London, March 29 : A pair of researchers from the Israel Institute of Technology in Haifa has discovered that it hurts dolphins to swim faster than about 54 kilometres per hour near the surface, but tuna do not suffer the same problem.
Gil Iosilevskii and Danny Weihs conducted a study to determining what limits the maximum speed at which fish like tuna and mackerel, and cetaceans like dolphins could swim.
The researchers carried out a series of calculations to model the tail and fins of these creatures for the purpose.
They found that, while muscle power limits the swimming speed of small fish, this is not the case for larger and more powerful swimmers like tuna and dolphins.
"There are certain limits on swimming speed that are imposed irrespective of power," New Scientist quoted Iosilevskii as saying.
The researcher said that one such limit is the frequency at which the swimmers can beat their tails to propel themselves forward, while the other is the formation of microscopic bubbles around the tail, a phenomenon known as "cavitation".
The researcher duo reckons that cavitation can be the most important limiting factor for animals like dolphins, which have nerve ending in their tails.
The bubbles form as a result of the pressure difference created by the movement of the fins, and this process produces the ribbons of tiny bubbles that stream behind a ship's propeller.
When the bubbles collapse, they produce a shockwave that eats away the metal in propellers, something that is painful for dolphins.
The researchers say that as per their calculations, within the top few metres of the water column, this happens when the dolphins reach 10 to 15 metres per second, or 36 to 54 kilometres per hour.
Tuna, on the other hand, have "bony" tails without nerve endings, which is why they may sometimes break the speed limit imposed by the pain barrier.
However, still, 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.
The study has been reported in the Journal of the Royal.