By Eric Freedman
Muskrats are less abundant in coastal wetlands directly connected to northern Lake Ontario than those in wetlands separated by unbroken barrier beaches from the lake.
And regulation of the lake’s water levels may be detrimental to a muskrat population that’s valuable both to fur trappers and the ecosystem.
Those findings by the Ontario Ministry of Natural Resources and Forestry have implications for the International Joint Commission’s new water regulation plan for the Lake Ontario-St. Lawrence River system.
Foraging and house-building by muskrats — nicknamed “ecosystem engineers” — are environmentally important because their activities “can drastically alter vegetation communities,” according to a new study published in the journal Wetlands. They can have a positive impact on the diversity and abundance of waterfowl and shorebirds in wetlands, and they eat invasive zebra mussels.
Muskrat harvests in eastern North America have plummeted by 75 percent during the past three decades, with a negative impact on Canada’s fur industry, the study said. Muskrat pelts, often used for clothing and fashion, account for 28 percent of fur exports by the $1 billion (Canadian)-a-year industry.
In addition to directly affecting muskrat abundance, regional water level control by the IJC has caused changes in wetlands vegetation, the study noted.
For the study, staff from the provincial ministry and several local conservation authorities surveyed 43 coastal wetlands along Lake Ontario’s northern shore between Hay Bay near Napanee and the Rouge River in Scarborough. They found muskrat houses in 19 of those wetlands.
The project incorporated aerial surveys and historical data for some of the wetlands.
While low water levels don’t seem to harm individual muskrats’ growth or reproduction rates, their population is less dense in “wetlands subject to regional control resulting in lower water levels,” the study said.
Low water levels can harm muskrats in several ways, such as impeding their ability to enter and exit their sub-surface houses, thus “leaving them vulnerable to predation,” the study said. Muskrats that construct their houses in shallower water have a higher winter death rate, while low water levels can make it more difficult to travel by water to find food.
Study co-author Jeff Bowman said muskrats, which are “highly fecund and can reproduce multiple times in a year,” are well-adapted to annual fluctuations, including drought and cold winters.
But the longer-term adverse impacts come from a reduced amount of wetlands habitat as the regulated water levels cause the shoreline to “harden” and invasive cattails to thicken, according to Bowman, a research scientist with the Ontario ministry and an adjunct professor in environmental and life sciences at Trent University.
The study said the new IJC regulation plan “more closely mimics historic water-level fluctuations” and may lead to “a renewal of open water patches and increased diversity in coastal wetland vegetation, which may result in more suitable habitat for muskrats.”
The binational IJC’s new plan took effect Jan. 7, replacing its original 1958 regional water regulation plan.
IJC public information officer Frank Bevacqua said the new plan “reduces the occurrence of extreme high and low water levels. It has more variability in between, which more closely represents the normal water level fluctuations,”
Under the new plan, there will be less severe water level drawdowns during the winters, he said.
“That will make conditions better for the muskrat populations,” Bevacqua said.
Bowman said the study will be useful in evaluating the effectiveness of changes in the regulation plan, adding that muskrat houses could be resurveyed in 10 years to assess the revised plan’s impact.
More muskrat research also is necessary, Bowman said.
“One thing we’re curious about and still studying is what are the effects of the invasive cattail and phragmytes on muskrats. We think phragmites will be detrimental because there seems to be less abundance in areas with lots of phragmites,” he said.