|Abstract or Summary
- Ecological resources available to freshwater fish shift spatially, temporally and across life stages. To better understand how spatial-temporal availability of resources influence fish, I examined the phenologies of hatching and emergence of Coho Salmon (Oncorhynchus kisutch) in streams with contrasting and strongly defined seasonal thermal variability. The study streams included groundwater dominated streams that were characterized by low spatial-temporal thermal variability, and surface-water dominated streams which had much higher variability in temperatures. In these streams, I quantified the timings of emergence and tracked individual fish and fish cohorts to understand the consequences of emergence phenologies for diets, bioenergetics (growth and consumption), body size and condition at the end of the growing season. Despite strong differences in thermal regimes hatch and emergence occurred at the same time among streams in the summer. In an attempt to understand the drivers behind this pattern I then evaluated spatial-temporal variability in thermal and trophic resources available to young-of year Coho Salmon post-emergence. Favorable growth temperatures (based on bioenergetic considerations) only occurred in the warmer surface-water stream in the summer months. However, individual growth (g·d-1) of young-of year Coho Salmon was not significantly lower in the colder groundwater stream in the summer, despite previous reports indicating such temperatures to be un-favorable for growth. Macroinvertebrate prey availability was highest overall in the groundwater stream in the summer, but there was significant variability among habitats, seasons and years in all streams. Coho Salmon fed on both larval and adult forms of macroinvertebrates from the benthos, drift, and riparian areas; with the dominant prey item being all life stages of the non-biting midges (F. Chironomidae, O. Diptera). In the last chapter, I incorporated the data collected from my earlier chapters into a bioenergetics model to employ a mechanistic approach in understanding how spatial-temporal resource variability affects juvenile salmon growth dynamics. The bioenergetics model was fit to empirical measurements of growth rates, diet composition, energy densities of the predator (fish) and prey (macroinvertebrates), and water temperatures experienced by fish. Estimated consumption rates (g·g-1·d-1) were higher in the surface-water stream in the summertime, due to the warmer temperatures and thus higher metabolic cost compared to fish growing in the groundwater stream. Further, there was a significant positive relationship between fish size and % lipid content in the groundwater stream only, suggesting that size is related to condition for fish surviving in the colder groundwater stream. Through this work I was able to quantify the consequences of synchronous emergence phenologies for young-of-year Coho Salmon in streams with contrasting thermal and trophic resources. Though fish emerged at similar times, they emerged into environments that offered dramatically different conditions for growth. In spite of this, fish in these streams realized similar rates of growth (g·d-1) and body sizes (mm) at the end of the growing season, with fish in a colder stream exhibiting higher condition. With respect to potential changes to thermal conditions in streams related to regional climate warming, my results highlight a high degree of flexibility in the response of young-of-year Coho Salmon. Understanding this flexibility from a detailed empirical and mechanistic perspective provided important and novel insights into precisely how early life stages of Coho Salmon will potentially respond to changing climates.