PEMBROKE (CBS) – It’s a unique sight at a unique time of year as herring churn up the water, making their run in Massachusetts rivers, streams and ponds.

Each spring they leave the ocean where they spend most of their lives, and move to freshwater to spawn. But if something blocks that migration, the next generation is in jeopardy.

There are hundreds of herring swimming against the rush of water in Herring Brook in Pembroke. It’s sheer instinct. The fish know they have to battle upstream to lay eggs.

“In spring, as we have the temperatures warm up and increased flows out of our rivers, that cues them to come in,” says Samantha Woods, the Executive Director of the North and South Rivers Watershed Assoc.

One of her organization’s missions is to make sure the herring can get to where they need to go so the species survives. The roadblock is usually dams, about 3,000 of them in the state.

“In most of our streams in Massachusetts we have dams left over from, many times, from colonial times and the industrial revolution,” she says.

Those dams have reduced the herring population. Conservation efforts are making a difference.

“This is a fish ladder along the Herring Brook,” Woods says, pointing to a metal structure angled upward with water rushing through it.

The herring pool below the ladder, eventually figure out where the entrance is and swim up the passageway, emerging into the pond above. The ladder even has a laser fish counter proving how successful it’s been.

“Just recently there were 19,000 fish in one day that went through the electronic counters her,” Woods said. That means hundreds of thousands of fish will pass through.

“That being said, in the time of the Wampanoags there would have been millions,” Woods added.

Why are the herring important? Because other species rely on them for food, from bigger fish like striped bass, to many birds and other critters.

“They’re still in their recovery phase this fish, and they need to have more help to have their populations recover,” Woods says.

There is an effort to remove some of the dams that are no longer useful, but it’s a slow and expensive process. In the meantime, fish ladders are helping.

Volunteers from The North and South Rivers Watershed Association play a role in keeping the ladders operational, and the town of Pembroke also does a lot of work to protect the fish.