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Humans are making fish swim faster: Intensive fishing triggers evolution in minnows to escape trawler nets

Fitness of 43 individual fish was measured before a trawling simulation
The fittest fish were consistently less vulnerable to being captured in nets
Over time the slower fish are removed from the ocean as they are caught
This means the fastest and fittest fish are able to pass on their genes leading to changes in the species

Published: 05:02 EST, 5 August 2015 | Updated: 11:23 EST, 6 August 2015

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Fish are evolving to survive intensive fishing practices, a new study has revealed.

Scientists have found that faster swimming marine creatures are better able to avoid trawler nets.

This consistent removal of slower fish from the world's oceans through intensive fishing is in turn favouring the survival of more agile, rapid fish.

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Fish being trawled will try to swim at a steady pace ahead of the mouth of the net for as long as possible (pictured), but a proportion will eventually tire and fall back into the net. The fitter fish that survive are able to pass on their successful traits to the next generation, slowly changing the descendant population

'Over time, the selective removal of poor-swimming fish could alter the fundamental physiological makeup of descendant populations that avoid fisheries capture,' said Dr Shaun Killen of the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow.

The study, led by Dr Killen, used simulated trawling to investigate how heavy fishing might effect wild minnows.

The researchers wanted to discover if certain individual fish were consistently more susceptible to capture in nets, and if these differences were related to speed and metabolism.

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.

Trawling simulation with minnows and miniature trawl net

HOW HEAVY FISHING DRIVES EVOLUTION

Humans are effective predators, and selective harvest of animals by humans represents one of the strongest drivers of evolutionary change for wild animal populations, according to Dr Shaun Killen.

Fishing is a selective process which removes 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,' he said.

'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.'

'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.'

Swimming ability, metabolic rate, and indicators of aerobic and anaerobic physical fitness of 43 individual fish were measured before they were placed in a trawling simulation tank.

The experiment was performed multiple times to identify which individuals were more susceptible to capture.

The results showed that the fish that consistently avoided capture and lived to pass on their genes showed higher rates of anaerobic capacity - the ability to engage is sharp bursts of movement that demand more oxygen than breathing alone.

Hunting and fishing are selective processes which often remove individuals that, under normal circumstances, may have the highest reproductive potential. Evidence suggests selective harvest can lead to genetic change within wild populations for specific traits, according to Dr Shaun Killen of the University of Glasgow (file image)

Maximum aerobic swim speed and metabolic rate was also highest among fish least vulnerable to trawling.

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.