Seasonal Density Dependence in Atlantic Salmon over Varying Spatial Scales

Abstract

Scale and ontogeny are important in understanding how various ecological processes structure populations. This is expected to be the case for density-dependent effects (DDE), which influence the population dynamics of many organisms. Our goal was to evaluate stage-specific DDE on the growth, movement, and survival of Atlantic salmon Salmo salar from the individual to the group level over varying spatial scales (20–160 m). We used passive integrated transponder tags and associated technology to assess individual growth, survival, and movement histories in a single stream in Maine undergoing restoration efforts via stocking of fry. We studied six life stages over 18 months for four cohorts of presmolt Atlantic salmon. Our findings show that the DDE on growth can be detected at multiple stages well past the young-of-the-year stage in juvenile salmon. We saw a clear development of decreasing DDE on size and growth as cohorts aged. This trend appears to coincide with a tendency for mean fish size to become more variable in low-density reaches. Density had limited effects on individual movements and mortality during most of juvenile rearing, with such effects largely being confined to the period from spring through fall of the age-1 year. Shifting the focus of analysis from individuals to group means spanning larger spatial scales had pronounced effects on both the detection of DDE, the magnitude of apparent density effects, and the amount of variation in fish size explained by density.