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- Executive summary
- Background: Species information
- Background: Distribution and Population
- Background: Species' needs
- Threats: Overview and Threats assessment
- Table 2: Detailed threats assessment
- Threats: Habitat Loss and Pollution
- Knowledge Gaps
- Species Recovery
- References and Glossary
3.2.2 Habitat Loss/Degradation
Habitat loss, either through degradation or fragmentation, is a significant threat to the survival of western silvery minnow in the Milk River. A number of existing or potential activities related to water use contribute to this threat, including: 1) changes in flow regulation associated with the diversion canal, 2) canal maintenance, 3) water storage projects, 4) groundwater extraction; and 5) surface water extraction. Degradation of shoreline habitat and water quality associated with livestock use of the flood plain may also impact minnow habitat.
Changes in Flow Regulation Associated with the DiversionCanal
Diverting water from the St. Mary River has reduced the effects of drought in the Milk River and may have extended the availability of suitable summering habitat for the western silvery minnow further upstream than under natural flow conditions (Willock 1969). The net effect of this change on the population is unknown, since upstream habitat gains may be offset by downstream losses, and other aspects of the species’ life history may be affected. Increased water velocities due to flow augmentation might, for example, adversely affect the species’ reproductive success by increasing larval drift downstream into unsuitable habitats such as the Fresno Reservoir (R. Bramblett, pers. comm.). Winter flows in the Milk River are considered natural and despite frequent low flow conditions there is no evidence of stranding (T. Clayton, pers. comm.). The likelihood of stranding, however, could increase if the rate at which flows are ramped down increases.
The St. Mary Canal is in need of maintenance and re-construction, and proposed changes include everything from abandonment to significantly increasing its flow capacity (Alberta Environment 2004; U.S. Bureau of Reclamation 2004). Due to its poor structural condition, the canal is not operating at its design capacity of 24.1 m3/s (850 cfs=cubic feet per second) but at a capacity of about 18.4 m3/s (650 cfs). Simply bringing the structure up to design capacity would increase flows by almost 27%. In addition, Montana has proposed increasing flow capacity to 28.3 m3/s (i.e. 1,000 cfs) during the irrigation period, and possibly extending the augmentation period. In either case, increased flows could have major implications for channel morphology, particularly in the lower Milk River where banks are already highly susceptible to erosion during high flow periods in the spring and summer. These changes could threaten western silvery minnow spawning and rearing habitat by increasing water velocities and thereby the drift rates of eggs and fry (R. Bramblett, pers. comm.). Changes to the flow regime of the Milk River should be preceded by detailed studies to determine how the various options might affect river morphology and western silvery minnow habitat.
Unexpected problems associated with the ageing canal can lead to temporary or premature closure to allow for maintenance activities. This has led to two interruptions to flow during the augmented period over that past 30 years; both were emergency situations where the integrity of the canal was at stake (K. Miller, pers. comm.). One of these interruptions occurred in 2001 when the canal was closed in mid-August to allow for emergency repairs. Combined with the extreme drought conditions, this reduced the lower Milk River and much of the minnow’s habitat to a series of isolated pools from August until the spring freshet.
Dam Construction and Operation
Although 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, and continues to be investigated. In reviewing any future proposal, the potential impacts on the western silvery minnow will need to be thoroughly considered. Particular attention should be paid to any modification of the flow regime. Changes associated with irrigation and impoundment may be a significant limiting factor to the western silvery minnow (Pfleiger and Grace 1987; Quist et al. 2004). More information on western silvery minnow ecology is likewise required for assessing such project impacts.
Impoundments alter habitat types, flow regimes, sediment loads, microbiota and water temperatures, and may also increase the risk of species introductions (Quist et al. 2004). These changes often produce systems that are narrower, less turbid, less subject to fluctuations in temperature and flow, and less productive with less substrate movement (Cross et al. 1986; Pleiger 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. In a recent study of an impounded river system in North Dakota, significantly more western silvery minnows of a broader size range were observed in natural river segments compared to the moderately altered segments downstream of a large dam (Welker and Scarnecchia 2004). Impoundments have had significant cumulative effects on fish fauna in the western Mississippi (Cross et al. 1986) and lower Missouri watersheds (Pfleiger and Grace 1987). In systems that were historically turbid, impoundment led to a shift in species abundance that favoured fishes that were not characteristic of turbid water (Pfleiger and Grace 1987; Quist et al. 2004). Instream habitats also changed, with the fine substrate typical of large plains streams being replaced by gravel, cobble and boulder. The effects of winter flow augmentation on western silvery minnow, through the release of impounded water, are not known at this time.
The loss of connectivity associated with dams may be responsible for the decline and highly endangered status of the Rio Grande minnow (Cowey 2002; Alò and Turner 2005), and for the upstream extirpation of several other prairie minnow species that follow a similar semi-buoyant, broadcast spawning strategy (Winston et al. 1991; Pringle 1997; Platania and Altenbrach 1998). Elevated sustained flows from the upstream Santa Rosa Reservoir in the Pecos River of New Mexico, combined with the relatively short reach length (89 km) to the Sumner Reservoir, have likely resulted in semi-buoyant eggs of these species being transported downstream into unsuitable reservoir habitat (Platania and Altenbrach 1998). Habitats in the lower Milk River have been fragmented by the Fresno Dam in Montana and numerous diversion dams downstream. The Fresno Dam prevents western silvery minnow populations downstream from re-colonizing habitats in Canada. Augmented summer flows may also reduce the species reproductive success in the lower Milk River by transporting eggs downstream into unsuitable habitat in the Fresno Reservoir.
Loss of surface water flow to groundwater occurs naturally along a section of the Milk River from Black Coulee (MacDonald Creek approx. 8 km upstream of Aden Bridge) to approximately 3 km downstream of the Aden Bridge (Highway 880 crossing) (Grove 1985). Subsurface losses may also occur in the lower Milk River downstream of the park to the eastern border crossing, but these losses are probably not permanent except for evapotranspiration.
Linkages between groundwater and surface water flow may have implications for western silvery minnow and other small fishes, especially during winter, low flow conditions. Excessive diversion of groundwater during this time could affect western silvery minnow habitat. More information regarding the species’ overwintering habitats is needed to determine the significance of this threat.
Surface Water Extraction - Irrigation
While water extraction for irrigation could seriously reduce habitat available for western silvery minnow, the threat in the Milk River within Alberta is considered low, since only a small proportion of the available flow is withdrawn and these withdrawals are regulated. Extraction of water for irrigation purposes only occurs while flows are augmented, from late-March or early April through to early September or mid-October. During this period about 5% (15,000 dam3 = cubic decametres) of the total flow (292,000 dam3) is licensed for use in Alberta, most of which (93%) is used for irrigation (T. Clayton, pers. comm.). Water removals under temporary diversion licenses (TDLs) are not included in this total. When the diversion is closed for maintenance, or during reduced flow conditions, withdrawals for irrigation are terminated or suspended on a priority use basis. Alberta Environment has initiated installing water meters on all irrigation pumps drawing water from the Milk River (K. Miller, pers. comm.). These meters would measure water removal four times a day to provide an accurate and up-to-date measure of water withdrawals.
Surface Water Extraction - Non-irrigation
In contrast to water licenses for irrigation, Temporary Diversion Licences (TDLs) for non-irrigation purposes are issued throughout the year, 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 western silvery minnow 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 diversion is prematurely or temporarily closed down. Under such conditions some TDLs may be revoked, as they were during the drought conditions in 2001 (S. Petry, pers. comm.). During the flow augmentation period, the Town of Milk River diverts about 0.3% of the total available flow for domestic purposes.
Livestock Use of Flood Plain
The Alberta Riparian Habitat Management Society (”Cows and Fish") has been actively engaged in the issue of livestock management in the Milk River flood plain. Several riparian and grazing management workshops, involving many ranchers along the river, have been held. 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 (Lorne Fitch, pers. comm.). Several riparian benchmark inventories have been completed, but there has not been any follow-up monitoring to date. Demonstration sites have been established and have shown riparian vegetation recovery, especially with woody vegetation. Riparian recovery usually becomes evident in three to five years after the first management changes are made, and it may be ten years before significant physical changes can be measured.
The likelihood of point source and non-point source pollution entering the Milk River at levels that would threaten western silvery minnow survival is considered low. Point sources of pollution include any stormwater and sewage releases, as well as accidental spills and gas leaks particularly at river and tributary crossings. 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.) making both of these a minimal risk. However, the inadvertent 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 western silvery minnow would depend on the substance released, the location of spill, 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 2,700 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.). Contamination of water from seismic or drilling activities is also a possibility. 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 in the vicinity of the Milk River are limited mainly to the runoff of agricultural pesticides and fertilizers. Overall, this threat is considered low. Most of the approximately 8,000 acres of cropland that is irrigated in the Milk River basin 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.). 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.) 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.), but nutrient concentrations can become elevated at downstream sites such as the Highway 880 crossing (W. Koning, pers. comm.). 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 (W. Koning, pers. comm.).
Reduced dissolved oxygen levels during the winter could seriously impact the survival of western silvery minnow and other fish species. A water quality study by Noton (1980) concluded that the most important water quality parameter potentially not meeting fish needs in the Milk River was dissolved oxygen. In one of the five winters sampled, oxygen concentrations under ice in the lower reach of the river were as low as 1.6 mg/L in January. Possible reasons for reduced oxygen concentrations at this time included an accumulation of organic debris which might oxidize or the inflow of anoxic ground water during low flows (Noton 1980). Further evaluation is required.
3.2.5 Natural Processes
The preceding sections outline threats to western silvery minnow survival and habitat caused by human activities. Two natural processes, drought and climate change, also have the potential to significantly impact these fish.
Southern Alberta is susceptible to extreme drought conditions, particularly during the summer and early fall. The severity of this threat will depend on the severity and duration of the drought but overwintering habitat is the habitat most likely to be threatened. Drought conditions in combination with water regulation, canal maintenance and extraction practices significantly reduce the amount of summer and overwintering habitat available to the minnow. In 1988 and 2001, for example, the surface flow of the Milk River was virtually eliminated in the fall and winter due to severe drought conditions, and the lower river was reduced to a series of standing pools (WSC 2006). Natural drought conditions alone may seriously stress minnow populations, but the combination with other anthropogenic stresses could compound the severity of drought effects significantly.
Climate change has the potential to impact water availability, temperature, and a broad range of other issues thereby affecting the availability and quality of western silvery minnow habitat. The extent to which this might affect the species is unknown.
3.2.6 Other Threats
Scientific sampling may also pose a threat to the western silvery minnow. This threat is rated as low as it usually involves live-sampling and has a high potential for mitigation as it is regulated through the issuance of permits under SARA.
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