The woods of Maine host the greatest abundance of fishless lakes in the Northeast, but their abundance is declining due to state-sanctioned fish stocking for recreational anglers and illegal releases of fish by others. When fish are introduced, not only does the lake ecosystem change dramatically but the surrounding upland community can, too.
According to University of Maine researchers Cynthia Loftin and Emily Schilling, lakes that lack fish are home to a wider variety of invertebrates than those that contain them, and these lakes provide better breeding habitat for frogs and other amphibians as well. When fish are introduced to historically fishless lakes, the fish quickly consume many of the larger, free-swimming invertebrates, including dragonfly larvae, midges, whirligig beetles, and water boatmen, as well as the eggs and larvae of frogs and salamanders.
“There can be a lot of consequences for the upland habitat, too,” said Schilling. “Researchers in the western U.S. have shown that loss of invertebrates may cause a decrease in songbird use of the area around the lakes because they lose that food source. It can cause changes in the plant communities in the riparian zone and garter snake population declines due to loss of their amphibian prey.”
To provide the Maine Department of Inland Fisheries and Wildlife with guidance on how to conserve the diverse aquatic communities that thrive in the absence of fish predation, Loftin and Schilling have studied how to predict which of the state’s 6,000 lakes are naturally fishless and which macroinvertebrates are indicators of fish absence. They identified two types of fishless lakes in the state: those that are fishless because they are hydrologically isolated, with no inflowing or outflowing streams, and those high-elevation lakes that have steep outflowing streams with waterfalls that fish cannot pass.
Using GIS data, the team developed a spatial model that predicted that 130 out of 2,500 lakes in two regions of the state would be fishless. They then visited 21 of the lakes to sample for the macroinvertebrates that indicate fishlessness to confirm that their model predictions were true. If they did find fish in a lake, they then scoured the lakebed for the mouthparts of a phantom midge, a species that does not survive in lakes with fish and indicates the lake was historically fishless.
“In this way we were able to tease out where our model didn’t work and where it did work and find where undocumented fish stocking has taken place over time,” Loftin said.
The biologists concluded that there are multiple ways of approaching the conservation of fishless lakes, and they recommend what they call a tiered approach based on the accessibility of particular lakes and whether or not a lake has ever been stocked before.
“If a lake is difficult to access or it hasn’t been stocked in a while, the state might consider not stocking it,” Loftin said. “And for lakes that have never been stocked, we recommend not stocking. But trying to keep people out of a lake that they’ve been fishing in for 50 years might not be worth trying, even if it was originally fishless.”