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

Passive integrated transponder (PIT) tags have been widely used as a tool for various monitoring and research needs, but the retention of PIT tags has rarely been tested in resident salmonids. We quantified the short‐term (≤1 week), annual (1 year), and long‐term (≥1 year) retention rates of PIT tags placed in the peritoneal cavity of small resident rainbow trout Oncorhynchus mykiss in 11 study streams and assessed whether fish size and gender affected tag retention. Short‐term retention rates were at least 92% and averaged 96% for all streams, but a paired t‐test nevertheless indicated that experienced taggers had significantly higher short‐term retention rates (mean, 98%) than did inexperienced taggers (mean, 95%). Annual retention rates for PIT tags averaged 81% among all study streams, ranging from a low of 67% to a high of 92%. Annual retention rates were lower for larger rainbow trout than for their smaller counterparts. Long‐term tag loss for females was the same as for males for fish smaller than 15 cm but significantly higher for fish 15 cm or more, suggesting that egg expulsion was the primary cause of tag loss. Received August 27, 2010; accepted January 3, 2011

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.