Freshwater rearing is a critical period in the life cycle of anadromous salmonids, known to produce carry-over effects mediating long-term growth and survival. Freshwater growth is primarily determined by food availability and temperature. While many species of salmon rear in freshwater for a full year or longer, most trophic ecology research is from a narrow window of time within the annual cycle. This work identified temporal biases in research, addressed gaps in understanding the trophic resources that support juvenile salmonids during key phases of freshwater rearing, and examined seasonal variability in asynchronous resource pulses and terrestrial-aquatic interactions. The first study in this thesis quantified temporal biases in the timing and duration of juvenile Pacific salmon and trout field research from the past 30 years. I found that across ecological topics, summer studies occurred 3x as commonly as winter studies and 57% of studies were focused on a single 3-month season. The second study in this thesis described the monthly variation in prey resources utilized by juvenile Steelhead Trout (Oncorhynchus mykiss) and Coho Salmon (O. kisutch) in the South Fork Skokomish River and its
tributary Vance Creek in Hood Canal, Washington, USA. The results from this study suggest that stable in situ prey resources and ephemeral allochthonous prey resources create an asynchronous energy portfolio supporting juvenile salmonid freshwater rearing throughout the full annual cycle. Aquatic resources from the benthos and drift provided a relatively stable source of energy to juvenile salmonids year-round but contributed most heavily to fish diet during summer somatic growth, overwinter survival, and early spring prior to smoltification. Terrestrial subsidies contributed to fish diet over the course of spring, summer, and autumn; however, within the aggregate of terrestrial inputs, I found multiple ephemeral pulses of specific prey items. These ephemeral terrestrial invertebrates contributed heavily to fish diets and coincided with stressful developmental phases including smoltification in spring and lipid storage prior to the onset of winter: 66% of Coho diet biomass in May came from rove beetles, 13% of Coho diet biomass in October came from spiders, and 30% of Steelhead diet biomass in October came from ground beetles. To an even greater extent, ephemeral marine subsidies in the form of salmon eggs provided energy to salmonids during a key development phase as they stored fat in preparation for overwinter survival. Multiple spawning events from three different species of salmon provided an extended resource pulse of salmon eggs utilized by juvenile Steelhead and Coho with 61-96% of prey consumption during spawning months coming from marine subsidies. Moreover, despite the depressed size of salmon runs in this system, consumption of marine subsidies resulted in substantially larger rations than seen at any other point in the year. While summer research may be more common, the largest energy fluxes in this field study came during spring and autumn. As salmon
populations in the Pacific Northwest continue to decline, conservation and recovery efforts could benefit from incorporating knowledge of temporal variability in the trophic resources that juvenile salmonids use throughout freshwater rearing.
Funding Statement (additional comments about funding)
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.