Predation by hatchery steelhead on natural salmon fry in the upper-Trinity River, California

Hatchery fish have been implicated in the decline of stocks of naturally produced anadromous salmonids in the Pacific Northwest. I investigated the extent of predation by hatchery steelhead on naturally produced salmonid fry in the upper-Trinity River, California. During spring of 2007, 315 residualized hatchery steelhead and 1,636 juvenile hatchery steelhead were captured and examined for the presence of salmonid fry in the gut. Residualized steelhead consumed 435 salmonid fry and 2,685 salmonid eggs. Juvenile hatchery steelhead consumed 882 salmonid fry. Predation by juvenile hatchery steelhead was significantly greater near a side channel where a high percentage of adult salmonids were known to spawn. I used mark-recapture techniques to estimate the population of residualized hatchery steelhead and PIT tag recoveries to estimate the population of juvenile hatchery steelhead. Using the population estimates and predation rates, I estimated that 24,194 [95% CI = 21,066-27,323] salmonid fry and 171,018 [95% CI = 155,272-186,764] salmonid eggs were consumed by 2,302 residualized hatchery steelhead in 21 days from 10 February to 2 March 2007. Excluding the results from the side channel, I estimate that 437,697 juvenile hatchery steelhead consumed 61,214 [95% CI = 43,813-78,615] salmonid fry in 30 days from 28 March to 26 April 2007. Assuming a constant population of 1,500 juvenile hatchery steelhead in the side channel during the 30 day period, an additional 49,445 salmonid fry were consumed. Managers should carefully consider all of the risks to naturally produced fish populations from hatchery fish in order to determine if the effects of hatchery releases are consistent with management goals.