Fall and Winter Movement Dynamics of Naturally Produced Spring Chinook Salmon Parr in Two Neighboring Interior Pacific Northwest Natal Rivers

Overwintering Pacific salmonOncorhynchusspp. are vulnerable to mortality. Ecosystem degradation exacerbates salmonid vulnerability to mortality; thus, knowledge of winter movement dynamics is essential for guiding holistic management and restoration to safeguard population viability. In northeast Oregon, long-term population performance metrics (e.g., survival) exist for parr of two neighboring Chinook SalmonO. tshawytschapopulations (Catherine Creek [CC] and Grande Ronde River [GRR]); however, knowledge of their winter ecology is lacking. Our objective was to spatiotemporally characterize the movements and rearing distributions of naturally produced CC and GRR spring Chinook Salmon parr during fall and winter. Using radiotelemetry techniques (October-February), we identified the movement dynamics of CC (2009-2012) and GRR (2014-2016) spring Chinook Salmon fall migrants (i.e., parr overwintering downstream from natal areas). For both populations, median weekly linear range (i.e., movement) was high during fall migration, while sedentary behavior was associated with anchor and surface ice. Fall emigration occurred during declining thermographs, while overwintering parr movement and water temperature peaks were proximate. Fall migrant CC parr predominantly overwintered in the upper Grande Ronde Valley (GRV), while GRR conspecifics primarily reared upstream from the GRV. Extensive portions of CC and the GRR were not included in either population's 99% kernel density estimate (KDE); however, each population's 99% KDE extended into the lower GRV. The GRR population's 50% KDE (i.e., core area) was small and proximate to several North American beaverCastor canadensisimpoundments, while the CC population's core area was relatively large and situated in the upper GRV. Overall, CC and GRR fall migrant populations exhibited divergent rearing and movement patterns; however, each population's winter core area was proximate to lentic-like habitats (e.g., beaver ponds). For Chinook Salmon parr, natal-stream-specific overwintering ecology knowledge appears critical to guide winter habitat conservation and restoration, while management for lentic-like habitats may be advisable throughout the Pacific Northwest.