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A Method to Compute Adult Anadromous Salmonid Abundance by PIT Tagging Juvenile Outmigrants

Jason Neuswanger, Andrew Murdoch, Daniel Rawding, Ethan Crawford

Washington Department of Fish and Wildlife, 600 Capitol Way North, Olympia, WA 98501

Abundance estimation of adult anadromous salmonids is sometimes made difficult by environmental conditions (e.g., turbid or high water) or logistical constraints that preclude effective direct counting of adults, redds, or carcasses. To overcome this limitation, we developed an approach to adult abundance estimation based on trapping and PIT tagging juvenile outmigrants and later detecting these tags when the fish return as adults. We present an example of this approach applied to steelhead in the Touchet River in southeastern Washington. Juvenile outmigrant abundance at weekly intervals is estimated using a Bayesian Time-Stratified Population Analysis (BTSPAS), which uses p-splines to interpolate abundance during weeks with missing data due to difficult sampling conditions. Juvenile estimates can be stratified by size and migration timing to account for associated differences in survival and capture probability. Survival of adults from Bonneville Dam to the Touchet River is estimated from detections of these tags at mainstem dams and instream arrays, using a modified Cormack-Jolly-Seber (CJS) model that accommodates multiple ocean ages. These methods are under development as a general-purpose R package expected to enable or improve abundance estimation for a variety of populations.