Evaluating common trends in Chinook density and the influence of temperature and salinity patterns among distributary channels in a large river estuary to aid evaluation, planning, and prioritization of restoration activities

Landscape context is critical in estuary restoration planning and assessment due to the complexity and size of estuaries, and the unique attributes and cumulative effects of individual restoration projects. In addition, the diversity and mobility of estuarine species, in particular juvenile salmon, highlights the importance of landscape position given certain locations in the delta are less accessible to salmon. The Snohomish River delta has been the focus of major estuary restoration efforts in recent years and efforts could result in the largest cumulative estuary restoration action in Puget Sound. While several large projects have been initiated/competed in recent years, information to help prioritization and planning for future projects is currently lacking. We used a time series analysis of Chinook densities from 2011-2015 to assess general patterns in fish use and the effect of temperature and salinity through the outmigration period among the mainstem Snohomish River and the three primary distributaries. Two common trends in Chinook salmon density among the distributary and mainstem channels reflect patterns attributable to potential life history variation interpreted as an estuary rearing/residence component and general freshwater rearing (parr) outmigration. Furthermore, peak densities occurred earlier when above average temperatures were observed throughout the estuary. The differential patterns in Chinook density and the apparent influence of temperature may aid restoration evaluation, planning and prioritization aimed at increasing capacity for estuary rearing Chinook salmon throughout the delta and provide input regarding the potential effect of changing temperatures due to climate change