Most experiments that look at how fish swim observe the animal's movements in smooth flowing water; however, the real world isn't like that. Fish routinely encounter swirls and eddies that threaten to throw them off course; but by how much and how do they cope? Hans Tritico now at Youngstown State University, USA, and Aline Cotel at the University of Michigan, USA, decided to find out how eddies affect creek chub by putting the fish in a turbulent water tunnel and filming the water's motion and the fish's responses ().
Disturbing the water's flow, with either vertical or horizontal cylinders, the duo generated small eddies (diameters up to 40% of the fish's body length) and large eddies (diameters of 92% of the fish's length) using 0.4 cm, 1.6 cm and 8.9 cm diameter cylinders. While the fish were unfazed by the smaller scale eddies, the larger eddies were more of a problem and horizontal eddies with diameters greater than 75% of the fish's length knocked the fish off course almost 2.5 times more often than the vertical eddies of the same size. In the most turbulent conditions the maximum swimming speed of fish was reduced by as much as 20%. The fish also followed a set recovery routine when they tumbled in the vertical eddies and added two additional body rolls as they righted themselves after spills in the horizontal eddies. Tritico and Cotel also noticed that the fish preferred swimming downstream of the gaps between the large cylinders when the water flowed slowly, but retreated behind the cylinders as the horizontally turbulent water speeded up, while settling behind the cylinder edge in high speed flows through the vertical cylinders.
So fast rotating horizontal eddies with a diameter similar to the fish's length pose the greatest challenge to fish swimming in the natural environment and Tritico and Cotel suggest that ‘the importance of eddy diameter, vorticity, angular momentum and orientation have the potential to improve future fishway and stream restoration design’.
