Abstracts Biological Sciences

Influence of body size, intra-and inter- specific salmonid densities, and habitat on overwinter survival of juvenile Coho salmon (Oncorhynchus kisutch) in Prairie Creek, California

by Peter Drobny

I evaluated the effects of fish length, habitat attributes, and densities of Coho Salmon and trout on overwinter survival of juvenile Coho Salmon in Prairie Creek, California. Survival of PIT tagged juvenile Coho Salmon was estimated using a Cormack-Jolly-Seber Model based on mark-recapture data from seine net captures, PIT tag antenna detections, and a rotary screw trap. A zero inflated Poisson binomial model, based on a 2-pass snorkeling methodology, estimated pool-specific abundance of juvenile Coho Salmon while accounting for variable snorkeler detection efficiency as well as estimating the effect of habitat attributes on density. Trout densities were obtained using raw snorkel counts. Overwinter survival of PIT tagged juvenile Coho Salmon during 2014-2015 was estimated to be 0.35 (95% CI 0.30-0.40), similar to survival estimates for Prairie Creek in previous years. Survival increased with fish length and decreased with intraspecific density. There was no evidence that density of small (<150 mm) trout, large (>150 mm) trout, or habitat attributes influenced survival. Estimated density of juvenile Coho Salmon in pools averaged 0.5 fish/m2 (n= 159) and ranged from 0- 2.2 fish/m2, consistent with regional density values within the Pacific Northwest. Juvenile Coho Salmon density estimates declined with cover complexity and watershed area, and increased with pool depth. Probability of detecting a fish while snorkeling varied with habitat features and among observers; estimated probability of detection averaged 0.63 (n=318) and ranged from 0.19 to 0.96 across snorkel passes. This study not only continued valuable monitoring of juvenile Coho Salmon overwinter survival in the southern portion of their range, but also determined factors that may be contributing to survival rates. In addition, this study successfully applied a relatively new hierarchical modeling technique (N-mixture model) to estimate juvenile Coho Salmon densities using non-invasive methods. Advisors/Committee Members: Wilzbach, Margaret A..