Swimming fish can hitch a ride upstream by riding the turbulent eddies of a burbling brook. The technique is likely to save a great deal of energy because the fish can do this without using their main swimming muscles.
鈥淚t鈥檚 a very low-energy way of moving through a turbulent environment,鈥� says George Lauder, a biomechanician at Harvard University in Cambridge, Massachusetts.
Lauder鈥檚 graduate student James Liao, together with colleagues at the Massachusetts Institute of Technology, also in Cambridge, used reflective glass beads to visualise the current as they watched trout and other fish swimming in a flow chamber.
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To their surprise, they found that the fish鈥檚 behaviour changed abruptly downstream of an obstacle that created turbulent eddies. Instead of swimming normally as they do in smooth currents, the fish began slaloming from side to side with an unusual, relaxed motion. 鈥淭hey look kind of floppy, not stiff,鈥� says Lauder.
When he looked more closely, Liao saw that the fish were bending their bodies into wing-like hydrofoils to catch the eddies that spun alternately off the two sides of the obstacle. In other words, the fish were tacking against the current in a way precisely analogous to the way a sailboat tacks upwind.
Muscle activity
Electrical recordings revealed that the only muscle activity required for this tacking is a slight turn of the head to catch each successive eddy. The large swimming muscles remain inactive. This presumably saves a huge amount of energy, although Liao has not measured exactly how much.
鈥淲e鈥檝e long suspected fish may do this,鈥� says Scott Hinch, a fisheries biologist at the University of British Columbia in Vancouver.
Several years ago, Hinch noticed that salmon sought out turbulent regions during their long upstream migrations. This must provide some sort of boost, he figured, because the fish move upstream more quickly than they could swim against the average current speed.
Liao鈥檚 study is the first to show exactly how the fish get this boost. 鈥淚t鈥檚 really critical,鈥� says Hinch. 鈥淲ithout that, they would not be able to get where they need to go.鈥�
Journal reference: Science (vol 302, p 1566)