Repatriated razorback sucker (Xyrauchen texanus) in Lake Mohave have been monitored for more than twenty 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.
Ten razorback sucker were handled at the Marsh & Associates camp during the March roundup from March 14 through March 18, 2016; no razorback sucker were captured at any other times of the reporting year. There were no original capture or stocking data in the Lower Colorado River (LCR) Native Fish Database for one of the 10 captures. Of the nine remaining individuals, all were PIT tagged repatriates with paired stocking and capture data in the database. Based on 2015 and 2016 monitoring data, we determined there is no effective wild razorback sucker population remaining in Lake Mohave. The repatriated razorback sucker population for 2015 based on 2015 and 2016 March roundup data is estimated to number 1,707 (95% confidence interval [CI] from 603 to 3,897).
Total deployment time for remote PIT scanners from October 1, 2015 through September 30, 2016 was 37,859 scan hours resulting in 230,666 PIT tag contacts, representing 3,244 unique PIT tags for which 3,128 had a razorback sucker marking record in the database (as of September 30, 2016). Among fish with a marking record, 3,110 were repatriates, 10 were wild, and eight 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. Post-stocking dispersal from zone to zone over the course of the study period was limited. The majority (>85%) of fish released in River and Basin were contacted in their zone of release, regardless of release year. Razorback sucker released in Liberty were generally contacted elsewhere (River and Basin). Among the three zones scanned in 2015 and 2016, River, Liberty, and Basin, remote PIT scanning detected little movement of razorback sucker between years with 96% of individuals (1,597 out of 1,668) contacted in the same zone both years. Dispersal in Katherine was not evaluated because there were limited stockings and captures, and no scanning in that zone.
Based on 2015 and 2016 remote PIT scanning, the 134.2 kHz tagged Lake Mohave repatriate population was estimated at 3,656 (95% CI from 3,418 to 3,912). Basin and River subpopulation estimates based on zone specific scanning in 2015 and 2016 also were calculated. The Basin subpopulation was estimated at 1,947 (95% CI from 1,761 to 2,151) and River at 2,158 (95% CI from 1,960 to 2,377). The subpopulation in Liberty zone was not estimated due to a lack of scanning effort there during the sampling season. All ten wild razorback sucker contacted in 2016 were also contacted in 2015. Too few wild fish were contacted to estimate Basin and River subpopulations separately (three and seven contacts respectively). The lake-wide estimate of the wild population based on PIT scanning in 2015 and 2016 was 12 fish (95% CI from 7 to 23).
A multi-state mark recapture model assessment in the computer program MARK, estimated 5.1% (95% CI from 3.5 to 7.3%) of razorback sucker transitioned from Basin to River from 2014 to 2015 and 4.2% (95% CI from 3 to 5.8%) from River to Basin. From 2013 to 2014, apparent annual survival in Basin was estimated at 94.3% (95% CI from 91.6 to 96.1%) and in River was estimated at 89% (95% CI from 85.3 to 91.8%). Apparent survival was estimated at 93.8% (95% CI from 91.2 to 95.7%) in Basin from 2014 to 2015 and 88.3% (95% CI from 84.7 to 91.2%) in River during the same time period. Survival and transition could not be accurately estimated for 2015 to 2016 due to confounding with recapture rates.
Bi-annual netting efforts continue to collect essential growth, health, census, and genetic data for razorback sucker. Combined collection efforts upstream of Willow Beach captured more than 1,500 larvae, indicating that an equal share of larvae from River and Basin could be collected if effort is increased and distributed throughout the upper reach. Deployment of remote PITscanners to monitor the two known subpopulation centers (River and Basin) should continue with a nominal effort like that applied during the past year. Additional scanning efforts have extended to the Liberty zone to determine if other aggregations exist and to further evaluate the dynamics of razorback sucker dispersal and distribution.