| Abstrakt: |
AbstractIn this study, we electrofished 961 study sites to estimate the abundance of trout (in streams only) throughout the upper Snake River basin in Idaho (and portions of adjacent states) to determine the current status of Yellowstone cutthroat trout Oncorhynchus clarkii bouvieriiand other nonnative salmonids and to assess introgressive hybridization between Yellowstone cutthroat trout and rainbow trout O. mykiss. Yellowstone cutthroat trout were the most widely distributed species of trout, followed by brook trout Salvelinus fontinalis, rainbow trout and rainbow trout × Yellowstone cutthroat trout hybrids, and brown trout Salmo trutta. Of the 457 sites that contained Yellowstone cutthroat trout, less than half also contained nonnative salmonids and only 88 contained rainbow trout and hybrids. In the 11 geographic management units (GMUs) for which sample size permitted abundance estimates, the number of 100-mm and larger trout was estimated to be about 2.2 ± 1.2 million (mean ± confidence interval); of these, about 1.0 ± 0.4 million were Yellowstone cutthroat trout. Similarly, the estimated abundance of trout smaller than 100 mm was 2.0 ± 1.4 million, of which about 1.2 ± 0.7 million) were Yellowstone cutthroat trout. Both estimates are almost certainly biased downward owing to methodological constraints. Yellowstone cutthroat trout were divided into approximately 70 subpopulations, but estimates could be made for only 55 subpopulations; of these, 44 and 28 subpopulations contained more than 1,000 and 2,500 Yellowstone cutthroat trout, respectively. We compared morphological assessments of purity with subsequent molecular DNA analysis from 51 of the study sites and found that levels of purity were positively correlated between methods (r= 0.84). Based on this agreement, we classified Yellowstone cutthroat trout (based on morphological characteristics alone) as pure at 81% of the study sites within these GMUs. Our results suggest that despite the presence of nonnative threats (genetic and competitive), Yellowstone cutthroat trout remain widely distributed and appear to have healthy populations in numerous river drainages in Idaho. |
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