Reproduction, Size, and Mortality in the Rocky Mountain Muskrat

Abstract

Although the muskrat has received considerable investigative attention throughout the United States and Canada in recent years, few studies have dealt with the Rocky Mountain muskrat, Ondatra zibethicus osoyoosensis (Lord), the endemic subspecies of the Rocky Mountain and Puget Sound regions. Two independent but similar studies at Gray's Lake and Dingle Swamp in southeastern Idaho afforded opportunity to obtain data on reproduction, sex and age ratios, subadult mortality, and size of the Rocky Mountain muskrat. A comparison of aging validity by external examination of reproductive organs (Baumgartner and Bellrose, 1943) and pelt primeness pattern (Applegate and Predmore, 1947; Shanks, 1948) with the internal examination of reproductive organs (Errington, 1939) was also undertaken on each area. Field work at Gray's Lake was conducted by Williams from June, 1949, to May, 1950, while a graduate student at the University of Idaho and at Dingle Swamp by Reeves from April, 1952, to December, 1953, while employed by the Idaho Fish and Game Department as leader of Pittman-Robertson Project W 106-R. The study areas .—Gray's Lake and Dingle Swamp remain two of Idaho's major muskrat producing marshes despite adverse environmental conditions created by irrigation water storage developments. Harvests of up to 50,000 pelts annually were formerly reported from each area, but recent harvests have seldom exceeded 10,000. Gray's Lake, a 22,000 acre marsh, is located in a shallow, ancient lake basin at an altitude of 6,386 feet in Bonneville and Caribou Counties. Emergent aquatic vegetation covers an almost solid expanse of 19,800 acres and is chiefly hardstem bulrush ( Scirpus acutus ) with a minor interspersion of common cattail ( Typha latifolia ). Except for small scattered potholes and channels, the open water area is concentrated in the southeastern corner of the marsh. Prior to 1924, excess runoff flowed from the marsh through a natural outlet. However, …