Population dynamics of razorback sucker, Xyrauchen texanus, in the Green River basin, Utah and Colorado, 2011’2013

Authors: Koreen A. Zelasko, Kevin R. Bestgen, Gary C. White

Year: 2016 (xlviii)

Abstract

Population dynamics of Upper Colorado River Basin (UCRB) Razorback Sucker, Xyrauchen texanus, a species federally listed as endangered, were unknown for many years because limited recapture data precluded a structured monitoring effort. However, recent increases in Razorback Sucker captures allowed survival rate estimation for propagated individuals stocked into the UCRB from 1996’2005 and 2004’2007, and identification of hatchery characteristics and stocking protocols that affected survival. Razorback Sucker survival rate and population abundance estimates were next generated specifically for the Green River basin using data from endangered Colorado Pikeminnow, Ptychocheilus lucius, abundance estimation sampling from 2006’2008. This study built on our previous results and employed Razorback Sucker capture data collected during Colorado Pikeminnow sampling from 2011’2013. We used multi-state, robust design models in Program MARK with Huggins’ closed captures to estimate Razorback Sucker abundance, as well as survival, transition, capture, and recapture rates among four reaches and nearly 700 r iver km of the Green River basin: upstream middle Green River, intermediate Desolation-Gray canyons, downstream lower Green River, and tributary White River, which flows into the Green River between the upstream and intermediate reaches. In our top - ranked estimating model, Razorback Sucker survival varied by time (year) and was almost twice as high from 2011’2012 (0.65, 95% confidence interval [CI]: 0.48’0.80) than from 2012’2013 (0.35, 95% CI: 0.26’0.46). Fish movement (transition rate) out of the upstream middle Green River was low or zero. Transition rates for other reaches indicated most movement was upstream (e.g., Desolation-Gray canyons to middle Green River [0.21, 95% CI: 0.11’0.38] and lower Green River to Desolation-Gray canyons [0.23, 95% CI: 0.12’0.40]). Abundance for the entire study area was similar across years (mean: 22,933, range: 21,099’24,010), but varied considerably by reach. Highest estimated mean abundance was in Desolation-Gray canyons (8,732, range: 6,922’11,741 annually), a reach where Razorback Suckers were only stocked in 2006 and where survival was estimated to be near zero in our previous analysis; that reach is located between heavily stocked middle and lower Green River reaches. Mean abundance was 5,865 in middle Green and generally increased during the study period (range: 3,633’8,592), and was 7,604 in lower Green but decreased during the study period (range: 3,459’11,068). No Razorback Suckers have ever been stocked into the White River, but mean estimated abundance across study years was 731 (range: 28’1,514 annually) and the transition rate from the Desolation-Gray reach to White River was 0.13 (95% CI: 0.04’0.33), demonstrating population expansion. No Razorback Suckers were stocked and few were recaptured in the Yampa River, an upstream tributary to the Green River, so abundance estimates were not possible. Low capture probabilities (0.02, on average, across reaches and years), primarily due to sampling efforts that did not specifical ly target Razorback Suckers, continued to hinder our ability to produce precise abundance estimates ’ a task imperative to evaluating success of recovery actions. However, precision of Razorback Sucker abundance estimates improved as captures of the species increased in the UCRB, and this study, along with our previous analyses, will improve understanding of Razorback Sucker population dynamics and aid managers in allocating monitoring and recovery resources.