requirements of the breeding migration and food declines in the ocean. 2. While the effects of these stressors separately are well documented, the cumulative and interactive impacts of them are poorly understood. 3. Here, we use a bioenergetics model recently developed for fish life history to investigate the individual life history and population responses to these stressors combined. 4. We find that food decline in the ocean can mitigate rather than exacerbate the negative effect of elevated migration costs imposed by infrastructure building in streams. This counterintuitive effect results from the highly nonlinear manner in which these stressors interact and affect the individual energetics. In particular, this effect arises from the fact that individuals growing in the ocean under higher food conditions reach larger sizes with concomitant larger migration costs but are leaner. As a consequence of their lower energy densities, they spend most of their energy reserves to transport their body upstream when migration costs are high, and little is left for reproduction, resulting in lower individual fitness. 5. Our results highlight the need of a mechanistic understanding integrating individual energetics, life history and population dynamics to accurately assess biological consequences of environmental change. A free Plain Language Summary can be found within the Supporting Information of this article.