Demography of Muskrats on the Upper Mississippi River

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 backwater. Harvest mortality was largely compensatory, suggesting that lower backwater population densities were the result of past overharvest decline and directly related to long-term habitat degradation.