Millions of years in the making

The story of England’s chalkstreams stretches back to before the dinosaurs.” Now one of the iconic denizens of this rare habitat is on the brink of extinction Fe

- Charles Rangeley Wilson

IN DECEMBER last year the IUCN (International Union for Conservation of Nature) revised its red list of endangered species and reclassified Atlantic salmon to a status of ‘near threatened’.

British salmon were rated more seriously as ‘endangered’. There should have been one more reclassification, however. Had British chalkstream salmon been singled out, they would surely have been labelled ‘critically endangered’. Chalkstream salmon are England’s white rhino: approaching extinction after two million years of residence. Last year only 287 adult salmon ascended the River Itchen to spawn, and in 2022 only 133 – far fewer than the lowest number deemed necessary for the survival of the species in that stream.

In 2018 Dr Jamie Stevens and a team of scientists from Exeter University proved that chalkstream salmon are genetically unique. Like the chalkstreams they breed in, they are therefore globally unique. On our watch we are losing one of Britain’s oldest native animals. Chalkstreams will still be around in 50 years but their wonderful native salmon may not. To understand why this would be an irreversible catastrophe, we need to scroll back in geological time to the origins of both chalk and salmon.

Chalk is made of the shell remains of a microscopic single-celled plankton called a coccolithophore. Almost 200 million years ago, as the supercontinent of Pangea began to fragment, sea levels were far higher than today: oceanic basins were shallow, seawater was warm and there was no ice at the poles. The world’s oceans flooded over its land mass.