Fish may be evolving to swim faster and evade capture in trawler nets, according to research.
Scientists at the University of Glasgow found that fitter fish are better at evading nets and believe that, over time, it could lead to physiological changes in future fish populations.
The group used simulated trawling with schools of wild minnows to investigate whether some individual fish were consistently more susceptible to capture by trawling and if that related to swimming performance and metabolism
The researchers measured the swimming ability, metabolic rate, and indicators of aerobic and anaerobic physical fitness of 43 fish. They then placed them in a tank with a simulated trawling net to identification those most susceptible to capture.
Dr Shaun Killen of the Institute of Biodiversity, Animal Health and Comparative Medicine, led the study.
He said: "There is a lot of concern on how overfishing is affecting the abundance of wild fish, consequences for the economy, employment and the ecosystem as a whole.
"But one aspect that is often overlooked is that intense fishing pressure may cause evolutionary changes to remaining the fish that are not captured."
He added: "Fish being trawled will try to swim at a steady pace ahead of the mouth of the net for as long as possible, but a proportion will eventually tire and fall back into the net.
"Fish that escape trawling are those that can propel themselves ahead of the net or move around the outside of the net. The key question is whether those that escape are somehow physiologically or behaviourally different than those that are captured.
"Most trawlers travel at the about same speed as the upper limit of the swim speed of the species they are targeting.
"While trawling nets can be in the water anywhere between 10 minutes to several hours, whether or not fish enter the net is generally decided within a few minutes of when they end up at the trawl mouth."
The study is published in the journal Proceedings of the Royal Society B and the researchers want to carry out a similar study on fish in the wild.
Dr Killen said: "Humans are effective predators, and selective harvest of animals by humans probably represents one of the strongest drivers of evolutionary change for wild animal populations.
"Hunting and fishing are selective processes which often remove individuals that, under normal circumstances, may have the highest reproductive potential. Available evidence suggests selective harvest can lead to genetic change within wild populations for specific traits.
"Using simulated trawling, our study provides the first evidence better swimming fish, and those with higher metabolic rates, are more likely to escape capture.
"Over time, the selective removal of poor-swimming fish could alter the fundamental physiological make-up of descendant populations that avoid fisheries capture."