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COSEWIC assessment and status report on the Rocky mountain ridged mussel Gonidea angulata in Canada

Limiting Factors and Threats

This taxon is declining, both in terms of area occupied and number of sites and individuals. In British Columbia, species such as G. angulata are not threatened by direct exploitation, but by loss or degradation of their habitat. In general, unionid mussels are very sensitive to environmental changes and consequently the group contains a high percentage of endangered species in North America, including Canada (G. Mackie pers. comm.). In British Columbia, changes to the physical and chemical composition of water bodies where these mussels are known to occur can adversely affect a given population. For example, increased siltation or significant changes in water temperature as a result of natural resource extraction or human development can negatively affect the persistence of this mussel. In other areas, such as in California, extensive diversion of rivers for irrigation, hydroelectric power generation, and water supply projects has greatly reduced the range of this species.

In British Columbia, the Okanagan River is likely to undergo significant realignment in the near future. The long-term goals for the realignment are to re-establish meanders, oxbows, wetlands, and marshes along the Okanagan River and to replace salmonid habitat. In-river treatments would include the removal of in-river flood control devices and the establishment of pools, riffles, quiescent waters, and other salmonid-related habitats required for the fulfillment of salmonid life requisites. In the long-term, this realignment could prove to be beneficial for G. angulata in terms of greater potential for dispersal and an increase in suitable habitat. However, the short-term risks are potentially significant and activities associated with in-river construction and structure placement/removal could seriously affect existing G. angulata populations.

Outside British Columbia, much of the middle Snake River in Idaho is rapidly becoming eutrophic, due to agricultural runoff, fish farms, and urbanization along the river corridor. Much of the river is impounded behind a series of small dams; this is also detrimental for cold-water species such as G. angulata. The area has been declared “water-quality limited” by the Environmental Protection Agency (EPA) and the State of Idaho. Fine sediment influx, generally from sources listed above, is also a major problem. A recent (1994) landslide adjacent to the Snake River adversely affected some historic sites. For discussion of threats to Interior Columbia Basin, California, and Idaho sites for this species, see references under Frest and Johannes (1999). In the lower Columbia River region threats to G. angulata include impoundments, continued siltation, eutrophication and other impacts on the few remaining sites with habitat characteristics approximating pre-impoundment conditions.

Pesticide and herbicide runoff from orchards and other agricultural operations, as well as runoff from lumber mills, and has led to increased nutrient loading in the Okanogan Physiographic Province of Washington State. Because unionids are filter feeders, they are commonly vulnerable to pollution from transition elements and heavy metals. The spread of the non-native Corbicula and zebra mussel Dreissena are also concerns. For discussion of threats to unionids, see Williams et al. (1993), Williams and Neves (1995), McMahon and Bogan (2001) and Dillon (2000).

Although it is uncertain which species is (are) the glochidal host for G. angulata, it is important to note that decline of fish populations in the lakes, rivers, and streams where G. angulata occurs could have detrimental effects on the distribution and occurrence of G. angulata. For example, the decline of a host fish species could lead to the decline of G. angulata populations resulting from limited dispersal of glochidia.