A giant carp, a critically endangered freshwater fish found in Southeast Asia. Zeb Hogan

Over the last half century, populations of migratory freshwater fish have dropped by 81 percent, according to a stark new U.N. report.

Hundreds of species of fish, which together sustain hundreds of millions of people, are imperiled by warming, pollution, dams, and intensive fishing, according to the report , launched at a U.N. meeting on migratory wildlife now underway in Brazil. As threats mount, populations of freshwater animals are declining faster than populations of animals on land or sea.

With more than 250 rivers and lakes worldwide crossing international borders, conservationists say that countries must work together to protect imperiled fish. “Rivers don’t recognize borders — and neither do the fish that depend on them,” said coauthor Michele Thieme, deputy lead of freshwater for World Wildlife Fund U.S.

The dorado catfish of the Amazon basin offers a prime example.…