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

Home , Coastal cutthroat trout, Cutthroat trout

North American Journal of Fisheries Management

ISSN: 0275-5947 (Print) 1548-8675 (Online) Journal homepage: http://www.tandfonline.com/loi/ujfm20

Kevin A. Meyer, Patrick Kennedy, Brett High & Matthew R. Campbell

To cite this article: Kevin A. Meyer, Patrick Kennedy, Brett High & Matthew R. Campbell (2017) Distinguishing Yellowstone Cutthroat Trout, Rainbow Trout, and Hybrids by Use of Field-Based Phenotypic Characteristics, North American Journal of Fisheries Management, 37:2, 456-466 To link to this article: http://dx.doi.org/10.1080/02755947.2017.1280572

Published online: 15 Mar 2017.

Submit your article to this journal

View related articles

View Crossmark data

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ujfm20

Download by: [Idaho Dept of Fish & Game] Date: 15 March 2017, At: 10:29 North American Journal of Fisheries Management 37:456–466, 2017 © American Fisheries Society 2017 ISSN: 0275-5947 print / 1548-8675 online DOI: 10.1080/02755947.2017.1280572

ARTICLE

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

Kevin A. Meyer* and Patrick Kennedy Idaho Department of Fish and Game, 1414 East Locust Lane, Nampa, Idaho 83686, USA Brett High Idaho Department of Fish and Game, 4279 Commerce Circle, Idaho Falls, Idaho 83401, USA Matthew R. Campbell Idaho Department of Fish and Game, 1800 Trout Road, Eagle, Idaho 83616, USA

Abstract Native and nonnative salmonids within the same genus sometimes hybridize, and the hybrids are often difficult to visually distinguish from parental species. We compared phenotypic delineations (based on several visual characteristics) and genotypic screening (using seven nuclear DNA loci) for 323 fish collected from an Idaho stream where Yellowstone Cutthroat Trout O. clarkii bouvieri were introgressed with Rainbow Trout O. mykiss to evaluate our ability to visually distinguish Yellowstone Cutthroat Trout from Rainbow Trout and hybrids. Assuming that the genotypes were 100% accurate, correct phenotypic classification was highest for Yellowstone Cutthroat Trout (the genotype confirmed the phenotype 94% of the time), followed by hybrids (79%) and Rainbow Trout (71%). All errors were between pure and hybrid fish. All of the measured phenotypic characteristics were useful for differentiating Yellowstone Cutthroat Trout from hybrids, but the most informa- tive characteristics were the lack of a white leading tip on the anal and dorsal fins, the presence of fewer than five spots on the top of the head, and the presence of a bright red-orange throat slash. Fish size did not influence correct phenotypic identification, although fry were excluded from our study. A logistic regression model predicted that biologists were more than 50% likely to visually detect Rainbow Trout traits in a hybrid when its level of hybridization was greater than 22% (95% confidence interval = 4–30%). For an additional 189 fish in three other Idaho streams, phenotypic differentiation of Yellowstone Cutthroat Trout from Rainbow Trout and hybrids was 91% accurate. These results can be used to screen for introgression in Yellowstone Cutthroat Trout populations or to selectively remove Rainbow Trout and hybrids from Yellowstone Cutthroat Trout populations. However, in most cases, genetic confirmation of purity or introgression would still be valuable for identifying low levels of hybridization in individuals or in the population at large.

Throughout the world, nonnative salmonids have been propa- many subspecies of Cutthroat Trout Oncorhynchus clarkii in gated to expand and improve recreational sportfishing opportu- western North America (Young 1995; Mayhood 2000;Behnke nities (reviewed by Cambray 2003). Unfortunately, when 2002). Such declines have led to numerous subspecies-specific nonnative salmonids are stocked in waters where native congeners management plans to help conserve as many unhybridized already exist, hybridization between native and nonnative species Cutthroat Trout populations as possible (e.g., CoffinandCowan often results (Leary et al. 1995). Hybridization with hatchery- 1995;Hirschetal.2005; Mayhood 2014). Common goals outlined stocked Rainbow Trout O. mykiss is a leading cause of decline in in such plans are to (1) identify pure Cutthroat Trout populations

*Corresponding author: [email protected] Received September 19, 2016; accepted December 29, 2016

456 DISTINGUISHING TROUT HYBRIDS BASED ON PHENOTYPE 457 and protect them from introgression and (2) eradicate or reduce fish’s body, have been used to differentiate Yellowstone introgression where feasible by removing Rainbow Trout and Cutthroat Trout from Rainbow Trout and hybrids (e.g., hybrids from the system. Both goals require an ability to accurately Campbell et al. 2002; Meyer et al. 2003, 2006). However, no differentiate between pure and hybrid fish. previous studies have provided the proportional incidence of Differentiation between Cutthroat Trout and Rainbow Trout is specific phenotypic traits in genetically identified Yellowstone usually straightforward; however, the identification of Cutthroat Cutthroat Trout, Rainbow Trout, and hybrids. Trout × Rainbow Trout hybrids (hereafter, “hybrids”)canbe Because hybrids can be difficult to identify accurately and can difficult. Historically, this was done using morphometrics and exhibit a range of parental phenotypes (Neff and Smith 1979), meristics, such as coloration; spotting patterns; the presence or combining genetic analyses with specific field-based phenotypic absence of basibranchial teeth; and counts of scales, finrays, traits that are easy to measure would help to create an accurate caeca, and vertebrae (Behnke 1979). Cutthroat Trout and metric for quickly differentiating Yellowstone Cutthroat Trout Rainbow Trout have separation in most of these characteristics, from Rainbow Trout and hybrids. Thus, our primary objective whereas hybrids tend to have intermediate characteristics, was to determine the phenotypic traits that could be used to most although they also often overlap in both parental species accurately differentiate Yellowstone Cutthroat Trout from (Behnke 1979). Recent advances in genetic tools, such as the Rainbow Trout and hybrids in the field, with an emphasis on identification of diagnostic molecular markers to differentiate characteristics that were easiest and fastest to measure. We parental species (Baker et al. 2002; Ostberg and Rodriguez determined these characteristics from one stream and validated 2002; Baumsteiger et al. 2005), have made the detection of them in three additional streams to verify their diagnostic utility hybridization in fish populations much easier and more accurate across a broader geographic range. As a secondary objective, we (Scribner et al. 2001). However, the cost of genetically analyzing evaluated whether the ability of biologists to identify Rainbow enough fish to confidently generate conclusions about the purity Trout traits in an individual hybrid was influenced by the degree of Cutthroat Trout populations prohibits sole reliance on this of hybridization within the fish. technique in many instances (Della Croce et al. 2016). Consequently, there is still a need for phenotypic differentiation of Cutthroat Trout from Rainbow Trout and hybrids in the METHODS field. The ability of biologists to visually differentiate such fish Fish sampling.—This study was primarily conducted in depends on the degree of overlap in phenotypic characteristics Palisades Creek, Idaho, a fourth-order (Strahler 1964;atthe exhibited by Cutthroat Trout and Rainbow Trout. Some 1:24,000 hydrologic scale) tributary of the South Fork Snake Cutthroat Trout subspecies, such as the Coastal Cutthroat Trout River near the Idaho–Wyoming border. The stream has a mean O. clarkii clarkii, are particularly difficult to phenotypically width of about 10 m and an average gradient of about 2% in the differentiate from Rainbow Trout or steelhead (anadromous area of study. Although the stream is 30 km long, a high-gradient, Rainbow Trout; Pollard et al. 1997; Baumsteiger et al. 2005; steep cascading section near river kilometer 11 presently appears Kennedy et al. 2009), probably because they evolved in sympa- to form a natural velocity barrier to fish, as Rainbow Trout and try. For other subspecies, such as the Yellowstone Cutthroat hybrids are absent upstream of this section (Meyer et al. 2006;K. Trout O. clarkii bouvieri, phenotypic differentiation has been A. Meyer, unpublished data). more successful. Seiler et al. (2009) recently summarized 29 Palisades Creek is one of four primary spawning tributaries for morphological measurements, which they used to distinguish Yellowstone Cutthroat Trout in the South Fork Snake River. By between groups of Yellowstone Cutthroat Trout, Rainbow the early 1980s, wild Rainbow Trout had begun to appear in Trout, and hybrids with a high degree of accuracy. However, Palisades Creek and the main-stem South Fork Snake River, their results were based on digitized photographs taken in the prompting cessation of all hatchery Rainbow Trout stocking. field but processed in the laboratory, and processing involved Since 2000, weirs have been installed and operated on Palisades numerous meristic measurements, such as eye diameter and the Creek in an attempt to halt upstream spawning migrations of length of each fin. This technique can be used to retrospectively Rainbow Trout and hybrids (Yellowstone Cutthroat Trout are identify pure or hybridized populations with high accuracy, but it passed over the weirs). However, most of the weirs were not cannot be used in real time. For example, at a spawning weir highly efficient or could not be operated effectively at very high installed to interrogate upstream-migrating fish, where Cutthroat spring flows during the critical period of the spawning migration Trout are passed upstream to spawn in natal tributaries but run (High 2010). As a result, a permanent electric weir was Rainbow Trout and hybrids are not (e.g., High 2010), a weir installed on Palisades Creek during 2009 at a site 0.7 km upstream operator cannot count fin rays and scales and measure tedious from the confluence with the South Fork Snake River. external characteristics on dozens if not hundreds of fish each For the present study, fish were collected via backpack elec- day. External characteristics that are more qualitative in nature, trofishing in September 2012 throughout the lower 11 km of such as body coloration, white-tipped fins, the presence of a Palisades Creek (but above the weir). Random samples of throat slash, the number of spots on the top of the head, and about 30 Oncorhynchus individuals from every 800 m of stream other spotting patterns or coloration on various regions of the were collected. Age-0 fish were excluded from our study for two 458 MEYER ET AL.

from Rainbow Trout and hybrids (Campbell et al. 2002; Meyer et al. 2003, 2006). Most of the characteristics are self-explanatory, but spot counts on the top of the head require further description. Biologists specifically counted the spots that were present directly on the top of the head, above the eyes, starting from just anterior of the nares and extending posterior to the point of scale formation (Figure 2). On pure Yellowstone Cutthroat Trout, there is often one spot next to each nare, and if present those spots were not counted. Based on the aforementioned characteristics, one of three experienced biologists classified the fish as a Yellowstone Cutthroat Trout, a Rainbow Trout, or a hybrid. A small fin clip was collected from each fish for use in genetic analyses. Genetic analyses.—All genetic samples were screened for Rainbow Trout hybridization/introgression by using seven diagnostic nuclear DNA (nDNA) markers (Occ34, Occ35, FIGURE 1. Box-and-whisker plots depicting the TLs of fish that were genotypi- fi Occ36, Occ37, Occ38, Occ42,andOM55; Ostberg and cally identi ed as Rainbow Trout, Yellowstone Cutthroat Trout, or hybrids in fi Palisades Creek, Idaho (line inside the box = median; boundaries of box = 25th Rodriguez 2002). Individual genetic classi cation was based on and 75th percentiles; whiskers = 10th and 90th percentiles; black circles = 5th and composite nDNA genotypes following procedures similar to 95th percentiles). Fish smaller than 60 mm TL were excluded from the study. those outlined by Ostberg and Rodriguez (2006) and Kozfkay et al. (2007). Samples were classified as pure Yellowstone Cutthroat Trout if they were homozygous for O. clarkii reasons. First, for all three taxa (i.e., Yellowstone Cutthroat bouvieri alleles at all loci, pure Rainbow Trout if they were Trout, Rainbow Trout, and hybrids), age-0 fish were small (i.e., homozygous for O. mykiss alleles at all loci, and hybrids if they <60 mm TL) at the time of sampling and were extremely rare in possessed a mixture of alleles from the two parental species. the catch. Second, some of the phenotypic traits that distinguish Hybrids were further classified as first-generation hybrids (F1) Cutthroat Trout from Rainbow Trout are not fully expressed in if they were heterozygous at all loci, or >F1 hybrids (i.e., either age-0 fish (Miller 1950; Martinez 1984; Seiler et al. 2009). Rainbow Trout or Yellowstone Cutthroat Trout backcrosses) if Fish were sedated in a solution of peppermint oil (Danner et al. they possessed a mix of heterozygous and homozygous loci. 2011) and water (using a 1:10 stock solution of peppermint oil and With seven codominant nDNA loci, our probability of ethanol; 0.3–0.5 mL of the solution was used per liter of water) and mistaking a backcross hybrid (>F1)asanF1 hybrid was less were measured for TL. Means and ranges of TLs were similar than 1% (Boecklen and Howard 1997). The use of seven loci between Yellowstone Cutthroat Trout, Rainbow Trout, and gave us a 90% probability of detecting a 15% or greater level of hybrids, although Rainbow Trout were slightly smaller on average introgression in an individual fish (Rasmussen et al. 2010). than the other two groups (Figure 1). A suite of phenotypic Statistical analyses.—Logistic regression was used to characteristics was then recorded for each fish (Table 1; determine which phenotypic characteristics (as the independent Figure 2); selected characteristics were largely based on the results variables) best distinguished Yellowstone Cutthroat Trout from of previous studies that differentiated Yellowstone Cutthroat Trout hybrids. We considered this differentiation the most important

TABLE 1. Variations in eight phenotypic characteristics that were used to distinguish Yellowstone Cutthroat Trout from Rainbow Trout and hybrids in Palisades Creek, Idaho. See Figure 2 for illustrations of these characteristics; letters correspond to the particular traits that are depicted in the figure.

Phenotypic characteristics by taxon Phenotypic metric Yellowstone Cutthroat Trout Hybrids Rainbow Trout Belly coloration Orange (A) Variable White Spots on side of body Large; clustered dorsally and posteriorly (B) Variable Small; distributed evenly Throat slash Bright red-orange (C) Dull or faint (E) Absent White leading edge of pelvic fins Absent Variable Present (F) White leading edge of anal fin Absent Variable Present (G) White leading edge of dorsal fin Absent Variable Present (H) Spots on top of heada Very few or no spots (D) Variable Many spots (I) Coloration on side of body No reddish hue Variable Reddish hue (J)

aOne spot is often present next to each nare, but those spots are not included in the count. DISTINGUISHING TROUT HYBRIDS BASED ON PHENOTYPE 459

B C A

F G

I G J F

D C E

FIGURE 2. Representative phenotypes for Yellowstone Cutthroat Trout (upper fish), Rainbow Trout (lower fish), and Yellowstone Cutthroat Trout × Rainbow Trout hybrids (middle fish), with various phenotypic characteristics indicated by uppercase letters that correspond to the letters in Table 1 (full descriptions are provided in the table). The lower three panels depict the head spot counting area and two throat slash variants. 460 MEYER ET AL. for two reasons. First, pure fish of either species were never stream. Fish were classified as Yellowstone Cutthroat Trout, confused with the opposite parental species, so the Rainbow Trout, or hybrids based on the phenotypic characteris- differentiation of Yellowstone Cutthroat Trout from Rainbow tics included in the top models developed for Palisades Creek Trout was unambiguous. Second, differentiating between fish. Fish were screened genetically via the same methods as Rainbow Trout and hybrids was considered immaterial since described above, and the genotype and phenotype were com- detecting either one would confirm the presence of Rainbow pared to evaluate the accuracy of phenotypically based Trout alleles in any Yellowstone Cutthroat Trout population classifications. being evaluated. Relating visual detection of Rainbow Trout alleles to We coded our response variable as 0 for hybrids and 1 for introgression levels.—A final logistic regression was used to Yellowstone Cutthroat Trout. Most of the independent variables evaluate whether visual detection of O. mykiss traits in were either “yes” or “no” categories (e.g., the leading tip of the individual hybrids was related to the degree of hybridization dorsal fin either had white coloration or it did not), so we coded in the fish. Hybrids from all four streams (n = 144) were them similarly. For the throat slash, we used only two categories combined for this analysis. We coded the response variable (i.e., “bright red-orange” or “other”). The number of spots on the as 0 for inaccurate phenotypic classification and 1 for accurate top of the fish’s head was categorized as either ≥5 or <5; although phenotypic classification of fish that were genetically the cutoff value was somewhat arbitrary, it was based on years of confirmed to be hybrids, and the independent variable was experience in differentiating these three taxa. All possible candi- the number of alleles (ranging from 1 to 13 out of the 14 date models were tested, and Akaike’s information criterion (AIC) alleles screened) that were identified as originating from was used to identify the best models. We only present the most Rainbow Trout. All statistical analyses were conducted in plausible models—those for which AIC values were within 2 units SAS (SAS 2009), with α set at 0.05. of the best-performing model (Burnham and Anderson 2004). We calculated AIC weights (wi) to judge the relative plausibility of each model within the set of most plausible models, and the RESULTS adjusted R2 for discrete models (R~2; Nagelkerke 1991)wasused During September 2012 in Palisades Creek, a total of 323 fish to assess the amount of variation explained by each model. The were randomly sampled for phenotype–genotype comparisons. Hosmer–Lemeshow goodness-of-fit statistic (Hosmer et al. 2013) Based on genotype, 218 fish were identified as Yellowstone was used to verify that the most plausible logistic regression Cutthroat Trout, 27 were identified as Rainbow Trout, 34 were models adequately fit the data. F1 hybrids, and 44 were >F1 hybrids. Assuming 100% genotyp- A separate logistic regression model was constructed to eval- ing accuracy, our phenotypically based classifications in uate whether fish TL (as the independent variable) was related to Palisades Creek were 94% accurate for Yellowstone Cutthroat the accuracy of our phenotypic classification; for the response Trout, 79% accurate for hybrids, and 71% accurate for Rainbow variable, a value of 0 was used for inaccurate phenotypic classi- Trout. All phenotypic errors were between pure parental fish and fications and a value of 1 was used for accurate classifications. hybrids (i.e., we never confused a Yellowstone Cutthroat Trout Validation of phenotypic characteristics in additional for a Rainbow Trout or vice versa). Fish that were misidentified streams.—During August and September 2015, we evaluated phenotypically as Rainbow Trout (n = 5) were all Rainbow Trout whether the phenotypic characteristics that best distinguished backcross hybrids, with each fish having at least 11 Rainbow Yellowstone Cutthroat Trout from hybrids in Palisades Creek Trout alleles (mean = 12.2 O. mykiss alleles out of the 14 that also worked effectively at other locations in eastern Idaho. This were screened). Fish that were misidentified phenotypically as evaluation was conducted in three rivers (Pine Creek, Toponce hybrids (n = 16) were all pure Rainbow Trout except for one Creek, and the South Fork Snake River) that were known to Yellowstone Cutthroat Trout. Fish that were misidentified phe- contain Yellowstone Cutthroat Trout, Rainbow Trout, and hybrids. notypically as Yellowstone Cutthroat Trout (n = 14) were all For each reach sampled in 2015, several-hundred meters of hybrids, and all but one were Yellowstone Cutthroat Trout back- each stream were electrofished to gather a sample of fish. cross hybrids. Of the 14 hybrids that were misclassified as Immediately after immobilized fish were netted, an experienced Yellowstone Cutthroat Trout, 29% had only one Rainbow Trout biologist examined the fish cursorily to select which individuals allele, and their average number of Rainbow Trout alleles was 2.9 to retain for our sample and which fish to return immediately to (out of 14). Phenotypic accuracy in differentiating Yellowstone the stream. During this initial screening, we retained fish from all Cutthroat Trout from Rainbow Trout and hybrids was 95%. taxa based on overall appearance rather than any specificdistin- All three biologists were accurate in phenotypically distinguishing characteristics. We selected fish in this manner—rather guishing between taxa. Two of the biologists were 92% and than genetically sampling all captured fish until an adequate 96% accurate; the third biologist was 83% accurate, with 15 of sample size was achieved—because it allowed us to obtain a 20 mistakes involving the misidentification of pure Rainbow sample with a wider range of fish sizes composed of all three taxa Trout as hybrids. of interest. From these retained fish, we sampled randomly until All of the field-based phenotypic characteristics we analyzed an adequate sample size (n ≥ 30 fish) was obtained for each were useful for differentiating Yellowstone Cutthroat Trout DISTINGUISHING TROUT HYBRIDS BASED ON PHENOTYPE 461 from Rainbow Trout and hybrids (Table 2). For example, 95% mostly Rainbow Trout, although some were Yellowstone of Yellowstone Cutthroat Trout had fewer than our a priori Cutthroat Trout (n = 5). Fish that were misclassified phenotypi- arbitrary cutoff of five spots on the top of the head, whereas cally as Yellowstone Cutthroat Trout (n = 12) were all Yellowstone only 8% of Rainbow Trout and hybrids had fewer than five Cutthroat Trout backcross hybrids; half of these fish had only one spots on the head (Table 2; Figure 3). In fact, the degree of Rainbow Trout allele (mean = 2.2 O. mykiss alleles out of 14 hybridization in individual hybrids directly affected the number screened). Phenotypic accuracy for differentiating Yellowstone of head spots (Figure 4). Least-squares linear regression pre- Cutthroat Trout from Rainbow Trout and hybrids in the three dicted that at the lowest level of hybridization we could detect streams was 91%. within an individual hybrid (i.e., 1 of 14 alleles, or 7%), the fish would have six spots on average. Relating Visual Detection of Rainbow Trout Alleles to The absence of white leading tips on the pelvic, anal, and Introgression Levels fi dorsal ns was also particularly useful for correctly classifying Including data from all four streams, it was evident that for Yellowstone Cutthroat Trout (Table 2). For example, no genetically confirmed hybrids, the ability of biologists to phe- fi Yellowstone Cutthroat Trout had white tips on the dorsal n, notypically detect Rainbow Trout traits in an individual was fi fi and only 1 of 71 sh with white tips on the pelvic ns and 2 of 81 affected by the fish’s degree of hybridization at the seven loci fi fi sh with white tips on the anal n were Yellowstone Cutthroat we screened (Figure 5). Using logistic regression (Wald χ2 = Trout (assuming that those individuals did not represent errors in 30.0, P < 0.001), we predicted that biologists were more than genotyping or data recording). However, these characteristics 50% likely to visually detect Rainbow Trout traits in a hybrid were less useful for correctly classifying hybrids: 45% of hybrids when the level of hybridization in the fish was greater than fi lacked white tips on the pelvic ns, 33% of hybrids lacked white 22% (95% confidence interval = 4–30%). tips on the anal fin, and 47% of hybrids lacked white tips on the dorsal fin. The throat slash, body spotting pattern, and side coloration were also good traits for separation, although some DISCUSSION hybrids exhibited Yellowstone Cutthroat Trout characteristics in Results from this study demonstrate that Rainbow Trout and all of the meristics we examined. hybrids can readily be distinguished from Yellowstone Cutthroat Phenotypic characteristics that were most commonly included Trout based on simple phenotypic characteristics. In Palisades (and usually statistically significant) in the best logistic regression Creek, only 0.5% of the captured Yellowstone Cutthroat Trout models for separating Yellowstone Cutthroat Trout from hybrids were mistakenly identified as hybrids, and only 41 of the 897 were as follows: (1) the absence of white on the leading tip of the Rainbow Trout alleles detected in the fish we analyzed geneti- anal fin, (2) the occurrence of fewer than five spots on the top of cally were missed in individuals that were misclassified as the head, (3) the presence of a bright red-orange throat slash, and Yellowstone Cutthroat Trout. Moreover, the characteristics that (4) spots on the side of the body that were clustered dorsally and worked best at differentiating between taxa in Palisades Creek posteriorly (Table 3). White on the leading tip of the dorsal fin(the were also effective in other streams, suggesting that our results absence of which was a distinguishing trait for Yellowstone can be used across a broad geographic scale—and perhaps across Cutthroat Trout) was included as a variable in all of the top the range of Yellowstone Cutthroat Trout—for differentiating candidate models, but this trait was not statistically significant in Rainbow Trout and hybrids from Yellowstone Cutthroat Trout. any of the models. In a separate logistic regression model, fish TL If the phenotypic characteristics that we identified as most useful was not related to the accuracy of our phenotypic classifications can be tested in other portions of the Yellowstone Cutthroat (Wald χ2 = 1.21, P = 0.27), indicating that correct classifications Trout’s range, this would reveal how broadly the present findings based on phenotype were not more difficult for smaller fish. can be applied. How easily these characteristics can be used by novice biologists or seasonal field staff to differentiate taxa Validation of Phenotypic Characteristics in Additional would also be a useful avenue for future research; considering Streams the simplicity of the characteristics we identified as most useful, The phenotypic characteristics that were most diagnostic in we expect that inexperienced field staff would have little diffi- differentiating taxa at Palisades Creek were used to phenotypically culty in applying our findings. classify 189 fish collected from three other streams in eastern Our phenotypic accuracy was highest for Yellowstone Idaho during August and September 2015. Assuming 100% gen- Cutthroat Trout. This concurs with the work of Campbell et al. otyping accuracy, our phenotypically based classifications in the (2002), who found that hatchery personnel at Henrys Lake, field were 89% accurate for Yellowstone Cutthroat Trout (n = Idaho, could visually distinguish Yellowstone Cutthroat Trout 110), 85% accurate for Rainbow Trout (n = 13), and 58% accurate (94% accuracy) much better than hybrids (48% accuracy) based for hybrids (n = 66). Two fish that were misidentified phenotypi- on a similar array of phenotypic characteristics, although we cally as Rainbow Trout were both hybrids with an average of 9 were more accurate at identifying hybrids (79% in Palisades Rainbow Trout alleles (out of the 14 screened alleles). Individuals Creek; 58% in other streams) than Campbell et al. (2002). One that were misidentified phenotypically as hybrids (n =16)were difference between the two studies was that Campbell et al. 462 MEYER ET AL.

TABLE 2. Frequency of occurrence (%) of the variations in phenotypic characteristics used to distinguish Yellowstone Cutthroat Trout from Rainbow Trout and hybrids (>F1 hybrids = backcross hybrids) in Palisades Creek and three additional streams in Idaho. Genotype Yellowstone Meristic Phenotypic character Cutthroat Trout >F1 hybrid F1 hybrid Rainbow Trout Palisades Creek Head spots Five or more 4.6 68.2 91.2 96.3 Fewer than five 95.4 31.8 8.8 3.7 Belly Orange hue 26.1 9.1 5.9 0.0 White hue 73.9 90.9 94.1 100.0 Pelvic fins White tips present 0.5 52.3 58.8 100.0 White tips absent 99.5 47.7 41.2 0.0 Anal fin White tips present 0.9 56.8 79.4 100.0 White tips absent 99.1 43.2 20.6 0.0 Dorsal fin White tips present 0.0 38.6 70.6 88.9 White tips absent 100.0 61.4 29.4 11.1 Throat slash Bright red-orange and prominent 89.9 34.1 17.6 0.0 Dull but continuous 4.1 11.4 17.6 0.0 Faint and barely visible 6.0 43.2 64.7 59.3 Absent 0.0 11.4 0.0 40.7 Body spots Smaller; distributed evenly on sides 3.7 70.5 76.5 100.0 Larger; clustered dorsally and posteriorly 96.3 29.5 23.5 0.0 Side coloration Presence of reddish hue 5.5 61.4 85.3 96.3 Absence of reddish hue 94.5 38.6 14.7 3.7

Other streams Head spots Five or more 12.6 32.6 85.7 91.7 Fewer than five 87.4 67.4 14.3 8.3 Belly Orange hue 41.7 8.7 42.9 5.7 White hue 58.3 91.3 57.1 94.3 Pelvic fins White tips present 1.9 52.2 42.9 91.7 White tips absent 98.1 47.8 57.1 8.3 Anal fin White tips present 0.0 63.0 57.1 100.0 White tips absent 100.0 37.0 42.9 0.0 Dorsal fin White tips present 2.9 52.2 85.7 88.9 White tips absent 97.1 47.8 14.3 11.1 Throat slash Bright red-orange and prominent 89.3 39.1 28.6 0.0 Dull but continuous 0.0 2.2 14.3 8.3 Faint and barely visible 10.7 47.8 42.9 61.1 Absent 0.0 10.9 14.3 30.6 Body spots Smaller; distributed evenly on sides 10.7 50.0 28.6 75.0 Larger; clustered dorsally and posteriorly 89.3 50.0 71.4 25.0 Side coloration Presence of reddish hue 95.1 58.7 57.1 91.7 Absence of reddish hue 4.9 41.3 42.9 8.3

(2002) did not record the phenotypic characteristics of each accuracy. In fact, across all four streams, none of the 27 hybrids individual fish. By comparing these characteristics to genotypic that were phenotypically misclassified as Yellowstone Cutthroat results via logistic regression, we were able to pinpoint which Trout had white leading tips on any of their fins; all but four had traits were most useful for separating taxa. Unfortunately, no fewer than five spots on the top of the head; all but four had spots combination of phenotypic characteristics could have differen- on the side of the body that were clustered dorsally and poster- tiated Yellowstone Cutthroat Trout from hybrids with 100% iorly; and all but five had a strongly discernible throat slash. Most DISTINGUISHING TROUT HYBRIDS BASED ON PHENOTYPE 463

1.00 Rainbow Trout TABLE 3. Top candidate logistic regression models that best differentiated Yellowstone Cutthroat Trout from Yellowstone Cutthroat Trout × Rainbow 0.80 Trout hybrids in Palisades Creek, Idaho, based on phenotypic characteristics ’ Δ 0.60 (AIC = Akaike s information criterion; AIC = difference in AIC value between the specified model and the best-performing model; wi = Akaike 0.40 weight; R~2= adjusted R2 for discrete models). Independent variables (defined α 0.20 in Table 2) that were statistically significant in the models (at = 0.05) are indicated by bold italic font, and the variables are listed in order of their Wald 0.00 χ2 value (highest to lowest).

~2 Model AIC ΔAIC wi R 1.00 Hybrids Anal fin + Head spots + Throat 128.38 0.00 0.23 0.68 0.80 slash + Body spots + Dorsal fin 0.60 Head spots + Anal fin + Throat slash 128.77 0.39 0.19 0.68 0.40 + Side coloration + Dorsal fin Anal fin + Head spots + Throat 0.20 129.14 0.76 0.16 0.68

Percent of fish slash + Body spots + Side 0.00 coloration + Dorsal fin Anal fin + Head spots + Throat 129.70 1.32 0.12 0.68 slash Yellowstone Cutthroat Trout + Body spots + Belly 1.00 coloration + Dorsal fin 0.80 Head spots + Anal fin + Throat 130.02 1.64 0.10 0.68 0.60 slash + Side coloration + Belly coloration + Dorsal fin 0.40 Head spots + Anal fin + Throat 130.07 1.69 0.10 0.68 0.20 slash + Dorsal fin 0.00 Head spots + Throat slash + Body 130.09 1.71 0.10 0.68 0 2 4 6 8 1012141618≥20 spots + Anal fin + Pelvic fins +

ﬁ Number of spots on the top of the head Dorsal n

FIGURE 3. Percentages of Yellowstone Cutthroat Trout, Rainbow Trout, and hybrids sampled in Palisades Creek that exhibited various numbers of spots on of these hybrids had only one Rainbow Trout allele or a few the top of the head. Rainbow Trout alleles at the loci we included in our genetic screening, so it is not surprising that they displayed phenotypic characteristics that were mostly (or sometimes entirely) the same as those of pure Yellowstone Cutthroat Trout. Surprisingly, our ability to phenotypically differentiate y = 23.9x + 3.8 80 R Yellowstone Cutthroat Trout from Rainbow Trout and hybrids ² = 0.16 fi 70 P < 0.001 was not diminished at smaller sh sizes. Although this was partly due to the fact that we excluded fry from our analyses 60 (since they were generally too small to capture), nevertheless 50 84 of the 511 fish examined across all four streams were 40 smaller than 150 mm TL, and error rates were essentially 30 equivalent between small fish (18%) and large fish (15%). 20 Spotting patterns and other phenotypic characteristics are highly variable or not fully expressed in juvenile trout Number of head spots 10 (Seiler et al. 2009), and small Cutthroat Trout are notoriously 0 difficult to distinguish from Rainbow Trout (Miller 1950; 0.00 0.20 0.40 0.60 0.80 1.00 Martinez 1984; Campton and Utter 1985), so we expected a Percent O. mykiss alleles marked difference in error rates between smaller and larger fish. However, previous studies have also demonstrated suc- FIGURE 4. Relationship between the level of introgression (percentage of cessful differentiation between small fish of these two species Rainbow Trout O. mykiss alleles among the 14 alleles that were screened) and the number of spots on the top of the head for Yellowstone Cutthroat Trout × based on phenotypic traits (e.g., Weigel et al. 2002; Seiler Rainbow Trout hybrids in Palisades Creek. The line, equation, and statistics et al. 2009). For fry, we suspect that white leading tips on were obtained by fitting a least-squares linear regression to the data. the anal, dorsal, or pelvic fins may be the most fully expressed 464 MEYER ET AL.

Yellowstone Cutthroat Trout (across all four streams) that were 1 genotyped as pure but that nevertheless might have been hybrids. Among the six Yellowstone Cutthroat Trout with at least 20 head traits) spots or with white tips on pelvic fins or the anal fin, all were 0.8 phenotyped as hybrids but no Rainbow Trout alleles were ampli- fied; this suggests that at least a few genotyping errors were made, O. mykiss but it is also possible that Yellowstone Cutthroat Trout exhibit 0.6 wider variation in some of the measured phenotypic traits. Considering that we screened only seven nDNA loci, some genotyping errors in individual fish should indeed be expected in our 0.4 results. Based on genetic sample size formulas provided by Boecklen and Howard (1997) and Rasmussen et al. (2010), the ability to detect a low level of introgression (5%) with a high level 0.2 of confidence (95%) for individual fish would require the screening of 29 diagnostic markers, and even then we would expect some

Pr (phenotypically detecting detecting (phenotypically Pr hybrids to be missed. The relationship we found between a fish’s 0 level of Rainbow Trout introgression and our probability of 0 0.2 0.4 0.6 0.8 1 visually detecting Rainbow Trout characteristics in that fish Percent of screened alleles of O. mykiss origin (Figure 4) suggests that any errors in genotyping were likely made on individuals with very low levels of introgression, thus FIGURE 5. Logistic regression relationship between the level of introgres- minimizing the consequences of mistakenly releasing those sion (percentage of Rainbow Trout O. mykiss alleles among the 14 alleles that hybrids back into the Yellowstone Cutthroat Trout population. were screened) and the probability of visually detecting Rainbow Trout Recent identification of single-nucleotide polymorphism loci characteristics in Yellowstone Cutthroat Trout × Rainbow Trout hybrids fi sampled from four study streams in eastern Idaho (dashed lines = 95% with xed allelic differences between Yellowstone Cutthroat prediction limits). The dotted vertical and horizontal lines depict the level of Trout and Rainbow Trout (e.g., Hohenlohe et al. 2011) should hybridization at which biologists were more than 50% likely to visually detect help to alleviate genotyping mistakes except for fish with extre- Rainbow Trout traits in an individual hybrid. mely low levels of introgression. When trying to phenotypically detect introgression in a Yellowstone Cutthroat Trout population, the misclassification characteristics for distinguishing Rainbow Trout and hybrids of a sampling reach as containing no fish with Rainbow Trout from Yellowstone Cutthroat Trout at that size; future research alleles would be highly unlikely unless very few fish are sur- to confirm or refute this suspicion would be useful. veyed or unless introgression is extremely low in that reach. Most of the phenotypic characteristics we used to distinguish However, because genotypic screening is more likely than phe- taxa were clear-cut, whereas the a priori cutoff for the number of notypic delineation to detect low levels of introgression in head spots required for phenotypic classification of fish as Yellowstone Cutthroat Trout, we encourage any broad-scale Yellowstone Cutthroat Trout was admittedly subjective, despite status surveys of this subspecies (e.g., Kruse et al. 2000;Meyer being based on years of experience in phenotypic separation of et al. 2006) to incorporate ample genetic sampling, in as many these taxa. The results of this study substantiated our preconceived locations as possible, to support the classification of reaches as notion that a count of about four to six spots on the top of the head containing unhybridized fish. For phenotypic detection of hybri- is a reasonable cutoff for separating Yellowstone Cutthroat Trout dization in individual fish, such as during a culling program or from hybrids, although a few Yellowstone Cutthroat Trout had when operating a weir at which fish with Rainbow Trout traits are more than six head spots. The gap between one Yellowstone blocked from spawning tributaries, it could be argued that acci- Cutthroat Trout with eight head spots and three Yellowstone dentally culling or blocking access to a few Yellowstone Cutthroat Trout with 20 or more head spots (Figure 3), in addition Cutthroat Trout would be more acceptable than accidentally to the three Yellowstone Cutthroat Trout with white leading tips on releasing or allowing passage to fish with Rainbow Trout alleles. either their pelvic fins or anal fin, suggests that a few errors in Unfortunately, our results indicate that the latter mistakes would genotyping or data recording were made. Because (1) phenotypic probably be more common. Regardless of the design and goal of differentiation of parental taxa was unambiguous, (2) errors in the program, any phenotypically based classification of classification between Rainbow Trout and hybrids were unimpor- Yellowstone Cutthroat Trout individuals or populations must tant (either one indicated O. mykiss alleles, which were to be acknowledge that some level of error is unavoidable. removed from the stream), and (3) errors on genotypically con- Removal of Rainbow Trout and hybrids from Cutthroat Trout firmed hybrids would be nonsensical (i.e., if alleles from both populations is a common conservation strategy across western parental types were identified, then the fish was unmistakably a North America to preserve native Cutthroat Trout populations hybrid), genotyping errors were only relevant for the 328 that are threatened with hybridization (e.g., Metcalf et al. 2008; DISTINGUISHING TROUT HYBRIDS BASED ON PHENOTYPE 465

Muhlfeld et al. 2009;High2010; Al-Chokhachy et al. 2014). Such Allendorf, F. W., R. F. Leary, N. P. Hitt, K. L. Knudsen, M. C. Boyer, and P. activities imply that hybridized populations of Cutthroat Trout Spruell. 2005. Cutthroat Trout hybridization and the U.S. Endangered – have conservation value, although debate on this subject has Species Act: one species, two policies. Conservation Biology 19:1326 1328. been fervent (Allendorf et al. 2004, 2005; Campton and Kaeding Allendorf, F. W., R. F. Leary, N. P. Hitt, K. L. Knudsen, L. L. Lundquist, and P. Spruell. 2004. Intercrosses and the U.S. Endangered Species Act: 2005). Many governmental agencies in the western USA collec- should hybridized populations be included as Westslope Cutthroat Trout? tively delineate Cutthroat Trout populations into three categories Conservation Biology 18:1203–1213. (UDWR 2000), which include core (<1% introgressed), conserva- Baker, J., P. Bentzen, and P. Moran. 2002. Molecular markers distinguish tion (1–10% introgressed), and sport fish (>10% introgressed) Coastal Cutthroat Trout from Coastal Rainbow Trout/steelhead and their – populations. According to this delineation, all three categories hybrids. Transactions of the American Fisheries Society 131:404 417. Baumsteiger, J., D. Hankin, and E. J. Loudeslager. 2005. Genetic analyses of are considered valuable for several reasons. First, at the scale of juvenile steelhead, Coastal Cutthroat Trout, and their hybrids differ sub- most existing data, it is usually difficult to describe the extent and stantially from field identifications. Transactions of the American Fisheries level of introgression throughout a particular Cutthroat Trout Society 134:829–840. population. Second, even in Cutthroat Trout populations where Behnke, R. J. 1979. The native trouts of the genus Salmo of western North hybridization is extensive, contrary to scientific perception America. Report to the U.S. Fish and Wildlife Service, Denver. Behnke, R. J. 2002. Trout and salmon of North America. Free Press, New York. (Allendorf and Leary 1988), empirical evidence suggests that Boecklen, W. J., and D. J. Howard. 1997. Genetic analysis of hybrid zones: hybrid swarms in Cutthroat Trout populations rarely develop numbers of markers and power of resolution. Ecology 78:2611–2616. (McKelvey et al. 2016). For populations in which hybrid swarms Burnham, K. P., and D. R. Anderson. 2004. Multimodel inference: under- have not formed and in which pure Cutthroat Trout still persist, the standing AIC and BIC in model selection. Sociological Methods and – removal of Rainbow Trout and hybrids should reduce the level and Research 33:261 304. Cambray, J. A. 2003. The global impact of alien trout species—a review with spread of introgression (e.g., Host 2003;Al-Chokhachyetal. reference to their impact in South Africa. African Journal of Aquatic 2014) and may eventually restore the Cutthroat Trout population Science 28:61–67. to a genetically pure or near-pure condition (Leary et al. 1995). Campbell, M. R., J. Dillon, and M. S. Powell. 2002. Hybridization and Genetic testing of each fish and releasing individuals only after introgression in a managed, native population of Yellowstone Cutthroat their genetic purity has been established will rarely be feasible. Trout: genetic detection and management implications. Transactions of the American Fisheries Society 131:364–375. Our results suggest that the use of several simple phenotypic Campton, D. E., and L. R. Kaeding. 2005. Westslope Cutthroat Trout, hybri- characteristics to visually identify Rainbow Trout and hybrids in dization, and the U.S. Endangered Species Act. Conservation Biology introgressed populations of Yellowstone Cutthroat Trout will 19:1323–1325. result in minimal failures to detect Rainbow Trout alleles. Such Campton, D. E., and F. M. Utter. 1985. Natural hybridization between steel- phenotypic characteristics should prove useful until more reliable head trout (Salmo gairdneri) and Coastal Cutthroat Trout (Salmo clarki clarki) in two Puget Sound streams. Canadian Journal of Fisheries and genetic analyses can be conducted within the population of interest Aquatic Sciences 42:110–119. to confirm or refute conclusions that have been drawn from Coffin, P. D., and W. F. Cowan. 1995. Lahontan Cutthroat Trout phenotypic classifications. (Oncorhynchus clarki henshawi) recovery plan. U.S. Fish and Wildlife Service, Region 1, Portland, Oregon. Danner, G. R., K. W. Muto, A. M. Zieba, C. M. Stillman, J. A. Seggio, and S. T. Ahmad. 2011. Spearmint (l-carvone) oil and wintergreen (methyl sali- ACKNOWLEDGMENTS fi fi cylate) oil emulsion is an effective immersion anesthetic of shes. Journal Numerous people were involved in obtaining n clips and in of Fish and Wildlife Management 2:146–155. laboratory processing of the genetic samples, most notably C. Della Croce, P., G. C. Poole, and G. Luikart. 2016. Detecting and quantifying Sullivan, E. Larson, C. Traughber, S. Elle, K. Nelson, K. Heindel, introgression in hybridized populations: simplifying assumptions yield over- and A. Boone. We thank K. Heim and D. Schill for providing confidence and uncertainty. Molecular Ecology Resources 16:1287–1302. manuscript comments and suggestions that greatly improved the High, B. 2010. Yellowstone Cutthroat Trout conservation efforts on the South Fork Snake River. Pages 275– 284 in R. F. Carline and C. LoSapio, editors. Wild paper. This study was funded in part by anglers and boaters Trout X: conserving wild trout. Wild Trout Symposium, Bozeman, Montana. through their purchase of Idaho fishing licenses, tags, and permits Hirsch, C. L., S. Q. Albeke, and T. P. Nesler. 2005. Range-wide status of and from federal excise taxes on fishing equipment and boat fuel Colorado River Cutthroat Trout, Oncorhynchus clarki pleuriticus. through the Federal Aid in Sport Fish Restoration Program. Colorado Parks and Wildlife, Fort Collins. Hohenlohe, P. A., S. J. Amish, J. M. Catchen, F. W. Allendorf, and G. Luikart. 2011. Next-generation RAD sequencing identifies thousands of SNPs for assessing hybridization between Rainbow and Westslope Cutthroat trout. REFERENCES Molecular Ecology Resources 11:117–122. Al-Chokhachy, R., C. C. Muhlfeld, M. C. Boyer, L. A. Jones, A. Steed, and J. Hosmer, D. W., S. Lemeshow Jr., and R. X. Sturdivant. 2013. Applied L. Kershner. 2014. Quantifying the effectiveness of conservation measures logistical regression, third edition. Wiley, Hoboken, New Jersey. to control the spread of anthropogenic hybridization in stream salmonids: Host, S. A. 2003. Reducing introgression with the use of a barrier in tribu- a climate adaptation case study. North American Journal of Fisheries taries of the South Fork Snake River, Idaho. Master’s thesis. University of Management 34:642–652. Idaho, Moscow. Allendorf, F. W., and R. F. Leary. 1988. Conservation and distribution of Kennedy, B. M., J. Baumsteiger, W. L. Gale, W. R. Ardren, and K. G. genetic variation in a polytypic species, the Cutthroat Trout. Conservation Ostrand. 2009. Morphological, physiological, and genetic techniques for Biology 2:170–184. improving field identification of steelhead, Coastal Cutthroat Trout, and 466 MEYER ET AL.

hybrid smolts. Marine and Coastal Fisheries: Dynamics, Management, and between native Westslope Cutthroat Trout and introduced Rainbow Trout. Ecosystem Science [online serial] 1:45–56. Transactions of the American Fisheries Society 138:1036–1051. Kozfkay, C. C., M. R. Campbell, S. P. Yundt, M. P. Peterson, and M. S. Nagelkerke, N. J. D. 1991. A note on a general definition of the coefficient of Powell. 2007. Incidence of hybridization between naturally sympatric determination. Biometrika 78:691–692. Westslope Cutthroat Trout and Rainbow Trout in the Middle Fork Neff, N. A., and G. R. Smith. 1979. Multivariate analysis of hybrid fishes. Salmon River drainage, Idaho. Transactions of the American Fisheries Systematic Biology 28:176–196. Society 136:624–638. Ostberg, C. O., and R. J. Rodriguez. 2002. Novel molecular markers differ- Kruse, C. G., W. A. Hubert, and F. J. Rahel. 2000. Status of Yellowstone entiate Oncorhynchus mykiss (Rainbow Trout and steelhead) and the O. Cutthroat Trout in Wyoming waters. North American Journal of Fisheries clarki (Cutthroat Trout) subspecies. Molecular Ecology Notes 2:197–202. Management 20:693–705. Ostberg, C. O., and R. J. Rodriguez. 2006. Hybridization and cytonuclear Leary, R. F., F. W. Allendorf, and G. K. Sage. 1995. Hybridization and associations among native Westslope Cutthroat Trout, introduced Rainbow introgression between introduced and native fish. Pages 91–101 in H. L. Trout and their hybrids within the Stehekin River drainage, North Schramm Jr. and R. G. Piper, editors. American Fisheries Society, Cascades National Park. Transactions of the American Fisheries Society Symposium 15, Bethesda, Maryland. 135:924–942. Martinez, A. M. 1984. Identification of Brook, Brown, Rainbow, and Cutthroat Pollard, W. R., G. F. Hartman, C. Groot, and P. Edgell. 1997. Field identifica- trout larvae. Transactions of the American Fisheries Society 113:252–259. tion of coastal juvenile salmonids. Harbour Publishing, Madeira Park, Mayhood, D. W. 2000. Provisional evaluation of the status of Westslope British Columbia. Cutthroat Trout in Canada. Pages 579–586 in L. M. Darling, editor. Rasmussen, J. B., M. D. Robinson, and D. D. Heath. 2010. Ecological Proceedings of the biological and management of species and habitats at consequences of hybridization between native Westslope Cutthroat risk. British Columbia Ministry of Environment, Lands, and Parks, Victoria, (Oncorhynchus clarkii lewisi) and introduced Rainbow (Oncorhynchus and University College of the Cariboo, Kamloops, British Columbia. mykiss) trout: effects on life history and habitat use. Canadian Journal of Mayhood, D. W. 2014. Conceptual framework and recovery guidelines for Fisheries and Aquatic Sciences 67:357–370. restoring Westslope Cutthroat Trout populations in Alberta. Freshwater SAS Institute. 2009. SAS/STAT 9.2 user’s guide, second edition. SAS Research Limited, Technical Report 2014/03-1, Calgary, Alberta. Institute, Cary, North Carolina. McKelvey, K. S., M. K. Young, T. M. Wilcox, D. M. Bingham, K. L. Pilgrim, Scribner, K. T., K. S. Page, and M. L. Burton. 2001. Hybridization in fresh- and M. K. Schwartz. 2016. Patterns of hybridization among Cutthroat water fishes: a review of case studies and cytonuclear methods of biolo- Trout and Rainbow Trout in northern Rocky Mountain streams. Ecology gical inference. Reviews in Fish Biology and Fisheries 10:293–323. and Evolution 6:688–706. Seiler, S. M., K. Gunnell, M. B. Ptacek, and E. R. Keeley. 2009. Metcalf, J. L., M. R. Siegle, and A. P. Martin. 2008. Hybridization dynamics Morphological patterns of hybridization between Yellowstone Cutthroat between Colorado’s native Cutthroat Trout and introduced Rainbow Trout. Trout and introduced Rainbow Trout in the south fork of the Snake River Journal of Heredity 99:149–156. watershed, Idaho and Wyoming. North American Journal of Fisheries Meyer, K. A., D. J. Schill, F. S. Elle, and W. C. Schrader. 2003. A long-term Management 29:1529–1539.

comparison of Yellowstone Cutthroat Trout abundance and size structure Strahler, A. N. 1964. Quantitative geomorphology of drainage basins and in their historical range in Idaho. North American Journal of Fisheries channel networks. Pages 4–40 in V. T. Chow, editor. Handbook of applied Management 23:149–162. hydrology, section 4, part II. McGraw-Hill, New York. Meyer, K. A., D. J. Schill, J. A. Lamansky Jr., M. R. Campbell, and C. C. UDWR (Utah Division of Wildlife Resources). 2000. Cutthroat Trout man- Kozfkay. 2006. Status of Yellowstone Cutthroat Trout in Idaho. agement: a position paper, genetic considerations associated with Transactions of the American Fisheries Society 135:1329–1347. Cutthroat Trout management. UDWR, Publication 00-26, Salt Lake City. Miller, R. R. 1950. Notes on the Cutthroat and Rainbow Trouts with a Weigel, D. E., J. T. Peterson, and P. Spruell. 2002. A model using phenotypic description of a new species from the Gila River, New Mexico. characteristics to detect introgressive hybridization in wild Westslope University of Michigan, Museum of Zoology, Occasional Paper 529, Cutthroat Trout and Rainbow Trout. Transactions of the American Ann Arbor. Fisheries Society 131:389–403. Muhlfeld, C. C., T. E. McMahon, M. C. Boyer, and R. E. Gresswell. 2009. Young, M. K., editor. 1995. Conservation assessment for inland Cutthroat Local-habitat, watershed, and biotic factors in the spread of hybridization Trout. U.S. Forest Service General Technical Report RM-256.