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Recovery Strategy for the Rocky Mountain Sculpin (Cottus sp.), Eastslope populations, in Canada

4. Threats

A number of threats to the Rocky Mountain Sculpin have been identified throughout its range. The most significant threats may be those that alter the natural flow regime of a river causing habitat loss or impairment. Such threats may include water removal (e.g., for irrigation, municipal, recreational, industrial and domestic use), impoundment, bank stabilization, channelization, and changes in flow conditions. Other threats to the species’ habitat and survival include pollution and degradation of riparian areas. Some of the above threats may also act indirectly by altering faunal communities which in turn threaten the sculpin’s existence.

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) (2005) identified water removal, diversions and reservoirs associated with irrigation, in combination with the frequent droughts as likely posing the greatest threat to the Rocky Mountain Sculpin in southern Alberta. Drought is a natural occurrence in prairie streams and one to which the sculpin is somewhat adapted. Man-made changes to flow can exacerbate the impacts of natural drought on sculpin habitat. The impacts of drought are difficult to mitigate, but habitat loss or degradation caused by man-made changes can be mitigated. The following sections summarize these and other sources of threats to the species and habitat.

4.1 Threat classification

The Recovery Team undertook a detailed assessment of threats to the species based on both published information and local knowledge. Threats were identified under the following broad categories identified by Environment Canada (2007):

  • habitat loss/degradation,
  • changes in ecological dynamics or natural processes,
  • exotic or invasive species,
  • pollution,
  • accidental mortality,
  • climate and natural disasters, and
  • natural processes or activities.

The methods and terminology used to assess threats to the Rocky Mountain Sculpin have been described by Environment Canada (2007). The results are discussed below and summarized in Tables 2 through 12.

4.2 Description of threats

4.2.1 Habitat loss/degradation

Habitat loss or degradation is a threat to the survival of Rocky Mountain Sculpin in the Milk River watershed. A number of existing or potential activities may result in habitat loss or degradation, including: 1) dam construction and operation, 2) changes in flow associated with restoration of the St. Mary Canal in Montana (from 18.4 m3/s to 24.1 m3/s), 3) groundwater extraction, 4) surface water extraction, and 5) livestock use of the floodplain.

Dam construction and operation

Dam construction and operation could pose a threat to Rocky Mountain Sculpin in the St. Mary and Milk river watersheds (Table 2, Table 3). The apparent absence of this species from the St. Mary Reservoir and reaches of the river downstream suggests that Rocky Mountain Sculpin could be extirpated from within the impoundment and possibly for some distance downstream. While there is no proposal at this time, the feasibility of developing a dam on the Milk River upstream of the Town of Milk River has been completed by Alberta. The purposes of a dam would be to improve the security of the water supply for existing withdrawals, and to provide water for the irrigation of additional acres.

 

Table 2. Threat to the Rocky Mountain Sculpin from dam construction.
 1. Dam constructionThreat Information
Threat CategoryHabitat loss or degradationExtent: Unknown
General ThreatWater impoundment (reservoir creation)Occurrence (Local): Unknown
Occurrence (Range-wide):
Frequency (Local): Continuous
Frequency (Range-wide):
Specific ThreatHabitat loss and fragmentationCausal Certainty (Local): High
Causal Certainty (Range-wide):
Severity (Local): Unknown
Severity (Range-wide):
StressReduced population sizeLevel of Concern: LowFootnote a

Footnotes

Footnote A

Comments: Rocky Mountain Sculpin are not found in existing impoundments, such as the St. Mary Reservoir. There is currently no plan to build a dam on the Milk River in Canada at this time. In the event that such a development were to occur the anticipated level of concern would be high as impoundment could lead to local or perhaps wider-ranging extirpation of the species.

Return to footnote a

 

Table 3. Threat to the Rocky Mountain Sculpin from dam operation.
 2. Dam operationThreat Information
Threat CategoryHabitat loss or degradationExtent: Unknown
General ThreatDownstream flow changesOccurrence (Local): Unknown
Occurrence (Range-wide):
Frequency (Local):
Continuous
Frequency (Range-wide):
Specific ThreatChanges in the seasonal flow regimeCausal Certainty (Local): High
Causal Certainty (Range-wide):
Severity (Local):
Unknown
Severity (Range-wide):
StressReduced productivity and/or population sizeLevel of Concern: LowFootnote a

Footnotes

Footnote A

Comments: The St. Mary Diversion exerts significant control over seasonal flow in the North Milk and Milk rivers. Dam construction on the Milk River would add another level of flow control. Flow changes could have positive or negative effects depending upon their timing and volume and the resultant effects on fish habitat. The effect would depend on how releases from a dam were controlled. The seasonality (e.g., during incubation) of any sediment addition may be an important consideration. In the event that such a development were to occur the anticipated level of concern would be high as impoundment could lead to local or perhaps wider-ranging extirpation of the species.

Return to footnote a

 

A reservoir in this reach of the river would inundate Rocky Mountain Sculpin habitat and alter flow and water quality downstream. Until such time as a proposal is advanced any assessment of potential threats is highly speculative and intended only to flag areas where further study or investigation may be required in the future.

Impoundments alter habitat types, flow regimes, sediment loads, microbiota and water temperatures, and may also increase the risk of species’ introductions (McAllister et al. 2000; Quist et al. 2004). These changes often result in a narrowing of stream channels, less turbid water, less subject to fluctuations in temperature and flow, and less productive with less substrate movement (Cross et al. 1986; Pflieger and Grace 1987; Quist et al. 2004). Water released from storage reservoirs is often withdrawn from near the bottom of the reservoir (hypolimnetic withdrawals), creating significantly cooler water conditions in downstream areas.

The effect of impoundment on sculpin habitat downstream would depend on how water releases are managed. Downstream effects noted elsewhere include low water flows, high summer temperatures, and silted substrates; all unfavourable conditions for the Rocky Mountain Sculpin (Alberta Sustainable Resource Development [ASRD] 2004). The species may be absent downstream of the St. Mary Reservoir, although the Spoonhead Sculpin is present (T. Clayton, pers. comm. 2007). Low flow conditions below a dam on the Milk River would increase the potential for extirpation of sculpin populations downstream, and the barrier posed by the reservoir might limit subsequent re-colonization by the upstream population, thus dam construction and operation could affect Milk River populations by reducing their range (T. Clayton, pers. comm. 2007).

In reviewing any future dam proposal, the potential impacts on the Rocky Mountain Sculpin will need to be thoroughly considered. Such a review would benefit from more information on the species’ ecology.

St. Mary Diversion

Diverting water in Montana from the St. Mary River to the North Milk River has reduced the effects of drought in the North Milk and Milk rivers (Willock 1969), and may have extended the availability of suitable summering habitat for the Rocky Mountain Sculpin further downstream than under natural flow conditions. The net effect of this change on the population is unknown, since downstream habitat gains may be offset by upstream losses, and other aspects of the species’ life history that may be affected. Winter flows in the North Milk and Milk rivers, while low, are considered natural and taken alone are unlikely to threaten the Rocky Mountain Sculpin.

The St. Mary Diversion Canal in Montana is in need of maintenance and restoration. Due to its poor structural condition, the canal is not operating at its design capacity of 24.1 m3/s but at a capacity of about 18.4 m3/s (Alberta Environment 2004; U.S. Bureau of Reclamation 2004). Work has begun to bringing the structure up to design capacity which would increase flows by almost 27% and could lead to flow surges during periods of peak runoff in June. In either case, increased flows could alter channel morphology, particularly in the lower Milk River where banks are already highly susceptible to erosion during high flow periods.

Unexpected maintenance has led to temporary or premature closures of the canal in the past. Two such interruptions have occurred over that past 30 years (K. Miller, pers. comm. 2006). One of these interruptions occurred in 2001 when the canal was closed in mid-August to allow for emergency repairs. Due to extreme drought conditions, the lower Milk River was later reduced to a series of isolated pools until the spring freshet. Flow persisted in the upstream sections of the Milk and North Milk rivers that support sculpins. It is not uncommon during the winter for the Milk River upstream from its confluence with the North Milk River to have no surface flow.

Threats posed to Rocky Mountain Sculpin by changes in flow related to the St. Mary Diversion are mostly relevant to the North Milk and Milk rivers (Table 4). Increased flows related to improvements in the diversion could alter water clarity and substrate composition, impact feeding and reproductive success, and facilitate the transfer of biota from the St. Mary River into the North Milk and Milk rivers. Reduced flows related to canal maintenance could cause water temperature to increase and/or dissolved oxygen level to decrease beyond the species’ preferred range. Changes to the flow regime that favour other species could be detrimental to sculpin populations. The rate at which flow is ramped down may alter the potential for stranding. The severity of these threats to Rocky Mountain Sculpin in the North Milk and Milk rivers is unknown. Anticipated changes to the flow regime of the North Milk and Milk rivers should be preceded by detailed studies to determine how the various options might affect river morphology and fish habitat, and Rocky Mountain Sculpin habitat in particular.

 

Table 4. Threat to the Rocky Mountain Sculpin from changes in flow.
 3. Changes in flowThreat Information
Threat CategoryHabitat loss or degradationExtent: Widespread
General ThreatSt. Mary DiversionOccurrence (Local): Anticipated
Occurrence (Range-wide): Anticipated
Frequency (Local): Seasonal
Frequency (Range-wide): Seasonal
Specific ThreatHabitat disruptionCausal Certainty (Local): Low
Causal Certainty (Range-wide): Low

Severity (Local): Unknown
Severity (Range-wide): Unknown
StressReduced productivity and/or reduced population sizeLevel of Concern: Low to Medium

Comments: The existing flow regime has been regulated for most of the past century. Changes to this regime that alter seasonal flow patterns and volumes could alter the seasonal availability of suitable sculpin habitat. At the population level, the net effect of any change could be positive or negative. The effects of flow changes have historically been greatest on the North Milk River, which receives water diverted from the St. Mary River watershed and is small relative to the diverted flows and to St. Mary River and Milk River mainstem.


Surface water extraction - irrigation

The threat to Rocky Mountain Sculpin in Alberta from the extraction of surface water for irrigation is considered low, since only a small proportion of the available flow is withdrawn and these withdrawals are regulated. Water is extracted from the Milk River for irrigation only while flows are augmented, from late-March or early April through to late September or mid-October. In 2008, Alberta irrigators were licensed to remove a total of up to 1.186 × 107 m3 of water from the Milk River (D. Hunt, pers. comm. 2008). This constituted about 92% of the total licensed annual water withdrawal, but only about 5% of the average annual flow at the Town of Milk River from April through September (2.423 × 108 m3; period of record 1909 to 2007 (WSC 2008b).

The total withdrawals allowable under water licenses are typically approached only during drought years (K. Miller, pers. comm.). When the St. Mary diversion in Montana is closed for maintenance, or during reduced flow conditions, withdrawals for irrigation are terminated, or suspended on a priority use basis. Alberta Environment has begun installing water meters on all irrigation pumps drawing water from the Milk River (K. Miller, pers. comm. 2006). These meters would measure water removal four times a day to provide an accurate and up-to-date measure of water withdrawals.

Sculpin populations in Lee Creek and the St. Mary River are unlikely to be affected by water withdrawals for irrigation at this time. There is little irrigation currently along these watercourses and further development that would require water diversions is unlikely due to the high elevation, topography, short growing season, and higher rainfall (Government of Alberta 2005). Very little water (27 140 m3) is withdrawn from Lee Creek for irrigation (D. Hunt, pers. comm. 2008). In 2008, Alberta irrigators were licensed to remove a total of up to 1.104 × 106 m3 of water from the St. Mary River (D. Hunt, pers. comm. 2008). This constituted about 80% of the total licensed annual water withdrawal, but only about 0.3% of the average annual flow at the Highway 501 Crossing during the irrigation period (i.e., 3.863 × 108 m3) April through September of 1998 through 2007 (WSC 2008d). If water withdrawals were to increase significantly in the future relative to the available flow, the impact of this potential threat to Rocky Mountain Sculpins should be reassessed.

Surface water extraction - non-irrigation

In contrast to water licenses for irrigation, Temporary Diversion Licenses (TDLs) for non-irrigation purposes are issued throughout the year by Alberta Environment, including during critical low flow periods. Oil and gas companies, for example, may be licensed to remove water from the river for activities related to well-drilling. Overwintering habitat for Rocky Mountain Sculpin may be particularly vulnerable to this type of extraction for reasons similar to those outlined under “Groundwater Extraction”. This kind of extraction also occurs during the augmented flow period, when it may not be an issue unless the St. Mary diversion is prematurely or temporarily closed down. The diversion of water during the non-augmented period is a concern and under such conditions, some TDLs may be revoked, as they were during the drought conditions in 2001 (S. Petry, pers. comm. 2006). TDLs are more prevalent for oil and gas development near the Milk River than they are near the St. Mary River and Lee Creek. Fish in the Milk River are most vulnerable during the non-augmented period. Restriction of TDLs in areas that would reduce sculpin habitat during critical low flows could be used to mitigate impacts.

During the flow augmentation period, the Town of Milk River diverts about 0.3% of the flow for domestic purposes (S. Petry, pers. comm.). The Town of Cardston draws its water from Lee Creek. In 2008, about 3.3% (1.500 × 106 m3; D. Hunt, pers. comm. 2008) of the average annual flow in Lee Creek at Cardston (4.433 × 107 m3; WSC 2008a) was licensed for municipal use. This constituted about 91% of the total licensed annual water withdrawal from Lee Creek. Water is not withdrawn for municipal use from reaches of the St. Mary River that are inhabited by Rocky Mountain Sculpin, unless flow in Lee Creek is insufficient to meet the needs of the Town of Cardston.

The threat to Rocky Mountain Sculpin in Alberta from surface water extraction for purposes other than irrigation is considered low, since only a small proportion of the available flow is withdrawn (D. Hunt, pers. comm.), and these withdrawals are regulated. If water withdrawals were to increase significantly in future relative to the available flow, the impact of this potential threat to Rocky Mountain Sculpins should be reassessed.

Livestock use of flood plain

The Alberta Riparian Habitat Management Society (“Cows and Fish”) has been working to improve livestock management practices in the North Milk and Milk rivers floodplain. Several riparian and grazing management workshops have been held, involving many ranchers along the North Milk and Milk rivers. There is a growing understanding of the value and vulnerability of the riparian area to degradation, and a greater understanding and adoption of management solutions by ranchers, including off-stream water development (L. Fitch, pers. comm. 2006). Several riparian benchmark inventories have been completed, but there has not been any follow-up monitoring to date. Demonstration sites have been established to show the positive effects of utilizing off-stream watering sites and have shown riparian vegetation recovery, especially with woody vegetation. Riparian recovery is usually evident within three to five years after the first management changes are made, but it may be ten years before significant physical changes can be measured. The Society has also conducted some work on Lee Creek and its tributary, Tough Creek. An aerial reconnaissance of the upper St. Mary River has been conducted, but nothing on the ground (T. Clayton, pers. comm. 2008).

Most Rocky Mountain Sculpin habitat is situated upstream of areas with cattle crossings (T. Clayton and M. Bryski, pers. comm. 2008). These crossings are more prevalent in the North Milk and Milk rivers and Lee Creek downstream of Beazer than in the St. Mary River. Outside the canyon sections (~4 km) much of the St. Mary River valley (~42 km) is relatively broad and accessible to cattle. Ranching is a primary agricultural activity and access to the St. Mary River by cattle is generally unrestricted. Balanced against this access is the natural resilience of the bed and shores of the river, where the gravel and cobble bed and underlying bedrock provide natural armour. While much of the shoreline is in good condition, some areas have been overused, resulting in shoreline degradation and man-made armouring may reduce the habitat value for sculpins and other fish if appropriate mitigation measures are not applied before, during and after construction. The threat to Rocky Mountain Sculpin and their habitat from livestock use of the floodplain can be largely mitigated through improving land management practices and is considered low.


4.2.2 Changes in ecological dynamics of natural processes

Blooms of the diatom Didymosphenia geminata (Bacillariophyceae) are an emerging threat to headwater rivers in Alberta with high water quality (i.e., low turbidity and nutrient levels) (Kirkwood et al. 2007) (Table 5). These blooms can create dense algal mats that cover kilometers of river bottom, impacting ecosystem structure and function and negatively affecting other trophic levels. The environmental factors and conditions that promote bloom events are not well understood. However, studies on the Bow and Red Deer rivers have found a negative relationship between the mean flow regime and diatom biomass. Flow regulation by dams may create the stable flow environment preferred by D. geminata. Together with other environmental factors such as water clarity, temperature, pH, conductivity, and total phosphorus this may promote diatom blooms.

If these algal blooms occur in river habitat occupied by the Rocky Mountain Sculpin they could alter the cover, food, and spawning habitats available to these fish and might displace them from these habitats. The likelihood of blooms occurring and negatively affecting Rocky Mountain Sculpin is unknown. Any impacts are likely to be periodic and to affect local areas, so the population as a whole is not likely at risk. The ability to prevent or mitigate the blooms themselves may depend upon altering the flow regime (Kirkwood et al. 2007).

 

Table 5. Threat to the Rocky Mountain Sculpin from Didymosphenia geminata.
 4. Didymosphenia geminataThreat Information
Threat CategoryChanges in ecological dynamics or natural processesExtent: Localized
General ThreatAlgal bloomOccurrence (Local): current
Occurrence (Range-wide): unknown
Frequency (Local): Recurrent
Frequency (Range-wide):
Specific ThreatHabitat impairmentCausal Certainty (Local): Low
Causal Certainty (Range-wide):
Severity (Local):
Unknown
Severity (Range-wide):
StressReduced productivity, displacement, reduced fitnessLevel of Concern: Low

Comments: Blooms are typically restricted to an area of a river.


4.2.3 Exotic or invasive species

Exotic or invasive species may be introduced into waters occupied by the Rocky Mountain Sculpin either intentionally by stocking (Table 6), or unintentionally in bilge water, on boat hulls, as bait, or by other means (Table 7). Introduced species can threaten native fish fauna through various mechanisms including: predation, hybridization, competition for resources, the introduction of exotic diseases or parasites, and habitat degradation (Taylor et al. 1984; Lassuy 1995; Courtenay 2007). The degree to which this threat is likely to occur depends on the suitability of sculpin habitats to potential invading species.

In Montana, authorized stocking of non-native fishes into the St. Mary River watershed began early in the 20th century and continued in Glacier National Park until mid-century (Marnell 1988; Mogen and Kaeding 2005a). It continues today in some waters of the Blackfeet Reservation, mainly isolated ponds and lakes. Non-native fishes that have established self-sustaining populations in the St. Mary River watershed in Montana include Brook Trout, Rainbow Trout, Yellowstone Cutthroat Trout, and their hybrids. Brook Trout have not been reported from Canadian reaches of the St. Mary River.

In Alberta, Kokanee, Rainbow Trout, and Walleye have been stocked into the St. Mary Reservoir but only Walleye have established a self-sustaining population (Clements 1973). The Milk River and its tributaries have not been stocked for at least 10 years, although Goldsprings Park Pond, an old oxbow of the river with no connection to the mainstem is stocked annually with Rainbow Trout (T. Clayton, pers. comm. 2006). The Alberta Fish and Wildlife Division does not plan to introduce sportfish species into the Milk River or St. Mary River watersheds, and is unlikely to do so in the future (T. Clayton, pers. comm. 2008). Unauthorized introductions have not been documented in these rivers. Such introductions are difficult to control and might increase the severity of this threat if a new species were introduced.

 

Table 6. Threat to the Rocky Mountain Sculpin from intentional fish stocking.
 5. Intentional fish stockingThreat Information
Threat CategoryExotic or invasive speciesExtent: Widespread
General ThreatFreshwater fishOccurrence (Local): Historic, current, anticipated
Occurrence (Range-wide): Historic, current, anticipated
Frequency (Local): Recurrent
Frequency (Range-wide): Recurrent
Specific ThreatResource competition, predationCausal Certainty (Local): Low
Causal Certainty (Range-wide): Low

Severity (Local): Unknown
Severity (Range-wide): Unknown
StressAltered productivity and/or reduced population sizeLevel of Concern: Low

Comments: Various fish species, typically those of interest to harvesters, have been introduced to both the Milk and St. Mary watersheds. Their effects on Rocky Mountain Sculpin are unknown and likely ongoing. Further stocking of already introduced species is unlikely to change the severity of this threat, however, new introductions could have a higher threat potential. The severity of this ongoing threat may be somewhat mitigated by the environment in which the Rocky Mountain Sculpins exist. Introductions to the Fresno Reservoir only affect the Milk River, whereas introductions to Lower St. Mary Lake in Montana can affect Rocky Mountain Sculpins in both the Milk and St. Mary watersheds. Illegal introductions are difficult to prevent and once established are difficult to eradicate. Education and regulation offer the best potential for mitigating this threat.

 

Table 7. Threat to the Rocky Mountain Sculpin from unintentional species introductions.
 6. Unintentional species introductionsThreat Information
Threat CategoryExotic or invasive speciesExtent: Widespread
General ThreatIntroduction of aquatic biotaOccurrence (Local): Historic, current, anticipated
Occurrence (Range-wide): Historic, current, anticipated
Frequency (Local): One-time, seasonal, recurrent
Frequency (Range-wide): One-time, seasonal, recurrent
Specific ThreatResource competition, predationCausal Certainty (Local): Low
Causal Certainty (Range-wide): Low

Severity (Local): Unknown
Severity (Range-wide): Unknown
StressAltered productivity and/or reduced population sizeLevel of Concern: Low

Comments: This threat has been ongoing in the North Milk and Milk rivers through inter-basin water transfers since the diversion began operation. Future introduction may also be of concern. Two organisms of concern are the New Zealand Mud Snail (Potamopyrgus antipodarum) and Northern or Virile Crayfish (Orconectes virilis). These species do not currently inhabit Canadian portions of the St. Mary or Milk river watersheds. Unintentional species introductions are most likely to occur during the open water period. Their occurrence may be sporadic, but the impacts will persist if an introduced species becomes established. The ability to prevent this threat is moderate but the potential to mitigate it once established is low. The overall level of concern for this watershed at this time is considered low. The focus should be on prevention through education and regulation (e.g., do not use crayfish as bait).


The effects of historical species introductions from the St. Mary River into the Milk River watershed via the diversion are unknown. Increasing the annual flow in the St. Mary diversion canal might further facilitate movement of biota from the St. Mary River into the Milk River. The potential impacts on Rocky Mountain Sculpins in the Milk River of controlling the entrainment of biota by the St. Mary Diversion are unknown.

To date, Trout-perch and Walleye are the only introduced fish species that have been observed in the lower Milk River where the Rocky Mountain Sculpin occurs (T. Clayton and D. Watkinson, unpubl. data). Further downstream, the Fresno Reservoir contains a number of introduced predatory species, including: Rainbow Trout, Walleye, Yellow Perch, Northern Pike, and Black Crappie (Pomoxis nigromaculatus), as well as other introduced species such as Lake Whitefish and Spottail Shiner (Notropis hudsonius) (http://www.ifished.com/montana/fresno-reservoir)). Spottail Shiners have also been observed in the river section between the international border and the reservoir (Stash 2001). While some species listed here have specific habitat requirements that may not be met in the lower Milk River of Alberta, others are generalists that might expand into Alberta.

There are no physical barriers to fish migration between the Fresno Reservoir in Montana, and areas of the North Milk and Milk rivers in Alberta that support Rocky Mountain Sculpin. Therefore, introduction of fish species to the Fresno Reservoir in Montana has allowed introduced fish species to migrate upstream into the North Milk and Milk rivers in Canada. At this time few introduced fish have been collected and the impacts to sculpin are likely low. Introductions of species to Lower St. Mary Lake in Montana have the potential to affect Rocky Mountain Sculpin in both the Milk and St. Mary watersheds.

The significance of species introductions would depend upon the species introduced. Introduction of the New Zealand snail (Potamopyrgus antipodarum), for example, can disrupt indigenous invertebrate populations and may cause a marked dietary shift in both sculpin and trout (Cada 2004). The potential impact of such an introduction into the Saskatchewan River watershed was assessed as unknown (Golder Associates Ltd. 2003). Crayfish have not been reported from the St. Mary River watershed above the St. Mary Reservoir or from the Milk River watershed in Canada (T. Clayton, pers. comm. 2008). Introduction of a crayfish into these areas could modify the aquatic macrophyte, macroinvertebrate and, ultimately, fish communities (Chambers et al. 1990; Hanson et al. 1990; McCarthy et al. 2006). These effects would likely be greatest in detritus-based littoral food webs (Usio and Townsend 2002, 2004).


4.2.4 Pollution

The likelihood of point source and non-point source pollution entering the St. Mary or Milk river watersheds at levels that would threaten Rocky Mountain Sculpin survival is considered low (Table 8). Point sources of pollution include any stormwater and sewage releases, as well as accidental spills and gas leaks particularly at river and tributary crossings. Over the past 12 years, fewer than 20 pollution incidents have been reported from the St. Mary River watershed, and some of these were in Lee Creek, Pothole Creek, or downstream of the reservoir (M. Bryski, pers. comm. 2008). These incidents included sewage discharges, feedlot run-off, sediment releases, small oil/gas spills and small chemical spills. Municipal sewage and industrial sediment were released into Lee Creek in the past at Cardston, affecting the lower four kilometres of the creek. The Town of Milk River has not released sewage into the Milk River for 20 years, and stormwater is surface run-off (K. Miller, pers. comm. 2006) making both of these a minimal risk.

 

Table 8. Threat to the Rocky Mountain Sculpin from point source pollution.
 7. Point source pollutionThreat Information
Threat CategoryPollutionExtent: Local
General ThreatAccidental spillsOccurrence (Local): Anticipated
Occurrence (Range-wide):
Frequency (Local):
Unknown
Frequency (Range-wide):
Specific ThreatRelease of contaminants from natural gas or oil pipeline leaks, at highway crossings and/or from sewage treatment plantsCausal Certainty (Local): Medium
Causal Certainty (Range-wide):
Severity (Local):
Unknown
Severity (Range-wide):
StressToxic effects, reduced productivity, increased mortalityLevel of Concern: Low


The accidental release of a toxic substance at any one of the river crossings including bridges or pipelines could have serious consequences. The extent and severity of any damage to the aquatic community including Rocky Mountain Sculpin would depend on the substance released, the location of spill, the time of year (flow augmentation or not), and the potential to mitigate the impacts. To date, no such spills have been documented for the Milk River. However, the possibility, although quite low, exists because traffic flow is significant at some crossings (e.g., average of 2700 crossings per day on the Highway 4 Bridge in 2003, 25% by trucks). A number of gas leaks have also occurred in recent years (S. Petry, pers. comm. 2006). The risk to the sculpin population from point-source pollution seems low based on both the low frequency and limited potential for a large-scale event that would affect a substantial length of river.

Contamination of water from seismic or drilling activities is also a possibility in the Milk River. Uncapped groundwater wells may also pose a problem although licensing and well capping programs help to minimize this threat (Alberta Environment 2001).

Non-point sources of pollution with the potential to affect Rocky Mountain Sculpin habitats are limited mainly to the runoff of agricultural pesticides and fertilizers (Table 9). Overall, this threat is considered low. The intensity of agricultural cultivation beside Lee Creek and the St. Mary River is low, lower than along the North Milk and Milk rivers (Government of Alberta 2005), and no feedlots are currently located on either the Milk or St. Mary rivers. Most of the approximately 8000 acres of cropland that is irrigated in the Milk River watershed is located within 50 km of the Town of Milk River, but there is another small section located upstream on the North Milk River near Del Bonita (K. Miller, pers. comm. 2006). The rough terrain near the river channel prevents crops in most areas from being grown within about 400 m of the river (K. Miller, pers. comm. 2006) and acts as a buffer, reducing the potential for direct contamination of the river. The growth period for most crops also coincides with the diversion period, when flows are usually at their highest, creating a significant dilution effect. Leaching of fertilizer residues has declined significantly in recent years due to the high costs of fertilizing and pumping of water (K. Miller, pers. comm. 2006), but nutrient concentrations can become elevated at downstream sites such as the Highway 880 crossing (W. Koning, pers. comm. 2006).

 

Table 9. Threat to the Rocky Mountain Sculpin from non-point source pollution.
 8. Non-point source pollutionThreat Information
Threat CategoryPollutionExtent: Unknown
General ThreatAgricultural fertilizers and pesticidesOccurrence (Local): Unknown
Occurrence (Range-wide): Unknown
Frequency (Local): Seasonal
Frequency (Range-wide): Seasonal
Specific ThreatContaminant and nutrient loadingCausal Certainty (Local): Low
Causal Certainty (Range-wide): Low

Severity (Local): Unknown
Severity (Range-wide): Unknown
StressToxic effects, increased mortalityLevel of Concern: Low


Water quality in the mainstem also changes seasonally in response to flow augmentation, with increases in the total dissolved solids, conductivity and salt (sodium) concentrations when the diversion is shut off in the winter months (Milk River Watershed Council Canada 2008; W. Koning, pers. comm. 2006).


4.2.5 Accidental mortality

Scientific sampling may pose a threat to the Rocky Mountain Sculpin (Table 10). Sampling occurs range-wide, but only affects a small area of habitat in any given year. Most fish caught are released alive but some are killed intentionally or accidentally. The severity and level of concern attached to this threat are low because scientific sampling is regulated through the issuance of permits under section 73 of the Species at Risk Act (SARA) and, consequently, the potential for mitigation is high. The extent of mortality is generally prescribed within any approvals granted so as to ensure that the survival or recovery of affected populations is not compromised.


4.2.6 Climate change

Climate change has the potential to impact water availability, temperature, and a broad range of other issues (Schindler 2001), thereby affecting the availability and quality of Rocky Mountain Sculpin habitat (Table 11). The extent to which this might affect the species is unknown.

Table 10. Threat to the Rocky Mountain Sculpin from scientific sampling.
 9. Scientific samplingThreat Information
Threat CategoryDisturbance or persecutionExtent: Localized
General ThreatScientific samplingOccurrence (Local): Historic, current, anticipated
Occurrence (Range-wide):
Frequency (Local):
Recurrent
Frequency (Range-wide):
Specific ThreatFish sacrifice, accidental mortalityCausal Certainty (Local): High
Causal Certainty (Range-wide):
Severity (Local):
Low
Severity (Range-wide):
StressIncreased mortalityLevel of Concern: Low

Comments: The threat from scientific sampling, while range-wide, only occurs periodically and only affects small areas of sculpin habitat in any year. It is regulated by permit.

 

Table 11. Threat to the Rocky Mountain Sculpin from climate change.
 10. Climate ChangeThreat Information
Threat CategoryClimate ChangeExtent: Widespread
General ThreatClimate changeOccurrence (Local): Anticipated
Occurrence (Range-wide): Anticipated
Frequency (Local): Recurrent
Frequency (Range-wide): Recurrent
Specific ThreatAltered flow and temperature regimesCausal Certainty (Local): Low
Causal Certainty (Range-wide): Low

Severity (Local): Unknown
Severity (Range-wide): Unknown
StressIncreased mortality, reduced fitnessLevel of Concern: Low

Comments: None.


4.2.7 Natural processes or activities

Recurring conditions such as droughts and anoxia also have the potential to significantly impact Rocky Mountain Sculpin.

Drought

Southern Alberta is susceptible to extreme drought conditions, particularly during the summer and early fall. The impact of this threat to Rocky Mountain Sculpin will depend on the severity and duration of the drought. Overwintering habitat is the habitat most likely to be threatened (Table 12). Drought conditions in combination with water regulation and water extraction practices have the potential to reduce the amount and quality of sculpin habitat. The severity of the combined effect could be significant. In 1988 and 2001, for example, the surface flow of the Milk River east of Aden Bridge and upstream of the confluence with the North Milk River was virtually eliminated in the fall and winter due to severe drought conditions with the lower reaches of the Milk River reduced to a series of standing pools (WSC 2008b).

 

Table 12. Threat to the Rocky Mountain Sculpin from drought.
 11. DroughtThreat Information
Threat CategoryNatural processes or activitiesExtent: Widespread
General ThreatDroughtOccurrence (Local): Historic, anticipated
Occurrence (Range-wide): Historic, anticipated
Frequency (Local): Recurrent
Frequency (Range-wide): Recurrent
Specific ThreatLow water, anoxiaCausal Certainty (Local): Medium
Causal Certainty (Range-wide): Medium

Severity (Local): Moderate
Severity (Range-wide): Low
StressIncreased mortalityLevel of Concern: Low

Comments: Natural drought is an ongoing, intermittent threat to Rocky Mountain Sculpins in Alberta. Fish in the North Milk and Milk rivers are most vulnerable, as the St. Mary River has higher seasonal flows and more water is withdrawn for irrigation from the Milk River. Water withdrawals or regulation could increase or decrease the severity of impacts from future droughts.


Anoxia

Low dissolved oxygen levels during the winter could seriously impact the survival of Rocky Mountain Sculpin and other fish species in the lower Milk River. In January, oxygen concentrations under the ice in the lower Milk River can decline to 1.6 mg/L, perhaps due to oxidization by organic debris or inflow of anoxic ground water (Noton 1980; R.L.&L. Environmental Services Ltd. 2002). The sculpin population as a whole is unlikely to be threatened by anoxia as similar declines have not been observed further upstream in the North Milk River, where measured winter levels are at or above 8.4 mg/L (Noton 1980), or at a number of isolated pools in the lower Milk River, where March levels are at or above 10.2 mg/L (R.L.&L. Environmental Services Ltd. 2002b). This suggests that oxygen exchange in these reaches, where there is continuous flow and possibly open water, is adequate to support sculpin. Anoxia is unlikely in the St. Mary River and Lee Creek for the same reasons. This parameter should be evaluated in future winter habitat surveys to confirm this conclusion.

Introduction