‘Improved survival’: How the Cold Brook eco-restoration project helps eels

In spring, herring get all the glory on Cape Cod. Folks line the runs and cheer them on, marveling at the feisty fish as they strive to return to the ponds to spawn. Seagulls cancel their regular appointments to show support for these heroic herring, or perhaps eat them, but still, it’s quite a show.

Meanwhile, another fish swims alongside the herring in relative obscurity. Perhaps it’s because American eels have a lackluster public relations team, a tepid social media presence or have trouble shedding an “icky” reputation. But we are here to elevate the eels, because it turns out they spend more time on the Cape than the herring that get all the headlines.

“I think eels are really cool,” said Karissa Collins, Fish Passage Project Manager for the Cape Cod Conservation District. “They look like little water dragons.”

An American eel in Brewster. Screen capture by Gerry Beetham

Collins also revealed the mind-blowing fact that the life cycle of eels and herring are completely opposite. Herring are born in fresh water, then return to the sea to grow up. Then, they head back to fresh water to spawn.

Eels are born in the sea, travel to fresh water to grow, then return to the sea to spawn.

And while herring can make the epic journey several times in their lives, eels only do it once. When eels make it to a freshwater locale, like a Cape Cod pond, they take up residence for years. Collins said male eels might stay in a pond for four to eight years, while females might stay in a pond for eight 20 years.

During the spring, herring and eels share the same watery highway on their way to the ponds. The herring are adults, but the eels arrive in a juvenile stage of development, with transparent bodies. They are called “glass eels” at this point in their life cycle.

Eels and the Robert F. Smith Cold Brook Preserve

Each spring, eels pass through the Harwich Conservation Trust’s Robert F. Smith Cold Brook Preserve on their way to Grassy Pond. An eel ramp at the western edge of the Preserve helps them along the way.

“Glass eels seek freshwater habitats to live and grow,” said Brad Chase, Diadromous Fisheries Project Leader for the state Division of Marine Fisheries. “They push up as far as possible, looking for suitable habitats. Most will settle in Grassy Pond. Some may go further into the wetlands upstream.”

And the recently completed eco-restoration project at Cold Brook may provide new habitat for eels.

The project transformed the site from a retired cranberry bog into a wildlife oasis, restored the free flow of Cold Brook through the Preserve for the first time in over a century, and included the construction of four ponds. Culverts were also removed from Cold Brook as part of the project.

“Some eels may now reside in the new ponds,” said Chase.

In Cold Brook, “this year’s eel presence is higher than any I have seen before,” said Chase. “The density of eels in the pool below the ramp, on the ramp and in the tank for a 10-day period certainly exceeded the highest count year on record or what I have observed before.”

This begs the question: Could the Cold Brook eco-restoration project have anything to do with the increase in eel numbers?

Chase thinks the increase was likely due to good year in the eel spawning grounds, located in the Sargasso Sea. But the eco-restoration project at Cold Brook is still good news for the eels that make the long trip to Harwich.

“The removal of the culverts likely made their passage to the eel ramp easier resulting in better survival for that journey in Cold Brook,” said Chase. “This improved survival should occur every year now. This would be a positive influence on the health of the Cold Brook/Grassy Pond eel run.”