Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
Habitat loss/degradation
Habitat loss, either through degradation or fragmentation, is a serious threat to the survival of western silvery minnow in the Milk River.The Milk River Fish Species at Risk Recovery Team (2007) identified a number of existing or potential activities related to water use contributing to this threat, including: 1) changes in flow associated with the diversion, 2) canal maintenance, 3) water storage projects, 4) groundwater extraction, and 5) surface water extraction.
Southern Alberta is susceptible to extreme drought conditions; water diverted from the St. Mary River has reduced the effects of drought during the augmentation period (March to October) when water is not available for irrigation. There have been discussions of maintenance and re-construction of the St. Mary Canal system. Proposed changes run the gamut of options from abandonment to increasing the capacity (Alberta Environment 2004; U.S. Bureau of Reclamation 2004) of the reservoir and flow rates. Whatever the results of these discussions, any change in the flow of the canal system will undoubtedly impact the available habitat in the Milk River (Milk River Fish Species at Risk Recovery Team 2007). It is difficult to comment on the precise nature of such impacts associated given the uncertainty of the canal’s future. However, three likely scenarios can be expected depending on the change. Increased flows could further impact channel morphology where the river banks are already prone to erosion during the high spring and summer flows. Although increased siltation and turbidity arising from bank erosion might benefit the species, increased water velocities might threaten spawning and rearing habitat. Specifically, it is predicted that any increase of flow above the existing 650 cfs capacity of the canal will significantly reduce the likelihood that drifting eggs settle in suitable riverine habitat; settlement in the reservoir could effectively act as a population sink for Alberta’s minnow population (Milk River Fish Species at Risk Recovery Team 2007; Clayton, pers. comm. 2008). On the other hand, abandonment of the canal could, combined with extreme drought conditions, reduce the lower Milk River, and much of the species habitat, to a series of isolated pools in late summer, as happened twice over the last 30 years (Milk River Fish Species at Risk Recovery Team 2007). As the canal continues to age, the threat of structural failure increases, and repair rate will undoubtedly increase (Clayton and Pollard, pers. comms. 2008). The 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; Schindler and Donahue 2006). In particular, given that the Milk River is situated in one of the most arid regions of Canada, continuing trends in reduced snow pack in the Rocky Mountains suggest that the frequency of drought conditions will increase (Rood et al. 2005). These conditions will be acerbated by increasing water requirements for irrigation.
The feasibility of developing a dam on the Milk River upstream of the Town of Milk River has been, and continues to be investigated. The potential impacts on the western silvery minnow will need to be taken into consideration, particularly in regard to flow regimes. Changes associated with irrigation and impoundments may be a significant limiting factor to the western silvery minnow.
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). Elsewhere in the Great Plains, modifications to habitat, particularly those associated with impoundments, have become a serious limiting factor for the western silvery minnow (Cross et al. 1986). Impoundments have probably had the greatest cumulative effects on fish fauna of the western Mississippi Basin, including H. argyritis (Cross et al. 1986; United States Geological Survey 2002).
These impoundments alter habitat type, stimulate introductions of exotic species and alter flow regimes, sediment loads, and microbiota (small, often microscopic organisms), resulting in streams that are generally narrower, less turbid, less subject to discharge and temperature variations (Cross et al. 1986), and less productive. Such changes to streams have resulted in changes to habitat diversity, and several species have declined, including the western silvery minnow, as they are adapted to shallow sandy streams with widely fluctuating flows, high turbidity and extreme summer temperatures (Cross et al. 1986). Such species that were once abundant and widespread are now out-competed by pelagic planktivores and sight-feeding carnivores, including introduced salmonids (Cross et al. 1986).
Ground and surface water extraction
Groundwater and surface water are connected, but their relationships are complex. An ongoing study on these relationships is expected to be completed in 2008 (Clayton and Pollard, pers. comms. 2008). Grove (1985) found that there was a natural loss of surface flow to groundwater in the Milk River. During winter when low flow conditions can persist, excessive diversion of groundwater could affect the availability and quality of western silvery minnow overwintering habitat (Milk River Fish Species at Risk Recovery Team 2007). However, at this time no information on overwintering habitat exists. Currently, there is no licence requirement for groundwater extraction.
Surface water extraction for irrigation could reduce habitat in the Milk River, but the threat is considered low as only a small portion of the available flow is withdrawn as it occurs during the augmentation period and these withdrawals are regulated (Milk River Fish Species at Risk Recovery Team 2007). In contrast, Temporary Diversion Licenses (e.g. for oil and gas related activities) are issued throughout the year including critical low flow periods, although they can be (and have been) suspended under extreme low flow conditions, such as when the canal has been shut down for repairs. Western silvery minnow overwintering habitat may be vulnerable to this type of extraction at a time when low flow conditions are already in effect. In addition to loss of flowing water and physical habitat, reduced dissolved oxygen levels during the winter could seriously impact the survival of western silvery minnow and other fish species (Milk River Fish Species at Risk Recovery Team 2007). Noton (1980) concluded that the most important water quality parameter potentially not meeting fish needs in the Milk River was dissolved oxygen.
Water withdrawal for irrigation for farming and ranching is currently the 4th largest consumptive use of water in Canada, and over 70% of irrigation withdrawals occur in southern Alberta and Saskatchewan. Over 18,000 ha are served from the Milk River, part of the larger St. Mary irrigation district, servicing 210,000 ha of southern Alberta farmland (Great Canadian Rivers 2007; Schindler and Donahue 2006). Recent studies (Dash 2008) indicate that total water withdrawals have almost doubled since the 1950s, principally in response agricultural demands. Water levels in the Milk River aquifer declined by over 30 m between the 1950s and 1980s, and ongoing data collection indicates that water levels continue to drop.
Grazing/agricultural and urban practices
The Milk River is characterized by heavy silt load associated with continuous erosion of the surrounding grasslands and river banks (Willock 1968). Willock (1968) stated that the increased rate of erosion associated with channelization for irrigation and overgrazing could result in the decline or extirpation of the western silvery minnow from its Canadian range, and may be the reason for its extirpation in some rivers in the United States. Similarly, Trautman (1957) believed that the western silvery minnow, like its eastern counterpart, has a limited tolerance for suspended sediment. However, given its abundance in highly turbid waters, high sediment loads do not likely limit western silvery minnow distribution in Alberta. The silt content and/or channel type does appear to be correlated with differences in abundance in the lower Milk River versus immediately upstream near the town of Milk River. Upstream, where minnow abundance is relatively low, the Milk River is characterized by a single meandering channel having more runs, riffles, rapids, and lower turbidity (RL&L 2001) flowing through more erosion-resistant sandstone formations (Willock 1969b). Immediately downstream of this section the river is more characteristic of the braided, shifting sand bottomed Missouri River. There is no information available to compare silt loads over time for the Milk River (T. Clayton, pers. comm.).
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 at present (Milk River Fish Species at Risk Recovery Team 2007). 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 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 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. No such spills have been documented for the Milk River, but the possibility, although quite low, exists; 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 (Milk River Species at Risk Recovery Team 2007). 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, limited mainly to runoff of agricultural pesticides and fertilizers, are not considered to be of major concern. Most of the cropland irrigated in the Milk River basin is located within 50 km of the Town of Milk River, although there is another smaller area located upstream on the North Milk River near Del Bonita. Usually, crops in most areas cannot be grown within 400 m of the river because of the rough terrain along the banks, thus 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 (Milk River Species at Risk Recovery Team 2007). Nevertheless, nutrient concentrations can become elevated at downstream sites, and 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 Species at Risk Recovery Team 2007).
Natural processes
The Milk River, Alberta is situated in a geographic region that is subject to extreme yearly and seasonal weather fluctuations that are likely to be exacerbated by climatic change. This variability, in addition to anthropogenic influences on the river system, may be responsible for limiting the distribution and abundance of western silvery minnow.
Although Canada is considered to have abundant fresh water (Gleick 2002), this can be misleading because of regional variability in supply. Southern Alberta, for example, lying in the shadow of the Rocky Mountains has relatively low annual rates of precipitation, and is one of the driest parts of the country (Schindler and Donahue 2006). Additionally the area is subject to periodic drought and has been identified as a prime area for further environmental degradation resulting from global warming (MEA 2005).
Archaeological evidence (see Schindler and Donahue 2006) suggests that severe and long-lasting droughts (lasting several decades) are not uncommon to the western prairies. The droughts of the 1930s, and the more recent warmer temperatures and lower precipitation of the 1998-2004 period were mild compared to droughts of the 18th and 19th centuries. Despite the apparently milder historic conditions of the 20th century, the average annual evapotranspiration exceeded average precipitation (Schindler and Donahue 2006). Annual precipitation has decreased by 14-24% in the southern prairies since the 1980s, while at the same time the area has experienced warming of 1-4 º C, most of which has occurred since the 1970s.
Several researchers (Déry and Wood 2005; Rood et al. 2005; Barnett et al. 2005), have determined long-term trends in flows of the major rivers of the area, but the analyses do not reflect trends during seasons of peak water demand, i.e., the summer months of May through August, when agricultural and urban use is at a maximum. Needs of aquatic flora and fauna are also greatest during this period. Warmer water temperatures, lower oxygen levels and low flows adversely affect the colder water organisms that inhabit the rivers and reproduce in the spring or fall (Schindler and Donahue 2006). Although annual flows in major drainages of the southwestern prairies have shown modest declines during the 20th century (Déry and Wood 2005; Rood et al. 2005), Schindler and Donahue (2006) have demonstrated that current summer flows are 20-84% lower than they were in the early 20th century. The longer-term trend for many rivers in southern Alberta over the summer is “stressed” or reduced below natural levels (Alberta SOE 2008). Damming, water withdrawals, and increased warming are attributed as causes of the decline. Watersheds without dams and/or water withdrawals showed less decline (20-30%), while those where impoundments and large-scale water withdrawals were in place demonstrated larger declines (40-80%) depending on the scale of impact (Schindler and Donahue 2006). All of the major rivers flow through semiarid and sub-humid zones where average annual evapotranspiration exceeds annual precipitation. Support of agriculture in these regions depends on reservoirs that trap spring snowmelt from the eastern Rocky Mountains and only about 20% of the runoff is returned to the rivers (Schindler and Donahue 2006).
Most climate models predict further warming of 1-2º C and slight increases in precipitation by the end of the 21st century (CCIS 2007). The forecasted increases are much lower than the predicted increase of 55% in evapotranspiration due to rising temperatures. The southern prairies are likely to be much drier (Schindler and Donahue 2006), and there will be less snowmelt to capture in the reservoirs. As a result it may become increasingly more difficult to maintain current summer flow regimes, and fish habitat.
Species introductions
Species introductions could threaten the native fish fauna of the Milk River through various mechanisms including: predation, hybridization, competition for resources, the introduction of exotic diseases and parasites, and habitat degradation (Milk River Fish Species at Risk Recovery Team 2007). So far, the yellow perch is the only non-native species that has been observed in western silvery minnow habitat, but the Fresno Reservoir contains a number of introduced predatory species, including: rainbow trout (Onchorhynchus mykiss), walleye (Sander vitreus), yellow perch, northern pike and black crappie (Pomoxis nigromaculatus), as well as other introduced species such as lake whitefish and spottail shiner (Notropis hudsonius) (Montana Fish, Wildlife and Parks 2004). Spottail shiners have also been observed in the river section between the International border and the reservoir (Stash 2001). Some of these species have specific habitat requirements that may not be met in the lower Milk River; others are generalists that might expand into Alberta.
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 (Milk River Fish Species at Risk Recovery Team 2007).