2005 WINNER

By Dr Yfke van Bergen
Company of Biologists, Cambridge

Winner of the 20-28 category

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 soon discovered that some of Aubin’s trout had oddly shaped hearts. These unfortunate creatures couldn’t cope with much excitement, and the everyday stress of life on a fish farm proved to be fatal for them. ‘These fish may appear to be healthy when young, but then suffer heart failure in later life,’ explains Claireaux ofCNRS in L’Houmeau, France. And spending months fattening up these fish only to watch them die as adults is a costly business for the industry. With Prof Farrell of the University of British Columbia, Vancouver, Claireaux 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.

Pondering how to develop an early warning system for future health problems on fish farms, Farrell reasoned that the answer might lie in the ability of a trout’s heart to deliver oxygen to active muscles while the fish swims along. In other words, if swimming performance is closely linked to cardiac performance, the swimming ability of young fish might predict which fish have the weakest hearts. Claireaux and Farrell didn’t waste any time. To test their idea, they loaded equipment into a van and drove to Aubin’s fish farm in Brittany. Here, David McKenzie, Gaylene Genge and Aurélien Chatelier joined the team. For some reason, workers at the fish farm thought Claireaux and his crew were very amusing. ‘Doing physiology in the field was a real challenge,’ Claireaux says. ‘The fish farm workers watched us set up all our physiological equipment in Brittany 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 100g. The tiny fish swam against a current in a swim-tunnel until they were exhausted, and Claireaux’s 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 had gained a hefty 1kg, the team placed the fish in the swim-tunnel for a second workout. To measure how hard the fish were working - their metabolic rate - the team measured how much oxygen each fish used up. They also inspected the pumping performance of the trout’s hearts by measuring arterial blood pressure and flow in each fish.

With these measurements, the team compared the fitness levels of poor and good swimmers. As they had suspected, cardiac and swimming performance are clearly linked; poor swimmers not only swim slower than good swimmers, they also have lower metabolic rates and their hearts pump blood at a lower rate. Most importantly, fish that were poor swimmers when they were young were still poor swimmers nine months later, which means that their cardiac deficiencies are clearly detectable at an early age.

But the poor swimmers’ misfortune didn’t end there. Adding insult to injury, the team found that poor swimmers are also fatter than good swimmers, and their hearts are abnormally rounded, whereas healthy fish have pyramid-shaped hearts. In other words, the poor swimmers may have poor genes.

Why is this important? Well, as Claireaux explains, ‘breeders select fish by placing them on a metal grid and keeping the fat fish that don’t fall through the holes.’ So all this time, fish farmers have cheerfully been rearing obese fish with abnormal hearts rather than sporty streamlined fish with healthy hearts. They have inadvertently turned ‘survival of the fittest’ into ‘survival of the fattest’. The traditional selective breeding that goes on in aquaculture - all in the name of providing plumper fish for humans with ever-expanding waistlines - would benefit from a return to basic evolutionary principles. Claireaux hopes to convince fish farmers that, when it comes to identifying the fittest fish, nothing can beat a simple swim test.