Amphipods Are Strong Interactors in the Food Web of a Brown-water Salmon River

Marine derived biomass from salmon carcasses is incorporated into coastal Pacific Rim salmon river ecosystems via the organisms and structures of the freshwater foodweb. In brown water rivers of Western Kamchatka, the foodweb is dominated by ubiquitous benthic amphipods (Anisogammarus kygi) that consume salmon carcass material. We hypothesized that A. kygi are a strong interactor in the feedback loop which links dead spawner biomass to juvenile salmonid growth. We found that A. kygi had a complex life cycle with anadromous and resident forms. A. kygi dominated the macrobenthos, comprising more than 88.0% (SE=.01, N=7) of invertebrate biomass, and were highly mobile within the system, exhibiting upstream migrations of ovigerous females (23 ind/m3 ± 5), drift of juveniles, and re-distribution during carcass loading. A. kygi was observed feeding on 97% of salmon carcasses examined (N=100), making up 98.8% (SE .007) of invertebrate consumers, at densities up to 3,000 carcass. Amphipods were an important food item for rearing salmonids, especially during the summer when fish diets reached a peak of 88.7% (SE=6.0%) amphipods in 2005, and 68% (SE=18%) amphipods in 2006. The condition factor of salmonid juveniles (K) increased from spring to summer, particularly in juvenile chum, whose spring diet was 76.83% (SE 0.05) amphipods, corroborating the importance of an amphipod based diet for salmonids in this river. We concluded that A .kygi is a strong interactor in the Utkholok system. We also observed abundance of A. kygi in six other brown water rivers of western Kamchatka which suggests that the amphipod-mediated feedback of marine derived nutrients described for the Utkholok, is typical of brown water systems with salmon.