Historical abundance and spatial distributions of spawners determine juvenile habitat accessibility in salmon: implications for population dynamics and management targets

Abstract

Spatial distribution of spawning may have important ramifications for population dynamics in species where early life stages suffer from low mobility and high density-dependent mortality. Here, we use time series of spatial spawning distribution in Atlantic salmon (Salmo salar) to test for density-dependent behavioural effects on the spatial utilization of spawning sites and resulting juvenile habitat availability. The probability of utilizing spawning sites in a given year increased both with increasing spawner abundance and proximity to sites used the previous year. The accessible area for juveniles increased asymptotically with both current and time-lagged spawner abundance. Several nonexclusive mechanisms may be responsible for the observed direct and lagged density dependence of spawner distributions, including social aggregation, asymmetric competition for space, local homing, and habitat modification by the previous year’s spawners. Time-lagged density-dependent spawner distributions can be predicted to reduce the realized population growth rate. If such effects are not accounted for, this may lead to a downward bias in estimates of spawning targets or other associated conservation or management measures derived from population abundance time series.