On a recent morning in May, a crew of scientists aboard a research vessel in the middle of Lake Champlain untangled a big commercial fishing net, preparing to drop it into the dark depths of the lake.
The crew was looking for young, wild lake trout — fish that are just about to make the transition to adulthood.
After decades of stocking, research and conservation efforts, Lake Champlain’s wild lake trout population appears to be sustaining itself. But scientists don’t know why the fish are suddenly thriving.
Annual surveys, like this one led by the Vermont Department of Fish and Wildlife, are part of how researchers monitor the populations.
Historically, the data has been used to help scientists from Vermont, New York and the federal government decide how many hatchery fish to stock in the lake.
For this survey, the crew uses a trawling net of the sort commercial fishing boats work with.
The technicians gradually lower the net hundreds of feet into the water from a boom on the boat’s stern.
Two metal weights called “gates” plunge into the water and drag it to the lakebed, where the boat drags the net along, like an open mouth.
After some time, the crew haul the net up and onto the boat, and dump the catch onto a laboratory table in the middle of the deck.
Along with a slew of fish, they catch a boat ladder — other trawls have yielded shopping carts, even a snowmobile.
From here, they count each fish by species, measuring their length and, if it's a lake trout, whether it’s stocked or wild.
Vermont Fish and Wildlife biologist Bernie Pientka holds up two small lake trout — one with a clipped fin and the other without.
“On this one, the adipose fin has been clipped, so that's a stocked fish,” Pientka said, pointing to a small, inconspicuous fin. “Where this fish of similar size, the adipose is there. All the other fins are intact and complete. So that's a wild fish.”
Pientka has been doing these surveys for decades. During that time he’s seen a lot change — less lake ice and several new invasive species.
When he first started, he said it was rare to see young, wild lake trout like this one.
But there was also a time long ago, when wild lake trout populations thrived in Lake Champlain.
Ellen Marsden is a fisheries biologist at the University of Vermont who has studied the fish for decades.
We're now getting to a point where they're more than 50% of the population out there.Ellen Marsden, fisheries biologist at the University of Vermont
She said in the early 1800s, Jesuit priests and other naturalists journaled about finding Lake Champlain full of lake trout, as well as Atlantic salmon and walleye. But the abundance of these cold water fish didn’t last.
“The populations of lake trout collapsed by 1900,” she said. “And the historical record said, ‘Yep, they were abundant. We fished for them.’ And at some point they said, ‘They've gone.’ And it's a mystery still [as to why].”
According to Marsden, no one really knows what happened to the lake trout.
Scientists thought it might have been sea lamprey that did them in.
When the wild population didn’t rebound in the early 2000s despite stocking, they thought a tiny invasive fish called alewife might be to blame.
After much research and many efforts at population control, Marsden said none of those theories fully explain the lake trout’s collapse.
Then, after decades and decades of stocking to the tune of about 80,000 fish a year, something started to shift about a decade ago. Out of nowhere, young wild fish began surviving.
“We're now getting to a point where they're more than 50% of the population out there,” Marsden said.
Scientists know the fish are spawning on their own because they set egg traps and scuba dived to look for eggs and yearling trout on the lake’s reefs and breakwalls — places where the fish like to spawn.
Surveys like the one Pientka’s crew does show those baby fish are making it into the adult population.
We've been able to restore a native species. That’s just superb news.Ellen Marsden, fisheries biologist at the University of Vermont
Still, Marsden said it’s a tenuous situation because scientists don’t really know why the wild fish are suddenly thriving again. It gives her and other researchers pause.
“What if we don't understand how this happened and something bad happened again?” she said. “We've got to watch them like a hawk.”
Marsden said it’s a bit nerve wracking to stop stocking, and she knows it might be unpopular with anglers.
But she said not stocking will protect the lake’s food web and make for better fishing in the future.
“Lake trout live for 25 to 30 years in this lake. When you put one in, you can't get it back,” she said. “So oversupplying or overstocking the predator population is a big concern, because we could crash the forage base — the smelt, the alewife, the sculpins — that are feeding these fish. And if that happens, we're in big trouble, because we can't say, ‘Stop eating for a little while until it gets better.’”
Marsden said humans won’t be completely walking away from Lake Champlain’s lake trout. They’ll continue to keep close tabs on their population through annual surveys like the one Pientka’s crew does.
It’s even more important now as lake trout face a warmer and more tenuous future thanks to human-caused climate change.
“This is a cold water fish, so this global warming has not got so bad that it cannot survive in the deep waters of Lake Champlain,” she said. “We've been able to restore a native species. That’s just superb news.”
And while their future may be uncertain, Marsden said the fact that they’re back shows there is still time to protect more species like them — even as the planet warms.
