Estuarine habitats are essential nursery grounds for outmigrating juvenile Pacific salmonids (Oncorynchus spp.), specifically those with an ocean-type life-history strategy (Simenstad et al 1982). The Skagit River system in northern Puget Sound, Washington is unique in that it supports all five species of Pacific salmon, including the largest runs of chum, pink, and chinook salmon in Puget Sound (Brown 2001). These three species (O. keta, O. gorbuscha, and O. tshawytscha, respectively) all share the ocean-type life history strategy, though chinook salmon, in particular, are known to utilize estuarine areas extensively for rearing (Reimers 1973).
Historically, the Skagit River delta was a large estuarine complex of over 29,000 acres. Agriculture and development, mostly during the last century, have eliminated or truncated tidal flows in much of the estuary, causing significant changes in geomorphology, and thus, habitat (Beechie et al 1994). Estuarine emergent marsh habitat alone has been diminished by 68% (of historic levels) to less than 5,000 acres (Beamer 2002). The elimination of functional marsh habitat has meant decreased rearing opportunity for outmigrating juvenile salmonids (Simenstad and Cordell 2000)
Past research has indicated that tidal wetlands are important sources of prey for juvenile salmonids (Congleton 1978, Wolf et al 1983, Shreffler et al 1992, Simenstad and Cordell 2000). Insects from the order Diptera, specifically chironomid larvae, pupae, and adults, and gammarid amphipods are known chinook prey items (Shreffler et al 1992). While
the success of chinook runs (adults returning to spawn) based upon estuarine rearing time is still unknown in the Skagit system, Reimers (1973) showed that chinook utilizing the Sixes River estuary for extended periods made up 90% of the returning spawners, while brief estuary users were absent in adult returns, suggesting that growth while in the estuary may provide some advantage once the fish migrate to sea. Estuarine residence time in the Skagit River estuary is estimated to be from 14 to 35 days (Beamer 2002), though it could be significantly longer. In 2000, a restoration project aimed at returning a portion of the delta to tidally influenced emergent marsh was undertaken in the Skagit system. This project (Deepwater Slough Restoration Project) removed an upstream agricultural dike and allowed flow to resume to 250 acres of pastureland (Fig. 1). Subsequent beach seine and fyke net sampling has shown adequate fish usage of the restored areas, suggesting that these areas are affording fish the ability to rear. Additionally, fish stomachs were collected for analysis and showed an abundance of chironomid larvae and adults as well as the mysid shrimp, Neomysis mercedis. These data, while descriptive in and of itself, was the basis for comparison between study sites using the Wisconsin fish bioenergetics model, as presented here. Bioenergetics models