Many fish appear to hang effortlessly in the water while they wait for prey, defend a nest or pause between bursts of activity. But our research shows that this quiet stillness is anything but effortless. Hovering, the behaviour that allows a fish to remain suspended in one place, is far more energetically demanding than scientists once believed.
In a comparative study of 13 near neutrally buoyant species, we found that metabolic rates during hovering were almost twice as high as during rest (when the fish supports its weight with the bottom of the tank). In some cases, they were even greater. These findings challenge the long standing assumption that fish can remain motionless in the water column at little physiological cost.
Most bony fishes possess a swim bladder, which allows them to regulate buoyancy and avoid sinking or floating. This ability has encouraged the idea that once a fish reaches neutral buoyancy it can stay at its chosen depth with minimal effort. Our results show that the story is more complex. A fish that hovers must do more than balance weight and buoyant force; it must also control its posture.
In many species, the centre of mass and the centre of buoyancy do not align perfectly. The slight offset between them creates a continual torque that would cause the fish to roll or pitch if no corrective action were taken. Even in still water, a hovering fish must repeatedly counter these small rotational forces. What looks like serene suspension is in fact the product of continuous and precise adjustment.
To understand the true energetic cost of these corrections, we combined metabolic measurements with detailed observations of movement. Each fish was placed in a respirometer chamber so we could measure oxygen consumption during hovering. We recorded its movements using synchronised high speed cameras. We also quantified important aspects of body form, including the positions of the centres of mass and buoyancy, using anatomical measurements and micro CT scans.
Although the fish were incredibly good at maintaining postural equilibrium, the recordings revealed an uninterrupted sequence of minor fin movements. Pectoral, pelvic, anal and tail fins all contributed to the task of maintaining position. The fin trajectories varied across species and often traced intricate three dimensional paths.
The energetic consequences of this activity were striking. Across the thirteen species, hovering metabolic rates ranged from about 158 to 351 milligrams of oxygen per kilogram per hour, always above resting levels. Most species nearly doubled their metabolic expenditure during hovering.
A few fish, such as gouramis, managed to hover with only a small rise in metabolism. Others, including giant danios, cichlids and glass catfish, expended far more energy. In these species, the tail played a particularly active role. Their tail fins moved through larger distances than those of the low cost species. This indicated that tail driven corrections, rather than pectoral fin use alone, were central to the task of staying still.
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Body shape had a clear influence on energetic demand. Deep-bodied fish, with their larger surface area, generate more resistance as water moves around them, making them naturally better at resisting unwanted rotations. These species relied less on fin movements and maintained position at comparatively low energetic cost.
Elongated or narrow-bodied fish were less inherently stable and needed more frequent corrections. Fin position mattered too. Species with pectoral fins set farther back on the body hovered more efficiently, because even small movements produced effective stabilising forces.
A hidden cost of everyday behaviour
So, hovering is far from a trivial activity. Many fish do it routinely throughout the day, whether guarding eggs, feeding on particles in the water, avoiding obstacles or keeping their place within a school. Understanding how much energy these routine actions require helps biologists build more accurate pictures of the daily lives of fishes and the ecological pressures they face.
The findings also shed light on the evolution of fish form and movement. Many teleost fish (bony fish, such as cod, salmon and goldfish) are inherently unstable. It’s a quality that allows them to manoeuvre rapidly when they need to turn sharply or evade predators.
But this same instability means they must make constant corrections whenever they stop moving. The balance between instability, control and energy use has shaped the extraordinary diversity of body shapes and fin arrangements found in modern fish.
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Our study has practical relevance beyond biology. Engineers designing underwater robots face many of the same challenges that fish have solved. A robot that needs to hold its position in moving water can waste significant power stabilising itself. By studying how fish coordinate multiple fins to correct minute disturbances, designers may be able to create more efficient vehicles capable of hovering for long periods while using far less energy.
The next time you see a fish suspended apparently without effort in an aquarium, it’s worth remembering what lies beneath that calm surface. Hovering may look simple, but it is a remarkably demanding feat of balance and control.
Our study shows that fish invest far more energy than expected simply to stay in place – a hidden cost in the daily lives of animals that spend much of their time looking as though they are doing nothing at all.
