Counting marmots…
AbstractWe used tag–recapture data to estimate apparent survival and capture probability for 119,129 hatchery‐reared, federally endangered razorback suckers Xyrauchen texanus stocked into upper Colorado River basin streams during 1995–2005. Effects investigated included reach, year, and season of stocking; fish total length (TL) at time of stocking; survival in the first year after stocking versus in subsequent years; and sampling effort. Recapture data were also used to describe poststocking movement. First‐year survival rate for stocked razorback suckers of average TL (252.5 mm) was low: 0.05 (95% confidence interval [CI], 0.042–0.071). Total length at stocking and first‐year survival were positively correlated; survival approached zero for fish smaller than 200 mm TL but increased to 0.75 or higher for the few fish larger than 500 mm. Season of stocking had a large effect on razorback sucker first‐year survival; the predicted rate for average‐length fish stocked in summer was less than 0.02 (CI, 0.012–0.022), but it was 0.07 (0.044–0.094), 0.08 (0.057–0.100), and 0.08 (0.057–0.118) for fish stocked in spring, autumn, and winter, respectively. The overall subsequent‐year survival rate for razorback suckers was estimated to be 0.75 (CI, 0.688–0.801). Capture probabilities were relatively low, ranging from 0.002 to 0.128 for fish of average TL. The mean minimum distance traveled, elapsed time, and rate of travel by razorback suckers between recaptures were 54.7 km (range, 0–514.9 km), 254 d (0–3,164 d), and 0.87 km/d (0–55.37 km/d), respectively. Movement was more frequent out of Colorado and Gunnison River stocking reaches (36.9%; range, 30.1–100%) than Green River stocking reaches (7.7%; range, 2.9–10.3%). Our recommendations include ceasing summer stocking, performing cost ‐benefit analyses of increasing TL at stocking, employing a standardized stocking protocol, and developing a comprehensive razorback sucker monitoring program, implementation of which should enhance recovery prospects for razorback suckers in the upper Colorado River basin.
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