Predators are growing hungrier in the rapidly heating seas

A warming planet means a warming ocean. The seas soak up over 90 percent of the heat humanity traps on Earth. Last year, ocean warming reached a record high.

These soaring marine temperatures have a multitude of problematic impacts, the likes of rising sea levels, destabilized Antarctic glaciers, and disrupted marine ecosystems. And as David Attenborough’s documentary Our Planet vividly captures, warmer oceans strip the algae off coral reefs, bleaching them. Now, new research recently published in the journal Science reveals yet another problematic repercussion: There's evidence, detailed below, that predator fish eat more in warmer waters. This could imperil many species lower in the food chain. 

"Unfortunately, I think we're going to experience a lot of loss," Gail Ashton, the study's lead author and a marine ecologist at the Smithsonian Environmental Research Center, an organization researching water and land ecosystems, told Mashable. The new study observed predation at 36 sites in coastal Pacific and Atlantic oceans, from Alaska down to South America.

The key reason for this upswing in predation is predators are burning more energy. The higher the water temperature, the more energy-demanding it becomes for animals high in the food chain to stay active. This motivates predators to hunt for more food.

The study covered 36 sites in the Pacific and Atlantic oceans. Credit: Nicolas Battini / CONICET

"How much they need to eat is going to depend on water temperature," Neil Hammerschlag, a marine ecologist at the University of Miami, told Mashable. "Under a [warming] climate change scenario, their feeding increases," Hammerschlag, who was not involved in the research, explained.

The effects of ocean warming on predators aren't uniform across the seas, as temperatures vary widely at different latitudes. The "predation intensity," as the authors call it, is lowest at higher latitudes (the poles), and more pronounced in warmer waters closer to the equator. But as the seas continually absorb more heat, predation might increase at the poles too, the researchers say. (The Arctic, for example, is a rapidly warming region.)

Previous research has also shown that ocean warming influences the way predators travel across oceans, and eat. Observations of tiger sharks, published in the science journal Global Change Biology earlier this year, revealed an interesting pattern: Waters closer to the poles (in the North Atlantic) warmed at a much faster rate than usual, thanks to ocean heating. This allowed tiger sharks to expand their range and move north from the equator. This has a direct impact on how much, and where, the sharks eat.

"Tiger sharks, if they're now spending more time in an area that they previously weren't because of warming, they are going to increase top-down pressure on that food web," Hammerschlag explained.

In recent decades, the oceans have continually absorbed massive amounts of heat. Credit: NOAA

Ashton’s team couldn’t pinpoint nearly all the predator species that grew more active in this single study. But they did find specific predators that likely consumed more prey in warmer waters, like triggerfish and pufferfish.

In the future, Aston and her team plan to research which prey were most impacted by hungrier predators.

Keeping tabs on predator activity

To track predatory activity in disparate parts of the ocean, the research team used squids as bait. An hour after leaving the bait underwater, biologists found more intense predation in warmer waters, meaning more bait was consumed. And as the researchers expected, the predation activity dropped to almost zero in the coldest waters (below 68 degrees Fahrenheit).

Then, the researchers tested the impacts of this heightened predator activity on prey. They temporarily caged prey creatures (like sea squirts) in the area and found the total number of living organisms, or biomass, in the warm waters to be higher. The prey species thrived. But when scientists uncaged the prey, their numbers once again fell.

One of the predators the study identified were triggerfish, who are voracious eaters. Credit: Smithsonian Institution

It's uncertain how energy-demanding, hungrier predators will impact both predators and prey in the long run, as major human activities come into play, too. For example, the fishing industry targets predator fish, meaning in certain regions of the ocean prey may be unaffected, or lesser affected, by the warming seas.

Yet the evidence suggests that some prey species will experience more predation, and declining numbers, in a rapidly warming world.

"Species have not been able to adapt at the rate that we're asking them to," said Ashton. "Unfortunately, the losses are gonna lead the way."