Eastslope sculpin COSEWIC assessment and status report: chapter 10

Limiting Factors

The “Eastslope” sculpin is considered locally abundant, but its range is extremely limited, being restricted to the St. Mary River, upper and mid-Milk River and North Milk River. A comparison of habitats occupied by the sculpin in these systems with habitats elsewhere in the Oldman River system (Paetz 1993) suggests that favourable habitat is available elsewhere (e.g., Upper Belly River, Waterton River above the Waterton Reservoir and the Oldman River mainstem upstream of Fort McLeod). However, expansion into these habitats is blocked by the presence of the St. Mary Reservoir, Waterton Reservoir and dam, and unfavourable conditions downstream of the reservoirs (e.g., low water flows, high summer water temperatures and silted substrate) (Paetz 1993; Roberts pers. comm.).

Under adequate flow conditions, spawning, rearing, feeding and overwintering habitats do not appear to be limiting throughout the St. Mary River above the reservoir, or the Milk River, except in the lower section (R.L. & L. 2002). The main threat to the populations of the “Eastslope” sculpin in Alberta appears to be the potential loss of flowing water through impoundment diversions and water removal, in combination with the frequent drought conditions experienced in southern Alberta (Paetz 1993). Within the Milk River, the removal of water in Montana from the upper section, in combination with the natural low flows, is likely responsible for the lack of favourable habitat conditions (i.e., depleted water flows) and the disappearance of sculpins here. Reduced water flow affects the availability of physical habitat, and it also increases water temperatures during the warm summer months. Indeed, Willock (1969) stated that temperature was the most important factor affecting the distribution of the sculpin in the Milk River. He believed that this was one of the reasons that sculpins only occurred in the upper sections of the north and south branches where temperatures are lower because of increased rainfall and higher elevations.

Southern Alberta is susceptible to extreme drought conditions during the summer, and naturally low flows at this time of year may be exacerbated by the seasonal operation of the St. Mary Canal, and by water removal for irrigation (Pollard 2003). In 2001 the August, October and December discharge were 50%, 7% and 6% of historic values, and the October and December rates in 2002 were 11% and 20%. Such low flows could seriously limit overwintering habitat, and in fact, during the late fall and winter of 2001/2002 the lower Milk River dried up completely, except for a number of isolated pools (R.L. & L. 2002). This severity of drought conditions in southern Alberta is not uncommon (Pollard 2003) and may be more common given predicted changes in aquatic ecosystems associated with global climate change (Poff et al. 2002). This may prevent populations from expanding and what is more significant is the higher temperatures that accompany the summer drought; all fish species, including the sculpin are exposed to increased risk which may be exacerbated by ongoing maintenance of the St. Mary Canal that results in closures of the canal for extended periods.

In addition to the potential effects on sculpin habitat downstream of impoundments, the habitat alterations associated with an impounded area of a river, such as the St. Mary Reservoir or the proposed reservoir on the Milk River, may be significant. No sculpins have been collected from the St. Mary Reservoir, although sampling has been extremely limited (Clayton pers. comm.). The reservoir has a very limited littoral zone, and water levels fluctuate widely throughout the year (Paetz 1993; Clayton pers. comm.). Other alterations to habitat would likely include elevated summer temperatures in shallow littoral areas, increased silting of substrate and the loss of riffle habitat, none of which are desirable for sculpin habitat (Peden pers. comm.). Finally, the aquatic insect community, which sculpins depend upon for food, would be altered from one suited to flowing water conditions to one suited to lake conditions (Clayton pers. comm.).

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