Mechanistic Linkage of Hydrologic Regime to Summer Growth of Age-0 Atlantic Salmon

Abstract

Significant reductions in juvenile stream salmonid growth have been observed in association with low summer flow, but underlying mechanisms are poorly understood and predictive power is limited. We conducted a stage-specific analysis of the relationship between summer flow and the growth of age-0 Atlantic salmon Salmo salar in two rearing sites in the upper Connecticut River basin, New Hampshire. We contrasted effects of variation in foraging habitat availability and temperature on individual age-0 Atlantic salmon mass during one high-flow year and two low-flow years and from high- and low-flow sites within years. Overall age-0 Atlantic salmon mass was positively correlated with the availability of model-predicted favorable foraging locations and negatively correlated with density during the summer. Individual Atlantic salmon mass and the proportion of temperature-predicted maximum mass were lowest during the two low-flow years and were lower in upstream than in downstream sections. Between-year variation in growth was not closely associated with temperature model predictions. However, some of the difference between upstream and downstream sections appeared to be associated with lower summer temperatures in the upstream section. Our case study provides a framework for combining empirical and modeling approaches to quantify the potential impact of hydrologic change on fish growth and for linking variation in stream discharge to juvenile Atlantic salmon performance across time and space.