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Linkages between temperature, macroinvertebrates, and young-of-year Coho Salmon growth in surface-water and groundwater streams

July 10, 2020

Ecological resources for fishes in stream food webs shift over space and time, providing a complex template of available resources that can be used for growth. We tracked water temperature in conjunction with young-of-year Coho Salmon size, growth, and diet in 2 streams with contrasting thermal regimes: a groundwater stream with colder temperatures and lower thermal variability all year and a surface-water stream with greater thermal variability and warmer summer temperatures more conducive to young-of-year salmon growth. We hypothesized that fry emergence would occur when rearing conditions are optimal for growth and that, all else being equal, summer fish growth will be greater in the surface-water stream. Previous work on Coho Salmon phenology in these streams showed that peak fry emergence occurred at the same time in early summer in both streams. We measured salmon fry emergence in relation to thermal variability and macroinvertebrate prey availability with subsequent tracking of somatic growth, diet, and body size during the 1st year of life in both streams. Macroinvertebrate prey availability was highest overall in the colder and thermally-stable groundwater stream than the surface-water stream. Prey availability was particularly high in the thalweg drift during peak fry emergence in the groundwater stream. There was no difference in Coho Salmon diet composition between streams, which included invertebrates from benthic, drift, and riparian habitats. We found no differences in young-of-year Coho Salmon body size, growth, or consumption between streams. Overall, our results suggest that large differences in thermal regimes do not necessarily translate to large differences in young-of-year Coho Salmon size, growth, or diet. Many variables can influence fish growth, and there is not always a direct connection between spatial and temporal dimensions of environmental variability and their cascading effects on young-of-year Coho Salmon growth during the 1st summer of life.