Summer Stream Temperatures Influence Sculpin Distributions and Spatial Partitioning in the Upper Clark Fork River Basin, Montana
The upper Clark Fork River basin of western Montana supports a poorly understood sculpin (Uranidea spp.) fauna that has perplexed ichthyologists and fish ecologists since the late 1800s. During our study, the basin contained three sculpin taxa whose taxonomy was under revision. All three taxa were formerly referred to the genus Cottus but are now treated as Uranidea. Our goal was to improve understanding of the distribution and ecology of two of the taxa. From 2006 to 2009, we sampled 144 reaches in 31 streams and rivers to determine distributions of each taxa across the study area and within streams. We collected habitat data in 2007 and stream temperature data from 2006–2009 to identify correlates of sculpin distributions. In streams where both taxa occurred, Rocky Mountain Sculpin Uranidea sp. cf. bairdii were downstream and Columbia Slimy Sculpin U. sp. cf. cognata were upstream with a syntopic zone in between. Summer stream temperatures strongly influenced sculpin distributions, with mean August 2007 water temperatures increasing in order of reaches characterized as: Columbia Slimy Sculpin-dominated, syntopic, Rocky Mountain Sculpin-dominated, and no sculpin. Columbia Slimy Sculpin occurred in cold tributaries of the Blackfoot, Clark Fork, and Bitterroot rivers and in two coldwater refugia in the mainstem Bitterroot River. In contrast, Rocky Mountain Sculpin occupied warmer downstream segments of many Blackfoot and Clark Fork river tributaries as well as some mainstem reaches of both rivers but were absent from the Bitterroot River drainage. Persistence of the taxa will likely depend, both directly and indirectly, on future water temperatures, and thus, sculpins are appropriate targets for researching and monitoring biological changes resulting from climate change.
Sculpin sampling (circles and triangles) and temperature recording (stars) reaches in the upper Clark Fork River drainage, Montana. Circles indicate reaches dominated by Columbia Slimy Sculpin (black) or Rocky Mountain Sculpin (white). Sculpin distributions represent combined results of summer samples from 2007--2009. Black triangles indicate reaches where we captured sculpin but could not positively identify them either because they were all small or because Cedar sculpin were also present. Open triangles indicate sculpin absence. Inset shows study area in Montana (shaded gray) and major rivers of the Columbia River basin.
Sculpin distributions in the Clark Fork River drainage downstream of confluence with Bitterroot River. Symbols are explained in Figure 1. Numbers indicate sample reaches on the mainstem of the Clark Fork rivers.
Sculpin distributions in the Clark Fork River drainage upstream of confluence with Bitterroot River. Symbols are explained in Figure 1. Pie charts represent the proportion of Columbia Slimy Sculpin (black area) and Rocky Mountain Sculpin (white area) in a reach. Numbers indicate sample reaches on the mainstem of the Blackfoot and Clark Fork rivers. Smaller panels better illustrate distributions in tributaries containing both taxa.
Sculpin distributions in the Bitterroot River. Symbols are explained in Figure 1. Numbers indicate sample reaches on the mainstem of the Bitterroot River.
Water temperatures relative to sculpin distributions in 2007. Mean± SE of daily average water temperatures in July and August for reaches in four sculpin categories: no sculpin, Rocky Mountain Sculpin-dominated (RMS), syntopic, and Columbia Slimy Sculpin-dominated (CSS). The “no sculpin” group includes only reaches in mainstem rivers or downstream of sculpin distributions in tributaries to avoid confounding the temperature pattern with reaches that sculpin could not access because of steep channel slopes. The CSS group does not include the Bitterroot River at Florence (reach BR2) where fewer than five sculpin were captured and those only in a thermal refugium away from our thermograph. Overall, category sample sizes ranged from two to 11, but from 21 July–31 August, all categories had at least six reaches.
Schematic of lakes and sample reaches along the Clearwater River, Montana. Symbols indicate sculpin status at each reach. Numbers above sample reaches are water temperatures recorded during sampling (see Table 3 for time of day). Vertical axis is to scale, but horizontal axis does not portray stream distances. Reach numbers as in Table 3.
Contributor Notes
Associate Editor: J. F. Schaefer.