Weaving their bodies from side to side while they take refuge behind an obstacle, most fish look as if they are working hard to stay put, whereas in fact they may just be surfing. James Liao, from the University of Florida, USA, explains that as water flows past an object, alternating vortices are shed from the structure, forming an oscillating jet of water that can sweep the fish's body back and forth as they maintain a stationary position behind the object. Explaining that this specialised pattern of movements is known as the Kármán gait, Liao says, ‘they literally let the water currents swim their body instead of powering it with their muscle’, adding that Kármán gaiting fish drop their energy consumption by half as they joyride the obstacle's wake. However, life in real rivers is rarely so simple, and he admits that interpreting the flow in such cluttered environments is extraordinarily challenging. So, instead of investigating how wake-surfing fish interact with the complex flows generated by tangled rocks and roots, Liao and his colleagues decided to analyse the wake generated by a pair of cylinders in a stream and find out how fish exploit this fluid motion to hold their position in more cluttered conditions.
Christina Walker and Liao selected trout that were just under 20 cm long and filmed them as they swam in water flowing at speeds from 20 to 120 cm s−1 behind a pair of 5 cm diameter D-shaped obstacles that were in line and separated by 1–11 cm. The trout adopted the characteristic Kármán gait weaving motion when the cylinders were closely spaced. However, as the separation between the cylinders increased, the fish performed the specialised motion less often.
Curious to find out why the fish were more reluctant to surf behind more widely spaced cylinders, William Stewart viewed the fluid motion behind the pair of objects and analysed the turbulence with Otar Akanyeti. Together, they saw that the obstacles produced a pattern of turbulence that was most similar to that of a single object when they were closely spaced; however, as the separation increased, the pattern of alternating spinning vortices that is essential if a fish is to remain in place became more degraded. And when Fang-bao Tian simulated the interaction between the vortices shed by the two cylinders, he and Stewart found that the turbulence generated by the upstream cylinder disrupted the spinning vortices produced by the cylinder at its rear as the separation increased, effectively destroying the clean flow pattern that was crucial for the fish to maintain its wake-surfing position.
‘Objects placed close together have stronger, more organised wakes that fish can identify and use… while widely spaced objects shed messy wakes that discourage Kármán gaiting’, says Liao, and he adds, ‘This study puts on the radar how important it is to consider how fish can exploit the wakes behind multiple objects to save in the cost of swimming’.