Dieter Willner went for his regular stroll around the Rossetti Natural Area just before the new year. It would have been quotidian save for the hordes of dead fish he saw coating the banks of the lake.

“What was so amazing was the density of it,” he said, recalling the sight. Thousands of small silvery cadavers lay in clusters of about 100 per square foot of waterline, per Willner’s estimate: “There was an awful lot of tiny little fish there, tremendous amounts.”

After receiving an inquiry from the Colchester Sun, the town investigated and sent photos to the Vt. Department of Fish and Wildlife, parks and rec director Glen Cuttitta said. The approximately 6-inch-long fish, identified as alewives, are not native to Lake Champlain, though they’ve lived in its waters since 2005. According to VT F&W fishery scientist Shawn Good, the die-off phenomenon is a natural one that can be brought on by fluctuations in water temperature.

Willner’s alewife spotting is the second report VT F&W received in as many weeks, fisheries biologist Bernie Pientka said.

Disconcerted in early years, VT F&W once collected alewife samples during die-offs. But they soon learned it was neither an indication of poor water quality nor a risk to human health. Alewives are simply not adapted to the variance in winter water temperature to which other Vermont fish have grown accustomed: “It’s kind of just the basic biology of that species,” Good said.

“It’s unsightly and it does look alarming to people,” he continued. “But, honestly, this is a natural thing that’s happened since [alewives] have been introduced in Lake Champlain, and it’s going to continue happening.”

Though it’s impossible to determine how the creatures—anadromous fish, like salmon, who move from the ocean to rivers to spawn—made their way to the lake, Good and his colleagues have their theories. Alewives’ introduction to Lake Champlain was likely a purposeful and illegal action by anglers looking to provide a “snack-size” food source for trout, bass and other local fish, Good said.

The effort was short-sighted, he added, as there are many negatives that accompany introducing a new fish to a body of water. Alewives eat both the young of native fish and of their food sources, like plankton. It’s for this reason VT F&W has laws against transporting live fish from one body of water to another.

People “think they know better … and they go off and [move the fish],” Good said. “Maybe they have good intentions, but they don’t think in the long-term of the repercussions that come along with that.”

And while die-offs eliminate droves of the invasive fish, they’re far from a solution to the overall alewife problem. The phenomenon kills young, old and weak fish but leaves plenty to repopulate, according to Good.

VT F&W first discovered alewives in Lake Champlain during fishery survey work. There were just a few then, he said, but within a year, the population exploded to thousands. They’ve now been found in Lake St. Catherine and Lake Carmi. Today, alewives are thriving and prolific, possibly the most abundant fish in the lake, Good said.

“We’re never going to be able to do anything about that at this point; that’s out of our hands,” he said, adding people can aid in preventing further spread and educate others on the harm of moving nonnative fish into Vermont’s waters.

“The best thing we can do is try to avoid them getting introduced to other waters,” Good said. “Let’s not spread them any further.”