The Influence of Release Strategy and Migration History on Capture Rate of Oncorhynchus mykiss in a Rotary Screw Trap

Rotary screw traps are used in rivers throughout the west coast of North America to capture emigrating juvenile salmonids. Calibrating the capture efficiency of each trap is essential for valid estimates of fish passage. We released PIT-tagged Oncorhynchus mykiss upstream of a rotary screw trap in the South Fork John Day River, Oregon, to estimate capture efficiency. We used three strategies for release of fish recently captured in the trap. We recaptured 28% of medium-sized fish (86-145mm FL) and 14% of large-sized fish (146-230mm FL) released during daylight 1.6km upstream from the trap. We recaptured 33% of medium-sized fish and 17% of large-sized fish released during daylight 4.8km upstream from the trap. We recaptured 42% of medium-sized fish and 23% of large-sized fish released at twilight 1.8km upstream from the trap. A PIT tag antenna detected summer-tagged parr (which were PIT-tagged upstream 1-5 months before migration) as they approached the trap to evaluate potential bias from reduced recapture of recently trapped fish. We captured 53% of the medium-sized first-time migrants and 40% of the large-sized first-time migrants. Although average capture efficiencies of first-time migrants were greater than those from any of the recently trapped fish from the three release strategies, twilight releases of recently trapped fish were the least negatively biased, especially for medium-sized fish.