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

Journal Article · Fri Mar 25 00:00:00 EDT 2016 · Ecosphere

DOI:https://doi.org/10.1002/ecs2.1248· OSTI ID:1619388

Collins, Scott F. ^[1]; Baxter, Colden V. ^[1]; Marcarelli, Amy M. ^[2]; Wipfli, Mark S. ^[3]; Peters, ed., D. P. C.

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1. Stream Ecology Center Department of Biological Sciences Idaho State University Pocatello ID 83209 USA
2. Stream Ecology Center Department of Biological Sciences Idaho State University Pocatello ID 83209 USA, Department of Biological Sciences Michigan Technological University Houghton MI 49931 USA
3. U.S. Geological Survey Alaska Cooperative Fish and Wildlife Research Unit Institute of Arctic Biology University of Alaska Fairbanks Fairbanks AK 99775 USA

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 bottom‐up, through aquatic primary and secondary production, with little consideration of reciprocal aquatic‐terrestrial 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.

• Sponsoring Organization: USDOE
• OSTI ID: 1619388
• Journal Information: Ecosphere, Journal Name: Ecosphere Journal Issue: 3 Vol. 7; ISSN 2150-8925
• Publisher: Wiley Blackwell (John Wiley & Sons)
• Country of Publication: United States
• Language: English