Each Sebasticook alewife undertakes a remarkable journey, beginning when it first wriggles out of a waterborne egg alongside tens of thousands of siblings. It heads downstream, past Benton, seeking the Atlantic Ocean, and then heads down the coast, sometimes traveling as far as South Carolina.

For three years, the alewife stays at sea, swimming thousands of miles while evading predators up and down the coast.

At age 4, instinct drives them to head back upriver to spawn with each inch of the 65-mile trip a hard fought battle against the current, won at a rate of about one mile per hour, said Nate Gray, a biologist with the Department of Marine Resources.

If 100,000 fish make it back to spawn in the Sebasticook, they can lay 15 billion eggs, but Gray said predicting the number of alewives that will successfully complete the cycle of life in any given year is impossible.