This is the first year of the current project to monitor the population status and distribution of razorback sucker (Xyrauchen texanus) and bonytail (Gila elegans) in the lower Colorado River downstream of Palo Verde Diversion Dam and upstream of the Imperial Diversion Dam. In support of this study, a total of 5,935 razorback sucker and 4,491 bonytail were stocked into backwaters in La Paz Co., AZ and Riverside Co., CA from October 2016 through January 2017. All fish released were implanted with a 134.2-kilohertz (kHz) passive integrated transponder (PIT) tag. Twenty subadults of each species were implanted with short-term (3 month) acoustic telemetry tags to examine dispersal patterns immediately following release. Ten adult razorback sucker were implanted with longer-term (36 month) tags to examine dispersal over a longer period. PIT tag sensing units were used to contact PIT tagged fish and were set monthly from October 2016-April 2017 for 1-2 weeks. Submersible ultrasonic receivers (SURs) were dispersed throughout the backwaters and river channel to detect fish movement.
Up to 20 PIT tag sensing units were distributed throughout backwaters and the main river channel for five days during each month from October-April. Effort between February and April was increased in the river channel in attempt to identify spawning sites and contact individuals during spawning. PIT tag sensing units were deployed for a total of 14,011.5 hours in this first year of study and recorded 671 unique contacts – 277 bonytail, 383 razorback sucker, and 11 individuals with no database record.
Based on the Lower Colorado River Native Fishes Database, totals of 15,795 razorback sucker and 11,696 bonytail were released with 134.2 kilohertz (kHz) PIT tags into the Colorado River downstream of Palo Verde Diversion Dam between 2007 and May 2017. Deployment of remote PIT tag sensing units since October 2014 (SY 2015) has resulted in contact with 900 razorback sucker and 438 bonytail from these releases, but only 186 and 76 of these contacts
respectively occurred outside of their release site. The greatest number of days at large (DAL) for a PIT tagged bonytail was 548 days, released in September 2015. A razorback sucker released October 2007 had been at large for 3,500 days.
Based on year to year PIT tag sensing contact records, a razorback sucker population estimate for SY 2016 was 216 (95% CI 173 to 271) with 130 encountered in SY 2016 (marking period October 2015 through May 2016), 130 encountered in SY 2017 (capture period October 2016 through May 2017), and 78 encountered in both periods (recaptures). More than 90% of contacts used in the population estimate were recorded in A10.
SURs were distributed throughout the backwater and river channels to detect movement of individuals implanted with an acoustic tag. Opportunistic manual tracking in backwaters was conducted to provide additional movement information. The maximum dispersal distance recorded was 20.4 kilometers (km) by an acoustic tagged adult razorback sucker released in February 2017. The greatest dispersal distance by an acoustic tagged subadult bonytail was 2.31 km, released in January 2017.