Incorporating passive antenna detections with physical recaptures improves survival rate estimates for Razorback Suckers, Xyrauchen texanus, stocked in the Upper Colorado River Basin

Authors: Koreen Zelasko, Kevin Bestgen, Gary White

Year: 2021

Abstract

Quantified demographic rates are useful for evaluating status and trajectory of an animal population. The most recent Recovery Goals (2002) for endangered Razorback sucker, Xyrauchen texanus (Abbott), family Catostomidae, require that two ““genetically and demographically viable, self-sustaining”“ adult populations, each exceeding 5,800 individuals, exist in the Upper Colorado River Basin (UCRB) before downlisting or can occur. Between 1995 and 2017, nearly 400,000 Razorback Suckers were stocked into the UCRB to help achieve that target. In the absence of sustained, measurable recruitment, survival estimation is a valuable metric to assess status of the stocked population. From 1995 through 2006, physical mark-recapture data and analyses yielded low first-year survival, particularly for smaller fish stocked during summer months. Since that time, physical captures of Razorback Suckers have increased, and the distribution and numbers of passive PIT-tag detection technologies (portable ““flat plate”“ and ““wagon wheel”“ antennas at congregation sites; fixed passive interrogation antenna arrays in tributaries, at fish ladders, and in diversion canals; and submersible ultrasonic receivers to detect radio tags associated with PIT tags) resulted in hundreds of thousands of Razorback sucker encounter records and documented movement between the UCRB and the San Juan River basin (SJRB) as well as first encounters of fish at large many years after stocking. We incorporated passive detection data alongside traditional, physical mark-recapture data to provide more up-to-date and robust survival rate estimates for hatchery-reared Razorback Suckers stocked into the UCRB. Here, an ““encounter”“ is any record of a Razorback sucker after initial stocking. A ““capture”“ is any encounter produced by physical sampling and handling after stocking. A ““detection”“ is any encounter produced by the passive technologies mentioned above. We used the Barker model to incorporate detections with captures that were collected from nearly 1,300 river miles of the UCRB, SJRB, and associated river inflows to Lake Powell, 2003-2017. Of the 321,233 fish stocked in the UCRB, 93% were never seen again and 7% were later encountered. Of those encountered, 62% were physically captured over 15 years, while 30% were detected during only the last five years of the study. The remaining 8% were encountered by both means. The survival portion of the top-ranked model included effects of time since stocking, season of stocking, and total length at stocking. Survival rates were low in a fish’s first year and lowest for smaller fish stocked in summer, supporting earlier analyses. Mean first-year survival rates for 350-mm-TL fish stocked in spring, summer, and autumn were 0.57, 0.18, and 0.46, respectively, with low mean coefficients of variation of 6, 9, and 5%. Mean survival after the first year was 0.80. The survival estimates were higher than those from our previous analysis from 1995-2006, which included only captures. Capture probabilities from physical encounters were low during first sampling occasions (mean: 0.06, range: 0.01-0.18) and declined thereafter. Detection probabilities from passive encounters were higher for fish stocked into the Green River subbasin (mean: 0.03, range: <0.01-0.16) than the Colorado River subbasin (mean: 0.01, range <0.01-0.04). This study was the first to employ the Barker model to estimate survival rates of hatchery-reared Razorback Suckers and include detections as year-round resights, rather than as additional physical captures within discrete capture occasions. Incorporating cost-effective passive detections alongside physical captures yielded the most robust estimates available for hatchery-reared Razorback Suckers, which will aid evaluation of stocking practices and progress toward recovery.