Steve Westerman

A FAIRLY still, sunny day and I was on the riverbank, looking for wildlife. Unexpectedly, a movement under the surface at the river’s edge caught my eye. There was a crayfish moving around. It didn’t look in the best shape – having only one claw and damage to an antenna (see photograph). Perhaps it had survived an attack. Crayfish are on the menu for a number of predatory species. I have seen them taken by both gulls and heron – and otters also will eat them. I remember one gull that seemed to have a favourite ‘crayfish-eating spot’. It was standing on a rock, a crayfish in its beak, with scattered, unattached claws, apparently from previous victims, lying around.

In the UK we have just one native species (the White-Clawed Crayfish – Austropotamobi pallipes), along with wild populations of seven other non-native species. It seems most of these non-native species are in other regions of the country, with the exception being the American signal crayfish (Pacifastacus leniusculus). Where both are present, the larger and more fecund signal crayfish generally displaces the white-clawed crayfish. The photograph shows a signal crayfish – but identification is easier with sight of the red underside to their claws.

Although non-native species should not automatically be regarded as problematic, complex, and potentially negative environmental effects can occur. One mechanism is through competition with native species for limited resources. This can be for food – but also for space in the environment. For example, in a previous nature notes I mentioned that Mandarin Ducks compete with native species for nesting locations in tree hollows. Similarly, crayfish compete for suitable refuges (e.g., rocks, tree roots) in the water. The presence of signal crayfish is thought to leave the native species particularly exposed and susceptible to predation.

Another means of effect is through disease. The introduction of the grey squirrel decimated the native red squirrel population, in large part by spread of ‘squirrel pox’, which the former can carry but are immune to. Similarly, signal crayfish can carry ‘crayfish plague’, to which they are resistant but that is always fatal for the white-clawed variant.

American mink are occasionally seen along the river. The hunting abilities and appetites of these non-native, predators of birds, mammals, and fish, (that tend to kill more than they need to survive) is thought to be an important factor in the serious decline in UK water vole numbers. Signal crayfish also bring different behaviours and proclivities to river environments – and concerns have been raised that burrowing activities and their feeding patterns, combined with higher population densities, produce negative effects on the health of river ecosystems.

So, some non-native species, including signal crayfish, cause concern. Of course, going back to an earlier environmental state isn’t so easy.