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


Habitat requirements

Gonidea angulata lives in freshwater habitats (lakes, streams, creeks, rivers) and in a variety of substrates ranging from gravel to firm mud. Presence of at least some fine material (e.g. sand, silt or clay) seems to be required, although very young specimens were noted in 1991 from coarse plant detritus at the mouth of the Okanagan River (= Okanogan in the U.S.). Sites preferred by this amniphilic (“stream/river loving”) species generally have constant flow, rather shallow water (typically < 3 m in depth), and well-oxygenated substrate, especially when occurring in finer substrates. Most known sites seem to lack macrophyte beds, although nearby stones may have a dense periphyton community. Mussels are typically found partly buried in finer substrates, often to at least half their length, and with the posterior end directed upstream. In coarser substrates, such as some Okanogan sites, mussels may be almost completely buried, with only the posterior end protruding slightly from the substrate. Generally, specimens of all non-larval age classes are found together in the same habitat. However, in exceptional cases, as near the mouth of the Okanogan River, only juveniles are found. In this area, larger mussels are likely to suffocate, so the dominance of juveniles at a particular site should be regarded as quite atypical. In many sites where G. angulata was recently collected (Table 2), only large adults were found.

In Vaseux Lake near Oliver, British Columbia, large specimens occur in muddy sand at a depth of 0.6 – 0.9 m along the shoreward edge of Potamogeton (pond weed beds). Elsewhere, G. angulata has been collected in streams ranging in size from large creeks (e.g., Toppenish Creek, a tributary of the Yakima River, Washington) to large rivers (e.g., lower Columbia River, Washington / Oregon). Most specimens collected have occurred at depths ranging from 0.2 – 3 m; but some specimens have been dredged alive from the Little Granite Reservoir (Washington) from depths of approximately 10 m and from the lower Columbia River (Oregon / Washington), from depths of approximately 20 m. This species is occasionally found in impoundments in which there is some continual flow. For example, G. angulata occurs in some river-lake systems such as Osoyoos Lake British Columbia (Taylor 1993) and the Little Granite Reservoir and lower Snake River, Washington (Frest and Johannes, 1992; this population is now believed extinct). Populations occurring in these systems appear to be sparse, consisting only of widely scattered adults, and rarely show well-rounded demographics. The species seems to avoid nutrient-rich waters, although some populations in the middle Snake River, Idaho, have been able to withstand considerable nutrient enhancement locally. However, some streams in the historic range may have originally lacked this taxon. Current distribution in southwest Washington and northwest Oregon is very sporadic despite persistence of seemingly good habitat. Regionally, the species appears to be quite uncommon, even if locally abundant.

Gonidea angulata prefers areas with stable habitat conditions and appears to avoid areas with shifting substrates, periodic dewatering or extreme water level fluctuations, or with seasonal hypoxia or anoxia. At some sites, turbidity changes markedly over a normal water year, but the species seems to be absent from streams with continuously turbid water, such as glacial melt water streams (e.g., the White Salmon River, Washington; the Hood River, Oregon) while being present in nearby streams that are only seasonally turbid, such as the Deschutes and John Day rivers of Oregon (Frest and Johannes, 1992).

In British Columbia, the Okanagan River has been significantly modified from a meandering river to a canal-like waterway with in-river flood control devices. These modifications have greatly reduced salmonid runs in the river and have altered bottom sediments. These in-river modifications have likely altered suitable G. angulata habitat and the greatly reduced salmonid (suspected hosts) runs may have interfered with glochidial dispersal. Given the reduced salmonid population in the Okanagan River and the instream barriers that exist, it is likely that upstream dispersal is severely hampered or impossible while downstream dispersal likely occurs as a result of the movement of fish downstream. In the U.S. and Canada, the Okanagan River is dammed creating a physical barrier to fish passage and effectively reducing upstream dispersal from U.S. to Canadian populations through glochidial movement via a fish host.



In British Columbia it is thought that G. angulata is restricted to very specific habitats in the southern Okanagan south of Penticton; however, wide-ranging searches for this species have not been conducted. Due to the limited amount of information available on its actual distribution and occurrence it is not possible to comment directly on the population status of this species; however, due to threats to its habitat, there is reason to suspect that G. angulata is habitat limited and declining. For example, G. angulata has a requirement for cold clear, oligotrophic waters (G. Mackie, pers. comm.) and only approximately 10‑20% of southern B.C. has lakes and/or streams that fit these requirements (M. Gaboury, pers. comm.).  As such, it is unlikely that that this species can/will increase its current range due to the limited availability of suitable habitat.

Within its historic range, G. angulata is presently scattered in distribution and must be presumed to be declining. In the middle Snake River, Idaho, finds of dead shells greatly outnumber living individuals and sizable stretches of the river now lack living specimens. Furthermore, some waterways from which living specimens have been reported (e.g., the Yakima and Wenatchee Rivers, Washington) also now lack living specimens (T.J. Frest, pers. obs.).  Some rivers within the range of G. angulata may lack historical records due either to absence of early collections or lack of the species during historical collections, so that modern absence may not reflect change from historical status (e.g., Sanpoil River, Washington). Frest (pers. obs.) has noted, in the Okanogan River in Washington and elsewhere, many populations in which recent recruitment seems not to have occurred, suggesting degradation from former status. Frest and Johannes (2001) termed it “locally common but decreasing” and suggested careful monitoring of extant populations for eventual protection in the state of Idaho.



Land ownership in the vicinity of known G. angulata sites in southern British Columbia varies from private to provincial, with the most recently encountered site on Park Rill Creek occurring on public property; however, only a dead specimen was found. In the U.S., areas occupied by G. angulata are on public land administered by the federal government (U.S. Department of the Interior, Bureau of Land Management or U.S. Department of Agriculture Forest Service).