THE heroic efforts of fish battling upstream to spawn are not quite what they seem. Many actually cheat a little, by hitching an energy-saving ride against the flowing current.
To give themselves a boost on their upstream journey, the fish use eddies in the current that fling them forwards, a new study shows. This saves so much energy that they do not need to use 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.
Lauder鈥檚 graduate student James Liao, together with colleagues at the Massachusetts Institute of Technology, used reflective glass beads to see how the current flowed around brook trout (Salvelinus fontinalis) as they swam against the current in a flow chamber. To their surprise, they found that as fish approached an obstacle that created turbulent eddies they began slaloming from side to side with an unusual, relaxed motion. 鈥淭hey look kind of floppy, not stiff,鈥� says Lauder.
Advertisement
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 effect, the fish were tacking against the current in the same way as a sailboat tacks upwind (Science, vol 302, p 1566). The researchers have confirmed that perch, sunfish and alewife also use the same trick.
Recordings of electrical activity in the fishes鈥� motor systems showed a slight turn of the head was all that was needed to catch each eddy. The large swimming muscles remained inactive. This presumably saves a huge amount of energy, though Liao has not measured exactly how much.
鈥淲e have long suspected fish may do this,鈥� says Scott Hinch, a fisheries biologist at the University of British Columbia in Vancouver, Canada. Hinch noticed several years ago that salmon seek out turbulent regions during their upstream migrations, and he figured this must give them a boost, because they move upstream more quickly than they could swim against the steady current.
Now Liao鈥檚 study has shown exactly what this boost is. 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�