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3.1 Overview

A number of threats to the western silvery minnow have been identified throughout its range, including those believed to be responsible for its extirpation from some systems. 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 and domestic use), impoundment, bank stabilization, channelization, and flow augmentation.  Habitat alterations, particularly the reduction in seasonal fluctuations in discharge and declines in turbidity related to channelization and impoundment, have been correlated with the precipitous decline of the western silvery minnow in the lower Missouri River (Pflieger and Grace 1987). 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 minnow’s existence.

In Canada, COSEWIC identified continuous erosion and siltation as a threat to the western silvery minnow.  Upon more detailed evaluation, the Recovery Team has determined that under current conditions, this is a natural occurrence in prairie streams and one to which the minnow has likely adapted.  However, changes to water flow resulting in habitat loss and degradation can pose a significant threat to minnow habitat.  The following sections summarize these and other sources of threats to the species’ survival and habitat.

3.2  Threats assessment

The Recovery Team undertook a detailed assessment of threats to the species based on both published information and local knowledge.  Four primary categories of threat were identified:

  • species introductions,
  • habitat loss/degradation,
  • pollution, and
  • natural processes.

A brief description of the methods and assessment of threats to the western silvery minnow is provided in Appendix A.  The results are discussed below and summarized in Table 2.

3.2.1 Species Introductions

Introduced species can threaten native fish fauna through various mechanisms including: predation, hybridization, competition for resources, the introduction of exotic diseases and parasites, and habitat degradation.  To date, yellow perch and walleye are the only introduced species that have been observed in the lower Milk River where the western silvery minnow occurs (T. Clayton and D. Watkinson, pers. comm.). Further downstream, the Fresno Reservoir contains a number of introduced predatory species, including: rainbow trout (Onchorhynchius mykiss), 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) (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).  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.  Given that there are no migration barriers upstream of the Fresno Reservoir in Montana, and that illegal fish transfers within the province can be difficult to control, the Recovery Team rated the likelihood of this threat occurring as moderate.

Fishes such as the western silvery minnow have adapted to the highly variable natural flow conditions and elevated turbidity that characterize the native prairie streams they inhabit. Elevated turbidity levels have less effect on the prey consumption of plains fish species adapted to turbid conditions than that of species not adapted to turbid conditions (Bonner and Wilde 2002). Activities such as water regulation and impoundment that alter these flow regimes and trap sediments, reducing turbidity downstream, can favour sight-feeding exotic piscivores such as bass, perch and salmonids, which historically were absent from these streams (McAllister et al. 2000; Quist et al. 2004).  Consequently, these activities may alter the faunal community and dynamics by encouraging the establishment of introduced species (e.g., northern pike) or by increasing the abundance of native predators that currently exist at low levels (e.g., sauger).

The Alberta Fish and Wildlife Division does not plan to introduce sportfish species into the lower Milk River, and is unlikely to do so in the future (T. Clayton, pers. comm.). The Milk River proper and its tributaries in Alberta 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.). Whether unauthorized introductions have occurred in the Milk River (e.g., bait fish releases) is unknown. 

The significance of possible species introductions is unknown at present but would depend upon the species introduced.  Under the worst case scenario, an introduced species could have serious implications to the survival of the western silvery minnow. The creation of reservoirs can raise interest in stocking non-native sportfish for recreational fishing, and might facilitate the introduction of these species into habitats up and down stream.