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

Abstract

Abstract. 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. bottom-up vs. top-down) of adding salmon-derived 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 3-year experiment across nine tributaries of the N. Fork Boise River, Idaho, USA, consisting of 500-m 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 1.5-fold 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 bottom-up and top-down responses to subsidy additions was asymmetrical, with top-down forces masking bottom-up 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.

Linking riparian shade and the legacies of forest management to fish and vertebrate biomass in forested streams

“…, Collins et al. ), and terrestrial invertebrate subsidies (Kawaguchi and Nakano , Saunders and Fausch ) can alter carbon quantity and quality at the base of the food web, which can resonate through higher trophic levels, ultimately influencing fish abundance, biomass, and growth. This concept—that consumers can be limited by food availability—is a fundamental concept in ecology, but as Naiman et al.…”

The roles of extrinsic and intrinsic factors in the freshwater life‐history dynamics of a migratory salmonid

“…, Collins et al. ). The lack of a numerical response after 1990 aligns with previous analyses from the Keogh River (Ward , Ward et al.…”

“…, Collins et al. ). At the population level, artificial subsidies have increased juvenile fish biomass and survival in some studies of sockeye salmon (Hyatt et al.…”

“…, Collins et al. ). It is worth noting that these studies were performed over relatively short periods of time (<12 yr) relative to the salmon life cycle and did not account for juvenile density dependence.…”

Incorporating food web dynamics into ecological restoration: a modeling approach for river ecosystems

“…Fish preferentially selected for and foraged upon carcass material in our model simulations because of the high quality of salmon carcass tissue relative to invertebrate prey (Gende et al 2002), as has been empirically illustrated in carcass addition experiments (e.g., Collins et al 2016). Fish preferentially selected for and foraged upon carcass material in our model simulations because of the high quality of salmon carcass tissue relative to invertebrate prey (Gende et al 2002), as has been empirically illustrated in carcass addition experiments (e.g., Collins et al 2016).…”