Fish don’t rest while hovering - it's actually hard work

It’s easy to assume that when fish hang still in the water, they’re taking a break. After all, they look motionless – just calmly suspended in place. But new research shows that this “hovering” is actually a demanding task that takes more effort than resting.

The study comes from scientists at the University of California San Diego’s Scripps Institution of Oceanography. It focused on how fish manage to stay still in open water without sinking or drifting away.

The team found that fish burn nearly twice as much energy while hovering as they do when resting on the tank floor. That means what looks like rest is really anything but.

Rethinking stillness in fish

Most bony fish have swim bladders – gas-filled sacs that help them stay neutrally buoyant, which make them neither float upward nor sink.

Because of this, researchers once thought that hovering was easy. But the new findings challenge that idea.

The scientists studied 13 different fish species with swim bladders. Each fish was placed in a special tank where its oxygen use was recorded during two behaviors: resting on the bottom and hovering midwater.

At the same time, high-speed cameras captured every fin twitch and tail flick.

The team also measured body features like the shape of the fish and the position of its fins. One key measurement was the distance between the fish’s center of mass and center of buoyancy.

These don’t always align. That small misalignment creates instability, which the fish must constantly correct.

“Hovering is a bit like trying to balance on a bicycle that’s not moving,” said Valentina Di Santo, an assistant professor at UC San Diego and study author.

Stability drains fish energy

Because of the mismatch between mass and buoyancy, hovering fish have to work against the natural tendency to roll, tilt, or drift.

The more misaligned those centers are, the more energy the fish burns trying to stay still. That constant effort means hovering is far from restful.

“What struck me was how superbly all these fishes maintain a stable posture, despite their intrinsic instability,” said Di Santo.

Fish with more compact, deeper bodies like goldfish or pufferfish were better at hovering efficiently. In contrast, longer, thinner fish – like giant danios or shell dwellers – had to work harder.

Fin position also played a role. Fish with pectoral fins placed farther back on their bodies generally hovered more efficiently. The researchers believe this may be due to improved leverage.

Goldfish have an edge

“This changes how we see hovering. It’s not a form of rest at all,” said Di Santo. “It’s an energetically costly activity but one that fish engage in anyway because it can be very useful.”

Fish may hover when guarding a nest, feeding in a specific spot, or holding their place in fast-moving water. These tasks require them to stay stable in one position, even when it’s exhausting.

The study also pointed out a trade-off. Fish with shapes that give them greater control and agility often burn more energy while hovering. Meanwhile, more stable shapes are less agile.

But that trade-off may be worth it. Being able to move nimbly through coral reefs or complex underwater environments is key to survival.

Fish inspire robot design

The findings could also influence the next generation of underwater vehicles. Most robots today are built to be stable. But like fish, that stability may limit their agility.

“By studying how fish achieve this balance, we can gain powerful design principles for building more efficient, responsive underwater technologies,” said Di Santo.

For engineers, this could mean building in a bit of instability and designing systems that, like fins, actively correct for it.

Doing so could allow underwater robots to better explore tight or cluttered environments, such as coral reefs or wreck sites.

“If you want a robot that can maneuver through tight spaces, you might have to learn from these fishes to design in some instability and then add systems that can dynamically maintain stability when needed,” said Di Santo.

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