When rainbow trout started dying mysteriously at Joël Aubin's fish farm, he called in fish physiologists Guy Claireaux and Tony Farrell to investigate. They discovered that some of Aubin's fish had oddly shaped hearts. `These fish may appear to be healthy when young, but then suffer heart failure in later life,' Claireaux explains, `so investing in these fish is a costly business for the fish farming industry.' He figured that if frail fish could be identified when they're young, fish farmers could weed them out to make sure that they only rear healthy fish. Searching for a suitable way to screen juvenile fish, Claireaux and Farrell devised a simple fitness workout that allows farmers to pick out the most athletic fish(p. 1775).

Pondering how to develop an early warning system for potential future health problems, Farrell reasoned that the answer might lie in the heart's ability to deliver oxygen to active muscles in swimming fish. In other words,if cardiac and swimming performance are closely linked, the swimming ability of young fish might predict which fish have the weakest hearts. To test their idea, Claireaux and Farrell loaded equipment into a van and drove to Aubin's fish farm in Brittany, where David McKenzie, Gaylene Genge and Aurélien Chatelier joined the team. `Doing physiology in the field was a real challenge,' Claireaux says. `Workers at the fish farm watched us set up our equipment and thought we were very strange,' he recalls.

Finally, the team were ready to give Aubin's fish a cardiovascular workout and find out if poor swimmers have weaker hearts than good swimmers. They began by testing the swimming ability of 600 young rainbow trout weighing in at around 100 g. The tiny fish swam against a current until they were exhausted. The team picked out the first 60 fish to tire and labelled them as poor swimmers. They labelled the last 60 fish to tire, which were still going strong almost an hour later, as good swimmers. Nine months later, when the fish weighed a hefty 1 kg, the team placed the fish in the swim-tunnel for a second workout. They compared poor swimmers' oxygen consumption and arterial blood pressure and flow with that of good swimmers. As the team had suspected,cardiac and swimming performance are clearly linked; poor swimmers not only swim slower than good swimmers, they also have lower maximum metabolic rates and their hearts pump blood at a lower maximal rate. Since fish that were poor swimmers when they were young were still poor swimmers nine months later,their cardiac deficiencies are clearly detectable at an early age.

But the poor swimmers' misfortune didn't end there; the team found that they are also fatter than the good swimmers, and their hearts are abnormally rounded, whereas healthy fish have pyramid-shaped hearts. Claireaux explains that breeders select fish by placing them on a grid and keeping the plump fish that don't fall through the holes. So all this time they've been rearing obese fish with abnormal hearts rather than sporty streamlined fish! The team hopes that its results will convince fish farmers that a simple swim test, and not a grid, truly identifies the fittest fish.

Claireaux, G., McKenzie, D. J., Genge, A. G., Chatelier, A.,Aubin, J. and Farrell, A. P. (
). Linking swimming performance, cardiac pumping ability and cardiac anatomy in rainbow trout.
J. Exp. Biol.