2012 Lake Mohave final report

Summary

Monitoring of repatriated razorback sucker (Xyrauchen texanus) in Lake Mohave has been conducted for twenty years, but low recapture rates have inhibited evaluation of factors contributing to highly variable post-stocking survival. To increase the number of encounters, deployment of remote passive integrated transponder (PIT) scanners able to detect 134.2 kilohertz (kHz) PIT tags was initiated in 2011 and expanded in 2012, while traditional capture methods were employed to continue to collect comparable long-term monitoring data and estimate abundance of all repatriated and wild PIT tagged razorback sucker (400 or 134.2 kHz).

Netting efforts from 23 November 2011 to 30 September 2012 resulted in the capture of 32 razorback sucker (31 captures, one short-term recapture). Sixty-five percent of captures occurred in March, and 35% during November. Five fish were captured with no tags and were presumed to be repatriates; all remaining individuals were PIT-tagged repatriates. No wild razorback sucker estimate was made due to a lack of captures. The repatriated razorback sucker population for 2011 is estimated to number 2,577 (95% confidence interval [CI] from 1,139 to 6,284) with a 2% estimated survival of all repatriates released as of 1 March 2011.

Total deployment time for remote PIT scanners from January through September 2012 was 8,392.6 scan hours resulting in a total of 46,855 PIT tag contacts representing 2,748 individual razorback sucker for which 2,704 had a marking record in the Lower Colorado River Native Fish Database (as of September 30, 2012). Of the fish with a marking record, 2,685 were repatriates, 13 were wild, and 6 were recorded as unknown.

Remote PIT scanning deployments were divided among three zones; River, Liberty, and Basin. Of the repatriated razorback sucker contacted in both 2011 and 2012, very little exchange was observed between the three zones with 94.5% of contacts (571 of 604 fish) occurring in the same zone in both years. Post-stocking dispersal between the three zones was also limited. Of the 1,070 razorback sucker that had been at large for at least one year and were released in the River zone after 1 October 2008, 93% (994 individuals) were scanned in the River zone. Razorback sucker released in Liberty were more likely to move elsewhere with 57% located in Basin (65 fish), and 41% in River (36 fish).

Population estimates for 134.2 kHz tagged razorback sucker were divided among River and Basin subpopulations based on remote PIT scanning in 2011 and 2012; 1,726 (95% CI from 1,507 to 1,976), and 958 (95% CI from 815 to 112) respectively. Wild razorback sucker were also contacted in Basin and River zones. Estimated abundance of wild fish that were tagged with 134.2 kHz tags was 3 (number of fish marked in the initial sample [M]=2, number captured in the second sample [C]=2, and number of marked fish in the second sample [R]=1) and 12 (M=7, C=11, R=7) respectively. A regression analysis of 134.2 kHz PIT tagged razorback sucker contacted in River from January through September 2012 was used to estimate the population there in 2012 at 2,174 (95% CI from 1,974 to 2,375).

For 2012, PIT scanning deployments in Lake Mohave increased the number of encounters with razorback sucker by more than a factor of ten (2,748 contacted compared to 170 fish captured in March roundup), established the separation of two subpopulations, and resulted in contact with 86% of the estimated population in the river; 1,866 razorback sucker contacted in 2012 that were released prior to 2012 out of an estimated population of 2,174 based on regression analysis. This level of annual encounter rate, if maintained for multiple years, will provide insight into the influence of stocking location, size, season, and temporal variations on post-stocking and adult survival.

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