Low river survival of juvenile steelhead in the Sacramento River watershed

摘要

Steelhead (Oncorhynchus mykiss) are a species of conservation concern in California's Sacramento River, and their poor status is hypothesized to be due in significant part to poor survival in freshwater. We used acoustic telemetry to examine the migratory survival of hatchery reared steelhead smolts through five contiguous regions of the Sacramento River and its estuary (upper river, lower river, delta, north bays, central San Francisco Bay) from 2006 to 2011. We consistently observed the highest losses in the upper river with only 20.5%, 33.7%, 43.3%, 21.3%, and 29.0% of the tagged population successfully passing through the 210-km region in years one through five respectively. Average migratory success was similar among the other regions with reduced rates of loss in the lower river (139.5-km, 8.4% +/- 4.2), delta (99-km, 7.9% +/- 3.4), and Estuary (69.5-km, 6.8% +/- 2.9). The average survival through the Sacramento River to the Pacific Ocean across all five years was 5.6% +/- 3.6 SE. In the first study year we observed tagged steelhead responding to an increase in river discharge. Sixty-seven percent of all steelhead that successfully migrated to the entrance to the Pacific Ocean began their migration during an increased discharge event. A similar pattern was observed in years two, four and five (77.8%, 77.8%, and 84%). Steelhead survival rates to the Pacific Ocean were highest in year five of the study (9.3% in January and 8.0% in December) when the largest discharges in the Sacramento River were observed (2200 m(3) s(-1)).