This is the third year of the current project to monitor population status and distribution of razorback suckers (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 12,686 razorback suckers and 6,490 bonytail was stocked into backwaters and main channel of the study area in La Paz Co., Arizona and Riverside Co., California from October 2018 through April 2019. An experimental release of 364 bonytail was implanted with 32 millimeter (mm), 134.2 kilohertz (kHz), half duplex (HDX) passive integrated transponder (PIT) tags. All other fish released were implanted with 12 mm, 134.2 kHz, full duplex (FDX) PIT tags.
Up to 20 portable remote PIT tag sensing units (scanners) were distributed throughout backwaters and the main channel for 5 days during each month from October to March. During the peak razorback sucker spawning season (January–February) PIT scanners were deployed for 10 days each month. Scanning effort in the river channel was increased during the active sample period (October 1, 2018 to March 30, 2019) to identify spawning sites outside of backwater habitat and contact individuals during spawning. Two, semi-permanent scanners were placed in culverts to monitor dispersal through and out of backwater habitat. These culvert scanners ran continuously throughout the field season. Marsh & Associates (M&A) personnel deployed PIT tag sensing units for a total of 16,973.2 hours in the study year. M&A and Bureau of Reclamation personnel contacted 346 bonytail and 1,836 razorback suckers through combined efforts. There were 344 razorback suckers contacted in the study year that had been released more than a year prior to their most recent contact; zero bonytail contacts fit this criterion. There were 337 razorback suckers and 106 bonytail contacted in the main channel during this study year. In previous years the largest numbers of river contacts were 15 razorback suckers and 9 bonytail. The increase in river contacts is largely due to the discovery of a razorback sucker aggregation site near the entrance to C7 McIntyre Park (hereon referred to as Aggregation Site) in January where 307 razorback suckers and 3 bonytail were contacted. No bonytail contacted during the marking period (January 1, 2018 to February 28, 2018) were contacted again in the capture period (October 1, 2018 to April 30, 2018), so no population estimate was possible. The razorback sucker population estimate for 2018 was 147 (95% CI 123 to 171).
Twenty subadult bonytail and 20 subadult razorback suckers were implanted with short-term (3-month) acoustic telemetry tags to examine dispersal patterns immediately following release. Ten adult razorback suckers were implanted with longer-term (36-month) tags and ten adult bonytail were implanted with 9-month tags to monitor long-term dispersal. This was the first year of the study where bonytail were implanted with long-term tags. Seven of the razorback suckers implanted with long-term tags were acquired by electroshocking over the Aggregation Site in the river.
Throughout the study year manual tracking of acoustic tags was conducted in backwaters to supplement submersible ultrasonic receiver (SUR) data used to track dispersal and to identify stationary tags. Tracking was also conducted in the main river channel January through March, and three razorback suckers were contacted at Aggregation Site in the river channel and one adult was contacted downstream of all backwaters. The maximum dispersal distance of any acoustic tagged fishes was 63.6 kilometers (km) by a subadult razorback sucker released in November 2018. Divers recovered six acoustic tags: three 3-month tags from subadult bonytail, two 9-month tags from adult bonytail and one 36-month tag implanted into an adult razorback sucker in the first study year.
For the first time in this study, adult razorback suckers were observed (contacted) aggregating in the mainstem Colorado River in the reach downstream of Palo Verde Dam. Population estimates for razorback sucker remain low, but continued stocking of large fish over the next few years will provide estimates of vital demographic rates. This in turn could be used to determine the potential for establishing a population of adult razorback sucker in the lower Colorado River.
Evidence of long-term persistence (more than one-year post-release) of bonytail in the study area is lacking, despite contacting thousands within the first few months post-release. Bonytail tagged with large (32 mm) HDX PIT tags had higher contact rates than those tagged with 12 mm FDX PIT tags during this study year (30.2% vs. 11.6% respectively), suggesting that this larger tag format may improve contact rates and increase the probability of observing long-term persistence. The impact of PIT scanner orientation on bonytail contacts was less clear. For the first three days of the study, PIT scanners with a bottom flat orientation contacted more bonytail with either tag type than PIT scanners with a bottom long orientation. On the fourth day the results were reversed with more bonytail contacts by PIT scanners with bottom long orientation regardless of tag type.