Comparative Survival of Repatriated Razorback Suckers in Lower Colorado River Reach 3 2014–2016

The razorback sucker (Xyrauchen texanus) is a fish endemic to the Colorado River drainage. It was once abundant throughout its range, but populations have steadily declined, and the species is listed as endangered. Under guidance of the Lower Colorado River Multi-Species Conservation Program), more than 75,000 razorback suckers have been stocked into Lower Colorado River Reach 3 (between Davis and Parker Dams) (Reach 3) since 2006. Contact rates of stocked fish have been low using traditional fisheries sampling methods (i.e., electrofishing and trammel nets); however, the use of remote passive integrated transponder (PIT) scanning technology has proven effective at contacting thousands of razorback suckers.

Remote PIT scanners were deployed in Reach 3 for 1 week per month for 4 months during the razorback sucker spawning season (January to April) from Davis Dam downstream to Park Moabi Regional Park, California, to target aggregations of razorback suckers. In addition, data were compiled from other projects and entities that scanned or captured razorback suckers in Reach 3. These annual collective efforts resulted in the contact of 1,972, 4,142, and 3,027 individual razorback suckers in 2014, 2015, and 2016, respectively. The razorback sucker population in Reach 3 was estimated at 4,935 (4,629–5,262; 95% confidence interval in 2014 and 4,923 (4,652–5,209; 95% confidence interval) in 2015.

Post-stocking survival appears to be higher for relatively small razorback suckers, with contact proportions as high as 10% for fish released at 13.8 inches (350 millimeters) compared to a 1.8% contact rate in Lake Mohave for similarsized fish. The season of release had a minimal impact on apparent survival. Population estimates for razorback suckers in Reach 3 are consistently higher than Lake Mohave. However, maintenance of both populations is wholly dependent on their respective stocking programs. PIT scanning continues to provide increased contacts of tagged fish compared to traditional means such as electrofishing and trammel netting, but the latter provides information not otherwise available. Monitoring of razorback suckers in Reach 3 should continue with both remote PIT scanning and biannual netting trips. The stocking regime should be coordinated with a sampling regime to test specific hypotheses about factors affecting post-stocking survival.

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