Smolt-to-adult return rates of juvenile chinook salmon transported through the Snake-Columbia River hydropower system, USA, in relation to densities of co-transported juvenile steelhead

To reduce mortality associated with passage of migrating juvenile salmonids through the Snake-Columbia River Federal power system, a large percentage of smolts migrating from the Snake River basin are currently transported downstream through the hydropower system in fish-transport barges. It has recently been suggested that transportation-associated stressors may reduce the fitness of juvenile chinook salmon Oncorhynchus tshawytscha and increase mortality after seawater entry. Because the major stressor for transported juvenile chinook salmon is believed to be co-transportation with larger and more aggressive juvenile steelhead O. mykiss, we tested the hypothesis that smolt-to-adult return rates (SARs) of transported yearling chinook salmon were negatively correlated with densities of co-transported steelhead. Our analysis, using SARs and barge loading data for groups of chinook salmon transported on a daily basis in 1995, 1998, and 1999, failed to confirm a relationship between chinook salmon survival and steelhead density. These results do not preclude the possibility of an undetected inverse relationship between post-release survival of transported chinook salmon and densities of co-transported steelhead, but do suggest that if such an effect exists it is less important than other factors, such as seasonal changes in estuarine and marine productivity or predator abundance.