Passive detection data aid estimation of Razorback Sucker Xyrauchen texanus survival rates

Authors: Koreen Zelasko, Kevin Bestgen, Gary White

Year: 2019

Abstract

Low probabilities of capture (<0.05), based only on physical recaptures of tagged fish, have hindered the precise estimation of survival rates for endangered Razorback Sucker, Xyrauchen texanus, in the upper Colorado River basin (UCRB). Because those populations are maintained by stocking of hatchery-reared individuals (N = 416,004 cumulatively through 2017), reliable evaluations of survival are essential to the species’ management. Since 2008, PIT tag detection antennas and ultrasonic receivers have been deployed throughout the UCRB, San Juan River Basin, and Colorado and San Juan river arms of Lake Powell to passively document tagged fish. Increased detections, along with physical captures from more intensive sampling, were used to determine if more robust estimates of survival were possible. Encounter records of 395,488 stocked Razorback Suckers from 1995 through 2017 were analyzed with the Barker model, in which passively-collected data are incorporated as “resight” events outside the physical sampling occasions. Annual capture probabilities remained low (mean: 0.05, range: 0.01–0.09), as in previous studies. Since antenna deployment began, resight probabilities increased from 0 to a high of 0.13 in 2017. Importantly, the empirical rate of physical recaptures for the entire 23-year study period was 3.9%, while rate of detections from antennas was nearly half that (1.8%) but almost entirely from just the most recent five years of antenna use, < 25% of the study period. Analyses supported previous findings that survival through the first year post-stocking was lower than subsequent years, first-year survival for fish stocked during summer (mean: 0.06, range: <0.01–0.12) was lower than other seasons (mean: 0.20, range: 0.01–0.39), and first-year survival of fish stocked in the Green River subbasin (mean: 0.27, range: 0.02–0.80) was higher than the Colorado River subbasin (mean: 0.12, range: <0.01–0.59). Both seasonal and subbasin survival rate estimates through the first year were higher and more precise than those from our previous studies using only records from 1995–2006 and 2004–2008. This analysis showed survival after the first year post-stocking increased to 0.83 (CV < 1%) and was higher and more precise than in 1995–2006 (0.75, CV = 4%). Incorporating passive detection data with physical recaptures provided more robust survival rate estimates, which are useful to evaluate hatchery stocking practices and recovery status of populations. Passive detections may also improve estimation of population parameters for other PIT-tagged fishes in large river systems where capture probabilities are low.