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Resource subsidies from adfluvial fishes increase stream productivity

Jones, Nicholas E., Mackereth, Robert W.
Freshwater biology 2016 v.61 no.6 pp. 991-1005
Catostomus catostomus, Catostomus commersonii, Oncorhynchus gorbuscha, ammonium, ammonium compounds, anadromous fish, aquatic food webs, biomass, carbon, dead animals, diet, ecosystems, eggs, energy, equations, excretion, freshwater, growing season, invertebrates, lakes, life history, microorganisms, milt, mortality, nitrogen, nutrients, organic matter, potamodromous fish, rivers, salmon, spawning, stable isotopes, statistical models, streams, subsidies
Anadromous fishes are well known to shape the structure and function of recipient ecosystems by introducing nutrients and rich organic matter from the ocean. In contrast, the importance of potamodromous migrations, confined to freshwater, and the subsidies they provide to stream ecosystems has received much less attention. Our objective was to determine the importance of excretion, eggs, milt, and carcasses as nutrient and energy sources from a large population (82 449 suckers) of migrating longnose (Catostomus catostomus) and common white (Catostomus commersonii) suckers into a small (wetted width c. 10 m) oligotrophic river system. We hypothesise that the adfluvial suckers provide a large material subsidy that increased the productivity of the Cypress River and that this resource subsidy rivals or exceeds those delivered by other native and non‐native fishes (e.g. Pacific salmonids). In total there was an estimated 5635 kg of eggs, 2025 kg of milt, and 1 kg of carcasses from suckers that spawn in the Cypress River. Relative to other mainly non‐native fishes, suckers provided 92% of the annual egg biomass and 95% of the milt. Suckers, however, only provided <1% (1 kg) of the annual carcass biomass, whereas, pink salmon provided 50% (600 kg). Overall, suckers provided 84% and 78% of the annual subsidies of N and P, or 212 and 14 kg respectively. Ambient NH₄ concentrations in the river were consistently below that predicted from excretion equations suggesting that microorganisms may have rapidly taken up much of the released ammonium. Downstream of the falls, epilithon biomass was over nine times more abundant, benthic invertebrate densities were approximately two times higher, and fish biomass was eight times greater compared to upstream. There were no upstream–downstream differences in substrate organic matter biomass. Fishes downstream of the falls had higher δ¹³C and δ¹⁵N values than biota upstream of the falls consistent with the anticipated effect of lake derived subsidies. Using stable isotopes values and a mixing model, we estimated that sucker eggs comprised 25–58% of the diet of stream fishes during the growing season. Suckers provided a large subsidy, greater than all other fishes, without mass mortality and significantly enhanced the productivity of the recipient river system. Stable isotopes revealed that subsidies were incorporated into the stream food web. Differences between salmon and sucker subsidies related to the type, magnitude, timing, and the life history of the donor highlight the potential importance of spawning migrations of adfluvial suckers, and other potamodromous fishes, in streams and rivers around the world.