This is the second year of the current project to monitor 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 Imperial Diversion Dam. A total of 5,950 razorback sucker and 6,779 bonytail were stocked into backwaters and main channel of the study area in La Paz Co. Arizona and Riverside Co. California from October 2017 through April 2018. All fish released were implanted with a 134.2 kHz Passive Integrated Transponder (PIT) tag.
Up to 20 portable remote PIT tag sensing units were distributed throughout backwaters and the main channel for 5 days during each month from October to March. During the peak spawning season (January–February) PIT tag sensing units were deployed for 10 days each month. Effort in the river channel was increased during the active sample period (October 1, 2017 to April 30, 2018) to identify spawning sites outside of backwater habitat and contact individuals during spawning. PIT tag sensing units were deployed for a total of 12,597.1 hours in the second year of study and recorded 1,792 unique contacts: 535 bonytail, 1,234 razorback sucker, and 23 individuals with no database record. One bonytail and 206 razorback sucker were contacted more than a year after release in study year (SY) 2018. Fifteen unique fishes, nine razorback suckers and six bonytail, were contacted in the main channel during SY 2018. No bonytail contacted during the marking period (October 1, 2016 to May 31, 2017) were contacted again in the capture period (October 1, 2017 to May 31, 2018), so no population estimate was possible. The razorback sucker population estimate for 2017 was 169 (95% CI 157 to 180).
Eighteen subadult bonytail and 18 subadult razorback sucker were implanted with short-term (3-month) acoustic telemetry tags to examine dispersal patterns immediately following release. Twenty tags for each species was planned, but four tags were inoperable. Ten adult razorback sucker were implanted with longer-term (36-month) tags to examine dispersal over a longer period. One adult was electrofished from the main channel of the Colorado River, the first razorback sucker capture from the river channel during this study.
During this study year nine Submersible Ultrasonic Receivers (SURs) were deployed, three of which were replacements for SURs lost during the study, five of which were supplemental, and one of which was moved to a more secure location in A7 upper. At the end of the field portion of the current study year, 18 SURs were active in the study area.
Throughout the study year manual acoustic tag tracking was conducted in backwaters to supplement dispersal data and identify stationary tags. During peak spawning months (January–February) and March, manual acoustic tracking was conducted in the main river channel, and one juvenile razorback sucker released in November 2017 was contacted. The maximum dispersal distance of any acoustic tagged fish was 80.9 km by a subadult razorback sucker released in November 2017. Divers recovered three, 36-month acoustic tags. One was implanted in an adult razorback sucker on January 18, 2018. The other two were implanted in adult razorback sucker on February 22, 2017.
The temporal and spatial limitations of the remote PIT tag sensing data in the reach continue. Evidence of long-term persistence (more than one year post-release) of either bonytail or razorback sucker in the study area is lacking, despite contacting thousands of bonytail and razorback sucker post-release. Most contacts with either species occur within 30 days post-release and are acquired from one backwater complex (A10 upper and lower).