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Muskrat CSI

What’s going on with muskrat populations?

Andy MacDuff and Scott Smith, Wildlife Biologists

pelt_patterns.jpgIf DEC biologists and technicians had a nickel for every time they have been asked that question, they would have a lot of nickels! One thing is clear—muskrat populations appear to be a fraction of what they were in the past. Theories abound as to why, with habitat loss, invasive plant species, increased predation and contaminants being common explanations. At this point, the driving forces behind this downward trend are not clear, but DEC staff, in cooperation with New York trappers, have been investigating factors that may be contributing to the decline.

Whenever populations decline, reproductive success and survival rates are often of primary concern. If fewer young are being produced and surviving over time, the population cannot persist at a stable level and will contract. With this in mind, during the 2008-09 trapping season, members of the Furbearer and Small Game Mammal Management Team reached out to fur trappers for voluntary assistance in collecting data on muskrat reproduction. Biologists sought trappers willing to allow access to their finished pelts from fall-caught muskrats and to provide carcasses from female muskrats trapped any time during the open season.

Muskrats have unique traits enabling both age and sex to be determined by examination of the finished pelt. Age can be determined by the patterns on the back of the pelt. Juveniles have an alternating light- and dark-striped pattern, and adults have a blotchy pattern of light and dark areas. Sex can be determined by viewing the belly side of the pelt for evidence of genitalia. Through these observations, data from hundreds of muskrats can be gathered quickly and provide a glimpse into sex and age ratios of the muskrat population.

not_uterYOU_but_uterUS.jpgThe submission of female muskrat carcasses enables biologists to collect data on reproduction by examining the uterus for scars. The uterus is removed from the carcass, placed on a light table and the scars counted. These scars indicate the previous presence of a muskrat embryo. Average uterine scar counts range from 12 to 14, consistent with data collected during the 1980s in New York. The highest number of scars counted from a single female was a whopping 37! So what have we learned? Over the previous five trapping seasons, age and sex ratios have been consistent, with about 10% adult females, 13% adult males, 31% juvenile females and 46% juvenile males in the population.

Biologists are continuing to refine and analyze the data collected from these efforts, including comparing high quality or primary habitat (e.g., large open marsh complexes) versus lesser quality or secondary habitat (e.g., forested or rocky streams) to determine whether significant differences exist in reproduction. Preliminary analyses suggest muskrats in secondary habitat have a lower reproductive capacity compared to those in primary habitat; however, this difference is not statistically significant.

We may know more about the role of habitat in muskrat declines after data collection and analyses are complete. It is encouraging that data collected from these recent efforts are similar to data collected during the 1980s. However, this suggests other factors are at the core of the decline in muskrat populations, and the search for a cause must continue. Plans for an investigation into “kidney spots,” including what causes them and their potential correlation with changes in muskrat populations, have been discussed by DEC’s wildlife biologists.

One thing is certain—without the voluntary assistance of numerous trappers, this study would have been difficult if not impossible to complete. DEC thanks all trappers who provided carcasses and granted access to their finished pelts during the course of this study.


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