Fish circling burns twice as much as energy scientists expect

Things that seem effortless aren’t always that easy. When fish are still on the water column, they seem to be resting, but new research shows that in fact their work is actually twice as much as scientists have imagined.

A comprehensive study of 13 fish species shows that hover burns almost twice as much energy as the real rest, overturning decades of assumptions about aquatic motion.

The findings, published in the Proceedings of the National Academy of Sciences, challenged a long-standing belief that there is little underwater fixation for fish equipped with swimming bladders, which is almost nothing. These gas-filled organs allow fish to achieve neutral buoyancy, leading researchers to believe that hovering is essentially free energy.

Static hidden mechanism

Scientists at the Scripps Oceanography Institute at UC San Diego found that fish are inherently unstable when wandering, like trying to balance on a fixed bike. Although their swimming bladders make them almost weightless, the fish keep tilting because their mass and buoyancy centers do not align perfectly.

“Horse is a bit like trying to balance on a bike that is still,” explains Valentina Di Santo, a marine biologist at Scripps. This misalignment creates a persistent tendency to force the fish to make continuous fin adjustments to maintain its position.

The team used a high-speed camera to capture fish that were hovering while measuring their oxygen consumption. The results were shocking – the thorny fish consumed about twice as much as the fish at the bottom of the tank.

Key findings include:

• The hovering metabolic rate ranges from 158 to 351 mg per kilogram of oxygen per kilogram
• Energy costs reach hover for up to 0.94 kg per kg for 10 minutes
• Separating larger fish between mass and buoyancy centers uses more energy
• Species with posterior pectoral fins proved to be more effective in hovering

The constant fin movement required for stability is not random. Fish exhibit complex three-dimensional fin movements, and the pectoral fins can reach 2.5 individuals per second in the complex Figure 8 mode. The caudal fin shows the biggest difference between energy-efficient and energy-intensive radiators.

Body shape determines energy costs

Studies have shown that fish anatomy significantly affects the efficiency of hovering. Long, slender species like giant Danios and shell resident Cichlids struggle with energy costs, while compact, dark fish such as goldfish and pufferfish linger more effectively.

This creates an evolutionary trade-off that explains fish diversity. “This changes the way we see hovering. It’s not a form of rest at all,” Di Santo notes. “It’s an energy-intensive activity, but fish can be engaged in anyway because it can be very useful.”

Energy consumption is biologically significant when fish behavior is considered. Hovering can achieve critical activities such as nesting, precise feeding, and maintain position in complex environments such as coral reefs. High energy costs represent the necessary investment in the special agility required for structurally complex habitats.

beyond biology

These findings go beyond fish biology and enter engineering applications. Understanding how fish achieve stability while maintaining operability can inform underwater robot design, especially for vehicles driving narrow spaces such as coral reefs or shipwrecks.

“By studying how fish achieve this balance, we can gain strong design principles to build more efficient, responsive underwater technology,” explains Di Santo. Current underwater robots prioritize built-in stability, which limits operability, which is the opposite approach to evolving fish design.

Research shows that future underwater robots may benefit from engineering instability pairing with dynamically stable systems, thus mimicking the method of fish trading constant energy consumption to improve agility.

This study fundamentally reshapes the scientific understanding of fish motor energy. The seemingly effortless hover actually represents one of the most energy-intensive behaviors performed by fish regularly, highlighting the significant adaptation to being able to survive in complex aquatic environments.

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