Mortality Rates of Beaver in Newfoundland: A Reply

Abstract

Muskrat (Ondatra zibethicus) populations were studied in backwater and open-water habitats of Pool Nine of the Upper Mississippi River to determine density, natality, survival, and harvest rate. Density in backwater habitats ranged from 0.3 to 3.7 ? 0.7 adults/ha, which was lower than open-water densities, which ranged from 1.0 ? 0.1 to 9.3 ? 1.3 adults/ha. Litter sizes (7.1 ? 0.2 young/litter) and numbers of litters (1.5-2.0 litters/female/year) were similar in each habitat. Monthly adult breeding-season survival in open-water habitats was 0.86 ? 0.06 in 1981 and 0.82 in 1982. Monthly summer survival for juveniles throughout Pool Nine was 0.80 in 1981 and 0.87 in 1982. The monthly overwinter survival rate in both habitats was 0.82 ? 0.04. Harvest rate averaged 0.79 during the 1981 trapping season; 0.92 in open-water habitats compared with 0.47 in backwaters. Harvest mortality was largely compensatory, suggesting that lower backwater population densities were the result of past overharvest decline and directly related to longterm habitat degradation. J. WILDL. MANAGE. 49(4):883-890 The importance of muskrats as a fur resource of the Upper Mississippi River (UMR) requires that adequate harvest management plans be adopted to ensure sustained recreational and commercial harvest. Demographic data are available for muskrat populations inhabiting inland marshes (Errington 1963), in coastal marshes and estuaries (Smith and Jordan 1976, Smith et al. 1981), on small streams (Errington 1937), and on moderate-sized rivers (Brooks 1980). There is little information on population density, survival, natality, and harvest of musk'Present address: Ducks Unlimited Canada, 1190 Waverley Street, Winnipeg, Manitoba R3T 2E2, Canada. This content downloaded from 157.55.39.181 on Thu, 29 Sep 2016 06:19:38 UTC All use subject to http://about.jstor.org/terms