LIMESTONE – Nestled between potato fields in central Aroostook County, Durepo Lake lies quietly – too quietly – under the spring rain.

This small lake is dead.

Because one person illegally stocked the lake with bass, upsetting the ecological balance, state biologists last fall killed all the fish and amphibians. The operation cost the state more than $10,000.

Cases like this are why the Department of Inland Fisheries and Wildlife calls illegal stocking of game fish “an epidemic that threatens a Maine heritage.”

DIF&W fisheries biologist Ken Warner has estimated that more than 500 illegal fish stockings have been made in Maine waters since 1980. About half of these introduced species are not native to Maine.

For three decades, the state has tried to assert control over fish introductions, but it largely has failed due to a lack of funding and a growing preference among anglers for catching large, showy exotic species, said Peter Bourque, director of the fisheries and hatcheries division of DIF&W.

“Personally, I think a lot of it comes from the types of fishing that are promoted on cable TV,” Bourque said. “They tend to show large fish, like bass and pike. When people watch this week in and week out, they get excited.”

Each year, Maine’s Department of Inland Fisheries and Wildlife stocks thousands of trout and salmon to boost native populations, but when anglers introduce other species, natural and managed fisheries alike are destroyed.

In the 1700s, bass did not exist in Maine. Today, smallmouth and largemouth bass, which are native to the southeastern United States, are found in hundreds of Maine fisheries, extending as far north as East Grand Lake. Meanwhile, brook trout fisheries have declined to the point that even with Maine’s aggressive stocking program, strict fishing limits have been instituted.

Bass, along with yellow perch, northern pike, white catfish and black crappie, are becoming common in Maine. But they also are considered dangerous exotic species because the state’s cold-water salmon and trout fisheries developed for millions of years without a niche for the large predatory fish that eat them and their food, Bourque said.

Exotic species simply out-compete Maine’s native trout and salmon. Within just a few years, the entire ecosystem is changed and in many cases the native fishery no longer exists.

“It’s biological pollution of the worst kind, and the most frustrating thing is that it’s irreversible,” said Paul Johnson, a DIF&W fisheries biologist based in the Moosehead Lake region.

The exotic species just have greater reproductive potential, Bourque said. A female brook trout can produce between 750 and 1,000 eggs during spawning. A female perch of the same maturity will produce about 150,000 eggs.

Bourque compares the threat of exotic fish species to that of exotic plant species such as milfoil, which recently has spurred a statewide control effort. Like milfoil, exotic fish cannot be eradicated easily once they infest a body of water.

“Clearly, in my mind, the influence of an exotic fish species has a greater impact than many of the aquatic plants. They can move a lot faster,” he said. “Most of these situations become pretty dire.”

Drastic action

In Limestone, the fear that bass might move from tiny Durepo Lake into some of the nation’s last remaining natural brook trout fisheries called for drastic action, said David Basley, a fisheries biologist with the DIF&W based in Ashland.

“There aren’t a lot of places like this left in the lower 48 states. This is the last stronghold for native brook trout fishing in Maine,” he said. “There’s a lot to be lost by [the actions of] a few selfish people.”

Fisheries biologists last fall began the lengthy and expensive procedure to “reclaim” Durepo Lake after a local fisherman called Basley to report an unusual catch.

Using an electroshocking device that stuns fish, as well as fishing the waters, biologists determined that a problem population of more than 1,000 largemouth bass had been established in the lake and several small feeder streams.

“We knew there were more than just three or four fish,” Basley said.

The fish had not reached sexual maturity, and biologists deduced that they had been introduced as fry just a few months earlier.

Because the fish had not reproduced and had little time to migrate into other waters, Basley recommended swift action.

The state approved what Bourque termed “a SWAT-team approach to fisheries management.”

Several tons of an organic pesticide called rotenone were applied to the lake and several local streams in September. Rotenone is an organic pesticide that targets fish and other gill-breathing organisms by interfering with how their bodies process oxygen. The organisms cannot utilize the oxygen they absorb from rotenone-treated water, and most die within 12 hours, Basley said.

Rotenone breaks down very quickly when exposed to sunlight, and although some studies have suggested that it may he harmful to humans when inhaled, eating the fish caught in a reclaimed pond is not considered dangerous, said Eric Sideman, director of technical services for the Maine Organic Farmers and Gardeners.

“Once it’s gotten wet and exposed to sunlight, it’s got about 12 hours of activity,” he said.

In low concentrations, it is not believed to harm warmblooded animals. In fact, a 132-pound person would have to drink more than 15,000 gallons of rotenone-treated water at one sitting to ingest a fatal dose, according to the American Fisheries Society.

Within days, bass were washing up on the shores of Durepo Lake, Silver Spring Brook, Limestone Stream and Limestone Community Pond. Thousands of trout were estimated to have died and sunk to the substrate, Basley said.

“It killed everything downstream, all the way into Canada,” he said.

The biologists held a public information meeting before the reclamation to prepare residents for the sight, but only a handful of people turned out, Basley said.

Most were anglers concerned with losing their favorite trout-fishing spots along Limestone Stream. Others feared the DIF&W was applying pesticide to the town’s drinking-water source. Silver Spring Brook and Limestone Stream had served as Limestone’s public reservoir for many years, but in 2000 the town shifted to using a series of deep wells in an effort to improve water quality.

Basley explained that the DIF&W had planned to restock the fishery with native brook trout from the Enfield fish hatchery. More than 1,000 adult fish will be introduced in late spring and early fall, while trout fry will be stocked during the summer, he said.

Within one year, Durepo Lake should revert to its natural state, biologists said.

“Most waters seem to recover fairly rapidly. Almost instantaneously, there are frogs and salamanders and aquatic insects of all kinds. They seem to recolonize these watersheds very rapidly,” Bourque said.

Reclamation rare today

Reclamation is rare today, and in some other states has sparked protest by environmental groups. But during the 1950s and ’60s reclamation was common in Maine and had a regular allocation in the DIF&W budget, Bourque said.

For about 30 years, state fisheries budgets were tight and practically no reclamation work was done. In recent years, the expensive procedure has been completed on about 10 ponds, typically funded through federal grants as no state money has been budgeted for control of exotic species in recent memory.

Only a small percentage of exotic species introductions have resulted in a prosecution, according to the Maine Warden Service.

State law provides for a minimum $1,000 fine and a maximum $10,000 fine for someone caught illegally stocking a body of water. The culprit also may face jail time and the loss of his fishing license for the criminal offense – if caught.

But even a $2,000 reward for the apprehension of those who introduce fish into Maine waters illegally has not led to the identification of the person who stocked Durepo Lake, so the state bore the full cost of the reclamation.

The pesticide alone cost more than $10,000. Add about two weeks of state biologists’ time and hatchery fish that have been diverted from other waters for restocking efforts in Limestone, and the cost skyrockets.

“Reclamation, as far as the state of Maine goes, is about the only option that we have, [but] it’s not a terrific option in terms of our budget situation,” Bourque said.

Biological control – introducing a native species to prey on the nuisance exotic fish – has not been an option in Maine. Preferred fish like chain pickerel that could be used to prey on warm-water species such as bass and perch also would prey on the native brook trout, likely causing as much damage as the exotic species.

State biologists also have considered the construction of barriers to contain exotic fish. But again, the process is costly, and many growing bass populations statewide simply have been accepted as an unnatural, but irreversible, ecological shift.

To protect the few remaining native fisheries, the state needs funding. But with recent budget shortfalls, this epidemic has not topped the DIF&W’s list of priorities. A grass-roots effort may be the fisheries’ best hope, Bourque said.

“It seems like we’re just struggling to keep our heads above water,” he said.

To report illegal fish introductions, contact the Warden Service’s Operation Game Thief at (800) 253-7887.

Misty Edgecomb is the outdoor reporter for the Bangor Daily News. She can be reached at medgecomb@bangordailynews.net.