Fishing hurts genetic diversity (Image: Sipa Press/Rex Features)

Plenty more fish in the sea? Maybe not for much longer. Overfishing is damaging the genetic diversity of fish to a greater degree than expected, leaving at-risk species vulnerable.

It was thought that even badly overfished species would remain genetically diverse, since millions of individual fish remain even in the most depleted species.

To test this assumption, Malin Pinsky and Stephen Palumbi of Stanford University in California gathered published data on genetic diversity in 37 overfished species and compared it with diversity in 51 of their lightly fished relatives.


To their surprise, they found that the overfished species carried, on average, about 18 per cent fewer genetic variants than their lightly fished relatives. “Contrary to what we expected, it looks like the [genetic] effects of overfishing are quite widespread,” Pinsky says.

At first glance, a drop in genetic diversity of just 18 per cent may seem small, given that some overfished species are thought to have suffered severe population crashes. However, widespread overfishing is just a few decades old, and if it continues it may lead to a further erosion of genetic diversity, says Pinsky.

Meanwhile, Michael Alfaro of the University of California, Los Angeles, and his colleagues have found another ominous sign for fish stocks.

Speedy evolution

They analysed the family tree of fish to see how quickly body size changes in each lineage, then noted which lineages are most heavily fished. The team found that species with unusually fast rates of evolution in body size are preferentially targeted by fishing.

Since changes in body size often lead to changes in other ecologically relevant traits, this means that fishing targets the most evolutionarily active groups of fish, Alfaro says.

“Humans are eating away the richest branches of the fish tree of life,” says Alfaro. “If you were going to come up with a plan to assault the fish tree of life, you would want to do it like this.”

The results add urgency to biologists’ call for more aggressive management of commercial fisheries. “These are not trends we would wish to see continue,” says Paul Bentzen, a fisheries geneticist at Dalhousie University in Halifax, Canada. “We need to be managing fisheries more effectively.”

That could involve measures such as shorter fishing seasons and an increase in the number of no-fishing zones. But the bottom line is that people need to catch fewer fish, Bentzen says.

Pinsky and Alfaro presented their findings at a meeting of the Society for the Study of Evolution in Norman, Oklahoma, last month