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Since 1995, California State Fish Hatcheries (Feather River, Nimbus, and Mokelumne) and Coleman National Fish Hatchery have raised approximately 29 million 4 fall run Central Valley Chinook salmon (Oncorhynchus tshawytscha) per season for stock enhancement. From April through June, fish are acclimated in net-pens prior to release at one of three sites: the Carquinez Strait at Conoco Phillips (CP), the mouth of the Napa River at Mare Island (MI), and the San Joaquin River at Jersey Point (JP). Striped Bass, Marone saxatilis, are known to congregate at the release location to feed on the hatchery fish as they enter the Delta and Bay, and are suspected to be reducing numbers of Chinook recruitment. Dual-Frequency Identification Sonar (DIDSON) was used to capture video-like images to enumerate and estimate sizes of potential predators in the area. Stomach analysis was used to obtain consumption rate data and a simple model was used to estimate predator impacts on the hatchery fish. Data was collected in 2011 and 2012. In 2011 the striped bass population at CP was significantly larger than MI (p=0.009) and JP (p=0.038) and in 2011 , and MI (p=0.046) in 2012. Predators were significantly smaller (range 11.8-61.7 em, mean 34.6 em in 2011 ; 21-67 em, 42.9 in 2012) atJP (p<0.001). Average size predator at MI was 47.3 em (range 31-59 em) in 2011 and 50.9 em (range 33-73 em) in 20 12; and at CP was 48.3 em (range 16-77 em) in 2011 and 52.7 em (range 31-78 em) in 2012. On average an estimated 2.2% of hatchery fi sh are consumed each year by striped bass and predator impacts are greatest at CP (p<0.001). Changing the release site often could improve salmon survival by decreasing predator attraction to the site and reducing immediate predator-prey encounters.
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