Length-Specific Survival of Out-Migrating Yearling and Subyearling Chinook Salmon in the Snake and Columbia Rivers

Studies examining survival of juvenile salmon as they emigrate to the ocean provide important information regarding the management of regulated river systems.  These studies are used to modify the structural make-up of hydroelectric dams and modify their operations.  Acoustic telemetry is a widely used tool for evaluating passage behavior and survival for juvenile salmonids in the Columbia Basin.  This methodology requires surgical implantation of transmitters into the coelomic cavity of study animals.  It is important to determine how the surgical tagging process and the presence of a transmitter affect survival so any biases can be anticipated, accounted for, or eliminated.  Thus, the effects of fish length and tag type [i.e., PIT tagged only (PIT fish) versus implanted with both a PIT and acoustic tag (AT fish)] on the survival of yearling and subyearling Chinook salmon were examined.  Data were collected at multiple locations within the Snake and Columbia rivers during three field seasons (2006–2008).  Fish length was positively correlated with the survival of both PIT and AT fish.  Fish length was a significant factor in modeling the post-release survival of yearling and subyearling Chinook salmon and affected the survival of fish implanted with both tag types in a similar manner.  Although significant differences in survival between tag types were detected in some models, these results were confounded by detection location; thus, making strong inferences regarding this factor would be inappropriate.  This research suggests that for the range of sizes that were evaluated, the presence of an acoustic transmitter did not influence length-specific survival rates as compared to PIT tagged only fish.  Results suggest that tagging effects may be due to the process of surgically implanting the transmitter rather than the presence of the transmitter.  More research is needed to determine whether the surgical implantation process affects the survival of AT fish as compared to PIT fish.  More research is also needed to reduce the effects of the process of surgically implanting transmitters into fish.