Hovering fishes aren’t resting, they’re working hard

At first glance, a hovering fish seems to be doing nothing at all, just resting in place. But our new study, published in PNAS, reveals a different story: hovering can nearly double a fish’s energy use compared to true rest.

Why would fishes pay such a steep price? Because holding position is vital. Whether guarding nests, capturing prey, or resisting currents on a reef, hovering allows fishes to stay exactly where survival demands.

The physics, however, are stacked against them. Unlike submarines, a fish’s center of mass and center of buoyancy rarely align, leaving them inherently unstable. Balancing in open water is a bit like trying to stay upright on a motionless bicycle: it requires constant, precise corrections.

High-speed 3D imaging showed that hovering fish are never truly still. Subtle, continuous fin and tail movements are at work 'behind the scenes', and these add up to significant metabolic costs. Some species manage better than others: deep-bodied fishes with more posterior pectoral fin position hover more efficiently, while long, slender species work much harder.

This study highlights a fundamental trade-off in fish design: agility often comes at the expense of stability—and with it, energy. What appears effortless in nature is, in fact, the product of hidden effort.

Read the paper here: https://www.pnas.org/doi/10.1073/pnas.2420015122

The paper was featured on the cover of PNAS with a photo by Phil Zerofski (SIO)