Tom Hardisky believes he knows the reason why muskrats have been on the decline in Pennsylvania.
Problem is, there might not be a solution.
Muskrat populations have declined over the last 25 years and Hardisky, who is a furbearer biologist with the Pennsylvania Game Commission, has studied a number of factors in an effort to determine the reason and if the trend could be reversed.
Disease, predation, parasitism and climate factors have all had an impact to varying degrees, but none is significant enough to result in an overall decrease of the muskrat population.
Disease can be an issue, Hardisky said, but only when populations are high. Parasite problems in muskrats are generally dictated by age, gender and habitat conditions and it isn’t a significant source of mortality.
Because muskrats are a food source for many predators, most mortality is the result of natural predation, Hardisky said. When high predation occurs from species such as mink, raptors and raccoon, it’s likely the result of poor habitat conditions.
“We tend to put total blame on predators for declines of prey populations,” Hardisky said. “However, we need to avoid treating symptoms and focus on root causes.”
That root cause, he suspects, is cleaner water. Muskrats thrive in aquatic environments — swamps, marshes, ponds and streams — with rich plant growth that serves as a food source and cover. Decades ago when strict regulations weren’t yet enacted, more nutrients entered the water and sparked the growth of aquatic plants. Water courses near farms comprised high-quality muskrat habitat at the time, Hardisky said.
But as the water entering swamps, ponds and other aquatic environments became cleaner, plant growth became less lush.
As a result, what was once high-quality muskrat habitat is lost.
“Prior to the 1980s, nutrient flushing was common in streams and rivers when human and livestock waste entered these water systems. Muskrats did very well under these conditions,” Hardisky said. “Muskrats disappear from these types of wetlands when nutrients are used up. Ponds and wetlands in agricultural areas that occasionally receive nutrients from manure runoff often have sustained muskrat populations.”
So is it possible to bring back muskrat populations when water pollution actually helped them thrive? Hardisky likens the muskrat decline to what happened to wild pheasants and bobwhite quail decades ago when farming practices changed the agricultural landscape in the state.
“There’s no easy fix,” Hardisky said. “The importance of managing nutrient loads in water systems to benefit muskrats should not be underestimated. The lack of nutrient flushing may be the root cause of the muskrat decline in Pennsylvania and throughout its North American range.”
Hardisky isn’t advocating for the intentional discharge of nutrients into waterways to create muskrat habitat, but he said there may be some smaller wetlands that could be managed for muskrats by controlling the nutrient flow into the water.
How much nutrients are needed and how to deposit them into the water remains a dilemma, he said.
Despite the challenge, Hardisky said there are enough muskrats remaining throughout the state that the population isn’t in danger of becoming threatened or endangered.
At least not yet.
“At this point I’m not too worried,” Hardisky said. “We’re keeping an eye on this. When we start seeing large areas devoid of muskrats, that’s a sign that they’re not holding on.”
There is also no reason to shorten trapping season for muskrats, he said, because the activity hasn’t increased to a level that would contribute to a population decline. If the regulated harvest of muskrats through trapping was eliminated, Hardisky said mortality from other factors would likely increase.
While the habitat changes from cleaner water, it’s possible that muskrats will continue to be found primarily in farmland areas and occur less in swamps and ponds in a forested setting.
In fact, muskrats might one day be considered as strictly a farmland wildlife species.
“That’s probably what we’ll have to live with,” Hardisky said. “I’m just hoping we can hang on to what we have.”