The world’s fishing industry relies on what’s called fisheries, the clusters of regional fish populations that people can catch economically. And on average, the researchers found that the numbers of fish in critical fisheries around the world have decreased by four percent since 1930.

Fisheries located in the Sea of Japan and the North Sea were the worst off. They experienced as much as a 35 percent drop in their numbers. Other fisheries, however, benefitted from warmer waters, and their populations grew, an expansion scientists warn could create unsustainable competition for resources.

“We were surprised at the strength the impact of warming has already had on fish populations,” says the study’s lead author, ecologist Chris Free at the University of California Santa Barbara.