Northwoods loon population in decline

New study suggests poor water clarity to blame

By Eileen Persike

MMC Staff

NORTHERN WISCONSIN – A study published this month in the journal Ecology connects water clarity in northern Wisconsin to survival and reproduction of the common loon.

The clearer the water, the better for one of the Northwoods’ most treasured birds.

A recent study suggested climate change could push the breeding range of the common loon hundreds of miles north in the next 30 years, possibly out of the U.S. entirely by 2080.

Studying loons in Oneida, Vilas and Lincoln counties since 1993, The Wisconsin Loon Project began with a focus on territorial and nesting behavior of loons. But findings such as these and others have led the project to look more closely at loon conservation.

“I started to notice it seemed the males were not as heavy the past five to 10 years as they were (10 to 25) years ago,” said Walter Piper, a professor of biology at Chapman University in Orange, Calif., and founder of the Wisconsin Loon Project. “And that also seemed to be the case for chicks. A lot of chicks seemed emaciated. This spawned the (new) study.”

Walter Piper, founder of The Wisconsin Loon Project, is pictured banding and weighing loons on a Northwoods lake. Photo by Rosemary Pugh

The study, entitled “Climate change-associated declines in water clarity impair feeding by common loons,” looked at water clarity and how it is affected by climate, and water clarity’s impact on the body mass of loons.

To take a step back, a 2020 paper authored by four researchers, including Piper, found the adult loon population in the Northwoods declined by 22% over the previous 27 years .The numbers of chicks and chick body mass declined “significantly,” and the population of young loons who do not yet reproduce fell by 46%.

There was also a noticeable decline in the number of chicks hatched each year.

“It used to be half would have two chicks, and these days that’s the exception,” Piper explained. “Today, 70-80% of the pairs have only a single chick. That was another warning sign.”

But perhaps the single most worrisome thing the 2020 paper found, Piper said, is the rate at which two to four-year-old adults come back. In what he calls “the roaring 90s and ‘00s,” researchers would see half of the loons banded as chicks come back as young adults. Not so these days.

“That is to say we would expect to band maybe 60 juveniles, and we’d see close to 30 of them back as adults in the old days,” Piper said. “Now we band 60 of them and we see 10 come back. It’s really that level of decline. It’s a really striking and alarming decline and we don’t have any idea what could be causing it.”

Lauryn Linsell and Jessica Ciske, field researchers for The Wisconsin Loon Project, work on banding loons. Photo by Walter Piper.

2024 research

Among countless potential environmental factors in the falling loon population and, more specifically, the declining mass of chicks, Piper considered that loons’ food sources had grown scarce.

Though measuring the fish population over the course of decades would be difficult, finding a large historical database of water clarity was not.

Piper reached out to Kevin Rose, an associate professor at Rensselaer Polytechnic Institute in New York who studies water clarity using remote sensing satellite data.

“I could look at mass data of chicks and adults and say, ‘Okay, well I know how old this chick was, and I have his mass, and now I can look at what the water clarity was … for that specific lake in that specific month where we were able to capture and weigh the chick.’”

 They found short-term water clarity is strongly correlated with chick mass: The clearer the water, the heavier the chick.

“So there are, I suppose, a few possible explanations, but the most obvious one, since loons are visual predators, they are chasing fish underwater and feeding their chicks,” Piper said. “If the water isn’t clear, that makes it difficult for loons to find food for chicks, and the chicks suffer as a result.”

Most loon chicks hatch in June, making July water clarity central to their survival.

“Chicks are very young in July, and that’s a critical month of growth for them,” Piper said. “So if the water is cloudy during the time their need is dire, then that hits them pretty hard.”

Adult mass also falls, he added, but not as dramatically.

They found that water clarity in the 127 study lakes declined steeply from 1995-2021. They found that water clarity fluctuates seasonally and in the short term.

But why?

The researchers looked at rainfall and temperature, noting a “tight correlation” between rainfall and water clarity. The question is: What is rain washing into the lakes? It could be dissolved organic material, fertilizer, sediment or a phytoplankton bloom. This is one of the topics Piper said will be researched in the next study.

“This is scary stuff for me and a lot of us up there,” Piper said. “We want to make something positive of it. We want to figure out what’s really going on. The rainfall and the temperature – we can’t do anything much about in the short term. Climate change is happening and it will continue to happen, but if it’s just fertilizer washing in from people’s lawns, then that’s something we could potentially fix and I think, if people understood the loons are being hurt by stuff that’s on their lawn, they would maybe say, ‘You know what? I can do better.’”

Piper said his hope is that whatever is causing a lack of clarity in lakes is fixable, so they can offer recommendations in the future and save the loon population in the Northwoods.

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