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Graduate students Darren Ward and Roxanne Karimi with Professor Carol Folt Captivated by Ecological Complexities

Graduate students Darren Ward and Roxanne Karimi with Professor Carol Folt (center) inspect the springtime emergence of insect life at Mink Brook in Hanover, N.H., one of many sites where young Atlantic salmon are released to the wild.

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Carol Folt and her team piece together puzzles of environment and biology

Carol Folt, Professor of Biological Sciences and Dean of Graduate Studies, likes big, intricate environmental puzzles.

Take Atlantic salmon. Folt and her colleagues are trying to figure out how to help Atlantic salmon survive and thrive in their once native spawning grounds along the Connecticut River. According to Folt, the 410-mile Connecticut, which runs from the Canadian border through four states to Long Island Sound, was the longest river system in the world that had been home to Atlantic salmon. These fish historically made their instinctive trip from where they were born in the tributaries of the Connecticut River out to the ocean, where they traveled as far as Greenland, and back into the Connecticut to spawn the next generation.

Unfortunately, but perhaps inevitably, Folt explains that use of the river and its surrounding habitat have changed over the past 150 years or so, and the Atlantic salmon have all but disappeared from the Connecticut River.

“When people started building dams on the Connecticut more than a century ago, the adult salmon were unable to return to spawn,” says Folt. “The original Connecticut River Atlantic salmon no longer exists. There are genetically related salmon that went to Maine or parts of Europe, but the original Connecticut River animal died out.”

Folt says that the government launched a restoration effort around 1965 to bring back Atlantic salmon and other anadromous fish (those that migrate from the sea to fresh water to spawn), hoping to reestablish the native aquatic wildlife.

“The government built three hatcheries for Atlantic salmon in the Northeast. A new species needed to be created—one that could inhabit the modern river. They created a new salmon by breeding salmon from the Penobscot and other rivers in Maine with salmon from Norway and other parts of Europe.”

image of salmon


The hatcheries give the salmon a head start in life safe from predators. In a typical hatchery, the salmon grow to about seven inches long, which is when they imprint on the stream providing the memory of where to return after their swim to the ocean. These cigar-sized animals are released into small streams for a few months before they begin their journey to the sea and back. In theory, it was a sound plan: help the salmon get started, and let nature take its course to bring them back to spawn. Unfortunately, it wasn’t working. Few salmon returned home.

In the mid 1980s, Folt and then undergraduate Wendy 9 Sanders ’86 teamed up to help a local hatchery in Bethel, Vt., figure out how to increase the success rate of their salmon stocking efforts.

“We were curious whether the habitat had changed so much over time that there would be something out of synch making it no longer suitable for salmon. We wondered whether the insects they eat would be available when the salmon were stocked, for example. We became the only people in this region doing that kind of research,” says Folt.

Over the past 20 years, Folt’s research in this area has included numerous undergraduates (including honors thesis students Marco Seandel ’94, Nina Perlroth ’96, Joe Bachman ’91 and Adam Sepulveda ’02), graduate students (such as Keith Nislow ’97 Ph.D., Brian Kennedy ’01 Ph.D., and currently Darren Ward), and colleagues from the Forest Service, the Fish and Wildlife Service and at many other organizations. According to Folt, these relationships make this research extremely productive and very collaborative. The group continues to learn about what makes a good habitat, not only for juvenile Atlantic salmon, but also for other fish in the region. They now know that salmon need sheltered places in the spring when the current speeds up. The fish also need access to calm, deeper water to catch their food—mainly passing insects and other stream invertebrates like snails. Later in the summer, when the temperature rises, the salmon need shaded, cool areas where they won’t overheat. Land use has a big impact on salmon habitat as it affects salmon growth and development.

“One thing humans do is remove the dead trees and other natural debris from streams, and that means when there’s a flood, the water rushes very fast down the entire stream channel, and there’s no place for fish to hide,” says Folt. “In the later part of the year, other ways people change the land become important. Streams tend to get very low in the summer, especially when we have a dry, hot summer. If you don’t have debris to create some pools and cool areas or streamside vegetation to provide shade, salmon and other fish can’t live.”

Folt and her collaborators develop models and search for particular types of habitats with a variety of vegetation, good water flow conditions and different depths where they predict fish will thrive. The quintessential site will allow the salmon to not only survive, but grow quickly. Recently Nislow,

Sepulveda and Folt reintroduced salmon to Mink Brook and Grant Brook, both near the Dartmouth campus, which were chosen with the team’s criteria in mind.

Over the years, new ideas and improved technologies have emerged, and Folt makes sure her projects take advantage of them through collaborations with scientists in other areas.

For example, the Atlantic salmon research got a boost about eight years ago during a conversation Folt had with fellow professor Paige Chamberlain from the Department of Earth Sciences. Now at Stanford, Chamberlain described his project with anadromous fish in California that revealed that a fish’s ear bone, or otolith, picked up an environmental chemical signature, which provided a road map back to where the fish had lived. An excited Folt launched a New England version of this project to determine features of the streams that successfully produced salmon that were hardy enough to survive and come back as adults. The effort continues today, and, according to Folt, their methods have been adopted by similar programs around the world.

Most recently, the researchers have embraced genetic technology to gain insight into the DNA of a robust, returning Atlantic salmon. Like discovering the chemicals in the otolith, knowing the genetic makeup of successful animals and who their parents were should help future breeding and stocking efforts. So the combined knowledge about what makes a good juvenile habitat and what makes a healthy and strong individual salmon should someday soon increase the odds on survival.

However, right now, even as the Atlantic salmon restoration project thrives, the salmon aren’t.

“No, there are not more Atlantic salmon returning to the Connecticut River,” Folt admits. “In fact it’s still declining. So I think we’re still in the discovery stage.”

The team, not discouraged, has learned a great deal about the nuances of the habitat and the perseverance of the salmon and other stream fishes. They have worked with the hatcheries to also tweak their program to release the salmon earlier, when they are only about three inches long, so the salmon live in the streams for a year or two before they swim to the ocean. Of course, more questions emerge to fuel their determination.

“We’re still trying to figure out if it’s in the genetics of an organism, the stream habitat conditions or if weather patterns are to blame,” says Folt, who points out that much of the program has taken place during the warmest and sometimes driest years in the last century. “We need to understand how their predators influence their survival as they swim to the sea, and how much the changing ocean environment plays a role. Atlantic salmon is a global species, and it will make it or not due to both local and global conservation efforts.”

Folt’s enthusiasm for the entire, interconnected web of ecosystem activities, often called biocomplexity, also includes her work to understand how metals impact life in lakes and ponds. Her research in this realm, which is part of Dartmouth’s Center for Environmental Health Sciences, centers on zooplankton, the conduit between fish and metals, some of which are naturally found in the environment and others atmospherically deposited from the discharge out of smokestacks and exhaust pipes. The basic scientific question is why similar fish from different lakes carry such different amounts of metal.

“We think that whatever controls the nature of the zooplankton community in lakes influences how much metal goes from the water into the fish,” she says.

Variations in the food web lead to an increase or a decrease in the metals in fish, Folt says. Some biological systems suppress and some support the transfer of metal from water to fish. The food web, already a crisscrossing maze of animal, plant and insect interaction, becomes more intricate because metals don’t respond to the environment uniformly. What happens to mercury isn’t necessarily what happens to arsenic.

“Zooplankton are the biological moderators. The zooplankton community determines the metal movement, the movement of other chemicals through the food web, and it can also influence the temperature and the amount of algae.”

Folt and Celia Chen ’78 B.A. ’94 Ph.D., Research Assistant Professor of Biological Sciences, along with numerous students and colleagues, study clean lakes and not-so-clean lakes throughout the Northeast. They find that, contrary to intuition, the biggest concentrations of metals, specifically mercury, are often in the fish from the cleaner lakes. The zooplankton and the algae that they eat are central players in this phenomenon. Paul Pickhardt ’02 Ph.D., together with Folt and Chen, discovered that when there is a lot of algae present, mercury is dispersed more widely throughout the algae. As a result, zooplankton that eat the algae are exposed to lower levels of mercury, so less goes into the zooplankton-eating fish. However, in systems with less algae, the opposite is true: The mercury is more concentrated in the algae, so the zooplankton eat more mercury with each meal.

Consistent themes run through Folt’s research projects. First, they all involve food web relationships, or as she says: “who eats what.” The second theme concerns the influence of biodiversity and density of organisms in any given habitat, from algae to insects to fish. Finally, she delights in working with a team of colleagues with interdisciplinary expertise, a combined set of skills from ecology and earth science to genetics and toxicology, to pursue her research.

“More than 10 years ago, I made a strategic decision to work on environmental problems that had global significance and human dimensions,” she says. “Plus, my students really want this kind of challenge; they want to do research that they think has some broader societal impact. We have fantastic students at Dartmouth, and they fuel us to be responsive to their interests. And given the interdisciplinary nature of our graduate program, we’ve been able to involve graduate students from several departments in our research. They learn to work across disciplines, which isn’t always easy at other institutions, but it’s essential for solving the complex environmental problems at hand.”

Progress can be measured, Folt says. When her team finds new, more suitable stocking locations for restoring and conserving salmon, it reflects on knowledge she helped gather. When the EPA and the National Institute of Environmental Health Sciences (at the National Institutes of Health) take an interest in her team’s findings about mercury and lake water quality, Folt feels rewarded. Successfully advancing the science and, in turn, informing strategic environmental management decisions motivates Folt and drives her career.


“We’re still trying to figure out if it’s in the genetics of an organism, the stream habitat conditions, or if weather patterns are to blame.”

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