Comparative Survival of Repatriated Razorback Suckers in Lower Colorado River Reach 3 2015 Annual Report

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 now listed as endangered. Under guidance of the Lower Colorado River Multi-Species Conservation Program, more than 60,000 razorback suckers have been stocked into Reach 3 (between Davis and Parker Dams) since 2006. Contact rates of stocked fish have been low using traditional fisheries sampling methods (i.e., electrofishing and trammel netting). However, the use of remote passive integrated transponder (PIT) scanning technology has proven effective at contacting hundreds of razorback suckers with minimal effort; 1 week of deployments with five PIT scanners during the peak of spawning (February – March).

Remote PIT scanners were deployed in Reach 3 for 4 months (with one additional trip in November 2014) from January to April 2015 from Davis Dam downstream to Park Moabi Regional Park, California, to target spawning aggregates of razorback suckers. In addition, data were compiled from other projects and from entities that scanned in Reach 3 from November 2014 to August 2015. These collective efforts resulted in the contact of 4,611 individual PIT tags. A total of 4,773 PIT tags were contacted through a combination of remote PIT scanning and standard sampling methodologies in 2015. Of these 2,232 individual razorback suckers were included in a 2014 population estimate of 4,795 (4,491–5,119; 95-percent confidence interval) individuals.

Relative contact rates of razorback suckers (number of fish contacted/number of fish released) were directly related to the size of fish at release. Contact rates (95-percent confidence interval) were lowest at 0.010 (0.001–0.019) for fish released at less than 11.8 inches (300 millimeters), increased at each size class, and were highest at 0.176 (0.140–0.213) for fish released at greater than 19.7 inches (500 millimeters). Differences in contact rates were statistically significant between stocking location, and the contact rate was lowest in zone 3-4 (between Copper Canyon and Parker Dam) at 0.006 (0.004–0.008) and highest for fish stocked in zone 3-2 (between Park Moabi and Lake Havasu Delta) at 0.069 (0.066–0.073). Contact rates were second highest for zone 3-1 (between Davis Dam and Park Moabi) and slightly lower for zone 3-3 (between Lake Havasu Delta and Copper Canyon) at 0.044 (0.041–0.047) and 0.027 (0.023–0.030), respectively.

Contact rates were higher for fish released in the spring months, slightly lower for fish released in winter months, and lowest for the autumn months at 0.071 (0.067–0.075), 0.050 (0.048–0.053), and 0.010 (0.008–0.012), respectively. Comparative Survival of Repatriated Razorback Suckers in Lower Colorado River Reach 3 – 2015 Annual Report ES-2 PIT scanning continues to provide increased contacts of tagged fish compared to traditional means such as electrofishing and trammel netting. Monitoring of razorback suckers in LCR MSCP Reach 3 should continue with both remote PIT scanning and biannual netting trips. As more data are collected, a more in depth analysis of post-stocking dynamics will provide additional information to assess post-stocking survival of razorback suckers in Reach 3

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