Diel Horizontal Migration: Juvenile Coho Salmon Exploit Trade-Offs Between Trophic and Thermal Resources in Streams

Diel movements are a ubiquitous behavior for fishes in lentic and marine systems, but are rarely documented in lotic systems. We researched the foraging behavior of coho salmon that rear in thermally heterogeneous streams in SW Alaska and exploit seasonally pulsed subsidies of sockeye salmon eggs. In our focal stream, upwelling groundwater generates a longitudinal thermocline, where summer temperatures are cold downstream (mean =6-7° C ) and warm upstream (mean=9-11° C). Sockeye salmon only spawned in the cold downstream reaches, so the resource pulse of eggs occurred in thermal habitat that limited the digestive capacity of coho salmon. We discovered eggs in the diets of coho salmon residing in warm water as far as 1200 m upstream of the nearest spawning salmon. RFID antenna arrays confirmed that these fish made nighttime feeding forays into the downstream reach—a round trip of at least 1 km.  Individuals spent 1-4 days residing in warm water between feeding forays. In situ measures of gastric evacuation combined with observed individual variation in growth support the hypothesis that these large-scale feeding forays provide adaptive behavioral thermoregulation to juvenile coho salmon. These results demonstrate how stream-dwelling fish benefit from and actively exploit habitat heterogeneity.