ORONO, Maine (AP) Salmon counters found fewer of the endangered fish in the Penobscot River last year than in any year since 2016.

Atlantic salmon are listed under the Endangered Species Act in the U.S., as the country’s only remaining wild populations of the fish are found in a few Maine rivers. The Penobscot River is vitally important to the future of the fish, and salmon returns there are watched closely.

Only 561 salmon were counted in the Penobscot last year. That was the lowest number since 2016, when 503 fish were found, the Bangor Daily News reported.

The number was a disappointment after more than 1,400 salmon were found a year ago. That was the highest return since 2011.

The low numbers likely reflect factors such as low survival at sea, the impact of drought during the fish’s time in the river and mortality from hydropower projects, said Sean Ledwin, sea run fisheries and habitat director for the Maine Department of Marine Resources.