Running on a treadmill in the gym isn't quite the real thing, and now it turns out that fish may have the same problem. David Ellerby, from Wellesley College, USA, is concerned that experiments in flow tunnels (the fish equivalent of treadmills), which had suggested that fish naturally prefer to swim at a speed that consumes the least energy, may not be telling the whole truth. ‘Almost all measurements of fish swimming costs are made at constant speeds imposed by the researchers, not those preferred by the fish. Although this is valuable information, we know very little about how the lab data actually relate to swimming behaviour in the field’, he says. Realising that they were going to have to get out of the lab and go swimming with free largemouth bass in Lake Waban to find out how realistic lab-based studies are, Ellerby and two undergraduate students, Angela Han and Caroline Berlin, devised an impromptu stereo camera system by strapping a pair of GoPro cameras to a camera head, donned their snorkels and went swimming.

Back in the lab, the trio estimated the length of each fish from the video and then measured each fish's speed, the directness of the path that they had taken and how fast they beat their tails. However, when the team compared the speeds that the fish had been swimming in the lake with the most economical speeds that had been recorded in a flow tunnel in the lab, they were astonished to see that the majority of the free fish were swimming at slower, less economical speeds (∼0.3–0.4 m s−1) than the optimal speeds that had been recorded in a flow tunnel (0.4–0.5 m s−1).

Wondering why the free fish might have opted for a more profligate swimming speed, Ellerby suggests that they may have to accept a more costly slower speed to increase their chances of snapping up tasty morsels as they pass by. Alternatively, it is possible that the fish that were swimming in the flow tunnel when their metabolic rates were measured were not behaving as naturally as the freely swimming lake fish. Either way, Ellerby warns that the behaviour of fish in the real world may be subtly different from that of fish pounding a metaphorical treadmill.

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Field swimming behavior in largemouth bass deviates from predictions based on economy and propulsive efficiency
J. Exp. Biol.