If you have ever ‘felt the burn’ after intense exercise, chances are you've experienced the build-up of lactic acid in your muscles as they run low on oxygen. For most of us mammals, simply having a good sit down and a breather will help us to remove the excess lactic acid. But if you happen to be a shark, your post-workout routine is a little more energetic. Gil Iosilevskii from the Israel Institute of Technology, and a team of colleagues including Nicholas Payne from Trinity College Dublin, Ireland, noticed something odd about the behaviour of large marine fish released after prolonged capture for scientific study. These fish, far from taking things easy after their tiring experience, appear to do the opposite: they swim off at high speed for several hours. Iosilevskii and Payne had an inkling that this might be something to do with how these fish are removing lactic acid and dived into the data to find out.
Firstly, Iosilevskii and Payne wanted to check whether or not these fish really were zooming away after release. They examined the recorded swimming speeds of 29 large fish, belonging to 10 species of shark and one tuna species. Crucially, these species happen to be obligate swimmers, meaning they need to actively swim to pump fresh, oxygenated water across their gills. Imagine if you and I could only breathe by opening our mouths and running; you wouldn't be far off the situation experienced by these animals. The fish were caught by line and hook and were then held immobile in water for periods of up to 2 h while their measurements were recorded and a speed sensor was attached to a fin. For an obligate swimmer, this is exhausting. Iosilevskii and Payne were interested in what happened next and examined the swimming speed data recorded by the bio-loggers.
When released, most of the fish sped off at double their usual cruising speed, only settling down to normal swimming 6 h later. Most studies refer to this period as a ‘stress response’, and effectively ignore the data collected during this time. Iosilevskii and Payne instead thought the behaviour could be linked to the build up of lactic acid, which can only return to normal once high enough oxygen levels are available again.
At these swimming speeds, Iosilevskii and Payne estimated that these tired fish could get just enough oxygen from the water to dispose of the lactic acid but weren't at risk of overexerting themselves. They surmised that once the fish returned to their normal cruising speed, all excess lactic acid had been removed or stored elsewhere. So for you or me, this process of shifting lactic acid usually requires us to rest and catch our breath, but for a shark or tuna, it means just keep swimming.