Scientists around Cooperstown, NY, are celebrating a rare victory: the slow return of a native species in Otsego Lake. There, in the upstate lake that spawns the Susquehanna River headwaters, the whitefish known as “Otsego bass” are making a comeback after having been decimated by predation and poor water quality.

Fresh from the State University of New York Cobleskill hatchery, 164 juvenile Otsego bass are placed in a plastic bag and laid on the ice of Otsego Lake to acclimate to the temperature before being slipped under the lake ice in January 2019. (Brent Lehman / SUNY Cobleskill hatchery)

The decline of the Otsego bass was so severe that a local outdoors columnist in 2012 pronounced the fishery dead, mourning the tradition of trolling for the popular fish. The loss began after 1986, when the alewife — a newcomer with a big appetite — began to appear in the lake. Prior to its arrival, scientists caught an average of 8.1 Otsego bass per net in fishing surveys. Surveys taken between 1990 and 2000 turned up an average of less than one per net.

The Otsego bass is actually a lake whitefish (Coregonus clupeiformis) native to and no different than those swimming in other coldwater lakes in the northern United States and Canada. The local fishing community dubbed it the Otsego bass, and the moniker stuck. Its abundance and popularity is well-documented back to the 1700s. An account in the State of the Otsego Lake, 1936 to 1996, described gill nets pulling in 5,000 fish at each haul.

Since the 1900s, local fisherman and farmers supplemented their income by catching and selling Otsego bass. The tasty fish was on the menus of mom-and-pop restaurants that lined the shores of the lake. Some fishermen sold them right off the boat, and one man sold them from a cooler at a hardware store.

During the winter, Otsego Lake was an ice fishing mecca. As soon as the lake froze, hundreds of ice shanties were occupied by men, women and children who were handy with an ice auger. Some of these mostly homemade, portable shacks had heaters and pictures on the walls, and anglers could drop their lines through holes in the ice from the comfort of recliners.

“It was quite a fishery 30 or 40 years ago,” said Cooperstown resident and fishing guide Tom Trelease. “The whitefish used to be part of the identity of the area.”

Otsego bass numbers began their decline by the late 1950s, but their massive slide began when the alewives arrived.

The alewife, a species of fish that lives part of its life in freshwater and part in the Atlantic Ocean, may have been introduced to the lake accidentally as bait or deliberately to provide prey for the lake trout.

If the latter is true, the strategy worked. The alewives quickly multiplied and, within a decade, became the dominant forage species in Otsego Lake. The trout, feeding on the alewives, put on size and weight, and trout fishing was unprecedented.

“Fisherman were getting used to, and loving, catching those large lake trout that were supported by an unusual abundance of prey,” said Mark Cornwell, chair of the Department of Fisheries and Wildlife at the State University of New York at Cobleskill. “They were caught up with the euphoria of catching big fat fish.”

But the alewives were also voracious feeders, and their large population was taking a toll on the lake’s ecosystem. Otsego bass declined rapidly as the alewives grazed on their spawn.

The alewives were eating another important species, too: Daphnia, a tiny crustacean that consumes plankton and keeps algae in check. By the 1990s, the alewives had drastically reduced daphnia in the lake. With the growth of algae unchecked, the lake was turning green, and it experienced summertime periods of low-oxygen “dead zones.”

“The lake was hammered,” said Scott Wells, a fisheries biologist with the state Department of Environmental Conservation. “When you get a prey species like the alewife, it changes the whole ecosystem of the lake, chemically and biologically.”

New York state fisheries staff and scientists joined with researchers from SUNY Cobleskill and SUNY Oneonta’s Biological Field Station to return Otsego Lake to its pre-alewife state.

The field station, with laboratories, offices and classrooms spread out over 2,600 acres along and near the lake, was a vital research hub for the project and the lake’s health as whole.

“Up until the 1990s, our management goal was to reduce [pollution from] nutrients where we could,” said Holly Waterfield, a researcher at the field station. “The impact of the alewives was similar to nutrient loading, but more extreme. We needed to work on management that dealt with alewives and increased the numbers of [daphnia].”

After years of study, researchers decided that yet another fish species might help rebalance the lake’s ecosystem: the walleye, which already had a small presence there. Walleye prey on alewives, and scientists believed that more walleye would mean fewer alewives.

“We were very cautious about the walleye,” Waterfield said. “Walleye made sense. They seem to fit our situation.”

Researchers began by adding approximately 80,000 young walleye to the lake in 2000, and the process continued for 14 years until the stocked fish began reproducing naturally.

Soon after the walleyes’ introduction, they began to eat their way through the alewife population. As a result, the daphnia began to rebound. Water clarity started slowly improving shortly after the walleye were introduced and greatly improved by 2009.

The trout and other larger species soon faced a shortage of forage food because the alewives, before their own decline, had eaten and depleted many of the other forage fish. It’s taking time for the ecosystem to adjust, but Wells said that some of those species are rebounding.

And so are the Ostego bass. In 2014, the first juveniles were observed in the lake in decades. State fisheries staff netted 20 adults in 2016 and 40 in 2018.

While alewives remain in the lake, they are so few in number that scientists describe them as almost undetectable.

The recovering population of Otsego bass has also been getting a boost from hatchery-raised fish. Students from SUNY Cobleskill have collected eggs and milt from the adult fish, and fertilized and reared them in the school’s hatcheries. Most recently, a group of 164 fingerlings were released under the ice of the lake in January 2019. They were marked with a dye that stains hard body parts and can be used to identify any that are captured — hopefully as adults — through surveys or recreational fishing.

Rearing the young fish in the hatchery has proven to be a delicate and difficult process, so partners in the program are taking a yearlong break with plans to continue the effort.

“It is rare and it is amazing that this is happening,” said Cobleskill’s Cornwell. “The future of [Otsego bass] in Otsego Lake is — hopefully — that people will be able to catch them again.”