You could call them ‘hanging fish’: fish that stay in one place for a while, for example to catch a prey or to look for a partner. That sounds sound relaxed but costs a lot of energy in practice, writes an international biologist team in Pnas. In order not to get out of balance, the fish must constantly adjust with their fins and their tail.
The principle of hovering – Stay still in air or water – occurs in nature with all kinds of animal groups. Think of praying birds of prey or gliding, not for nothing hoverflies mentioned in English. For fishing with a swim bladder that is relatively easy, biologists thought. The fact that gas -filled organ occurs in the majority of the fish (sharks and rays excepted) and ensures that a fish can quickly adapt to differences in pressure and depth. By linging in one place, hardly any energy would be used in theory.
The practice shows something else, according to the Pnas-Publicication. The researchers studied the energy consumption of thirteen hanging fish with a very different construction and came to the conclusion that hanging always costs much more energy than resting on a fixed surface. In general, the amount of oxygen that the fish consume when they move from resting to hovering. This also increases the required energy and the required amount of food. However, it differs from species to species how much extra energy is needed.
Vulnerable to disruption
That hanging is so complicated appears to have to do with that swimming bladder. It forms their ‘buoyancy power point’, while their ‘mass tool’ is determined by the distribution of the most important muscle groups and their skeleton. The difference between the two makes them vulnerable to external disturbance: at the least swirl they would spin around their own axis – teh if they make adjustments with their bodies.
With all species, fish with their fins during the hover. Especially the breast fins are constantly on the move: that is how horizontal 8s are turned into the water, the researchers write. Thin, long fish appear to use the most energy while hanging. Their breast fins are further forward, far away from the tail, and contribute less to stability. That is why the elongated fish must adjust more with their tail, resulting in higher energy costs.
For example, a sidderal that stylish uses no less than 464 percent more energy than if it were to swim at a speed of half a body length per second.