A brook trout (Provided photo — state Department of Environmental Conservation)

A study recently published by the National Academy of Sciences shows a poor outlook for the future of Adirondack brook trout populations in lakes.

The study, titled “Concurrent warming and browning eliminate cold-water fish habitat in many temperate lakes,” explores how trout habitat is impacted by two water quality variables — temperature and dissolved organic carbon.

By studying these factors, the research group from Cornell University and Rensselaer Polytechnic Institute learned that in 2023, only an estimated 4.8% of 1,468 Adirondack lakes contain suitable habitat for trout. That suitability is a significant decrease from the 1980s, when 23.4% of those lakes contained suitable habitat.

The report attributes much of this loss of habitat to browning, which is measured by dissolved organic carbon, DOC. This is the plant-derived material that gives many Adirondack lakes their dark tint.

In very clear lakes, a small amount of browning and keeping the rest of the water column cool during the critical months of summer. But most lakes in the Adirondacks do not fall into the “very clear” category.

According to the study, in lakes with moderate to high browning, heat absorption benefits of DOC are outweighed by its oxygen depletion effects. This organic matter creates more microbial respiration, which uses up oxygen. In addition, browning shades photosynthetic benthic algae and phytoplankton from the sun’s rays, leaving only the surface for these oxygen-producing organisms to do their work.

To study browning and its impact, researchers closely monitored 15 lakes with sensors that recorded dissolved oxygen and temperature. They then used the data to create maps of the lakes’ layers.

During the summer, the water separates into different zones, with warm water on the surface and cold water on the bottom. The layer suitable for trout is called the oxythermal layer, and it is located somewhere in the middle. Oxythermal means warm and well-oxygenated, and the greater the span of this layer, the better it is for the trout.

The researchers determined that lakes are becoming increasingly “squeezed” by browning. This means that the deep, hypoxic bottom layer is expanding upward, cutting into the oxythermal layer and reducing the trout habitat.

According to the report, “Browning-induced heat trapping at the surface of Adirondack lakes has led to surface warming rates more than double the average rates reported for temperate lakes worldwide and more than 180% faster declines in deep-water dissolved oxygen.”

The study suggests that most lakes right now are already poorly suited for trout, and further disruption of habitat will continue to threaten the future of trout populations.

“Only three out of 15 of our intensively monitored lakes currently retain large amounts of cold-water habitat throughout the warm season, such that brook trout would not face oxythermal habitat challenges in the foreseeable future,” the study found. “Mining the extensive historical data for Adirondack lakes data suggests that future prospects of oxythermal habitat are even bleaker because deep lakes are rare and DOC concentrations in most lakes were high enough to produce a substantial net loss of suitable oxythermal habitat under observed rates of browning.”

Evan Bottcher, owner of Hungry Trout Fly Shop in Wilmington has noticed significant changes to lakes after many years of fishing the same waters.

One of the changes he has noticed is a difference in the flora and fauna of lakes.

“I’ve got a couple ponds that I have been fishing for 15 years, one of which was Twin Ponds in Malone. That pond in particular, I’ve noticed in my lifetime just acting different and changing, and trout populations shrinking. All of the not-sought-after species, like sunfish, never used to exist there, and now they are there in droves. Once those come in, in the thousands, you’re not getting those out of there,” Bottcher said.

This change in fish populations affects other species.

“Because of this you no longer see leeches and salamanders. It is a pretty obvious food chain adjustment,” he said.

Plants in the pond also started behaving differently.

“There was a weed growth in the pond that looked like a ‘tall cabbage’ that would break down by the time fish would spawn. After 10 to 15 years in there, the weed growth stopped receding like it used to,” he said.

Bottcher’s fly shop business includes guided fishing trips in the Adirondacks, so he pays attention to research and studies, including the browning study, published Jan. 2.

“I thought it was just a really small control group, to compare. There are so many different variables that go into ponds,” he said.

Although Bottcher believes that a study utilizing a control group greater than the 15 lakes monitored in this study would be more conclusive, he said that it remains useful by helping people understand the changes that are occurring in Adirondack fisheries.

“Do I take it as a grain of salt? No. Because I trust that they know, and that they are monitoring. Any study like that, even if it’s not the most grand study that they do, does help the cause that we all know is accurate,” he said.