Are fish evolving to outswim trawlers?

A new study suggests that by catching the slowest swimmers, commercial fishermen are inadvertently helping fish species evolve to swim faster.

Ann Hermes/The Christian Science Monitor/File
Fisherman Baldassare Noto unloads the daily catch from the 'Jenny G' commercial fishing vessel onto the dock on February 16, 2013, in Gloucester, Mass.

Is high-volume fishing changing the genetic makeup of fish populations? In other words, if all the slow fish are caught, leaving only the fastest fish to breed, will fish species actually evolve to swim faster?

Yes, according to new research.

The "harvest of animals by humans may constitute one of the strongest evolutionary forces affecting wild populations," says lead author Dr. Shaun Killen, with the University of Glasgow.

"Evidence suggests selective harvest can lead to genetic change within wild populations, for specific traits," he added. His team's research can be seen in the August issue of scientific journal Proceedings of the Royal Society B.

Dr. Killen and his colleagues investigated whether commercial fishing was prompting physiological changes, as faster fish had better chances of evading capture by commercial trawlers. "Over time, the selective removal of poor-swimming fish could alter the fundamental physiological makeup of descendant populations that avoid fisheries capture,” explains Killen. 

Though controversial, trawling nets play a large role in the commercial fishing industry; the speed of a trawling net is species-specific, typically determined by the maximum speed of the fish. "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,” said Killen in a press release from the University of Glasgow. The study mimicked commercial trawling practices with schools of wild minnows in a laboratory, to observe how the fish interacted with the nets. 

The researchers measured many traits in the 43 fish studied, including swimming ability, metabolic rate, and indicators of aerobic and anaerobic physical fitness. They found that high anaerobic capacity – which allows fish to swim rapidly for short bursts – was most helpful for fish in evading capture. 

Conservationists and fishermen alike are concerned about rapidly decreasing fish populations.

"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," said Killen in the press release. "But one aspect that is often overlooked is that intense fishing pressure may cause evolutionary changes to remaining the fish that are not captured."

This “intense fishing pressure” largely comes from Asia, responsible for catching 48 percent of all captured fish, according to the United Nations. Worldwide, commercial fishing generates $63 billion in household income, according to a 2010 report.

The research team now plans to study fish in the wild to see if they find the same behaviors that they observed in laboratory conditions.

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