Geography Outweighs Effects of Freshwater Rearing on Early Marine Survival of Puget Sound Steelhead Smolts

Steelhead smolts migrating through Puget Sound experience very low survival rates each year, likely limiting the recovery of depressed populations in the region. It is unclear whether poor survival to the ocean is a function of direct effects within the marine environment, or caused by watershed-specific characteristics like habitat conditions or hatchery introgression. We transplanted steelhead smolts from a highly urbanized, hatchery-influenced watershed feeding into central Puget Sound (Green River) to a relatively undisturbed south Puget Sound watershed free of hatchery influence (Nisqually River), while an identical number of smolts were transferred from the Nisqually River to the Green River. Smolts from both populations released in the Green River survived at similar rates, as did smolts from both populations released in the Nisqually River, and smolts from both release sites survived at similar rates through congruent migration segments. However, smolts from both populations released in the Nisqually River experienced lower early marine (all segments combined) survival rates than smolts with a 65 kilometer shorter migration from the Green River, indicating an effect of release location over any population effects. These results are consistent with a mortality mechanism independent of freshwater conditions.