MANCHESTER, N.H. — There was a surprising increase in the number of river herring passing the Amoskeag Fishways Learning Center in Manchester this spring where the herring run was already the biggest it’s been since 2000, biologists say.

As of May 15, a total of 17,333 river herring were counted passing through the fish lift at Essex Dam in Lawrence, Mass., the first dam upstream from where the Merrimack River empties in to the Atlantic Ocean.

The figure is almost twice the total of all last year’s run and more than 20 times the run in each of the two years before that.

“They were getting close to getting listed as an endangered species last year. To see them coming back in these numbers is really fantastic,” Rachel Brown, a naturalist at the Amoskeag Fishways, told the Nashua Telegraph. “These are the best numbers that I’ve seen in years.”

It isn’t clear what has led to the increase.