Biggest year for wild Atlantic salmon returns to U.S. rivers since 2011

PORTLAND, Maine — The last wild Atlantic salmon that return to U.S. rivers have had their most productive year in more than a decade, raising hopes they may be weathering myriad ecological threats.

Officials counted more than 1,500 salmon in the Penobscot River in Maine, home to the country’s largest run of Atlantic salmon, state data show. That is the most since 2011, when researchers counted about 2,900.

The salmon were once abundant in American rivers, but factors such as overfishing, loss of habitat and pollution reduced their populations to only a handful of rivers in Maine. The fish are...