Abundance and Size Distribution of Ocean-Type Juvenile Chinook Salmon in the Upper Sacramento River Margin before and after Hatchery Releases

—If hatchery-reared salmon delay emigration after release, they may compete with wild salmon in freshwater rearing habitat. We measured the densities and size distributions of ocean-type juvenile Chinook salmon Oncorhynchus tshawytscha in two rearing areas downstream of a hatchery in the Sacramento River, California, before, during, and after two large releases in both 2001 and 2002. Densities of juvenile salmon followed a unimodal trend through time, peaking at about 0.5–1.5 fish/m2 during late March or early April but declining by the time hatchery fish were released in mid to late April. Hatchery releases did not increase densities above the underlying trend, except after one release at one site. Density increased by 0.83 fish/m2 (SE, 0.30 fish/m2) after this release but returned to the baseline level within 3 d. Although hatchery fish were much larger than most wild fish, the mean size of fish captured did not increase appreciably after hatchery releases, even after the release when density increased. These data suggest that the strategy of delaying hatchery releases until many hatchery fish were smoltifying and many wild fish have emigrated was relatively effective in reducing potential interactions in freshwater rearing areas of the stream margin in the upper river. Stocking hatchery fish has become increasingly controversial because stocked fish may have negative effects on naturally spawned fish (hereafter referred to as wild) through direct ecological interactions (e.g., White et al. 1995) or alter the genetic composition of wild stocks, thereby reducing their ability to persist in the long term (e.g., Busack and Currens 1995). Competition between hatchery-reared and wild juvenile salmon in streams may be an important negative ecological interaction that is causing the gradual replacement of some stocks of wild salmon by hatchery salmon (e.g., Nickelson et al. 1986; Flagg et al. 1995). One strategy to reduce interactions between the two groups is to minimize their spatial and temporal overlap (U.S. Fish and Wildlife Service 2001). * Corresponding author: [email protected] 1 Present address: Versar, Inc., 9200 Rumsey Road, Columbia, Maryland 21045-1936, USA. Received November 5, 2003; accepted March 8, 2004 In the Sacramento River, California, the fall run of Chinook salmon Oncorhynchus tshawytscha is the most abundant of the four seasonal runs (Yoshiyama et al. 1998) and is also stocked in the greatest numbers. Wild fall-run Chinook salmon have an ocean-type life history (Healey 1983). They emerge from the gravel during December through March, and emigrate to the estuary during March through July (Yoshiyama et al. 1998). Shasta Dam, completed in 1945, greatly reduced the amount of habitat available for salmon to spawn and rear in the Sacramento River. To mitigate for the loss of natural production in sport and commercial fisheries, Coleman National Fish Hatchery (CNFH) releases approximately 12 million fall-run Chinook salmon smolts/year into the upper river during late April. A small fraction of the hatchery fish released are marked by excising the adipose fin (12% in 2001 and 18% in 2002), but the remaining fish are unmarked and indistinguishable from wild fish except by differences in their size