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Summer movements and habitat use by Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) in small, montane streams Michael K. Young
Abstract: Radio telemetry was used to assess the mobility of, and habitat use by, 29 adult Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) in the North Fork Little Snake River drainage in south-central Wyoming from 27 May to 27 August 1992. Median home range (233 m) and median total movement (332 m) were larger than expected for cutthroat trout in small streams, and all but two fish used more than one primary habitat type during the study. Median weekly movement and the number of primary habitat units used generally declined as summer progressed. Of the available habitats, those used by Colorado River cutthroat trout tended to consist of deeper water in pools, especially pools formed by large woody debris. Fish were significantly farther from stream banks and not significantly closer to cover than would be expected by chance. These patterns of mobility and habitat use may be influenced by the connectivity and productivity of the study streams. Résumé : Du 27 mai au 27 août 1992, nous avons employé des dispositifs télémétriques pour estimer la mobilité de 29 truites fardées adultes (Oncorhynchus clarki pleuriticus) du fleuve Colorado, et l’utilisation qu’elles faisaient de l’habitat, dans le bassin hydrographique de la rivière North Fork Little Snake de la partie centre-sud de l’état du Wyoming. Le domaine vital médian (233 m) et les déplacements totaux médians (332 m) sont supérieurs à ce que nous attendions pour ce poisson dans de petits cours d’eau, et tous les sujets sauf deux ont utilisé plus d’un type d’habitat principal au cours de l’étude. Les déplacements hebdomadaires médians et le nombre d’unités d’habitat principal utilisé diminuaient généralement à mesure que l’été passait. Des habitats disponibles, ceux qu’utilisait la truite fardée du Colorado étaient plutôt des milieux en eaux plus profondes dans des trous d’eau, particulièrement ceux formés par l’accumulation de gros débris ligneux. Les truites se tenaient éloignées des berges de manière significative, mais ne se rapprochaient pas des zones de couvert plus que le hasard ne pouvait l’expliquer. Ces habitudes de déplacement et d’utilisation de l’habitat peuvent être influencées par la connectivité et la productivité de ces cours d’eau. [Traduit par la Rédaction]
Introduction Trout in small streams are generally considered to be sedentary. This assumption has been thought to apply to many species, including brook trout (Salvelinus fontinalis) (Leclerc and Power 1980), brown trout (Salmo trutta) (Hesthagen 1990), rainbow trout (Oncorhynchus mykiss) (Klein 1974), and cutthroat trout (Oncorhynchus clarki) (Miller 1957; Heggenes et al. 1991a). However, these and many other studies on trout movement relied on marking and infrequently locating fish in selected reaches of streams; such techniques may underestimate movement by failing to account for marked fish that are never recaptured because they move outside the selected reaches (Gowan et al. 1994). With the advent of radio telemetry and studies using intensive electrofishing, large-scale tagging, and upstream–downstream fish traps, the paradigm of restricted movement (Gerking 1959) has been challenged. Biweekly electrofishing revealed changes in the abundance of several trout species in a California stream, which implied that fish movement was Received April 19, 1995. Accepted December 13, 1995. J12880 M.K. Young. Rocky Mountain Forest and Range Experiment Station, 222 South 22nd Street, Laramie, WY 82070, U.S.A. e-mail: /s=m.young/
[email protected] Can. J. Fish. Aquat. Sci. 53: 1403–1408 (1996).
common (Decker and Erman 1992). Using mark–recapture techniques and upstream–downstream traps, Riley et al. (1992) found that a large proportion of brook trout in four Colorado streams were mobile. Similarly, in three radiotelemetry studies, the majority of adult brown trout were mobile, with movements of over 90 km in a Wyoming drainage (Young 1994) and over 30 km in Michigan (Clapp et al. 1990) and Wisconsin streams (Meyers et al. 1992). Movement of adult cutthroat trout in small streams in summer, however, has not been examined using these techniques. The use of radio telemetry might also resolve some questions regarding the characteristics of micro- and meso-habitats of adult cutthroat trout. For example, models of trout habitat quality consistently include variables representing pool depth and overhead cover (see Fausch et al. 1988 for many examples). Both generic models (those treating all species of trout as ecological equivalents; Binns and Eiserman 1979) and models specific to cutthroat trout (Hickman and Raleigh 1982) include these variables. However, many models are based on electrofishing data, which might emphasize the importance of refuges provided by deep water and overhead cover. Also, the acknowledged failure of either model to successfully predict either the abundance or biomass of cutthroat trout (Persons and Bulkley 1984; M. Oberholtzer, Wyoming Game and Fish Department, personal communication) suggests that habitat use should be reevaluated. © 1996 NRC Canada
1404 Fig. 1. Study reaches on the North Fork Little Snake River drainage in south-central Wyoming. Dumbbells mark water diversion barriers.
Can. J. Fish. Aquat. Sci. Vol. 53, 1996
structed in the 1960s and 1980s, block fish passage in both directions on every perennial tributary and the main stem near 2650 m above mean sea level (U.S. Forest Service 1981). Currently, 32 km of the drainage upstream from the weir supports Colorado River cutthroat trout (Oberholtzer 1990). Harrison Creek, Green Timber Creek, four other tributaries, and the lower North Fork Little Snake River form part of the largest connected portion (27.8 km). The upper North Fork Little Snake River (i.e., above the barrier) and Rhodine Creek constitute the next largest fragment (2.3 km), with the remainder scattered among the headwaters of five tributaries.
Methods
In this study, I used radio telemetry to assess the mobility and habitat use of Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) during summer in a high-elevation, montane watershed.
Study area Colorado River cutthroat trout were studied in three tributaries and two reaches (divided by a human-made barrier) of the main-stem North Fork Little Snake River in south-central Wyoming from 2487 to 2865 m above mean sea level (Fig. 1). Mean low-flow wetted widths were as follows: lower North Fork Little Snake River, 5.3 m; Harrison Creek, 2.1 m; Green Timber Creek, 2.3 m; upper North Fork Little Snake River, 3.2 m; and Rhodine Creek, 1.7 m. The study reaches were characterized by predominantly rubble–boulder substrates, moderately confined channels, and abundant riparian conifers. Also, riparian trees had been cut, toppled across the channel, and cabled to their stumps in an attempt to increase pool habitat along part of the lower North Fork Little Snake River in 1988 (Nick Schmal, U.S. Forest Service, personal communication). Mottled sculpin (Cottus bairdi) was the only other fish species present, and it was absent from the upper North Fork Little Snake River and Rhodine Creek (M.K. Young, personal observation). Human activities have isolated parts of the watershed. A weir constructed in 1977 to prevent invasion of non-native trout into the main stem 9 km below the mouth of Harrison Creek is the lowermost barrier, and water diversions, con-
Adult Colorado River cutthroat trout (n = 34, total length 190–237 mm) were collected by hook and line, implanted with transmitters (see Young 1995a for details), released where captured, and monitored by radio telemetry between 27 May and 27 August 1992. Most captured fish were in spawning colors and several had not yet spawned. Observers usually located fish twice each week during daytime by walking parallel to and within 50 m of the stream bank (but out of sight of fish) until a signal was detected. In pilot tests, hidden transmitters were easily found by using telemetry gear; I concluded that fish positions were usually identified by triangulation to within 1 m2. To improve precision, observers tried to see each fish before approaching its position to measure habitat characteristics. Twelve habitat variables associated with each fish location were assessed: (i) whether the stream was split into several channels, (ii) primary habitat type (Hawkins et al. 1993), (iii) primary structural association, (iv) secondary habitat type, (v) secondary structural association, (vi) distance to the nearest bank, (vii) distance to and (viii) kind of nearest cover, (ix) water depth, and (x–xii) water velocity at three depths. Primary structural associations (large woody debris, boulders, meanders, or structures) created primary habitat types (riffles or obstruction, plunge, or dammed pools). Secondary habitat types consisted of pockets, edges, and main channel sites. Pockets were areas of lower water velocity in the main channel surrounded by higher water velocities. Edges had reduced water velocity influenced by a feature of the bank and higher water velocity only toward the center of the channel. Main channel sites had no local feature that reduced water velocity at the water surface. Secondary structural associations (large woody debris, root mats of riparian vegetation, boulders, meanders, banks, or structures) created secondary habitat types. I classified cover (i.e., an object providing overhead protection) as overhanging vegetation within 15 cm of the water surface, instream or suspended (within 15 cm of the water surface) large woody debris, boulders (greater than 30 cm along one axis), or undercut bank. All distances and depths were measured to the nearest centimetre. Water velocity was measured to the nearest millimetre at 10 cm from the surface, at 0.6 of depth from the surface, and at 10 cm from the bottom to estimate maximum, mean, and focal point water velocities. Habitat available to fish was measured at five equally spaced points across transects set perpendicular to the flow at 50-m intervals. Transects (68 on the lower North Fork Little Snake River, 19 on Harrison Creek, and 18 on the upper North Fork Little Snake River) were surveyed once between 7 July and 4 August 1992. Habitat measurements were the same as those collected at fish positions. As a scale to measure trout movements, one bank was staked at 50-m intervals on a line parallel to the thalweg. After identifying a fish’s position, observers moved to the bank on a course perpendicular to the thalweg and measured the distance to the nearest stake. Weekly movement consisted of the difference in locations during a particular week, plus interpolated portions of the difference between the last location during the previous week and the first location during the week in question, and the last location during the week in question © 1996 NRC Canada
Young and the first location during the following week. Total movement was the sum of all weekly movements. I defined home range as the difference between a fish’s most upstream location and most downstream location. Positions more than 2 m apart were considered distinct. Data for nine continuous habitat variables (four in used habitats, five in available habitats) were non-normal, as were data for weekly fish movement (one-sample Kolmogorov–Smirnov tests, P < 0.05). Hence, I compared used and available habitat by using log-likelihood ratio tests for discrete variables and Mann–Whitney tests for continuous variables. To determine whether there were differences in movement among weeks, I used a one-way Kruskal–Wallis test. Associations between variables were assessed with Spearman rank correlation. I used SPSS/PC+, version 4.01 (Norusis 1990), for all analyses. Throughout, I considered P ≤ 0.05 as indicating significance. The experimental protocol could have influenced fish movement and habitat use, but I consider this unlikely. As recommended by Winter (1983), transmitters never exceeded 2% of body weight. After using a similar surgical and recovery procedure, Moore et al. (1990) concluded that implanted transmitters had no effect on survival, growth, and swimming and feeding behavior in young Atlantic salmon (Salmo salar). Martin et al. (1995) concluded that transmitter implantation had no effect on growth, condition factor, or gonadal development of rainbow trout, even when implanted shortly before spawning. Trout similarly handled in other studies (Young 1994, 1995a) had completely healed externally within 3 weeks. Finally, implanted fish occasionally resumed feeding immediately (e.g., less than 15 min) after surgery. For comparable reasons, the twice-weekly measurement of habitat at fish positions seemed unlikely to have affected choice of location by implanted Colorado River cutthroat trout. Implanted fish would often continue feeding less than 5 m away during habitat measurements, then return to the original site after measurements were taken. In addition, despite these disturbances, many fish were located in the same positions for several consecutive observations. Thus, I inferred that neither surgery nor repeated measurements of habitat dramatically altered the behavior of implanted fish.
Results Movement Of the 34 Colorado River cutthroat trout implanted, 29 were tracked for 16–85 days. Five fish were never relocated, but three of these were believed to have carried faulty transmitters that failed immediately (caused by sensitivity to cold temperature). Several fish were initially captured on or near redds, and many movements through 26 June appeared to be associated with spawning or postspawning behavior. Also, Colorado River cutthroat trout were rarely concealed during the day; 73% of the relocations were based on sightings of implanted fish. Monitored fish were more mobile than expected. Median home range length over the 3-month study was 233 m (range, 0–1792 m; Fig. 2). Median total movement throughout the study was 332 m (range, 0–2443 m; Fig. 2). Movement declined in early July (after spawning was apparently completed) but was still greater than previously reported. For the 12 fish tracked until late August, median home range length was 45 m (range, 11–652 m) and median distance moved was 125 m (range, 39–728 m) after 1 July. There were significant differences in trout movement among weeks from late May to late August (χ2 = 62.6, P < 0.0001). Median weekly movement peaked in mid-June then declined as summer progressed
1405 Fig. 2. Histogram of home range lengths (A) and total distances moved (B) for Colorado River cutthroat trout above (open bar) and below (solid bar) the main-stem barrier in summer 1992.
(Fig. 3); this seasonal trend was significant (r = –0.87, P < 0.0001). Consequently, total movement was not correlated with the number of days a fish was tracked (r = 0.30, one-tailed P = 0.055) or the number of relocations (r = 0.23, one-tailed P = 0.13). Finally, no fish in the upper North Fork Little Snake River (including Rhodine Creek) had home ranges over 300 m, but 10 of the remaining 25 fish below the barrier did (Fig. 2). After fish were implanted, their eventual location was unpredictable. Seven of 13 fish initially captured in Green Timber and Harrison creeks descended to the lower North Fork Little Snake River; 4 then moved downstream and 3 upstream within the main stem. Of the six fish remaining in these tributaries, three moved downstream, two moved upstream, and one stayed where captured, but four of the six were monitored for less than 1 month (probably because of predation or premature transmitter failure), hence movement for the entire season was unobtainable. Of the 16 fish captured elsewhere, 8 went downstream (although 1 ascended Harrison Creek first), 5 went upstream, and 3 moved less than 10 m. Movements by some fish were especially erratic. In particular, a 195-mm fish originally captured in Green Timber Creek on 2 June descended 706 m to its mouth, 150 m down the lower North Fork Little Snake River to the mouth of Harrison Creek, and 497 m up this tributary by 18 June. After 1 week, this fish had descended Harrison Creek, then ascended the lower North Fork Little Snake River 435 m past the mouth of Green Timber Creek, and remained in this vicinity until it was lost after 21 July. © 1996 NRC Canada
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Table 1. Characteristics of used (n = 313) and available (n = 525) habitats in reaches of the North Fork Little Snake River watershed. Used Multiple channels (%)* Primary habitat type* Obstruction pool (%) Plunge pool (%) Dammed pool (%) Riffle (%) Primary structural association* Large woody debris (%) Boulders (%) Meanders (%) Structures (%) Secondary habitat type Main channel (%) Edge (%) Pocket (%) Secondary structural association* Large woody debris (%) Rootmats (%) Boulders (%) Meanders (%) Bank (%) Structures (%) Undercut bank (%) Large woody debris (%) Overhanging plants (%) Boulders (%) Cover distance (m) Bank distance (m)* Water depth (cm)* Water velocity (cm/s) 10 cm from bottom* 0.6 of depth 10 cm from surface*
Available
10
3
42 9 13 37
14 1 6 80
34 23 36 7
7 10 81 3
55 33 12
61 29 11
38 2 37 14 3 7 13 22 6 58 0.3 1.5 37
7 1 34 50 6 2 11 10 6 73 0.4 0.9 13
10 13 15
14 13 20
Note: Proportions are presented for discrete variables and medians are presented for continuous variables. *Significant comparison.
Habitat use As a consequence of their mobility, most fish used several primary habitat types (median, 4; range, 1–6) and several distinct positions (median, 5; range, 1–9), often within a single primary habitat type. Only two fish occupied just one primary habitat type during the study, and one of these fish was only relocated twice, both times at the same position. The number of distinct positions occupied was correlated with the number of relocations (r = 0.45, P = 0.015) and the number of primary habitat types occupied (r = 0.79, P < 0.0001), but the number of relocations was not correlated with the number of primary habitat types occupied (r = 0.14, P = 0.48). The number of primary habitat types used tended to plateau between three and six despite increasing numbers of observations, possibly because long-range fish movement declined in late summer. Of the 12 fish tracked until late August, 8 were found using only one primary habitat unit and none used more than 3 during August. Short-range movements were common, however, because these fish occupied a median of 2.5 distinct positions (range, 1–6) during that month.
Colorado River cutthroat trout did not use habitats in proportion to their availability (Table 1). Although pools constituted only 21% of primary habitat types, over 60% of fish relocations were in pools. Use of primary habitat types, particularly pools, created by large woody debris was disproportionately high. For example, 39% of the available pools were formed by large woody debris (or single logs that had been cut and cabled to their stumps), but 64% of the fish relocations in pools were in pools created by debris. Furthermore, over 80% of primary habitat types were created by meanders, but less than 40% of the used habitat types were so formed. There was little difference between the proportions of used and available secondary habitat types, but the use of those formed by large woody debris (and log structures) was disproportionately high, and the use of habitats formed by meanders was disproportionately low. Finer scale characteristics of used and available habitats also differed (Table 1). Boulders were the most abundant cover type, yet proximity to large woody debris tended to be disproportionately high. This could be coincidentally related to the frequent use of habitats created by debris, because the importance of nearby cover was equivocal; there was no significant difference in the distance to nearest cover between used and available sites, and used sites were significantly farther from stream banks than were available sites. Used sites were also significantly deeper and had significantly slower water velocities near the bottom and surface than did available sites. There was no difference in mean water velocity between used and available sites.
Discussion Movement Previous assessments of the average home range of cutthroat trout in small streams (4 m, Heggenes et al. 1991a; 18 m, Miller 1954, 1957) were exceeded by more than an order of magnitude by Colorado River cutthroat trout in the North Fork Little Snake River drainage. Much of the movement apparently involved fish returning from spawning areas to feeding habitats, yet the papers mentioned above considered those home ranges to represent a fish’s entire life history. The relative lack of movement by cutthroat trout in other studies may have been caused by differences in their study designs that could have obscured or overlooked movements (see Gowan et al. 1994; Young 1994). For example, Miller (1957) failed to relocate 41% of the marked fish, and attributed their absence to mortality rather than movement. Similarly, Heggenes et al. (1991a) could not account for 40% of their tagged fish; also, their sample included very few adults (T.G. Northcote, University of British Columbia, personal communication); thus, spawning migrations could have been overlooked (Fausch and Northcote 1992 (p. 688) found several fish apparently marked in this earlier study to have moved at least 300 m and concluded that this movement was related to spawning). Finally, Quinlan (1980) concluded that Colorado River cutthroat trout in the North Fork Little Snake River drainage were sedentary, but they based this on the recovery of marked fish in August 1979 that had been tagged in late July to early August 1978. Any movements in the interim e.g., spawning runs, would have been undetected (also see Miller 1957). © 1996 NRC Canada
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Colorado River cutthroat trout above the main-stem water diversion barrier cannot undertake long-distance migrations because the barrier limits their movement. Other streams with apparently sedentary trout populations also may contain such barriers. For example, a population of Paiute cutthroat trout (Oncorhynchus clarki seleniris) that were thought to be immobile (Diana and Lane 1978) were restricted by a downstream barrier and unsuitable water quality upstream to a 3.3-km reach. In such streams, trout movement may be disadvantageous because it is energetically expensive and fish passing downstream barriers, such as weirs, diversions, or even hostile non-native fishes, are lost to the upstream population. However, in portions of watersheds with less isolation (such as the lower North Fork Little Snake River), mobility may be favored if habitat suitability varies in different parts of the watershed and if survival and reproduction can be enhanced by moving (Northcote 1992). Much larger connected watersheds support even more mobile life history forms of other subspecies of cutthroat trout (Middle Fork Salmon River, Idaho, Bjornn and Mallet 1964; Logan River, Utah, Bernard and Israelsen 1982; Carnation Creek, British Columbia, Hartman and Brown 1987; Yellowstone River, Montana, Clancy 1988). Habitat use Several aspects of habitat use by Colorado River cutthroat trout are consistent with results from other studies. In the North Fork Little Snake River drainage, Colorado River cutthroat trout tended to occupy pools, especially those created by large woody debris (cf. Fausch and Northcote 1992). Jespersen (1981) found that the abundance of Colorado River cutthroat trout was significantly correlated with the amount of cover, but he defined cover as deep pools created by large woody debris or other objects. In the U.S. Pacific Northwest, the abundance of adult coastal cutthroat trout (Oncorhynchus clarki clarki) peaked in pools (Glova 1987), especially in plunge and scour (equivalent to obstruction) pools (Bisson et al. 1982, 1988). As in the present study, Heggenes et al. (1991b) noted a penchant for cutthroat trout to use deep water, be closer to cover provided by large woody debris, and use slower water velocities. Although overhead cover has been thought to characterize sites used by cutthroat trout (Hickman and Raleigh 1982), fish in this study appeared to occupy cover infrequently. Similarly, only 27% of west-slope cutthroat trout occupied feeding positions beneath cover in two pools in a Montana stream (Nakano et al. 1992). Wilzbach et al. (1986; see also Wilzbach and Hall 1985) noted that cutthroat trout fed less efficiently in habitats with overhead shade and boulders (and by analogy, cover) and demonstrated that this species would abandon locations with overhead cover when food availability was low. High-elevation streams are usually unproductive (Ward 1986), which could lead to the reduced use of cover by feeding fish. For example, cutthroat trout in the present study were often observed to continually cruise throughout a primary habitat type when foraging, suggesting that drifting food may have been insufficient to induce individuals to establish feeding territories. This may also explain why fish were located in different positions within one primary habitat type on different days in late summer. Given that most populations of cutthroat trout are now restricted to low-order, high-elevation streams (Young 1995b) that are likely to be infertile, these patterns of cover use and movement may be common.
Fig. 3. Median weekly movement of Colorado River cutthroat trout in summer 1992. Broken lines represent the 25th and 75th percentiles of the distribution.
In conclusion, habitat use by Colorado River cutthroat trout was similar to that of cutthroat trout in other small streams, but these fish were more mobile than expected. If mobility is more prevalent than previously reported, it could affect conclusions about population size and distribution, the success of angling restrictions, the influence of habitat improvement, and the genetic isolation of populations (Gowan et al. 1994; Fausch and Young 1995; Riley and Fausch 1995).
Acknowledgements I thank my field crews for assistance with data collection and Chas Gowan and Kurt Fausch for stimulating discussions about trout movement. Reviews by Peter Bisson, Chas Gowan, Casey Harthorn, Rudy King, Thomas Northcote, Nick Schmal, Margaret Wilzbach, and an anonymous reviewer substantially improved the manuscript.
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