Overwinter survival and movement of juvenile coho salmon (Oncorhynchus kisutch) in relation to large woody debris and low-velocity habitat in Northern California streams

Some studies suggest that Coho Salmon populations are limited by overwinter survival as a result of insufficient winter habitat. While many small-scale projects aim to define reach and basin-level habitat requirements for Coho Salmon, large-scale studies that assess multiple independent populations remain few. For my research, I quantified large woody debris (LWD) by volume and low-velocity rearing habitat (LVH) as percent area in three coastal watersheds of similar size in northern California to untangle the relationships between Coho Salmon overwinter survival, emigration timing and specific winter habitats. I used mark-recapture techniques with PIT tags to formulate Cormack-Jolly-Seber models for each of three years (2013-2015) to (1) estimate apparent overwinter survival of juvenile Coho Salmon populations, (2) determine to what extent outmigration timing varies among basins, and (3) evaluate the relationships between reach-specific survival, movement and winter habitat. LWD volume ranged from 47.8 to 109.9 cubic meters per kilometer among stream reaches while LVH area spanned from 9.3% to 23.6% of total stream area per reach. Effects of LWD on apparent overwinter survival and early emigration were absent during all three years of the study. Effects of LVH were not observed during 2013 and 2014. In 2015, LVH correlated positively with apparent overwinter survival and negatively with emigration. Larger Coho Salmon had higher apparent overwinter survival rates than small fish, whereas smaller fish had greater emigrations rates before spring. Mean apparent overwinter survival varied by basin from 0.052 to 0.567 but basins maintained consistency across years. Early emigration rates ranged even further by basin (0.023-0.773). Variation in both apparent overwinter survival and early emigration was much greater among basins than within basins. A lot remains to be learned regarding how habitat affects the migratory behavior of Coho Salmon in California and these results suggest the effects may vary significantly by stream. The drastic life history differences observed in neighboring Coho Salmon populations demonstrate the plasticity in a species once thought to be relatively inflexible. Moving forward, incorporating multi-basin approaches should be considered when evaluating freshwater survival and movement to inform large-scale restoration and conservation