Scientists have raised the possibility that fish which escape a trawler's nets may one day give rise to a breed that could be all but uncatchable.

A team of researchers at the University of Glasgow, investigating whether commercial trawling is driving evolutionary change, have found that fitter fish are better at evading capture.

They speculate that, over time, this could lead to physiological changes in future fish populations as these fish pass on their genes through breeding while those that are caught do not.

There are many examples of an evolutionary ‘arms race’ between predator and prey in nature, where adaptations that help hunted animals avoid capture prompt changes in hunters that help them become more deadly.

But the concern is that better adapted fish will drive down the size of the catch and make fishing more difficult, even with industrial trawlers.

Dr Shaun Killen of the unversity's Institute of Biodiversity, Animal Health and Comparative Medicine, 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.”

A study led by Dr Killen, published in the journal Proceedings of the Royal Society B, used simulated trawling with schools of wild minnows to investigate two key questions around fisheries-induced evolution.

The researchers wanted to know whether some individuals within a fish shoal were consistently more susceptible to capture by trawling than others, and if so, was this linked to individual differences in swimming performance and metabolism?

The researchers measured the swimming ability, metabolic rate, and aerobic and anaerobic physical fitness of 43 individual fish.

They then placed them in a tank with a trawling net in a simulation that was repeated several times, enabling the identification of individuals which were more susceptible to capture.

Dr Killen said: “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."

The experiment revealed that some fish were more susceptible to capture than others, and this was strongly related to their ability to endure short periods of intense physical activity which demand more oxygen than is available from breathing alone.

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 makeup of descendant populations that avoid fisheries capture.”

The researchers now want to study fish in the wild to see if they get the same results.