Can steelhead ‘ocean mortality' be explained by survival in lower rivers and estuaries?

We studied a means of improving upon estimates of ocean survival and identifying potential sources of mortality in lower rivers and estuaries of steelhead in the Oregon coastal distinct population segment (DPS). Steelhead smolt survival to the ocean is currently estimated using data from smolt traps located well upstream of the estuary. Mortality incurred in this zone has previously been incorporated into survival models under the category of ocean mortality. Our data from the Nehalem and Alsea rivers in Oregon suggest: 1) wild steelhead smolts spend little time in the estuary, 2) typically only 40-50% of the wild steelhead smolts reaching the estuary actually enter the ocean, 3) most mortality occurs in the lower estuary, and 4) smolts tagged during the peak of the run appear to have higher survival rates. Our research provides information to strengthen life cycle models used in recovery plans by providing missing data on mortality locations and rates in the riverine and estuarine portions of smolt migration. Ultimately, we will propose the design of a monitoring plan for estimating survival of migrants to the ocean within currently accepted errors.