Getting even with the crays

Richard Hardy issues a call to crustacean arms against the invasive signal crayfish

- Richard Hardy

It is early June 2017 and on my little chalk stream an irregular spate of sulphur mayflies are hatching, tearing and pushing through the surface tension before coursing downstream. Bobbing, racing and spinning, faster then slower, it is almost as though they are riding an aquatic waltzer, dodging hungry wild brown trout and passing through the midday sunlight.

This pastoral drama has been re-enacted for thousands of years on our fertile southern rivers, but on this particular day it was overshadowed by my first brush with an illegal alien. A single crushed claw, the leftovers from an otter’s midnight feast, was the clue to an invader from the other side of the pond. It is most certainly not good news for my precious chalk stream beats.

The signal crayfish, Pacifastacus leniusculus, hails from North America and, at 3in to 8in, it is far larger, typically brasher and considerably more voracious than our native white-clawed crayfish, Austropotamobius pallipes. Both share similar freshwater habitats, preferring an adequate depth of moderately flowing oxygenated water, feeding predominately on freshwater invertebrates such as mayfly, caddis and mollusc and spending most of their time sheltered among rocks, tree roots or under the riverbank edge.

Dastardly

The dastardly invader was purposefully introduced to our freshwaters during the mid-1970s as a fast-growing cash crop, with the intention of supplying the lucrative Scandinavian market, which was suffering from a disease known as crayfish plague in its native stocks.