Fish populations are declining as oceans warm, putting a key source of food and income at risk for millions of people around the world, according to new research published Thursday.

The study found that the amount of seafood that humans could sustainably harvest from a wide range of species shrank by 4.1 percent from 1930 to 2010, a casualty of human-caused climate change.

“That 4 percent decline sounds small, but it’s 1.4 million metric tons of fish from 1930 to 2010,” said Chris Free, the lead author of the study, which appears in the journal Science.

Scientists have warned that global warming will put pressure on the world’s food supplies in coming decades. But the new findings — which separate the effects of warming waters from other factors, like overfishing — suggest that climate change is already having a serious impact on seafood.