2017 Lake Mohave Annual Report

Repatriated razorback sucker (Xyrauchen texanus) in Lake Mohave have been monitored for more than 20 years, but low recapture rates have inhibited evaluation of factors contributing to highly variable post-stocking survival. In 2011, deployment of remote passive integrated transponder (PIT) scanners able to detect 134.2 kilohertz (kHz) PIT tags was initiated to increase the number of encounters with marked fish. The program was expanded in 2012 and 2013, while traditional capture methods continued to be employed to collect comparable long-term monitoring data and estimate abundance of all repatriated and wild razorback sucker marked with either 400 or 134.2 kHz PIT tags.

Eleven razorback sucker were handled by Marsh & Associates during FY2017; seven fish on December 5, 2016 with assistance from Arizona Game and Fish Department (AZGFD) and Arizona Department of Environmental Quality (AZDEQ), and four fish during spring monitoring from March 14 through March 16, 2017. One fish was a first-time capture, not previously PIT tagged, and 10 fish were previously PIT tagged repatriates of which nine had paired stocking and capture data in the database; one fish with unknown stocking or capture data was omitted from further analysis. Based on 2016 and 2017 monitoring data, we determined there is no effective wild razorback sucker population remaining in Lake Mohave. The repatriated razorback sucker population for 2016 based on 2016 and 2017 March monitoring data was estimated to number 1,291 (95% confidence interval [CI] from 531 to 3,436).

Total deployment time for remote PIT scanners from October 1, 2016 through September 30, 2017 was 54,850 scan hours resulting in 71,434 PIT tag contacts, representing 3,707 unique PIT tags for which 3,490 had a razorback sucker marking record in the database (as of September 30, 2017). Among fish with a marking record, 3,462 were repatriates, nine were wild, and 19 were of unknown origin.

Lake Mohave was subdivided for analytical purposes into four stocking zones; up- to downstream these were River, Liberty, Basin, and Katherine. Poststocking dispersal from zone to zone over the course of the study period was limited. The majority of fish released in River and Basin were contacted in their zone of release, regardless of release year. Razorback sucker released in Liberty and Katherine were generally contacted elsewhere (River and Basin). Among the four zones scanned in 2016 and 2017, remote PIT scanning detected little movement of razorback sucker between years with 83% of individuals (1,762 out of 2,125) contacted in one zone for two years.

Based on 2016 and 2017 remote PIT scanning, the 134.2 kHz tagged repatriate population in 2016 was 3,815 (95% CI from 3,573 to 4,073). Basin and River subpopulation estimates based on zone specific scanning in 2016 and 2017 also were calculated. The Basin subpopulation was estimated at 2,008 (95% CI from 1,848 to 2,181) and River at 2,213 (95% CI from 1,976 to 2,479). The subpopulation in Liberty zone was not estimated because there were no recaptures there. Too few wild fish were contacted to estimate Basin and River subpopulations separately (two and seven contacts respectively). The lake-wide estimate of the wild population based on PIT scanning in 2016 and 2017 was 10 fish (95% CI from 5 to 23).