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Fish with a more compact body, such as this damselfish, expend less energy while hovering compared to fish with a long, slender body. Credit: Philippe Bourjon, CC BY-SA 4.0
Often, scientific research confirms what scientists already suspected, but other times, science reveals a twist that no one saw coming. That’s the kind of study that recently showed how much energy fish burn while hovering in place instead of resting on the ground.
Scientists have long thought that lingering motionless in the water is just as restful as it looks for fish. After all, most bony fish have a swim bladder—a sac of air that lets fish manage their buoyancy by inflating or deflating the bladder. When the bladder expands with gas, it makes the fish more buoyant. When fish deflate the bladder, they are less buoyant. So just hanging out in the water seems like it should be effortless.
The reality is a different story. Staying in place actually requires a lot of energy! Biologists from the University of California San Diego tested the energy cost of hovering versus resting on the bottom of a tank in 13 species of fish by measuring how much oxygen they used in each position. The researchers also measured each fish’s body size and shape and filmed them to observe their fin speed and movement. They used all these data to calculate each fish’s stability and found that remaining in place in the water column uses about twice as much energy as resting on the bottom.
The scientists also learned that a fish’s shape and the placement of its pectoral fins (on either side of its body) affect how efficiently it can hover. The closer its pectoral fins are to the middle of the body, the less energy it uses to hover. Long, thin fish, like a halfbeak, are less efficient than more compact fish like damselfish and pufferfish. So if hovering uses so much energy, why do it? Staying still can help with guarding nests, feeding and other activities. It just doesn’t help with getting rest!
