The Role of an Intermittently Closed, Northern California Estuary for the Feeding Ecology of Juvenile Steelhead

A new focus has been placed on estuaries as a potential rearing habitat for juvenile Oncorhynchus mykiss, once thought only to be utilized as a corridor for migrating to the ocean.   Previous research (2008) by Hayes and Bond has found that steelhead rearing in an intermittently closed small estuary had greater growing opportunities and thus were more successful in their recruitment back to the adult population. The University of Washington, School of Aquatic and Fishery Sciences’ Wetland Ecosystem Team conducted a study from 2009-2010 designed to evaluate how different natural and managed ocean entrance conditions in the Russian River estuary (CA) affect juvenile salmon foraging and ultimate performance as a function of prey availability.  The study was designed to integrate both- systematic sampling coincident with juvenile salmon entry to and residence in the estuary, and “event” response to stochastic (and programmed) changes in estuary entrance conditions.  Systematic sampling was intended to capture the natural ecological responses (prey composition and consumption rate) of juvenile steelhead and availability of their prey resources (insect, benthic and epibenthic macroinvertebrates, zooplankton under naturally variable water level, salinity and temperature stratification in the estuary. The formation of a freshwater lagoon when the estuary entrance is closed increases the water level from an average of 1 meter to a managed 2.3 meters.

 This flooded lagoon provides juvenile steelhead and other salmonids the opportunity to feed in peripheral habitats, and potentially allowing for new or expanded prey resources and increased consumption and growth rates.  Analyses of the diet composition of steelhead captured in 2009-2010 indicate that epibenthic crustaceans (amphipods, isopods, mysids) and aquatic insects (water boatmen) were the typical and dominant prey in most samples.  Early observations suggest that when the estuary is open to the ocean, the river currents restrict the invertebrates to the more protective shoreline, limiting the more profitable foraging area for steelhead; when the estuary entrance is closed and becomes a freshwater lagoon, invertebrates can more effectively occupy the entire estuary, increasing the potential rearing habitat for juvenile steelhead.   This study will help management decide if an estuary can serve as an alternative rearing habitat for juvenile steelhead.