Bed texture, food web structure, and juvenile salmonid rearing in North Coast California rivers

Excessive loading of fine sediments into western rivers has degraded spawning and rearing habitat for salmonids, and contributed substantially to their declines. Impacts on salmon redds have been studied extensively, but effects on juvenile rearing are less well documented. In a field experiment in the South Fork Eel River, we investigated the impacts of deposited fine sediment on juvenile steelhead. Our experimental design allowed us to isolate the effects of fine bed sediments from other covarying factors and to reveal the mechanisms of their effects. Increasing levels of embeddedness with deposited fine sediment (from zero to 100%) decreased growth and survival of juvenile steelhead trout. The nearly linear decreases in growth resulted from decreased food availability and metabolic costs of increased activity and intraspecific aggression. The invertebrate community changed from one of more available prey to one of unavailable burrowing taxa with higher levels of deposited fine sediment. Steelhead in more heavily embedded channels showed more continuous movement and aggression and higher incidence of injury. This study shows a direct impact of riverbed composition on salmonid rearing success, which has been identified as a life history bottleneckby models informing efforts to recover these populations.