Fish using more energy to stay still than first thought, research finds

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Fish staying still in water are using considerably more energy than first thought, new research has found.

Remaining stationary in the water is essential for fish as it helps them look out for predators, access prey in small crevices and feed on hard-to-reach resources like aquatic plants.

Fish achieve this by so-called “hovering” which includes corrective fin and body movements. Without these movements, they would naturally flip sideways or upside down.

The research, carried out by a team including an academic from Aberystwyth University’s Department of Computer Science, discovered that hovering requires twice as much energy compared to resting.

The reason for the excess energy use is instability, which is caused by the swim bladder, a gas filled sac that sits within nearly all bony fishes. Although the swim bladder provides the buoyancy that stops a fish from sinking, it also creates a stability problem. Due to the location of the swim bladder, there is a separation between the centre of buoyancy and centre of mass, and this separation makes fish inherently unstable. This is why dead fish are seen floating on their sides or upside down because they naturally tip.

The findings could have implications for the design of more manoeuvrable and stable miniature aquatic robots. The research involved the team placing 13 species of bony fish in a specialised tank and recorded oxygen consumption while they were hovering and while they were at rest at the bottom of the tank. High speed cameras were used to track fin movements.

Body size and shape were also measured from each fish to investigate what influences instability during hovering. This data was then used to create a simple model to link the body properties of fish to metabolic cost of hovering. The model suggests that fish with deep compact bodies, such as goldfish, were more efficient in their hovering, while long slender fish such as rummy-nose tetra had more difficulty.

Dr Otar Akanyeti, from Aberystwyth University’s Department of Computer Science, said: “You would have thought that staying still would be easy but this research has upended this assumption. Hovering is essential for fishes to eat, avoid predators or hunt prey but it has energetic consequences that are not negligible. It is similar to trying to balance on an inflatable tube in the swimming pool or sea, which is harder than it first looks.

“The findings have important real-life implications for sea exploration. First, they help us with ecological modelling to better understand fish behaviour. Second, they can be used to improve the manoeuvrability of undersea robots, raising the possibility of them being able to access previously unreachable areas.

“Undersea robots are traditionally compact for stability but by using these findings, we can in effect build in some instability to help the robots move in the same way as fishes who hover more effectively.”

The findings were published in a paper in the Proceedings of the National Academy of Sciences. In addition to Dr Akanyeti, the study was co-authored by seven institutions. The research was led by the University of California San Diego’s Scripps Institution of Oceanography.

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