Summer Movements within the Fish Community of a Small Montane Stream

—We studied movements by fishes in Chamberlain Creek, Montana, from 24 July to 16 August 2001. We operated six weirs with two-way traps and one additional upstream trap, separated by 14–1,596 m, to quantify the timing, direction, and distance of movements and to estimate fish populations in the study reaches. We trapped and marked 567 fish of seven species, including 368 westslope cutthroat trout Oncorhynchus clarkii lewisi and 172 sculpin (slimy sculpin Cottus cognatus and an unidentified species similar to mottled sculpin C. bairdii). We recaptured 173 westslope cutthroat trout and detected net movements as long as 1,581 m (median, 91 m). Bidirectional movements for 116 westslope cutthroat trout ranged from less than 18 to more than 1,581 m (median, 64 m). Sculpin moved as far as 209 m (median, 26 m). We estimate that 14% of sculpin and 48% of westslope cutthroat trout were mobile during the study. We captured all species more frequently at night or twilight (n 5 296) than during the day (n 5 83) and more frequently moving downstream (n 5 419) than upstream (n 5 277). These results demonstrate considerable summer movement by the fish community in a small stream. Movements are critical to the ecology and demography of many lotic fish populations. Movement of fish can affect population gene frequencies, create metapopulation connectivity, and contribute to the flux of energy and nutrients through stream networks (Hall 1972; Rieman and Dunham 2000). The spatial and temporal scales over which fish movements occur vary widely, and at all scales, movements can vary within and among individuals and populations (Kahler et al. 2001; Rodrı́guez 2002), as well as among taxonomic groups. Quantifying animal movements, including within-population variability, is fundamental to understanding community ecology and effectively conserving populations in the face of anthropogenic impacts. In addition to more subtle effects, barriers to fish movements can lead to loss of life history types or even population extirpation, accompanied by loss of the potential for recolonization (Hanski 1999; Rieman and Dunham 2000; Neraas and Spruell 2001). Although sculpin are often important components of communities in small, coldwater streams, making up as much as 85% of the fish abundance (McCleave 1964; Freeman et al. 1988), the ecology, movements, and population structure of most * Corresponding author: [email protected] Received February 6, 2003; accepted December 18, 2003 strictly freshwater sculpin species are poorly understood. Spawning movements by adult sculpin ranged from 0 to more than 100 m in one freshwater species (Natsumeda 1999) and up to 5 km in an amphidromous species (Goto 1988). Summer home range movements typically average less than 20 m (McCleave 1964; Greenberg and Holtzman 1987; Natsumeda 1999), although Shetter and Hazzard (1939) suspected greater vagility. Moreover, some of the conclusions regarding limited movements probably resulted from study designs biased against finding long-distance movements (see Gowan et al. 1994; Gowan and Fausch 1996). Some early and much recent work suggests that many stream-resident trout move extensively (Gowan and Fausch 1996; Young 1996; Hilderbrand and Kershner 2000); other work, however, implies that movements are more limited (restricted movement paradigm; see Gowan et al. 1994 for review; Rodrı́guez 2002). Movements are usually attributed to spawning, dispersal, or seasonal habitat changes (Young 1996; Young et al. 1997; Hilderbrand and Kershner 2000; Gowan and Fausch 2002). For example, many cutthroat trout Oncorhynchus clarkii move soon after spawning, when flows remain high (Young 1996; Hilderbrand and Kershner 2000); generally, however, movement of salmonines (including cutthroat trout) decreases greatly during late summer (Gowan and Fausch 1996; Hilderbrand and Kershner 2000). Yellow1164 SCHMETTERLING AND ADAMS FIGURE 1.—Study area on Chamberlain Creek, Montana, showing lettered weir locations (circles), upstreamonly trap (triangle), and irrigation diversion dams. Weirs were operated at locations F, G, and I for 1 week and then moved to locations A, B, and C. stone cutthroat trout O. c. bouvieri and westslope cutthroat trout O. c. lewisi commonly make autumnal movements to overwintering habitats (e.g., Northcote 1992; Brown and Mackay 1995; Jakober et al. 1998).Without an impetus for movements, such as spawning or habitat changes, summer movements of cutthroat trout are reportedly minimal (Shepard et al. 1984; Schmetterling 2001; Hilderbrand and Kershner 2000). On the basis of previous research, we expected summer fish movement to be minimal in a small coldwater stream. Our objectives were to quantify the timing, direction, and distance of summer movements by fish in a small Montana stream. We used mark–recapture with weirs to sample the mobile fraction of the entire community and estimated population sizes by electrofishing.