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West Brook, a tributary that is part of the Connecticut River Basin with a fish tag antenna: Photo by Todd Dubreuil The Connecticut River flows more than 400 miles; past Vermont and New Hampshire, through Massachusetts and into Connecticut. A new study is looking at the tens of thousands of culverts and dams in the river, to see if they’re preventing fish from getting to their spawning grounds, and reproducing.

Aquatic Ecologist Ben Letcher of the US Geological Survey has been assessing the health of fish populations by following individual fish. A lot of them.

“Since 1997 almost 25,000.”
“Different fish?”
“Different fish Yeah. We’re a little insane”

Letcher is standing next to Jimmy Nolan Brook in West Whately, Massachusetts which feeds the Mill River which feeds the Connecticut River. Letcher says the life histories of individual fish tell the story of an entire population.

“Who survives who mates with who how they move around what their body growth rates are…basically kind of get medical histories of individual fish over a long time scale and we put all that stuff together into statistical mathematical models that help us explain why the numbers go up and down. But the key is to be able to tag these individuals and keep track of them over a long time.”

A culvert on Jimmy Brook with an antenna: Photo by Todd Dubreuil So how do they track the fish? They insert a rice-grain-sized transponder in each one. The transponder in the fish is like an E-Z-pass on a car going through a toll booth. When a fish passes through a culvert-those are the pipes that divert streams under roads. A nearby antenna sends out an electromagnetic wave.

“It sends out a signal. The tags don’t have any batteries in them. They’re waiting for a signal from an antenna and once they get that signal they send back the unique code for that tag.”

The antenna is connected to a receiver that stores not only the tag number, but the location of each fish along with the date and time it passed through a culvert. But I wondered about those tags inside the fish.

“Do you ever worry about someone eating one of these if someone catches the trout?

“They’re not in the muscle. They’re in the intestine. When someone guts the fish the tag would go with the guts.”

Tyler Evans, a fisheries field technician, who works with Letcher is opening a waterproof suitcase. Evans explains inside is a receiver which records which fish swam into a nearby culvert.

“So we had 1300 encounters. So the fish swam that 1300 times. Usually we have a fish….See the red light? There’s actually a fish down there right now hitting it…. Yup. Beta 163.”

“What’s his name?”

“Beta 163. That’s the last 4 digits of its ten digit tag number.”

Culvert on Mitchell Brook perched above the streambed: Photo by Todd Dubreuil Beta 163, as it turns out spends a lot of time near this culvert which sits fairly low, not far above the stream bed, making it easy for fish to pass through. But other culverts that are too high or dry present challenges to fish. Down the road, another tributary, called the Mitchell Brook, has a culvert that sits several feet above the water.

“People think there’s no way fish could get up that, but based on the antenna surrounding the culvert, we know which fish that try to get up here actually make it all the way through.”

Letcher says the fish make almost a running start from a pool below to get past the rush of water.

“They can really push their little tails and just zoom right up and get a good jump almost like salmon.”

But only 40 % of the fish that try to make it up are successful. They’re trying to get to stretches of stream that are good places to spawn. And getting to a spawning area is what it’s all about. Without reproduction populations decline. The Brook trout here are in good shape. If they can’t get up one culvert there are other tributaries they can get to. But Letcher worries about other places.

“The problem is more and more houses are being built in places like this in hillsides that means more roads, more road crossings more culverts.”

Letcher says because the trout are healthy here it’s a good place to study. He says if scientists understand how a functioning system works then that knowledge can be applied to places where fish are having trouble. Letcher’s data will be used to build a model of the impact of culverts on brook trout in small streams in the entire Connecticut River Basin. The Nature Conservancy is funding the modeling study. Kim Lutz of the Conservancy says the model has never been built before

“We’ve always known these breaks in connections between rivers and streams have been a problem, but we’ve never really known how much habitat is enough. Is 20 miles of connected river enough? 50? 100? And because Ben Letcher and his crew from U.S.G.S. have amassed this huge data set we can use that to create a model to answer just how many river miles need to be reconnected for healthy fish populations.”

The model is expected to be built within three years. The Nature Conservancy hopes to use it to figure out which culverts are hampering fish passage and need to be replaced