U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version

October 2005


Published by International Biodeterioration & Biodegradation 52 (2003) 101-106.


The native range for muskrats (Ondatru zibethicus) includes much of North America, but they also have been introduced beyond their native range, including into the Fall River, California, where they have come into conflict with human interests. An easily applied method to assess their abundance is an important need for their management. We developed a muskrat visual index (MVI) to provide the information necessary to address this need. Observations were made at randomly located sites along the river. The number of muskrats observed during a 45 min period was recorded during the late afternoon peak activity time at each site on multiple days. The mean number observed over sites was calculated for each day. The index was the mean of the daily means. These design and measurement methods present valuable advantages over most traditional muskrat indexing methods in this environment. Traditional methods usually involve counting burrows or houses. However, in a relatively stable environment such as along the Fall River, muskrat burrows and houses tend to be long-lasting structures, making acute changes in population difficult to detect by there methods. Examining these structures for activity can be time-consuming and labor-intensive. Of particular importance, the statistical properties inherent to the MVI data structure permit calculation of standard errors, confidence intervals and statistical tests allowing quantitative comparisons among MVI values. Development of a management program for muskrats on the Fall River will require understanding of muskrat population fluctuations and densities, as well as knowledge of the effectiveness (short- and long-term) of control strategies. Here we develop a useful method, derive its statistical properties, and present baseline information for managing muskrats along the Fall River.