Abstract.-In the western United States, exotic brook trout Salvelinus fontinalis frequently have a deleterious effect on native salmonids, and biologists often attempt to remove brook trout from streams by means of electrofishing. Although the success of such projects typically is low, few studies have assessed the underlying mechanisms of failure, especially in terms of compensatory responses. A multiagency watershed advisory group (WAG) conducted a 3-year removal project to reduce brook trout and enhance native salmonids in 7.8 km of a southwestern Idaho stream. We evaluated the costs and success of their project in suppressing brook trout and looked for brook trout compensatory responses, such as decreased natural mortality, increased growth, increased fecundity at length, and earlier maturation. The total number of brook trout removed was 1,401 in 1998, 1,241 in 1999, and 890 in 2000; removal constituted an estimated 88% of the total number of brook trout in the stream in 1999 and 79% in 2000. Although abundance of age-1 and older brook trout declined slightly during and after the removals, abundance of age-0 brook trout increased 789% in the entire stream 2 years after the removals ceased. Total annual survival rate for age-2 and older brook trout did not decrease during the removals, and the removals failed to produce an increase in the abundance of native redband trout Oncorhynchus mykiss gairdneri. Lack of a meaningful decline and unchanged total mortality for older brook trout during the removals suggest that a compensatory response occurred in the brook trout population via reduced natural mortality, which offset the removal of large numbers of brook trout. Although we applaud WAG personnel for their goal of enhancing native salmonids by suppressing brook trout via electrofishing removal, we conclude that their efforts were unsuccessful and suggest that similar future projects elsewhere over such large stream lengths would be costly, quixotic enterprises.

Length and age at sexual maturity for Yellowstone cutthroat trout Oncorhynchus clarki bouvieri vary across their historical range, but the factors that influence this variation are poorly understood. We collected 610 Yellowstone cutthroat trout from 11 populations across southeastern Idaho from streams and rivers with a variety of physical characteristics to determine length and age at sexual maturity and other reproductive demographics. The oldest Yellowstone cutthroat trout captured (age 10) was from the South Fork Snake River; most fish (90%) were between ages 2 and 4, and only three (Ͻ1%) were older than age 7 (all from the South Fork Snake River). Cutthroat trout from the South Fork Snake River did not mature until they were 300 mm long and 5 years of age, whereas cutthroat trout from other migratory and resident sites began maturing at ages 2-3 and lengths of 100-150 mm. Fish 100-250 mm long were much more likely to be mature if they were from sites with resident rather than migratory reproductive life histories. The sex ratio (expressed as the percentage of females) averaged 46% and varied from 27% to 66% among sites. At all but one study site, males matured at a smaller size than females. For both male and female Yellowstone cutthroat trout, length at maturity was directly related to stream order and width, negatively related to gradient, and weakly correlated with conductivity, elevation, mean aspect, and mean summer water temperature. Length-at-maturity models were stronger and fit the data better than age-at-maturity models. Our results enable prediction of length at maturity for Yellowstone cutthroat trout by using readily derived physical data from streams. As such, the results could be useful in estimating risk assessment parameters, such as the number of breeders in and the genetic effective population size of Yellowstone cutthroat trout populations.

Mountain whitefish Prosopium williamsoni are a broadly distributed native salmonid in western North America, but comparatively little investigation has been made regarding their population characteristics. We surveyed 2,043 study sites to assess whether physiochemical stream conditions affected mountain whitefish distribution and abundance in southern Idaho, and at 20 of these sites life history characteristics were also estimated. A total of 581 sites were dry or contained too little water to support any fish species. Mountain whitefish were captured at 106 sites; for these sites only, mean abundance was 2.2/100 m2. They were rarely present when mean wetted width was less than 10 m but were almost always present when wetted width was greater than 15 m. We estimated that within the study area there were approximately 4.7 ± 1.8 million mountain whitefish, mostly in fifth‐ to seventh‐order streams, which comprised only 13% of the total stream kilometers but accounted for 93% of the total abundance of whitefish. Growth was positively related to mean annual water temperature and negatively related to site elevation. Mountain whitefish were long lived, most (90%) populations containing fish estimated to be at least 10 years old. This longevity produced total annual survival rates averaging 0.82 (range = 0.63–0.91). In general, the growth, fecundity, and survival of mountain whitefish were higher in the upper Snake River basin than in other areas for which data have been reported. Whitefish matured at about 250 mm and about age 2, with little variation in length and age at maturity between sites; males matured at a smaller size and younger age than females. The disproportionate use of larger (i.e., >15‐m‐wide) streams by mountain whitefish in southern Idaho differs from the situation in more northerly locations, where they apparently are more common in smaller streams.

From 2006 to 2009, we tagged and released 22,202 fish with T‐bar anchor tags valued at US$0 to $200 if returned. Our intent was to assess angler tag reporting rates in Idaho and to determine whether reporting rates declined over time or differed between species. A total of 4,643 tags were reported by anglers. Assuming a reporting rate of 100% for $200 tags, weighted mean reporting rates were 54.2% for $0 tags, 69.7% for $10 tags, 91.7% for $50 tags, and 98.9% for $100 tags. By combining $100 and $200 as high‐reward tags to increase sample size, nonreward tag‐reporting rate was 54.5%. Tag reporting rates varied between groups of species, being highest for harvest‐oriented species, both coolwater and warmwater, such as walleye Sander vitreus ($0 = 68.3%), yellow perch Perca flavescens (58.5%), and crappie Pomoxis spp. (59.7%), and lowest for largemouth bass Micropterus salmoides (39.2%). There was little variation in tag‐reporting rates over time, weighted means being 53, 56, 50, and 56% from 2006 to 2009, but reporting rate did appear to decline for some species (most notably crappies). There was some evidence of a slight violation of the assumption of independence in tag‐reporting, indicated by nonreward tag‐reporting rates being marginally higher for anglers reporting both nonreward and reward tags than for those reporting only one or the other (signifying possible batch‐reporting of tags). No batch‐reporting was evident from differences in reporting rates for households reporting multiple tags compared with those reporting only one tag. Our results suggest that anglers in Idaho reported over half the nonreward tags they encountered, but rates appeared to vary among species, and this knowledge is being used to estimate angler exploitation across Idaho. Received November 22, 2011; accepted April 5, 2012

In 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 bouvierii and 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 3 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 6 1.2 million (mean 6 confidence interval); of these, about 1.0 6 0.4 million were Yellowstone cutthroat trout. Similarly, the estimated abundance of trout smaller than 100 mm was 2.0 6 1.4 million, of which about 1.2 6 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 ¼ 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.