Spatial and Temporal Variability in Chinook Salmon Abundance Reflects Opportunity to Support Life History Diversity in an Estuarine Landscape

Understanding fish-habitat associations and distributions are important for making informed management decisions relevant to habitat restoration and species recovery. Chinook salmon, in particular, rely heavily on estuaries, and restoring these habitats has become an important component of recovery plans for threatened populations. Information regarding general distribution patterns, while often lacking, could be useful for restoration evaluation and planning. We used time series analysis to identify common trends in weekly Chinook salmon abundance among seven separate zones within the Snohomish River delta while evaluating the effect of temperature and outmigration abundance on observed patterns. We found two unique trends described seasonal Chinook salmon density and a significant negative effect of temperature on observed density throughout the estuary. The first trend indicated Chinook salmon density increased steadily from February through early June before decreasing into mid-summer. The second trend was characterized by an early spike and a gradual increase in density into early April before leveling off then decreasing into mid-June. The association of each zone with these trends was consistent with the distribution of tidal wetland habitat area among each zone. We postulate that these trends in seasonal Chinook density reflect patterns of movement and rearing, respectively, throughout the estuary and highlight how and where estuary habitat in the Snohomish delta supports life history diversity. Confirmation of seasonal rearing patterns may aid current restoration project evaluation, while a migratory seasonal trend may help prioritize future restoration in the Snohomish estuary by identifying where opportunity exists to support estuary rearing for Chinook salmon. Finally, our methodology and analytical approach for evaluating spatial and temporal variability in Chinook salmon distributions and abundance may be applied to other tidal deltas to make informed decisions or evaluations of both current and future restoration actions.