Big Fish Are in Trouble, Little Fish Are Fine, but They’re All Leaving the Tropics

A new study finds that big declines in fish populations could have a catastrophic impact on local food supplies.

(Photo: Franco Banfi/Getty Images)

Oct 24, 2014· 1 MIN READ
Taylor Hill is an associate editor at TakePart covering environment and wildlife.

Fish are drawn to tropical coral reefs like tourists gathering around Paris crepe stands. But thanks to climate change, tropical hot spots are getting too hot.

That’s according to a new study from the University of British Columbia, which finds that fish could start migrating toward the poles. If water temperatures keep rising, the tropics could see whole populations of fish species pack up and leave by 2050.

Scientists used the Intergovernmental Panel on Climate Change’s models to predict how 802 fish species and invertebrates, such as crabs and lobsters, would react to warmer waters.

The worst-case scenario—in which ocean temperatures rise by as much as 3 degrees Celsius by 2100—showed fish could move as far as 16 miles per decade away from the equator.

“The tropics will be the overall losers,” said William Cheung, associate professor at the UBC Fisheries Centre and coauthor of the study.

That would have a catastrophic impact on the region’s food supply.

“This area has a high dependence on fish for food, diet, and nutrition,” said Cheung. “We’ll see a loss of fish populations that are important to the fisheries and communities in these regions.”

Bluefin Tuna (Photo: Sue Flood/Getty Images)

Large predatory fish are already in hot water. Thanks to nearly a century of industrialized fishing, the world’s predatory fish population has declined by 54 percent since 1960, according to another UBC study.

Researchers looked at more than 200 food-web models—which chart interacting food chains—and found that the total biomass of predatory fish species has fallen by more than two-thirds in the past century.

Top-level species that people like to eat, such as tuna, swordfish, grouper, and shark, have been hit the hardest.

The biggest change now that there are fewer big fish? More little fish. Small prey species such as anchovies and sardines have seen their total volume by weight more than double in the past century, according to the study.

“Overfishing has absolutely had a ‘when cats are away, the mice will play’ effect on our oceans,” Villy Christensen, UBC professor and the study’s lead author, said in a statement. “By removing the large, predatory species from the ocean, small forage fish have been left to thrive.”

Still, disrupting the food web generally has negative effects. Take the decline of sea otters off the California coast. The otters kept herbivorous sea urchins in check, which allowed kelp forests to thrive and provide habitat for a plethora of marine species. In areas where otters have disappeared, urchins have exploded in population as they devour kelp forests.

“This should be a warning bell for the rest of the world,” Christensen told The Independent. “We are still fishing way too much. Predators are important for maintaining healthy ecosystems."