A Long-Term Comparison of Yellowstone Cutthroat Trout Abundance and Size Structure in Their Historical Range in Idaho

Abstract

We compared estimates of population abundance and size structure for Yellowstone cutthroat trout Oncorhynchus clarki bouvieri obtained by electrofishing 77 stream segments across southeastern Idaho in the 1980s and again in 1999–2000 to test whether populations of Yellowstone cutthroat trout had changed. Sites sampled in the 1980s were relocated in 1999–2000 by using maps and photographs or by finding original site‐boundary stakes, so that the same reach of stream was sampled during both periods. Abundance of Yellowstone cutthroat trout longer than 10 cm did not change, averaging 41 fish/100 m of stream during both the 1980s and 1999–2000. The proportion of the total catch of trout composed of Yellowstone cutthroat trout also did not change, averaging 82% in the 1980s and 78% in 1999–2000. At the 48 sites where size structure could be estimated for both periods, the proportion of Yellowstone cutthroat trout that were 10–20 cm long declined slightly (74% versus 66%), but the change was due entirely to the shift in size structure at the Teton River sites. The number of sites that contained rainbow trout O. mykiss or cutthroat trout × rainbow trout hybrids rose from 23 to 37, but the average proportion of the catch composed of rainbow trout and hybrids did not increase (7% in both the 1980s and 1999–2000). Although the distribution and abundance of Yellowstone cutthroat trout have been substantially reduced in Idaho over the last century, our results indicate that Yellowstone cutthroat trout abundance and size structure in Idaho have remained relatively stable at a large number of locations for the last 10–20 years. The expanding distribution of rainbow trout and hybrids in portions of the upper Snake River basin, however, calls for additional monitoring and active management actions.

Distinguishing Yellowstone Cutthroat Trout, Rainbow Trout, and Hybrids by Use of Field‐Based Phenotypic Characteristics

“…Means and ranges of TLs were similar between Yellowstone Cutthroat Trout, Rainbow Trout, and hybrids, although Rainbow Trout were slightly smaller on average than the other two groups (Figure 1). A suite of phenotypic characteristics was then recorded for each fish (Table 1; Figure 2); selected characteristics were largely based on the results of previous studies that differentiated Yellowstone Cutthroat Trout from Rainbow Trout and hybrids (Campbell et al 2002;Meyer et al 2003Meyer et al , 2006. Most of the characteristics are self-explanatory, but spot counts on the top of the head require further description.…”

“…For example, at a spawning weir installed to interrogate upstream-migrating fish, where Cutthroat Trout are passed upstream to spawn in natal tributaries but Rainbow Trout and hybrids are not (e.g., High 2010), a weir operator cannot count fin rays and scales and measure tedious external characteristics on dozens if not hundreds of fish each day. External characteristics that are more qualitative in nature, such as body coloration, white-tipped fins, the presence of a throat slash, the number of spots on the top of the head, and other spotting patterns or coloration on various regions of the fish's body, have been used to differentiate Yellowstone Cutthroat Trout from Rainbow Trout and hybrids (e.g., Campbell et al 2002;Meyer et al 2003Meyer et al , 2006. However, no previous studies have provided the proportional incidence of specific phenotypic traits in genetically identified Yellowstone Cutthroat Trout, Rainbow Trout, and hybrids.…”

Biology, Status, and Management of the Yellowstone Cutthroat Trout

“…Numerous studies document more specific trend information. For example, for almost 20 years after the original samples of 77 stream sites from southeastern Idaho, relative abundance and size structure remained quite consistent (Meyer et al 2003b). During this period, the number of sites with introgressed populations rose from 23 to 37 (30-48% of the total number of sites), but it appeared that most of the changes occurred at 17 sites located in the Blackfoot River and South Fork Snake River drainages (Meyer et al 2003b).…”

“…Over the past century, anthropogenic activities have resulted in a substantial reduction in the historical distribution of the Yellowstone cutthroat trout, and many unique local populations have been extirpated (Meyer et al 2003b;May et al 2007). Numerous federal and state resource management agencies and nongovernmental organizations have designated the Yellowstone cutthroat trout as a "species of special concern" or a "sensitive species."…”

A Long‐Term Comparison of Redband Trout Distribution, Density, and Size Structure in Southwestern Idaho

“…Quantitative information is often available for use to delineate further trends in abundance within the known occupied range or to refine qualitative estimates of distribution. For example, Meyer et al (2003) recently quantitatively examined the long-term trend in abundance of Yellowstone cutthroat trout O. clarkii bouvieri in Idaho by resampling population abundance at sites that were originally sampled in the 1980s. Strange and Habera (1998) resampled streams 7-16 years after initial surveys to examine potential changes in the elevational distributions of native populations of brook trout Salvelinus fontinalis.…”