Salmonid thermal habitats in Lake Superior tributaries

Abstract

For some potamodromous salmonid species, such as the Brook Trout, streams are a critical spawning habitat but also habitat for juveniles and stream residents to grow. Salmonids are coldwater fishes and cooler stream water temperatures are essential for individuals to avoid temperature stress and in some cases death. My goal was to study relationships between salmonid relative abundance and stream thermal habitat characteristics observed at multiple spatial scales within Lake Superior tributary streams. These spatial scales included the microhabitat (~1 m2 ), reach (50 m) and segment (2 km) spatial scale. At the microhabitat scale, the ability of unbaited underwater cameras to obtain estimates of relative abundance comparable to the estimates of relative abundance obtained through electrofishing surveys was evaluated. The estimates of relative abundance obtained using visual surveys and electrofishing surveys were found to be comparable, and thus microhabitat relative abundance and reach catch-per-minute were the measurements of relative abundance used to study salmonid ecology at the three spatial scales. Results showed that salmonid relative abundance at the microhabitat scale was lower when surface water temperature was higher, higher when the temperature variation observed within a reach was higher and was higher stream segments had a higher density of predicted flow pathways. However, at all spatial scales, these temperature variables did not explain variations in salmonid relative abundance to the same degree as certain random variables such as stream, date and year sampled. Use of landscape spatial scale analyses generally performed well in locating areas of thermal refugia within a stream network. Most reaches containing predicted flow pathways exhibited a greater range in water temperatures than non-flow pathway reaches with multiple reaches contributing cold water to localized regions of a stream throughout the entire summer. Though patterns of salmonid distribution and abundance within a stream network cannot be solely explained using thermal habitat characteristics, sustainable land use practices that maintain the thermal integrity of streams should allow Lake Superior and its tributary streams to continue supporting healthy salmonid populations.