limate and conspecific density trigger pre-spawning mortality in ockeye salmon ( Oncorhynchus nerka ) ichael

Pre-spawning mortality (PSM) has been occasionally observed in association with high densities of adult Pacific salmon (Oncorhynchus spp.), but because large escapements are rare, the phenomenon remains poorly understood. A large spawning escapement (∼12 times the 54 year median, and 3X the previous maximum) to a small stream in Alaska provided a unique opportunity to explore the factors that contribute to density-driven spawning ground mortality. After comparing patterns of mortality in 2014 with over 20 years of prior abundance and environmental data, we identified low dissolved oxygen (DO) as likely contributing to PSM. We then utilized a fish habitat-DO model to explore the roles of density-dependent and -independent factors in reducing DO. Stream flow and spawning density were identified as primary drivers of oxygen availability. Despite suboptimal oxygen levels the salmon did