Estimating Overwinter Survival of Juvenile Coho Salmon in a Northern California Stream: Accounting for Effects of Passive Integrated Transponder Tagging Mortality and Size‐Dependent Survival

Abstract

AbstractWe used multiple resightings (live recoveries) of passive integrated transponder (PIT) tagged fish to estimate overwinter (October‐March) survival rates of juvenile coho salmon Oncorhynchus kisutch in a near‐pristine northern California headwater stream. Cormack‐Jolly‐Seber models were used to estimate period‐specific survival rates and recapture probabilities from three tag groups released during October and November 1999 and March 2000. Parameter estimates from the best‐supported models provided strong evidence of initial tagging mortality and also suggested that subsequent survival rates were affected by fish size at tagging. We found statistical evidence for initial PIT tagging mortality, suggested by the lower estimated period‐specific survival rates for tagged fish in their first period at large compared with estimated survival rates for tagged fish that had survived through a previous period. Estimated overwinter survival rates (November‐March) for fish tagged in October that were uncontaminated by initial PIT tagging mortality ranged from about 21% for 55‐mm fish (fork length at tagging) to about 84% for 106‐mm fish (overall average = 45.5%). Lower apparent survival rates for smaller juveniles may reflect (1) size‐dependent movement out of the study reach, (2) chronic size‐dependent mortality related to PIT tagging, or (3) size‐dependent winter mortality unrelated to PIT tagging. We believe that development of successful recovery strategies for coho salmon stocks will require similar stock‐specific (ideal) or region‐specific (minimally adequate) estimates of survival rates through preadult life stages.