Effects of experimentally added salmon subsidies on resident fishes via direct and indirect pathways

Artificial additions of nutrients of differing forms such as salmon carcasses and analog pellets (i.e. pasteurized fishmeal) have been proposed as a means of stimulating aquatic productivity and enhancing populations of anadromous and resident fishes. Nutrient mitigation to enhance fish production in stream ecosystems assumes that the central pathway by which effects occur is bottomup, through aquatic primary and secondary production, with little consideration of reciprocal aquaticterrestrial pathways. The net outcome (i.e. bottomup vs. topdown) of adding salmonderived materials to streams depend on whether or not these subsidies indirectly intensify predation on in situ prey via increases in a shared predator or alleviate such predation pressure. We conducted a 3year experiment across nine tributaries of the N. Fork Boise River, Idaho, USA, consisting of 500m stream reaches treated with salmon carcasses (n = 3), salmon carcass analog (n = 3), and untreated control reaches (n = 3). We observed 2–8 fold increases in streambed biofilms in the 2–6 weeks following additions of both salmon subsidy treatments in years 1 and 2 and a 1.5fold increase in standing crop biomass of aquatic invertebrates to carcass additions in the second year of our experiment. The consumption of benthic invertebrates by stream fishes increased 110–140% and 44–66% in carcass and analog streams in the same time frame, which may have masked invertebrate standing crop responses in years 3 and 4. Resident trout directly consumed 10.0–24.0 g·m−2·yr−1 of salmon carcass and <1–11.0 g·m−2·yr−1 of analog material, which resulted in 1.2–2.9 g·m−2·yr−1 and 0.03–1.4 g·m−2·yr−1 of tissue produced. In addition, a feedback flux of terrestrial maggots to streams contributed 0.0–2.0 g·m−2·yr−1 to trout production. Overall, treatments increased annual trout production by 2–3 fold, though density and biomass were unaffected. Our results indicate the strength of bottomup and topdown responses to subsidy additions was asymmetrical, with topdown forces masking bottomup effects that required multiple years to manifest. The findings also highlight the need for nutrient mitigation programs to consider multiple pathways of energy and nutrient flow to account for the complex effects of salmon subsidies in stream-riparian ecosystems.