Life History Observations of Adfluvial Chinook Salmon prior to Reintroduction of Anadromous Salmonids

AbstractPacific salmon Oncorhynchus spp. vary in life history and degree of anadromy, but information on populations inhabiting only freshwater throughout their life cycle is limited. We confirmed the presence of a self-sustaining population of adfluvial Chinook Salmon O. tshawytscha upstream of century-old hydroelectric dams in the Skokomish River system, Washington. Snorkel, redd, and juvenile-trapping surveys revealed their life history attributes prior to planned reintroductions of anadromous salmonids above the dams. Adult Chinook Salmon in Lake Cushman (a reservoir) were large-bodied fish (mean length = 610 mm; mean weight = 5.4 kg), up to 4 years of age, and migrated into the inlet river to spawn in October and November. Annual peak counts of adult spawners were chronically low (35 or less) based on interannual snorkel surveys since 1994. Chinook Salmon parr (n = 780; mean length = 105 mm) of a single cohort were captured (June–August) at the upper dam and had faded parr marks and bright, silvery c...