If you choose to live in fast water, you have to get used to being buffeted about, and that is exactly what stream dwelling trout have done. They appear unaffected by turbulence that would disturb fish used to more tranquil waters. So what adaptations have stream dwelling species made to their turbulent life styles? Anja Przybilla from the University of Bonn explains that trout seem to be able to hold their position just to the side of and behind obstacles in rivers using only a gentle swimming action and this behaviour is known as entraining. Knowing that some fish take advantage of turbulence generated immediately behind rocks to save energy, Przybilla and her colleagues Horst Bleckmann and Christoph Brücker decided to find out how trout apparently defy their turbulent environment and hold a steady station when entraining ().
Placing a 5 cm diameter semicircular cylinder in a flow tank with water flowing at 42 cm s–1, Przybilla allowed individual trout to find a location where they were happy to hold station near the D-shaped obstacle and filmed them swimming with high speed video. Reviewing the movies, Przybilla saw that the fish spent almost 30% of their time swimming in the entraining positions, either to the left or to the right of the D-shaped cylinder's corners. She was also struck that instead of swimming continually, the trout interspersed periods of undulating swimming with brief periods, lasting less than a second, of inactivity. Przybilla explained that during the period when the fish was inactive it drifted back slightly, recovering its position quickly when it restarted swimming. The fluid dynamic forces acting on the fish must have been almost perfectly balanced while the fish was almost stationary.
Having measured the trout's position, body angle relative to the flow direction and other movement parameters, Przybilla described her discovery that the fish stopped swimming while entraining to Brücker and Alexander Rudert from TU Bergakademie Freiberg. They used computational fluid dynamics to build a mathematical model to explain the unexpected behaviour. Analysing the model, the team could see that the angled fish's body essentially behaved like an aerofoil, with the forces generated by the fast flowing water between the fish's body and the D-shaped obstacle cancelling out the lift and drag forces exerted on the body by the water flow. ‘The calculations pointed out that this is an energetically beneficial way of swimming because it is low cost,’ says Przybilla.
Having shown that entraining fish take advantage of the fluid dynamics for an almost free ride, Brücker suggested replacing the D-shaped cylinder in the flow tank with a long thin D-shaped plate as his calculations had shown that this should improve the trout's stability. Trying the trout out in the flow tank with the D-shaped plate, Przybilla found that the fish used their pectoral fins less for stability than they had with the D-shaped cylinder, so entraining next to a long obstacle is even more energetically efficient than entraining by small rocks.
Finally, the team decided to directly visualise the fluid flows around the fish, but as the trout would not swim in the thin plane of laser light that the team needed to reveal the fluid motion, Sebastian Kunze from Brücker's lab built a model of a fish's body and positioned it in the water like an entraining trout. The flows behaved exactly as Rudert and Brücker had predicted. So trout that live in fast flowing water take advantage of the turbulence to save energy while entraining.
