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The Bureau of Reclamation's Tracy Fish Collection Facility was constructed in the mid- 1950's as part of the Central Valley Project. The facility uses a louver-bypass type of fish diversion system to separate fish from the exported flow. The fish are collected and held in holding tanks where they await transport back to the Sacramento-San Joaquin Delta. The louver system design was based on field tests conducted with young striped bass and chinook salmon and is generally believed effective for fish large enough (>38 mm) to detect them. However, its current efficiency is not well known not only for these species but also for the other 35 + fish species that are entrained in the export flows. We began to re-evaluate louver efficiencies for juvenile chinook salmon and striped bass using mark-release-recapture techniques. We defined louver efficiency as the proportion of fish recovered in the holding tanks relative to the number released upstream of each louver system. A total of 12 groups of juvenile striped bass and chinook salmon were released at four to six sites within the facility at various flow, tide, and day/night conditions.