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

Abstract

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 gor-buscha (0, 1, 2, 3, and 4 carcasses/m 2 or 0, 1.9, 3.7, 5.6, and 7.4 kg wet mass/m 2) were replicated six times in once-through artificial channels, then each channel was stocked with three live age-0 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/m 2 . Further, in a small stream in which we added salmon carcasses to a cumulative density of 0.54 carcasses/m 2 , 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 com-munity nutrition and productivity may be important considerations for salmon stock protection and restoration and for freshwater and marine ecosystem management.