Does broodstock source affect post-release survival of steelhead? Implications of replacing a non-native hatchery stock for recovery

Abstract

Steelhead trout (Oncorhynchus mykiss) reared at the Nimbus Fish Hatchery (NFH) and released into the Lower American River (LAR) in California are not part of the native California Central Valley Distinct Population Segment (CCVDPS); listed as threatened under the Endangered Species Act (ESA), and therefore cannot contribute to recovery. In response to this situation, we evaluated the potential of using a CCVDPS stock from the Coleman National Fish Hatchery (CNFH) to aid in recovery. Steelhead from the CNFH were reared in 2015 at NFH along with NFH-origin steelhead. Smolts from both stocks were implanted with acoustic transmitters and released into the LAR. This study assessed: 1) reach-specific, post-release survival of each stock from release to the ocean; 2) the degree to which either stock residualized; and 3) the spatial distribution of post-release mortality in the LAR. Multistate mark-recapture models were constructed to estimate reach-specific survival and entrainment into alternate migration routes. Four models were constructed to test hypothesized relationships between survival and covariates. Covariates included: 1) broodstock origin; 2) release date; 3) fork length; and 4) no covariates. Fork length was the best predictor of reach-specific survival and exhibited a positive relationship with survival in each reach. Juveniles from both stocks emigrated quickly, did not residualize, and exhibited similar survival while emigrating to the ocean. Results suggested the need for additional research to further evaluate CNFH-origin steelhead performance at NFH-origin during additional life stages to support recovery of CCVDPS Steelhead.