Marine Subsidies in Freshwater Ecosystems: Salmon Carcasses Increase the Growth Rates of Stream-Resident Salmonids

—We tested the hypotheses that marine-derived resource subsidies (salmon carcasses) increase the growth rates of stream-resident salmonids in southeastern Alaska and that more carcasses translate into more growth. Five carcass treatments of pink salmon Oncorhynchus gorbuscha (0, 1, 2, 3, and 4 carcasses/m2 or 0, 1.9, 3.7, 5.6, and 7.4 kg wet mass/m2) were replicated six times in once-through artificial channels, then each channel was stocked with three live age0 coho salmon O. kisutch. The experiment spanned more than 9 weeks: 16 August to 24 October 1998. The body mass and fork length of the young coho salmon significantly increased from carcass additions, but the incremental increases sharply diminished at carcass-loading levels above 1 carcass/m2. Further, in a small stream in which we added salmon carcasses to a cumulative density of 0.54 carcasses/m2, both cutthroat trout O. clarki and Dolly Varden Salvelinus malma grew significantly faster during the 2 months in which carcasses were added (September–October) compared with fish in control reaches. Fish maintained their assimilated body mass through winter into the following spring. This study illustrates that marine nutrients and energy from salmon spawners increase growth rates of resident and anadromous salmonids in streams. This elevated growth should translate into increased survival and reproduction, ultimately elevating freshwater and marine salmon production. Ecological relationships between salmon runs and aquatic community nutrition and productivity may be important considerations for salmon stock protection and restoration and for freshwater and marine ecosystem management. Each year, tons of marine-produced biomass are spread throughout freshwater and riparian ecosystems in Alaska and other coastal regions when salmon migrate to their natal habitats to mate (Mathisen et al. 1988; Levy 1997; Cederholm et al. 1999). Pacific salmon sequester marine carbon and nutrients while maturing at sea and transfer this accumulated biomass to freshwater habitats where they spawn and die (Mathisen et al. 1988; Groot et al. 1995). The effects of this biomass influx include transfer of marine nutrients and energy to plants and animals in freshwater and riparian ecosystems, resulting in increased densities, biomass, and marine isotopic signatures of many species (Kline et al. 1997; Bilby et al. 1998; Wipfli et al. 1998). Although some evidence shows that more spawners equates to increased stream pro* Corresponding author: [email protected] 1 Present address: Pacific Northwest Research Station, U.S. Forest Service, 1133 Northwestern Avenue, Wenatchee, Washington 98801, USA. Received March 5, 2002; accepted September 30, 2002 ductivity (Wipfli et al. 1999), the broader ecological effects of this marine subsidy remain poorly understood (Gende et al. 2002; Naiman et al. 2002). These anadromy-driven linkages may be crucial for sustaining the trophic structure and productivity of freshwater food webs that ultimately support the populations responsible for this mass influx (Wipfli et al. 1998). A variety of populations are involved in that food web (Michael 1995), including invertebrates (Chaloner and Wipfli 2002) that are food for these fishes (Wipfli 1997). The sharp declines of Pacific salmon stocks in many regions of North America (NRC 1996; Stouder et al. 1997) may create important losses of nutrient and energy inputs to many river systems (Gresh et al. 2000). In Alaska and parts of western Canada where historic salmon run sizes have been largely maintained (Baker et al. 1996; Slaney et al. 1996), millions of salmon return to streams, lakes, and other freshwater habitats providing nutrientand energy-rich biomass to consumers (Levy 1997; Willson et al. 1998; Wipfli et al. 1998). In southeastern Alaska high rainfall and