Fish in the Northeast Atlantic – including cod, the prime ingredient in fish-and-chips – saw a dramatic drop of 34 percent in the past several decades as the Earth warmed.

And it's not only cod: many other species of fish are in hot water – literally.

Warming oceans from human-caused climate change has shrunk the populations of many fish species around the world, according to the study released Thursday.

Overfishing and poor fisheries management have only intensified the problem.

Some of the biggest drops were In the seas near China and Japan, where fish populations dropped by as much as 35 percent from 1930 to 2010, the decades analyzed in the study.

"We were stunned to find that fisheries around the world have already responded to ocean warming," said study co-author Malin Pinsky, a Rutgers University ecologist. "These aren't hypothetical changes sometime in the future."

Globally, the drop is 4.1 percent for many species of fish and shellfish, according to the study, which was led by Chris Free, formerly of Rutgers and now a post-doc at the University of California, Santa Barbara.

Keeping fish stocks plentiful is vital, the study says, since Earth's oceans have become a crucial source of food for the planet's rapidly growing population. In fact, more than 50 million people around the world earn a living by fishing, and seafood provides about half of the protein eaten by people in developing nations, according to the study.

“We recommend that fisheries managers eliminate overfishing, rebuild fisheries and account for climate change in fisheries management decisions,” Free said.

In the study, Free and his team studied the impact of ocean warming on 124 species in 38 ecological regions around the world.

It's not necessarily all bad news, however: While most fish populations will see a downturn as the seas warm, some, like black sea bass along the Mid-Atlantic coast, saw an increase.

However, “fish populations can only tolerate so much warming,” said study co-author Olaf Jensen, also a Rutgers scientist. “Many of the species that have benefited from warming so far are likely to start declining as temperatures continue to rise.”

And worldwide, more fish populations dropped than rose during the period studied.

Looking ahead, "future fisheries production may be at even greater risk considering that, owing to (human-caused) climate change, the oceans are continuing to warm even faster than originally predicted," said Australian scientist Éva Plagányi in a commentary that accompanied the study.

Additionally, the study only looked at how warming oceans affect fish and did not take into account other climate-driven impacts, such as ocean acidification, which can also lead to marine populations declines. The world's seas are becoming increasingly acidic because of the rising levels of atmospheric carbon dioxide.

The study was published in the peer-reviewed journal Science.