Signal crayfish reach a stretch of the Ter still holding natives
September 4, 2026
A signal crayfish was recorded in the Ter at Sant Joan de les Abadesses on August 5, in the middle of town where the old bridge crosses the river. The following day three white-clawed crayfish were logged at the same point, a few metres away. Signal crayfish had already been seen lower down the valley this year, at Ribamala a day earlier and at Les Llosses in June, but this is the first one logged on the stretch of the Ter running through the town, and the first time the two species have been recorded side by side there.The (Austropotamobius pallipes) is the only crayfish native to these rivers. It is a modest animal, rarely much over a hand's length, olive to brown with a rough shell and pale, whitish to pinkish undersides to its claws, which is where the name comes from. It wants cool, clean, well-oxygenated water with stones and undercut banks to hide beneath, and it has been losing ground across Europe for half a century.The (Pacifastacus leniusculus) is the main reason. It is a North American species brought to Europe for farming in the 1960s and 70s, larger than the native, with smooth claws that carry a white or pale blue patch at the hinge of the pincers and red on their undersides. It also carries crayfish plague, a water mould (Aphanomyces astaci) that lives in its shell without doing it much harm. The mould sheds swimming spores into the water, and a European crayfish that picks them up dies, typically within weeks. There is no treatment. Where carriers and natives share a reach, the native population usually vanishes within a season or two, and the signal crayfish, which also breeds faster and competes harder for the same hiding places, takes over the river bed.Two sightings on consecutive days do not show that plague is present in the Ter here, and no die-off should be read into them. What they do show is that the carrier has reached a stretch where the native is still living. That is the moment when what people do on the riverbank matters most. Plague spores travel in water and on anything wet: boots, nets, dog fur, a bucket. Never move crayfish or water between streams, let gear dry fully before using it in another river, and if a crayfish with white-blotched, red-bottomed claws turns up in the Ter or its side streams, photograph it and report it to the Generalitat's wildlife service rather than handling or releasing it.
