What is the fastest a dolphin can swim? Near the surface, no more than 54 kilometres per hour. Why? Because it hurts it to swim faster.
Those are the findings of a pair of researchers from the Israel Institute of Technology in Haifa. But tuna, they say, do not suffer the same problem.
Gil Iosilevskii and Danny Weihs carried out a series of calculations to model the tail and fins of fish such as tuna and mackerel, and cetaceans such as dolphins. The aim was to determine what limits the maximum speed at which these creatures can swim.
The researchers found that although muscle power is the limiting factor for small fish, this is not the case for larger and more powerful swimmers such as tuna and dolphins.
“There are certain limits on swimming speed that are imposed irrespective of power,” explains Iosilevskii. One of these is the frequency at which the swimmers can beat their tails to propel themselves forward.
The other is the formation of microscopic bubbles around the tail, a phenomenon known as “cavitation“. According to Iosilevskii and Weihs, for animals such as dolphins that have nerve endings in their tails, cavitation can be the most important limiting factor.
The bubbles form as a result of the pressure difference created by the movement of the fins. This process is what produces the ribbons of tiny bubbles that stream behind a ship’s propeller (see image).
When the bubbles collapse, they produce a shockwave, which eats away the metal in propellers. To dolphins, it is painful. According to the researchers’ calculations, within the top few metres of the water column, this happens when the dolphins reach 10 to 15 metres per second (36 to 54 kilometres per hour).
Tuna have “bony” tails without nerve endings, which is why they may sometimes break the speed limit imposed by the pain barrier. Tuna have been known to have lesions typical of the damage caused by cavitation.
Despite this, cavitation does slow tuna down: when the bubbles collapse, they break the flow of water over the fish’s fins and tail, causing it to stall.
But for real speed, head deep. Cavitation events decrease as fish or dolphins swim deeper, and the local pressure increases.
The theoretical top speed of dolphins and tuna is unknown, however, as the animals’ intrinsic power and maximum “tail beating rate” is unknown.
Either way, Iosilevskii says reports of dolphins overtaking speed boats are likely to be a result of the dolphins “cheating” by surfing the bow waves.
Journal reference: Journal of the Royal Society Interface (DOI: 10.1098/rsif.2007.1073)