Use of Eelgrass Habitats in Puget Sound Large River Deltas by Forage Fish and Juvenile Salmon

Seagrass beds support many other species through food production, refuge from predation, and provision of recruitment substrate and nursery habitat.  Beds of eelgrass Zostera marina occur on the outer edges of large river deltas in Puget Sound, Washington.  These beds can be impacted by focused river flow and sediment delivery resulting from diking and river channelization, so knowing the importance of the beds for marine fish, including forage fish, and juvenile salmon emigrating from the rivers into the Sound is relevant to management decisions.  In 2004 we initiated studies to investigate physical processes affecting delta habitats including eelgrass, and in 2008 fish sampling was added to characterize habitat use.  Effort has been concentrated in the Skagit River delta, the largest delta in Puget Sound, and the Nisqually River delta where recent dike removal has restored tidal inundation to extensive lowland marshes.  We conducted fish sampling monthly from April to September with a lampara seine, chosen because it slides over the top of eelgrass causing minimal damage.  A key finding is that Pacific herring Clupea pallasii show an association with eelgrass.  Herring catch rates in eelgrass were at least twice those in paired non-eelgrass habitats.  Herring catch rates in one eelgrass bed were at least three times those in the other three beds sampled in the Skagit River delta.  Furthermore, recently metamorphosed age-0 herring occurred almost exclusively in the bed where catch rates for mixed ages were high.  The eelgrass bed with high herring catch rates and presence of age-0 herring also had finer underlying sediments, a different benthic macroinvertebrate community, more marine water column properties, and less fragmentation compared to the other three Skagit delta beds.  These data indicate an association between Pacific herring and eelgrass beyond the well known use of eelgrass for spawning substrate by herring.  Whether the association is based on foraging opportunities, predator avoidance, or some other mechanism awaits further research.  Catch rates in eelgrass and other delta habitats were higher for juvenile Chinook salmon Oncorhynchus tshawytscha than for juveniles of the other salmon species.  Juvenile Chinook salmon catch rates were similar between eelgrass and paired non-eelgrass habitats.  Our results are being integrated with studies of fish use of estuary habitats farther inshore and offshore to refine our knowledge of habitat connectivity and function.  Understanding the factors driving fish use of eelgrass will help guide land and river management decisions affecting eelgrass.