Comparative survival of razorback sucker in lower Colorado River

Authors: Kristen Patterson, Brian R. Kesner, Paul C. Marsh

Year: 2012 (xliv)

Abstract

Razorback sucker (Xyrauchen texanus) is one of four ‘big river’ fishes endemic to the Colorado River basin. Once abundant in the system it is now listed as endangered. Today, its persistence in many reaches, like Lake Havasu, Arizona and California, relies almost entirely on stocking. Biannual netting and electrofishing efforts produce low recapture rates , providing limited insight into current population status, and what factors may be driving low survival in stocked fish. Recent telemetry studies in Lake Havasu have found large spawning aggregations of razorback sucker in habitat unsuitable for standard net-based sampling that occurs in this reach. We deployed remote PIT scanners biweekly for the spawning period (January-early April, 2012) in fast flowing waters from Davis Dam downstream to Needles CA to contact a greater proportion of the population. We detected 763 individuals with PIT scanners and estimated the 2011 population to be 2659 (2069 to 3414, 95% confidence interval) individuals. Fish released in the largest size class, ≥ 500 mm, were contacted at a rate 3.2 to 10.3 times greater than fish released in any other individual 100-mm size class. Individuals also were significantly more likely to be contacted if released in spring than autumn (ANCOVA: P = 0.03, F-value =5.99). Results from multiple iterations of these combined efforts will form the basis for recommendations to adjust the stocking program to enhance razorback sucker survival in Lake Havasu.