Razorback sucker monitoring in Lake Mohave 2015-2019 Final Report

Repatriated razorback suckers (Xyrauchen texanus) in Lake Mohave have been monitored for more than 25 years, but low recapture rates have inhibited evaluation of factors contributing to highly variable post-stocking survival.  In 2010, 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 (i.e., trammel nets) continued to be employed to collect comparable long-term monitoring data and estimate abundance of all repatriated and wild razorback suckers marked with either 400 or 134.2 kHz PIT tags. Lake Mohave was split into four distinct zones, listed from upstream to downstream as: River, Liberty, Basin, and Katherine.

Trammel netting efforts at Carp Cove from October1, 2014 to September 30, 2019 resulted in capture of 83 PIT-tagged repatriated razorback suckers that had single captures during the monitoring period and one fish that had two captures.  Sex was determined at time of capture for all fish and most were female (n=68) along with 15 males and one juvenile.  Of the 84 repatriates, 80 had paired stocking-capture data (i.e., fish with stocking and capture data).  Overall total length (TL) at release ranged from 265–531 millimeters (mm) and TL at capture ranged from 342–741 mm with an overall mean TL at release and capture of 426 and 595 mm, respectively.  The shortest time at large, from stocking to capture, was 10 days, and years at large ranged from less than one to 22 with an overall mean of 6.  Year classes ranged from 2001–2016; the 2004-year class was not represented.  Mean growth rate of females was 6 mm TL per month and for males approximately 4 mm TL per month.  Most captured fish were released in Basin (n=59), followed by Liberty (n=12), River (n=7), and Katherine (n=2); 36 were raised in lakeside backwaters and 44 in offsite facilities.

Based on routine monitoring data (i.e., trammel netting) from the last 5 years we estimate there is no self-sustaining wild razorback sucker population remaining in Lake Mohave.  The repatriated razorback sucker population in 2014, based on 2014 and 2015 March monitoring data, was estimated at 2,230 fish (95% confidence interval [CI] from 922 to 5,963), which was more than double the current (2018) estimate, based on 2018 and 2019 March monitoring data, for razorback sucker in Lake Mohave of 994 fish (95% CI from 602  to 1,639).

Total deployment time for PIT scanners (remote monitoring) from October 1, 2018 through September 30, 2019 was 37,257 scan hours resulting in 96,575 PIT tag contacts, representing 4,408 unique PIT tags for which 4,225 had a razorback sucker marking record (i.e., implanted with a PIT tag and associated data recorded), and 60 had a bonytail (Gila elegans) marking record in the Native Fish Database (as of September 30, 2019).  Among razorback suckers with a marking record, 4,165 were repatriates, 10 were wild, and 50 were of unknown origin.  All bonytail with a marking record were repatriates.

Total deployment time for remote PIT scanners from October 1, 2014 through September 30, 2019 was 201,382 scan hours resulting in 627,411 PIT tag contacts, representing 6,304 unique PIT tags for which 5,876 had a razorback sucker marking record (i.e., implanted with a PIT tag and associated data recorded), and 106 had a bonytail marking record in the Native Fish Database (as of September 30, 2019).  Among razorback suckers with a marking record, 5,799 were repatriates, 13 were wild, and 64 were of unknown origin.  All bonytail with a marking record were repatriates.

Based on 2018 and 2019 remote PIT scanning, the 134.2 kHz PIT tagged Lake Mohave repatriate population for 2018 was estimated at 3,649 individuals (95% CI from 3,552 to 3,745).  Population estimates using zone specific scanning for 2018 estimated Basin population at 1,963 (1,904 to 2,021) and River at 2,120 (2,012 to 2,227).  Too few wild fish with 134.2 kHz PIT tags were contacted to estimate Basin and River subpopulations separately (four and seven contacts in 2019, respectively).  Wild fish continue to be contacted by PIT scanners, although the majority were originally tagged with 400 kHz tags.  The lake-wide estimates calculated from PIT scanning data likely underestimate the wild population due to PIT scanners only contacting 134.2 kHz tags.  Still, an estimated nine (5 to 19) wild razorback suckers with 134.2 kHz tags survive in Lake Mohave in 2018.

Lake-wide repatriate population estimates based on PIT scanning increased each year from 2010 (coinciding with increased scanning effort) until peaking in 2016 at 3,871 fish. The population of razorback sucker with unknown history (tagged at capture) has increased from a low of 7 in 2013 to a high of 32 in 2018.  No estimate was calculated for 2015 due to low recaptures (3). 

A robust mark-recapture model was applied to accumulated PIT scanning data from sample years 2013 to 2019 to estimate annual survival for River and Basin subpopulations of razorback sucker.  Estimates of adult survival were similar between River and Basin, with all but one annual estimate (Basin 2016 to 2017) more than 95%.  Emigration rates were more disparate with all but one estimate lower than 6% for Basin, and all but one estimate more than 6% in River.  This suggests that temporary emigration out of River is slightly higher than out of Basin.  Capture probability ranged from a low of 0.0012 on the last sampling occasion in May of 2019 (Basin) to 0.508 on the first sampling occasion in January 2013 (Basin).  All years had at least one occasion with capture probability above 0.250, with the highest value for a given year typically in February in Basin and December, January or February in River.

Stocking displacement was examined to determine distance traveled from stocking locations and to identify movement between zones.  Fish that were released in River or Basin zones were mostly contacted exclusively within their zone of release (64.3% and 69% of contacted fish, respectively).  However, a considerable portion of fish released in River or Basin zones were detected in the other zone (33.1% and 29.5% 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 all were contacted upstream of their release location.

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 and to further evaluate the dynamics of razorback sucker dispersal and distribution.  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.  Together, these efforts continue to contribute to the maintenance of this endangered species.