Spatial Contraction and Shifting Density Dependence: Implications for Conservation in Depressed Populations of Anadromous Fish

The density-dependent relationship between spawner abundance and recruitment is the foundation of understanding and managing populations of fish such as wild salmon. A core tenet of spawner-recruit modeling is the expectation of compensation, higher per-capita productivity at low abundance. However, some depressed populations have shown stronger than expected density-dependent limitation. We examined the possibility that spatial contraction during population declines can undermine compensation and strengthen watershed-level density dependence. We tested whether changing population structure could alter density dependence using 40 years of steelhead population data from the Keogh River, British Columbia, Canada. Steelhead populations have declined dramatically due to a prolonged period of poor smolt-to-adult survival. The resultant low spawner abundance has been associated with lower production of smolts than expected based on models of compensatory dynamics parameterized pre-decline. Juvenile densities in the Keogh show changes in spatial structure associated with strengthening density dependence. Using a population-dynamics model in a simulated river network, we demonstrated that spatial contraction during a period of poor survival can strengthen density dependence, reducing smolt production at the watershed scale. Our results highlight a novel linkage between smolt-to-adult survival and spatial structure, leading to changes in density-dependent relationships, recovery trajectories, and sustainable harvest levels.