Eastslope sculpin COSEWIC assessment and status report: chapter 6
Distribution
Previously, the mottled sculpin was thought to have a wide but discontinuous distribution in North America (Figures 2, 3), where it was represented by two geographically isolated groups separated by the Great Plains, where it is absent. The eastern group ranges from at least the Tennessee River system in Georgia and Alabama to Labrador on the north, west to the Great Lakes basin. In eastern Canada, the mottled sculpin is found in discontinuous locations in Labrador, Ungava Bay in Quebec west to the Hudson Bay drainages, through the St. Lawrence-Great Lake system and in the James and Hudson Bay drainages of most of Ontario to southern Manitoba (Scott and Crossman 1973; Lee 1980). However, it now appears that there are no “mottled” sculpin, i.e., Cottus bairdii in western North America (see Species Information).
The distribution of the western group from the Rocky Mountains to the Pacific coast is discontinuous and unclear, particularly in the United States (A. Peden, Liparis Consultants, Victoria, B.C.; personal communication 2003). In western Canada, two distinct forms of “mottled” sculpin have been identified: (1) the Columbia sculpin, C. hubbsi, originally described by Bailey and Dimick (1949); and (2) the Rocky Mountain form, C. bairdii punctulatus, found in southeastern British Columbia and southwestern Alberta (Peden 2000).
The Columbia sculpin, designated by COSEWIC as a species of Special Concern in 2000 (Peden 2000), is found only in the western Columbia River system (Figure 2). Specifically, it has been noted in a few systems in southern British Columbia, including the Similkameen River (above Similkameen Falls), Kettle River (below Cascade Falls), various streams tributary to the Columbia River, as well as the lower Kootenay River below Bonnington Dam (Peden 2000). South of the international border, the distribution of the Columbia sculpin is much less well known. It was originally described in the Columbia River system in Washington and Idaho (Bailey and Dimick 1949).
The Rocky Mountain sculpin, Cottus bairdii punctulatus, was originally noted in southwestern Alberta in 1925-26 (Bajkov 1927) [however, there is some doubt as to the authenticity of the Bajkov specimen (Nelson and Paetz 1992)] and in the Flathead and upper Missouri River systems in Montana in the late 1930s (Schultz 1941). According to Neely’s research (Neely 2002, Neely unpublished), Cottus punctulatus is a valid species confined to the Colorado River. As discussed earlier in this report, recent genetic and molecular studies indicate that the form straddling the Continental Divide is also a separate taxon, provisionally named “Eastslope” sculpin, Cottus sp. In Canada, the only other known population that appears to share the same taxon as the “Eastslope” sculpin (Figure 4) occurs in the Flathead River in British Columbia (Troffe 1999; Peden 2000; McPhail pers. comm.). Other closely related populations in the United States are found in the upper Missouri River system in Montana and Wyoming (Neely pers. comm.). Interestingly, the Milk, St. Mary and Missouri populations of sculpins on the eastern slopes of the Rocky Mountains are separated from the Flathead population of sculpins on the western side by the Continental Divide (Peden 2000). This may not be surprising given that the geological evidence suggests that portions of the South Saskatchewan River, including the St. Mary River and portions of the Flathead River, shared post-glacial drainage connections with the upper Missouri River drainage during the retreat of the last glacier approximately 10 000-13 000 years ago (Troffe 1999).
Figure 4: Distribution of the “Eastslope” Sculpin in Alberta
Shaded grey line indicates distribution based on recent surveys in the St. Mary and Milk rivers (R.L. & L. 2002; P & E 2002), as well as on earlier studies (R.L. & L. 1987; Paetz 1993) where they agree with the most recent work. Open diamonds indicate sites on the upper Milk River where sculpins were found by Willock (1969), Clayton and Ash (1980), and R.L. & L. (1987), but not in the more recent surveys. The inset illustrates the Flathead River population that appears to be the same taxon (distributional information for these sculpins in the upper Missouri system was not available).
In the western United States (Figure 2), proposed taxa within what was referred to as the the C. bairdii complex are found in the Snake River (upstream of Shoshone Falls), specifically the Bonneville Basins, the Malheur Basin in Oregon, and the Colorado River basin (Neely pers. comm.). In addition, disjunct populations are also found in Arizona, New Mexico, Nevada and Missouri (Lee 1980; Peden 2000).
The only likely source of gene flow between the “Eastslope” sculpin in Alberta and populations outside of the province is from the most upstream section of the St. Mary River mainstem that flows through Montana.
The zoogeography of the “Eastslope” sculpin is complex, and theories regarding glacial refugia and dispersal routes vary. The present distribution suggests that this species survived in two, possibly three, refugia (i.e., Missourian, Mississippian and Columbian) [Bailey and Allum 1962; Crossman and McAllister 1986]. The extent of ice coverage during the last glaciation period (which began approximately 18 000 years ago) of the Pleistocene epoch is somewhat uncertain, but it is believed that much of Alberta was covered, with possibly some ice-free areas occurring in southern Alberta (summarized in Nelson and Paetz 1992). Nelson and Paetz (1992) suggested that during that last deglaciation period various post-glacial connections between the Oldman (e.g., St. Mary River) and the Milk drainages may have permitted movement from the Missouri drainage to the Saskatchewan system. Movement of the “Eastslope” sculpin from the Missouri drainage could have occurred during this time. Willock (1969) proposed that the isolated occurrence of the “Eastslope” sculpin outside of the Missouri system in the St. Mary River reflects a post-glacial arrival here, probably occurring fairly recently. An alternative view is that the “Eastslope” sculpin may have entered the Milk River system from the St. Mary River through the irrigation canal system in Montana (Nelson and Paetz 1992, Paetz 1993; W. Roberts, Zoology Museum, University of Alberta, Edmonton, Alberta; personal communication 2003). This movement may happen annually and could stimulate the apparent downstream expansion of sculpins observed in the Milk River over a recent 20-year span (Paetz 1993). Several other upper Milk River fish species are also found in the St. Mary River (T.B. Clayton, Sustainable Resource Development, Lethbridge, Alberta; personal communication 2003). Given this observation, and the unresolved taxonomy of the “Eastslope” sculpin, it is impossible to determine whether the sculpin expanded from the St. Mary River to the Milk River or from the Milk River into the St. Mary River. Crossman and McAllister (1986) proposed that the present-day occurrence of species such as the “mottled” sculpin might depend on the availability of deeper, colder water habitat rather than be explained just based on refugia. Therefore, the current distribution of the “Eastslope” sculpin may be explained by its preference for colder waters upstream, in addition to movement by way of irrigation canals and reservoirs and post-glacial dispersal.
Extensive fish sampling since the 1960s has delineated a fairly well defined distribution range for the “Eastslope” sculpin in Alberta. The “Eastslope” sculpin was first identified (as C. bairdii) in the Milk River in 1966 (University of Alberta Zoology Museum record 3771). Its presence in Alberta appears to be limited to the St. Mary River above the reservoir and to the North Milk River and Milk River mainstem, except for the furthest downstream section (i.e., the lowermost 85 km in Alberta) (Roberts 1988; Nelson and Paetz 1992; Paetz 1993; see Figure 4). This distribution is primarily within the Dry Mixedgrass, as well as the Mixedgrass and Foothills Fescue subregions of Alberta (ANHIC 2002a). The Dry Mixedgrass Subregion is considered the driest area in the province, with the warmest summers, cold winters and extreme variability in the amount of annual precipitation (ANHIC 2002a).
The earliest published study within the Milk River found the “Eastslope” sculpin only in the upper reaches of the North Milk River and at the international border in the upper Milk River above the confluence with the North Milk River (Willock 1969). A later study also documented the presence of sculpins at three locations in the upper Milk River (Clayton and Ash 1980). In 1986, a survey documented the sculpin throughout the North Milk River as far downstream as a site approximately 100 km upstream of the international border and at one upstream site in the upper Milk River (R.L. & L. 1987). Paetz (1993) confirmed the sculpin’s presence in the North Milk River and mainstem, but for the first time noted an absence in the upper Milk River. He believed that the sculpins in the Alberta portion of the upper Milk River had been extirpated as a result of the depletion of water flows south of the international border in Montana. Water withdrawal for irrigation is unregulated there (Clayton pers. comm.). Furthermore, comparisons of the earliest work (Willock 1969) with work conducted in 1979 (Clayton and Ash 1980) suggest that sculpins occurred some 130 km further downstream in the later study.
Most recent assessments have found that the “Eastslope” sculpin is widely distributed throughout most of the North Milk River and Milk River mainstem, except for the lowermost section (0-85 km upstream of the international border) where it is absent (R.L. & L. 2002) (Figure 4). This is consistent with earlier studies (Clayton and Ash 1980, R.L. & L. 1987; Paetz 1993), suggesting that the distribution in these sections has not changed in recent years, with the exception of the upper Milk River above the confluence with the North Milk River. Studies in 2000-2001 found this section of the Milk River dry as a result of severe drought conditions and the operation of the St. Mary Canal (R.L. & L. 2002), reflecting findings similar to Paetz (1993).
With regards to the St. Mary River, provincial fisheries’ catch records prior to 1980 did not document any “Eastslope” sculpins downstream of the St. Mary Reservoir (summarized in Paetz 1993). Paetz (1993) confirmed the sculpin’s presence in the St. Mary River above the reservoir and in the lower 10 km of Lee Creek. He also noted its absence in the St. Mary River downstream of the reservoir and in upper Lee Creek, as well as the Belly, Waterton and Oldman rivers. Similarly, the “Eastslope” sculpin has not been observed in the reservoir (Clayton pers. comm.; Roberts pers. comm.). The St. Mary Reservoir is likely a major obstacle to downstream dispersal of sculpins in the St. Mary River (Paetz 1993). Paetz (1993) suggested that the absence of sculpins downstream of the reservoir reflected a relatively recent movement of the “Eastslope” sculpin into Alberta waters. However, it has also been suggested that the “Eastslope” sculpin likely did occur downstream of the reservoir before its construction and that current habitat conditions (e.g., silty substrate) have resulted in its extirpation here (Roberts pers. comm.). Unfortunately, no historical records are available to support either hypothesis. The recent studies conducted in 2000 also found the “Eastslope” sculpin to be common throughout the entire section of the St. Mary River above the St. Mary Reservoir to the international border (R.L. & L. 2002) (Figure 4). However, in Lee Creek, a major tributary of the St. Mary River, distribution was limited to the lower sections, with the uppermost extent found 6 km upstream of the settlement of Cardston (R.L. & L. 2002).
Overall, the “Eastslope” sculpin occupies approximately 80 km of stream in the St. Mary system and 220 km of stream (with a probable maximum width of 0.02 km) in the Milk River in Alberta (Paetz 1993). However, habitat availability within these lengths can vary significantly depending on water flow, particularly in the Milk River, where availability becomes significantly reduced during periods of extreme drought when certain sections are completely dry. Some changes in distribution appear to have occurred in the Milk River since the 1960s, with significant downstream expansion in the mainstem, and extirpation in the upper Milk River as a result of the consistent lack of adequate water flow. In contrast, no changes in distribution are apparent in the St. Mary River, but the possibility exists that sculpins were present downstream of the reservoir before its construction.
No genetic or movement data are available to describe population substructure or the number of genetically discrete populations that may exist in Alberta. However, some postulations can be made based on other studies and knowledge of the river systems. Peden (2000) noted fairly small-scale differences among populations for both “mottled” and shorthead sculpins in British Columbia, suggesting little gene flow even among small tributaries within drainage. In addition, Bailey (1952) noted very little movement for tagged C. bairdii punctulatus in Montana over a period of one year. It is now generally agreed that the Bailey (1952) reference is to the same taxon (S. Pollard, BC Ministry of Water, Land and Air Protection, personal communication 2005). Conservatively, there may be at least two genetically distinct populations of “Eastslope” sculpin: one in the St. Mary River immediately upstream of the St. Mary reservoir, and one in the Milk River system that is similar to the St. Mary River in Montana upstream of the St. Mary Canal intake. Some movement of larvae from the upper St. Mary River in Montana to the furthest downstream section immediately above the reservoir is possible but probably limited, allowing for some genetic isolation. The St. Mary Canal becomes operational during the spring high flows and it is at this time that some sculpin larvae are likely drawn downstream through the canal into the North Milk River, resulting in an annual influx of genetic material (Paetz 1993; Clayton pers. comm.).
Based on genetic and movement studies of this and closely related sculpins (Bailey 1952; Peden 2000, 2001), it is likely that some sub-structuring within the St, Mary and Milk rivers occurs. Lee Creek, St. Mary River and North Milk River (below the confluence with the North Milk) all contain some water year-around and thus can sustain sculpin populations.
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