PLYMOUTH – Move over, herring; American eels are making a seasonal run as the wiggly darlings of local rivers and streams.

The slippery and mysterious fish have been making their way up Town Brook this month in record numbers and are getting an assist in their migratory habits from the town’s natural resource officers.

An eel ladder installed by the town in 2019 is credited with enabling tens of thousands of the creatures to make their way into the brook’s upper watershed, where they will spend the next decade or two slithering around under rocks and in the mud.

The migration unfolds outside the Plimoth Grist Mill, Plimoth Patuxet museum’s reconstruction of the Plymouth colonists’ original 1636 mill on Town Brook. At the base of the dam forming Jenney Pond, the town has installed the ladder, which, like the nearby fish ladder for herring, enables eels to continue their journey upstream.

The upstream migration generally coincides with the annual herring run, but can start earlier and end later. Visitors can watch it happen from the deck outside the grist mill. On one side is the fish ladder. On the other is the eel ladder.

A group of visitors from Virginia recently took in the slippery spectacle as the town documented a record number of migrating eels.

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“Every so often you see one slither by,” John Telfer, of Chesapeake, Virginia, said while watching the 2-inch eels wiggle their way over the lip of the ladder. “It’s amazing all the work they do.”

In that one day last week, more than 20,000 young eels made their way up the ramp and into a small collection pool, where resource officers counted them and then gave the squirming creatures a lift over the dam into the pond and the watershed beyond.

The one-day total is more than double the amount of eels recorded at Town Brook in all of 2020 and almost half the number from last year. As of last Wednesday, 39,000 eels had made their way up the ladder and across the dam this spring.

Before the ladder, David Gould, the town’s director of marine and environmental affairs, said handfuls of eels at a time could be pulled from the stonework of the dam. The flow of water through the fish ladder used by herring is considered too swift for the little eels, but the ladder works well.

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Located near the 14-foot-diameter waterwheel, the slim metal ramp contains a fabric mesh that Gould compares to a convoluted mess of dental floss. Young eels that find the base squirm up the mesh and enter  a tank of water.

Two different stages of eels make their way up the ladder. Glass eels are about a year old and are translucent. Elvers are a little older and a little bigger, have pigmentation and are a precious commodity.

Prized as food, especially in Asia, when they reach adulthood, elvers are raised in fish farms and sell for about $2,200 a pound. The only legal fisheries in America are in Maine and South Carolina.

Gould said his resource officers sometimes find evidence of people trying to poach elvers in Town Brook and other rivers and streams. They monitor the watersheds and occasionally find and destroy elver nets.