PORTLAND – After 20 years of slow and steady growth, Maine’s loon population may have stopped rising.

The Maine Audubon Society’s annual summer count shows that the number of adult loons in the southern half of the state has remained virtually the same for the past three years. This year’s count also documented a drop in the number of loon chicks for the second year in a row.

Susan Gallo, a Maine Audubon wildlife biologist, said it’s not clear if the population has peaked or merely paused, or whether the numbers are cause for concern, she said.

The halt in population growth could mean the population has reached its natural peak. But it also could be an early signal that loons are under pressure from sprawl and pollution. A nesting loon needs about 100 acres on average, Gallo said.

“There’s certainly a question out there about habitat – how much habitat there is and whether it is full. Some people have speculated there isn’t room,” she said. “It probably is a combination of multiple factors.”

Maine Audubon has organized an annual loon count in Maine for the past 20 years, with an army of volunteers surveying lakes and ponds one day each July to monitor the state’s loon population. Maine has the largest loon population in New England.

Based on the count last July, Maine Audubon estimated there are 2,558 adult loons and 208 chicks in the southern half of the state. There are no recent figures for the northern half of the state, where the loon count is less intensive and periodic aerial surveys of lakes and ponds provide the best snapshot.

Biologists focus on loons because they provide a good indication of the health of their environment. Mercury that contaminates a lake will build up in fish and affect the behavior and reproduction of loons that eat them, for example.

“It’s very high on the food chain and pretty susceptible to environmental contaminants,” said Tom Hodgman, a biologist with the Maine Department of Inland Fisheries and Wildlife.

The results of the latest annual census may show a real shift, but are not cause for alarm, Hodgman said.

“It’s been a general feeling amongst folks who work with loons that there is an abundance of loons and shortage of territories,” he said. “You’re seeing the first indications probably that things are starting to level off.”