Adult salmon provide literally tons of marine derived energy and nutrients to freshwater ecosystems annually throughout the world, but their runs have been severely reduced in many places, particularly over the past century. Scientific evidence largely supports the hypothesis that many species and communities rely upon this marine subsidy to partly sustain their productivity. We are conducting a multi-watershed nutrient supplementation, adaptive management project in Idaho, to measure differences between two nutrient supplementation strategies (salmon carcasses and salmon carcass analogs) in headwaters where anadromous fishes have been extirpated, to mitigate, at least in part, for the loss of marine nutrients once delivered by large runs of Pacific salmon (Oncorhynchus spp.).
Results/Conclusions

Inorganic soil nitrogen levels increased 500-1500 μg g^-1 within the immediate vicinity of riparian-deposited salmon carcasses.  Soil nitrogen levels increased as deep as 10 cm and remained elevated for almost one year following carcass decay.  Two weeks after treatment application, stream algae increased three and six times in the analog and carcass streams, respectively, compared to control streams; total microbial biomass increased two-fold in both the carcass and analog streams. Two weeks after treatment application adult aquatic insects, predominantly chironomid midges, were most abundant along analog-treated streams, and terrestrial Dipterans (Calliphoridae and Muscidae flies) most abundant along carcass-treated riparian habitats.  Resident fish growth doubled in both the carcass and the analog treatments within six weeks.  Fish diet data suggests this short-term increase in fish growth might be due to direct consumption of analog and carcass material. To date, treatments have not increased bat foraging frequencies, activity, and species diversity.  And we have found no increase in isotopic carbon signatures of bats, suggesting that bats are either not consuming insects that have taken up marine-derived nutrients (¹³C, ¹⁵N), or not consuming them in quantities high enough for marine signals to be detected.  Bear data are still undergoing analyses. Beginning 2010, an inorganic (i.e., no carbon) treatment will be added to the study, to additionally investigate the role of carbon on these headwater food webs. These data support the hypothesis that nutrient supplementation can help mitigate for the loss of marine-derived nutrients once provided by anadromous fishes in places where runs of those fishes have declined.