Fisheries Research 68 (2004) 259–270 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 Tyler Wagner a,* , James L. Congleton b , Douglas M. Marsh c a Department of Fish and Wildlife Resources, Idaho Cooperative Fish and Wildlife Research Unit, University of Idaho, Moscow, ID 83844-1141, USA b Department of Fish and Wildlife Resources, US Geological Survey, Idaho Cooperative Fish and Wildlife Research Unit, University of Idaho, Moscow, ID 83844-1141, USA c NationalMarine Fisheries Service, Northwest Fisheries Science Center, Fish Ecology Division, 2725 Montlake Boulevard East, Seattle, WA 98112-2097, USA Received 3 January 2003; received in revised form 14 November 2003; accepted 28 November 2003 Abstract 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. © 2004 Elsevier B.V. All rights reserved. Keywords: Barge transportation; Columbia Basin; Salmon recovery; Snake River dams; Delayed mortality 1. Introduction In the early 1980s, the US Army Corps of Engi- neers implemented a fish transportation program to * Corresponding author. Department of Fisheries and Wildlife, Michigan State University, East Lansing, MI 48824, USA. Tel.: +1-517-353-2267; fax: +1-517-432-1699. E-mail address: wagnerty@msu.edu (T. Wagner). reduce exposure of migrating juvenile salmonids On- corhynchus spp. to sources of mortality associated with hydrosystem development in the Columbia Basin, USA (Ward et al., 1997). Juvenile fish are collected and loaded into fish-transport barges (>90% of fish transported) or trucks (<10%) at four dams on the Snake and Columbia Rivers, transported around the remaining downstream dams, and released below Bon- neville Dam, 235 km upstream from the mouth of the 0165-7836/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2003.11.006