Juvenile Coho Salmon Exhibit Compensatory Mechanisms in a Large Volcanic Spring-Fed River

Southern Oregon/Northern California Coast (SONCC) coho salmon are currently listed as threatened under both the federal Endangered Species Act (ESA) and the California Endangered Species Act (CESA). Consequently, there is an urgent need to identify and understand the habitats and ecological processes that can assist recovery planning and enhance viability under a rapidly changing climate.  We quantified the growth and production of juvenile coho in five stream segments that differed in their spatial proximity to cold water spring sources on the Shasta River.  We found strong differences in mean weekly maximum temperatures (MWMT), invertebrate prey availability, and the growth and condition factor of juvenile coho salmon.  Coho salmon reared in close proximity to springs experienced MWMTs ranging from 14.8°C to 16°C, exhibited an apparent growth rate of 0.13 mm/day, and a 26% increase in mass over the nine week study period.  Conversely, individuals reared six kilometers downstream from cold water spring sources experienced MWMTs ranging from 17.6°C to 21°C, exhibited a growth rate of 0.27 mm/day, and a 161% increase in mass during the same period.  Our results indicate that juvenile coho salmon may have the ability to metabolically compensate for elevated water temperatures when food resources are abundant.