Survival and predation of Atlantic salmon smolts in anadromous watersheds with lakes: a case study from the Skienselva watershed

Abstract

Atlantic salmon (Salmo salar) exhibit marked niche shifts between spawning and juvenile recruitment in running freshwaters and growth in marine waters. These habitat shifts necessitate long migrations which may come at a substantial cost to the population. As smolts, Atlantic salmon migrate downstream, from freshwater rivers to salty ocean environment, transitioning between two very distinct aquatic ecosystems. This downstream migration is a critical life stage. In some watershed, this migration passes through lakes, transitional habitats where behaviour and survival strategies are not well studied. The preliminary studies based on limited sample sizes through small lakes suggest that migration through lakes can be prolonged and non-directional, potentially resulting in higher mortality rates. When coupled with typically higher predator diversity and density in such habitats, vulnerability to predation increase. A striking example of smolt mortality in lakes is the Lake Tange Sø where the mortality rate reached more than 90%, mainly driven by pike (Esox lucius) and bird predation. The present case study examined survival of Atlantic salmon smolt from upstream riverine recruitment areas, through two major lakes, L. Heddalsvatnet and L. Norsjø in the Skienselva river system in south-eastern Norway, before entering the sea. To assess the risk of predation by pike, stomach content analysis was performed on 14 pike samples during peak migration period. Results indicated nearly half had empty stomachs, while others contained fish remains, including one Atlantic salmon smolt Radio tagged smolt tracked through potential pike habitat in the lower rivers also indicated pike predation. These findings suggest that predation by resident piscivores may contribute to elevated smolt mortality during lake passage.

KEYWORDS: migration cost, Atlantic Salmon smolt, telemetry, predation, Pike,