The World's Fish Are Shrinking

The ocean's fish are getting smaller, posing a threat to global food security for the 3 billion people whose meals depend on them. And while scientists have a name for the puzzling phenomenon—the temperature-size rule—they cannot pinpoint the mechanism that's being triggered to cause it. "We're blinded to fixing problems if we don't understand what's causing them in the first place," Deakin University professor Timothy Clark of Australia tells the Washington Post.

• Factors: Ocean fish samples suggest that almost three-fourths of fish populations shrank in body size between 1960 to 2020, according to a study in Science last year. One factor is likely overfishing, because both recreational and commercial fishers tend to keep big fish and toss back smaller ones. Another is rising water temperatures, although the reason is unclear.
• Warm vs. cold: In one experiment cited by the Post, two scientists raised 400 brook trout in tanks at different temperatures, and those in warmer water averaged less than half the size as the cooler trout. "You look at the fish, it's so obvious," says University of Massachusetts conservation biologist Lisa Komoroske. But why it's happening remains a topic of study.
• Theory debunked? One leading theory revolves around gill capacity. Fish need more oxygen in warmer water, and the idea is that it's easier for gills to handle the load for smaller bodies. However, Komoroske and collaborator Joshua Lonthair cast doubt on that in their trout study, which showed sufficient gill size to take in sufficient oxygen. They say a combination of factors, not just oxygen levels, are likely at play, per a release from the UMass Amherst College of Natural Sciences.
• Changing ecosystems: Larger fish such as the North Atlantic skate are steadily getting smaller, while smaller species such as mackerel are becoming more plentiful, reports the Guardian. Multiply changes like that throughout the ocean, and it adds up to major shifts in the ecosystem.

(In the world's oceans, new temperature records have become a regular occurrence.)