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Rocky Mountain Ridged Mussel (Gonidea Angulata)

Population Sizes and Trends

Clarke (1981) collected four individuals from Vaseux Lake, British Columbia (6 August 1972) and none were gravid. More recent collections of G. angulata have been restricted to Okanagan Lake and Skaha Lake; however, specimens have been limited to shells and it is generally believed that this species is declining and threatened by habitat degradation and loss. Currently there are two distinct, severely fragmented populations of G. angulta: 1 in the Okanagan River with approximately 8 locations and another in the Kootenay River with an undetermined number of locations.

Frest and Johannes (1995b) regarded this taxon as declining in numbers at regional sites in the western U.S. and made similar comments for Idaho populations (Frest and Johannes, 2001). Recent searches indicate that the species is absent from some streams in which it was found historically (e.g. the Yakima River) (TJF).

Gonidea angulata was recorded from 25 sites during a 1991 survey of some western U.S. states (Neitzel and Frest 1993) (Table 2). Out of more than 500 survey sites, only 25 were positive for this species (5%), with live occurrence (likely breeding populations) confirmed at only 14 (2.8%).

Between 1998 and 2001, Deixis Consultants of Seattle Washington collected G. angulata from a number western U.S. sites not included on Figure 3 which were scattered over the range of G. angulata (i.e., from southern California to the B.C.-U.S. border).


Table 2.  Recent records of G. angulata from the Columbia River drainage system in the western U.S. and the collection in which they are held (from Neitzel and Frest).

River system and site number from
Neitzel and Frest (1993)

DeixisOkanoganRiver 9, 13WA
DeixisDeschutes 7, (37)*, 38, (45), (49)OR
DeixisWillamette 8, (19)OR
DeixisOwyhee 1AOR
DeixisClearwater River, South Fork 22AID
Deixislower Snake River 14, (17), (18), (24), (25)ID
Deixismiddle Snake River 9, 10, (24)ID
Deixisupper Snake River 38ID
DeixisSalmon River (1). (3), 14, 70ID
DeixisWeiser 3ID

*Indicates sites where only shells were collected.


Table 3.  Other confirmed sites for G. angulata, 1988 – 2001.
Drainage/StateRiverStateNo. SitesDatabase Site No.
Snake River WY-ID-WASnakeID627, 433, 434, 500, 1147, 1148
 Crooked Ck.OR14424
Pit River, CA CA51039, 1111, 1265, 1610. 5189
Umpqua R. OR OR21488, 1593
Klamath River, OR-CAWilliamsonOR11860
 SpragueOR51895, 3142, 3145, 3147, 3149
 LostOR23120, 3122

The sites listed in Table 2 represent 24 sites out of approximately 2,500 visited where G. angulata was detected. All 24 sites likely represent extant populations. One of the authors (TJF) estimated that the population in the Lower Granite Reservoir consisted of only a few hundred individuals while populations in the middle Snake and John Day systems certainly range into the thousands or tens of thousands of adult individuals. Population density estimates were obtained by marking out the population by snorkeling and direct visual observation; then ¼ m2quadrats were distributed randomly around the population and direct counts were made (20-30 quadrats for each population). Total population size was inferred from the quadrat area results vs. the area occupied by the whole population (Okanogan, middle Snake, John Day). Because it can be difficult to identify the entire spatial extent of a given population, population estimates are given as ranges rather than precise figures. For the Lower Granite Reservoir, transects were walked along the dewatered bottom of the reservoir in relatively undisturbed areas (i.e., not disturbed by birds, mammals, or rodents) and direct counts were made along each transect. The total area of transects walked was determined, and an estimate of the population derived.