Lake Mohave Native Fish Monitoring Annual Report 2020

Repatriated razorback suckers (Xyrauchen texanus) in Lake Mohave have been monitored since the first releases in 1992.  The most significant change to monitoring techniques occurred in 2010 with the advent of remote passive integrated transponder (PIT) scanners able to detect 134.2 kilohertz (kHz) PIT tags.  Utilization of this technology was initiated to increase the number of fish encounters and it has proven successful in that regard.  The program was expanded in 2012 and 2013, while traditional capture methods (i.e., trammel nets) continued to be employed to collect comparable long-term monitoring data and estimate abundance of razorback suckers marked with either 400 or 134.2 kHz PIT tags.  Monitoring methods remained relatively unchanged from 2015–20 apart from the occasional addition or subtraction of remote PIT scanning effort in certain areas or locations of the reservoir, which is split into four distinct zones, listed from up- to downstream as: River, Liberty, Basin, and Katherine.

Trammel netting at Carp Cove in December 2019 and March 2020 resulted in capture of 80 razorback suckers, 57 female and 23 males.  Seven of the fish captured had no detectable PIT tag and one had a PIT tag but no previous record of tagging.  The 72 razorback suckers captured with a tagging record represented 70 unique fish (two were same-trip recaptures).  All were repatriates.  Overall total length (TL) at release ranged from 310–570 millimeters (mm) and TL at capture ranged from 355–695 mm with an overall mean TL at release and capture of 453 and 553 mm, respectively.  The shortest time at large, from stocking to capture, was 95 days, and years at large ranged from less than 1 to 12 with an overall mean of 3.  Sixty-six fish had year class information ranging from 2006–16; most (52 fish) were from the 2014–16 year classes.  Mean growth rate was 5 mm TL per month.  Most captured fish were released in Basin (n=62), the rest were released in River (n=8); 11 were raised in lakeside backwaters and 59 in offsite facilities.

Total deployment time for PIT scanners (remote monitoring) October 1, 2019 through September 30, 2020 was 27,330 scan hours resulting in 294,574 PIT tag contacts, representing 5,960 unique PIT tags for which 5,914 had a matching record in the Lower Colorado River Native Fish Database, including 43 double PIT tag contacts (two tags in the same fish).  Of the unique fish contacted, 5,870 were razorback suckers; 5,796 repatriate, 8 wild, and 66 unknown origin.  One repatriate bonytail (Gila elegans) also was encountered. 

 Wild razorback suckers were not encountered during SY 2020 routine monitoring events (i.e., trammel netting).  The repatriated razorback sucker population in 2019, based on 2019 and 2020 March monitoring data, was estimated at 1,559 fish (95% confidence interval [CI] from 967 to 2,653).  Based on SY 2019 and SY 2020 remote PIT scanning, the 134.2 kHz PIT tagged Lake Mohave population for 2019 was estimated at 3,906 individuals (95% CI from 3,789 to 4,022).  Population estimates using zone-specific scanning for 2019 estimated Basin population at 1,901 (95% CI from 1,856 to 1,945) and River at 2,014 (95% CI from 1,896 to 2,131). 

Stocking displacement was examined to enumerate fish contacted in multiple zones (contact histories by zone) and to determine farthest distance traveled from stocking locations (movement distance).  Fish that were released in River or Basin zones mostly were contacted only within their zone of release (64.9% and 67.3% of contacted fish, respectively).  However, a considerable portion of fish released in River or Basin zones were also detected in the other zone (31.9% and 31.6% of contacted fish, respectively).  Razorback suckers released in Liberty zone were mostly contacted upstream in River zone or downstream in Basin zone in approximately equal proportions, and fish stocked in Katherine zone were mostly contacted upstream of their release location.  Movement distance analysis results were comparable to those of previous evaluations in that the distribution of apparent range limits of fish stocked in River and Basin were bimodal with peaks in both zones.  Fish stocked in Liberty exhibited a more unimodal distribution of range limits, with fish traveling to join the River and/or Basin subpopulations.

Deployment of remote PIT scanners to monitor the two known subpopulation centers (River and Basin) will continue to be an effective means of contacting razorback sucker aggregates.  Additional PIT scanning efforts have continued in Liberty to determine if other aggregations exist.  Bi-annual routine monitoring efforts in Basin zone continue to collect essential growth, health, census, and genetic data for razorback sucker.  These data continue to provide long-term insight into population dynamics and demographics and contribute to the conservation of this endangered species.