Transportation of Snake River Fall Chinook Salmon 2008: Final Report for the 2004 Juvenile Migration

Abstract : The National Marine Fisheries Service began annual studies in 2001 to evaluate the efficacy of transporting Snake River fall Chinook salmon Oncorhynchus tshawytscha smolts from Lower Snake River hydropower projects. From 2001 through 2003, we tagged hatchery subyearling fall Chinook salmon at Lyons Ferry Hatchery and released them 81 km above Lower Granite Dam at Snake River kilometer 254. In 2004, this tagging routine was interrupted due to a lack of available subyearling fall Chinook salmon at Lyons Ferry Hatchery. Instead, we tagged river-run fish at Lower Granite Dam in 2004. During September and October 2004, we also tagged a fall transport index group of subyearling Chinook salmon (no fish were released to migrate inriver) for the third year at Lower Granite Dam. Final adult returns from tagging in both summer and fall 2004 are reported here. Results from 2001-2003 have been reported and are summarized and cited here in Appendix B. Our original study was designed to compare smolt-to-adult return rates (SARs) of fish transported as juveniles from Lower Granite Dam with those of fish released to migrate inriver and not detected at any collector dam. However, recent data has shown the model used to estimate numbers of nondetected fish in studies using spring Chinook salmon cannot be used for Snake River fall Chinook salmon. This model relies on the assumption of equal probabilities of detection for fish from each cohort. However we now know that some fall Chinook delay their downstream migration for several months, passing dams during the winter when bypass systems are dewatered. Thus, since there is no way to reliably estimate numbers of nondetected fish, we report only the SARs of fish with known passage histories from the fish initially released.