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In big cities even the fish are always rushing around the place

In big cities even the fish are always rushing around the place

All kinds of animals, from rats and pigeons to weeds and mosquitoes, are evolving so they can survive and often thrive in cities. Now it has been shown that even city fish are evolving in unique ways.

“Even though urban fish are widely separated, they are consistently different from rural fish,” says formerly at North Carolina State University and now at Ewha Womans University in South Korea.

The big difference between urban and rural streams is the speed of water flow, she says. In rural areas, rain soaks into the soil. In urban and suburban places, it flows off hard surfaces straight into streams.

That means peak water flows are much stronger, even if the stream bed hasn’t been altered. “It’s not necessarily the concrete bed, it’s the parking lots and the roads and buildings,” says Kern.

 

Speedy fish

She and her colleague Brian Langerhans studied two fish common in North Carolina: western blacknose dace (Rhinichthys obtusus) and creek chub (Semotilus atromaculatus). They caught hundreds of them in 25 different streams.

They found the chubs in urban streams are more streamlined than the rural fish. The populations in different streams have all independently evolved in the same way in response to the same pressure.

The urban dace have adapted in a different way: rather than becoming more streamlined, they have deeper, thick tails enabling them to swim faster.

Crucially, when fish from urban and rural streams were raised in the lab in identical conditions, the differences in body shape persisted. This shows that the differences are a result of genetic changes rather than, say, the dace getting more muscular because they have to swim harder.

Adapted to humanity

In the UK, city fish may have evolved in the opposite way. An extensive network of canals with slow-moving water was built from the 18th century onwards. “That is more than enough time for such evolutionary changes to occur,” says Kern.

Other studies have shown that the shapes of fish living in the reservoirs behind dams are changing. “Still, fish sometimes ignore scientists’ predictions and evolve in a different direction than expected,” says Kern.

Building cities is just one of the ways in which we are forcing wildlife to evolve and adaptPlants and animals are also evolving in response to other human pressures, from pollution and pesticides to climate change and fishing.

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