Can well-managed lakes in the Adirondacks provide important refuges for lake trout in the face of climate change?

That’s the focus of a new intensive research effort being conducted at Follensby Pond, a 1,000-acre lake purchased by The Nature Conservancy in 2008.

The pond offers the perfect opportunity to research lake trout at the southern end of their range, to determine how these large and ecologically important fish could best be managed and protected given rising temperatures and other environmental changes.

As the longest-lived member of the salmonid family, lake trout can also serve as a long-range indicator of how well Adirondack deep water habitat is meeting the needs of other fish species as well.

Lake trout have already disappeared from an estimated 20 percent of Adirondack lakes that held them, mainly due to competition with northern pike and other introduced species, acid rain, waste water, and overfishing.

Climate change could pose a more severe threat. Could deep lakes like Follensby make a difference for the fish?

The Philosophers’ Camp

Follensby Pond is best known not for its lake trout, but for a group of influential citizens that met there in 1858, most notably Ralph Waldo Emerson.

This gathering of writers, artists, scientists and poets met at a rough camp, called Camp Maple, along Follensby’s banks. They brought different perspectives and ideas, but all shared an appreciation for nature. They wrote and painted and discussed the mysteries of the universe.

For many years, understanding lake trout seemed as elusive as discerning the meaning of life. Their deep water habits mean they’re difficult to study much of the year.

When the Conservancy purchased the property, scientists recognized this was an opportunity to better understand lake trout in the Adirondacks. The lake had been privately owned and was only lightly fished for decades.

Research has shown that lake trout require water that is well oxygenated and that remains 55 degrees or less in the summer. In the Adirondacks, you won’t find them in lakes that are less than 30 feet deep. They’re large fish, but slow to mature; lake trout usually don’t reproduce until they’re between five and eight years old.

They can live to be 20 to 30 years old and grow to immense sizes – the largest recorded from Follensby Pond weighed 31 pounds.

“Could Follensby be an important refuge for this species?” asks Michelle Brown, director of conservation science for the Conservancy’s Adirondacks chapter. “We think it has that potential. Follensby could make an important contribution to conservation. It has it all—it’s a deep lake, it’s cold, it’s large.”

First, though, researchers have to better understand the population, age structure and health of the fishery.

Beneath the Surface

To study fish that live at depths of thirty to a hundred feet, researchers have to use a wide variety of methods. Dr. Chris Solomon, assistant professor of aquatic ecology at McGill University, is leading an effort to assemble an accurate population model for Follensby.

“We are trying to gain as complete a picture of the lake trout in Follensby as possible,” he says. “It is neat that the Conservancy is being forward thinking about this so that this resource can be managed well into the future.”

The first part of that picture comes into focus through hydro-acoustics: essentially a high-powered version of the electronic fish finder used by sport anglers. It uses sound energy to locate fish, which show up as blips on a screen.

This method helps researchers to estimate the abundance of fish in the lake. Hydro-acoustic surveys also show where fish are in the water column and where they congregate in the lake.

This provides an estimate, but Solomon and his students use other techniques. They capture fish using a number of methods – gill nets, fish traps and angling – and mark individuals with tags, enabling researchers to track growth rates and also provide more information on the population size.

This technique is known among fishery researchers as mark and recapture. It basically works like this: researchers collect a sample of fish and mark them, so there is a known number of marked fish swimming around the lake.

They later return and catch a second sample of fish; a proportion of the caught fish will be marked. The proportion of marked fish in the second sample is an estimate of the proportion of marked fish in the entire population. Researchers can use this to help estimate the population.

The researchers also collect the otoliths – or inner ear stones – on some fish. These ear stones tell stories similar to tree growth rings, and when combined with other data like size and weight, can provide crucial data on growth rates and age.

Solomon is only six months into the research but believes that he is already acquiring great information on the lake trout. “Follensby has a pretty impressive population,” he says. “Once, fisheries managers did the best they could with limited information. Using techniques like hydro-acoustics and otolith analysis gives us much better data, and will allow better management of fisheries.”

This research will ultimately enable researchers and managers to predict how the size structure and abundance of the population will respond to various scenarios of fishing regulation and fishing pressure.

Lake Trout in a Warming World

The lake trout’s need for cold, well-oxygenated water might appear to make it highly vulnerable to climate change, especially in southern parts of its range like the Adirondacks. Will lakes lose their biggest predator? Is there anything that can be done?

To answer those questions, science writer and researcher Mary Thill conducted a thorough literature review of studies done on lake trout and climate.

“There are so many issues to investigate with a species that has such specific needs,” she says. “Will the oxygen levels at the bottom of the lakes be enough to sustain lake trout through the summer? Will lake trout move up in the water column and fall prey to introduced species? What places will be most resilient?”

Thill found that lake trout declines have already happened in the Adirondacks, and demonstrate just how at risk the species is from climate.

“Restricted habitat, slow growth, late maturity and slow replacement rate have historically made Adirondack lake trout vulnerable to overfishing, competition from introduced species and pollution—stressors that are magnified by increased temperature,” her report states.

These threats are not restricted to the Adirondacks; in fact, lake trout fisheries are in better shape here than in many other parts of their range.

Scientific reports estimate that lake trout could be lost from 30 to 40 percent of lakes in Minnesota and Ontario. No such study has been conducted for the Adirondacks, but given similar latitudes, it seems likely there will be losses.

Shallower lakes are the most vulnerable. While there isn’t evidence that temperature is affecting Adirondack lake trout populations yet, there is evidence that some shallower lakes are failing to provide adequate oxygen.

That means that deep lakes like Follensby Pond, with depths of at least 100 feet, could be vital for the long-term survival of lake trout in the Adirondacks.

Gathering this information now not only provides a baseline, but enables managers to make the best decisions for the long-term health of the fishery.

“If you are going to invest in strongholds for important species, you want it to be an investment that is still worthwhile in 100 years,” says Thill.

“Follensby is well positioned to serve as refugia for lake trout,” agrees Michelle Brown. “By studying these fish, it helps us get our head around what impacts this region will face from climate change, and how we can protect the areas that will be more important for long-term resilience.”

Photos (from above: New York State Department of Environmental Conservation aquatic biologists Rob Fiorentino (left) and Jim Pinheiro inspect a lake trout netted during a survey of Lake Placid in 2013 (Mary Thill); Follensby Pond (Carl Heilman II); Researchers prepare to survey lake trout in Follensby Pond (Erika Edgley/TNC); and Follensby Pond lake trout survey underway in 2013 (Chris Solomon).

Note: This post first appeared on the Nature Conservancy’s blog Cool Green Science.