Eastslope sculpin COSEWIC assessment and status report: chapter 7

Habitat

Habitat Requirements

Sculpins are nocturnal and tend to remain under cover (usually rocks) during daylight hours (McPhail 2001). “Mottled” sculpins are found in moderately cool streams with riffle habitat, rocky or gravel substrate and weak to fast currents (Peden 2000). An older study by Bailey (1952) on the Rocky Mountain sculpin (C. bairdii punctulatus), which may be the same taxon as the “Eastslope” sculpin (see Taxonomy), in southwestern Montana also found these fish to be most abundant in riffle habitat where rubble and boulders were predominant and provided refuge. They were usually absent from pools where bottoms were entirely sand or clay (Bailey 1952). Little information is available regarding temperature preferences for western “mottled” sculpins, but shorthead sculpin populations were found to inhabit streams with spring and summer temperatures averaging 15°C (range 8°C-21°C) in Oregon (Bond 1963), and in streams with summer temperatures of 12°C to 18°C and winter temperatures of 0°C to 4°C in British Columbia (Peden 2000).

The only study describing spawning habitat was for the Rocky Mountain sculpin in southwestern Montana (Bailey 1952). Nests consisted of holes under rocks that ranged from 0.12 m to 0.38 m in diameter. Eggs were usually attached to rocks, but other substrates including aquatic vegetation, wood and other debris were also utilized (Bailey 1952). Water depth of nests was over 0.3 m, and surface water velocities ranged from 0 m/s to 1.6 m/s.

Since the 1960s, a number of studies on the St. Mary and Milk rivers have described the habitat features preferred by the “Eastslope” sculpin. Willock (1969) stated that the colder temperatures and increased water clarity in the upper Milk River accounted for the presence of species such as the “Eastslope” sculpin. These characteristics are associated with higher rainfall, higher elevation and gradient, more vegetation and less erosion because of the presence of more resistant sandstone substrate in the upstream reaches of the Milk River compared to further downstream reaches (Willock 1969). In particular, Willock (1969) stated that water temperature was the single most important factor affecting sculpin distribution in the Milk River. In addition, he found that sculpins were most numerous in sections of the Milk River with little or no current, and were at least as common at creek mouths as in the mainstem proper. Similarly, Paetz (1993) noted that sculpins in the North Milk River and the St. Mary River were most common in silt-free rocky substrate near the stream margin where currents were slower, whereas no sculpins were found in the main river current. In Lee Creek (a tributary of the St. Mary River), sculpins appeared to prefer the slightly silty stream margins where currents were slower, compared to the mid-creek section, which was silt-free but had higher velocity (Paetz 1993). Paetz (1993) also noted that sculpins used areas where instream sedges and bankside shrubs trailed in the slower current in the middle Milk River if rocky substrates and cobbles were absent, particularly near the Town of Milk River. Other habitats utilized consisted of debris anchored by an obstruction such as a root in the streambed. Clayton and Ash (1980) noted that the “Eastslope” sculpin appeared to prefer clean substrates, but lower numbers were also found in quiet pools with silty substrate.

A detailed habitat evaluation for the “Eastslope” sculpin was conducted in 2000-2001 in the St. Mary and Milk river systems (R.L. & L. 2002). Some variability in habitat selection appeared to be drainage-specific and dependent on habitat type availability, as well as water levels. In general, sculpins were present predominantly in shallow runs and riffles, as well as run/boulder gardens (larger boulders providing in-stream cover). A statistical analysis of microhabitat characters found that rather than being associated with a particular type or range of character values, the “Eastslope” sculpin appeared to be more of a generalist (R.L. & L. 2002). However, water depths in capture locations tended to be shallow (range 0.05-0.42 m, mean of 0.19 m), and velocities were low (range 0-0.6 m/s, mean of 0.22 m/s). Silt depths tended to be low (range 0.0-0.02 m deep), rock was the predominant cover type (10%-40%), turbidity was low (range 0-5%), and substrate consisted mainly of gravel and cobble (R.L. & L. 2002).

Little information is available regarding habitat features associated with any life history stage for the “Eastslope” sculpin. Spawning, rearing and feeding habitats are not believed to be limited throughout the St. Mary River or the upstream sections of the Milk River where sculpins are found (R.L. & L. 2002). Interestingly, Willock (1969) noted a disproportionately large number of young sculpins in muddy areas with little gradient in the Milk River, suggesting that these areas might be used for rearing. A similar observation was made by Bailey (1952), who noted that some small specimens of Rocky Mountain sculpin were found in quiet waters near the shore. He proposed that these small sculpins could stir up clouds of silt for cover. Overwintering habitat is also believed to be well represented in both river systems, provided that adequate water flow is available (R.L. & L. 2002).


Trends

The greatest alterations to sculpin habitat in the St. Mary and Milk rivers are related to water diversions, reservoirs and water removal for irrigation. These factors, in combination with the frequent droughts experienced in southern Alberta, seriously affect the availability of sculpin habitat. The construction of the St. Mary Reservoir, completed in 1951 (Clayton pers. comm.), significantly altered the type of habitat available to fish species in the St. Mary River (see Figure 4 in Distribution section for dam location). Currently, “Eastslope” sculpins are not known to be present in the reservoir or downstream of the reservoir (Roberts pers. comm.; Clayton pers. comm.). Although it is not known whether they once inhabited these sections, future expansion into downstream habitats is not possible because of the presence of the dam. Furthermore, the absence of sculpins in the reservoir suggests that conditions (i.e., temperature regimes and bottom type) here are unfavourable.

The biggest alteration to habitat in the Milk River occurred after 1917 when the St. Mary Canal was constructed in Montana to divert water from the St. Mary River to the North Milk River for irrigation purposes. In most years, the canal diverts water from April to September, increasing the water volume in the North Milk River and the Milk River proper. Before construction of the canal the Milk River was probably a typical small prairie stream, possibly intermittent in times of drought, and generally less turbid (Willock 1969). Although the volume of water may have increased downstream of the canal outflow in the North Milk River, this has become a highly managed flow, which may be turned off temporarily or prematurely during the open water months if canal repairs are required. This occurrence, in combination with the drought conditions common to this region, can result in the severe reduction in the availability of sculpin habitat in the Milk River. In addition, the ongoing removal of water in Montana from the upper Milk River, which is above the confluence with the North Milk River, may be partly responsible for the disappearance of sculpins in this upstream section of the mainstem (Paetz 1993). In 2000-2001, the upper Milk River was dry during the summer and fall sampling seasons (R.L. & L. 2002). In fact, this section is often dry during the summer months (Clayton pers. comm.). Any use of this section of river by sculpins is at most temporary. Similarly, the tributaries of the North Milk River are considered ephemeral, and are dry to damp most of the year under average conditions (Clayton pers. comm.).

No other major changes to habitat have been observed since the construction of the St. Mary Reservoir. Instead, the availability of habitat, particularly overwintering habitat in the Milk River, is highly variable from year to year and dependent on adequate water flows. The combination of severe drought conditions and water flow management associated with the St. Mary Canal can lead to extremely low flow conditions, as observed in the late summer and fall of 2000 and 2001 (R.L. & L. 2002). A potential threat to existing sculpin habitat in the Milk River is the recurring (still under consideration) proposal to construct a dam on the Milk River upstream of the Town of Milk River for irrigation purposes. Such a dam would flood approximately 19 km of the North Milk River and 11 km of the Milk River mainstem (R.L.& L. 1987), resulting in approximately 10.5% of existing habitat being lost above the dam, as well as possible effects downstream of the dam (Paetz 1993), such as altered flow, turbidity and temperature levels.

In terms of re-colonization potential, the St. Mary irrigation canal is a probable source of migrants from the upper St. Mary River in Montana to the North Milk River on an annual basis. The suspected eastward expansion of sculpins in the Milk River since the species was first documented in the 1960s (see Distribution section) suggests that the “Eastslope” sculpin is capable of expanding into new habitat, particularly into downstream areas. In contrast, Peden (2000) claimed that genetic differences between “mottled” sculpin populations in British Columbia, in combination with a relatively sedentary lifestyle, suggest that dispersal is slow and movement among streams is limited. Given this information, re-colonization of an extirpated population in the North Milk River by annual immigration through the canal from the St. Mary River is probable and could happen relatively quickly (i.e., within an estimated 10 years); however, given recent drought conditions and other factors it is doubtful that this would happen under recent conditions. Re-colonization of the Milk River mainstem from the North Milk River would likely be a slower process (i.e., 10 or more years), based on previous reports documenting changes in distribution (Willock 1969; Clayton and Ash 1980). Natural re-colonization of the Milk River from the upper Missouri system in Montana is not possible given the absence of sculpins in the Milk River downstream of the international border and the presence of six or more impassable dams (Stash 2001). Similarly, natural re-colonization of the St. Mary River from the North Milk River would likely be impossible given the design of the St. Mary Canal (Clayton pers. comm.).


Habitat Protection/Ownership

The St. Mary and Milk river habitat is largely under private ownership and none is legally protected. The Crown owns the river-bed, and shores up to 6 ft from the high water mark. Lands bordering the Milk and North Milk rivers are only under 38% private ownership -- the remainder is public land, although a good portion of that is under grazing lease. Within the basin itself, probably most of the land is deeded (Clayton. pers. comm. 2005).

Although protection could be available under Fish Habitat sections of the Federal Fisheries Act of 1867 or the provincial Wildlife Act no provisions have been made as yet. A provincial management plan was developed by Paetz (1993) to aid in protecting existing populations. More recently, surveys have been commissioned in the Milk River (2000 to 2002) to help determine the status of several non-game fish species, including the “Eastslope” sculpin, and to provide recommendations with regards to protection (see R.L. & L. 2002; P& E 2002).

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