Swimming fish stick to same Strouhal number

Abstract

Fish swimming at a certain speed appear to adopt a narrow range of swimming styles instinctively. Robert Nudds and colleagues from the University of Manchester, UK, explain that fish seem to adapt their undulations so that the product of their tail beat frequency and the maximum displacement of the tail tip when divided by the fish's forward velocity almost always converges to a number ranging from 0.2 to 0.4, known as the Strouhal number. This means that, at a certain swimming speed, if the fish beats its tail faster it compensates by dropping the amplitude of the tail's undulation, or if the tail beats become broader, they slow down. The same phenomenon, where movements are conserved to a narrow range of Strouhal numbers during locomotion, has also been found in bird flight, leading scientists to conclude that animal movements have been tightly constrained by evolution to maximise propulsive efficiency. However, Nudds points out that none of the observations that have directed scientists to this conclusion had been collected systematically, so Nudds, Emma John, Adam Keen and Holly Shiels decided to resolve the problem by investigating how trout adjust their swimming styles in warmer waters (p. 2244).The team explains that ectothermic fish are susceptible to the environmental temperature, so when the temperature rises, the fish's metabolic rate rises also, reducing the amount of energy that they can invest in swimming at higher speeds and affecting how they beat their tails. With that in mind, the scientists filmed young trout swimming at speeds from 0.28 to 1.11 m s−1 in water at temperatures of 11 and 20°C while recording the fish's tail beat frequency and amplitude.After painstakingly analysing hours of swimming data to calculate the fish's Strouhal number at different speeds and temperatures, the team could see that the fish maintained the same Strouhal number when swimming at both the low and high temperatures at any given speed. The hotter fish beat their tails faster than the cooler fish, but they compensated by reducing the amplitude of each tail beat to maintain a constant Strouhal number at that speed. ‘This is the first experimental evidence for an apparent adherence to a preferred (perhaps optimum) Strouhal number for an animal using oscillatory propulsion’, the team says.However, when they calculated how the fish's Strouhal number varied as the animals speeded up, they saw that the number gradually increased from 0.19 at the lowest speeds to 0.22 at the highest. Although the fish continued beating their tails at the same rate, they increased their speed by sweeping the tail further. The team suggests that the fish's apparent inability to increase their tail beat frequency indicates that their muscle contraction frequency might be finely tuned to a very narrow range at any given temperature. And when they measured the metabolic rate of the fish, although the animals' basal metabolic rate increased with temperature, they did not have to increase the amount of effort they put into fast swimming at higher temperatures. The team suggests that small increases in water temperature may even benefit trout swimming performance. They conclude by saying, ‘Future predictions of changes to water temperature of lakes and rivers are only 5–10°C over the next 100 years and it appears that rainbow trout, at least, can cope with this easily in terms of swimming biomechanics.’