Restoring productivity of salmon-based food webs: Contrasting effects of salmon carcass and salmon carcass analog additions on stream-resident salmonids

We tested the hypotheses that salmon carcasses and salmon carcass analogs (dried, processed hatchery salmon) increase the condition factor, production, and whole-body lipid content of stream-resident salmonids and that stream shading affects responses to enrichment. Two enrichment treatments (salmon carcass, salmon analog) and a control, each with and without simulated riparian shading (95% shade), were replicated six times in once-through artificial channels (mesocosms). Each channel was stocked with three live young-of-the-year coho salmon Oncorhynchus kisutch. The experiment spanned 30 July through 18 September 2002. Production and lipid content significantly increased in both carcass and analog treatments. Condition was generally unchanged. Stream shading significantly reduced the magnitude of production and lipid responses. In addition, in a small stream where salmon carcasses and analogs were added to two separate tributaries, cutthroat trout O. clarki had higher mean condition, production, and lipid content than fish in stream sections that were not enriched (6 August to 20 September 2002). Furthermore, coho salmon production was also higher in an analog-enriched reach than in control reaches. This study illustrates that marine subsidies from salmon spawners and salmon analogs increase condition, production, and lipid concentrations of resident and anadromous salmonids. Larger and more lipid-rich fish may subsequently have higher survival and reproduction, thereby elevating freshwater and marine salmonid production. Salmon analogs appear to hold promise for restoring nutrients and productivity to freshwater ecosystems suffering reduced salmon runs and may prove to be a useful, albeit temporary, tool for improving aquatic productivity.