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 tshanytscha 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/m(2) 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/m(2) (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.