NE Portugal is one of the most isolated and low populated regions of
Western Europe. Recently, two American crayfish species reached this
area: signal crayfish Pacifastacus leniusculus coming from Spain and red
swamp crayfish Procambarus clarkii in its northward expansion. The dispersion
of both species was followed throughout the last decade in river
Maçãs (Douro catchment). Our results support the hypothesis of faster
downstream colonisation. The mean spread rate of signal crayfish was
faster for the downstream expansion, 2.8 km·yr−1, while the upstream rate
was 1.7 km·yr−1. Exceptionally, in one period, the rate of spread reached
6.7 km·yr−1, which was apparently caused by downstream washing in extreme
conditions of flow. The mean rate of spread is higher than the observed
in other studies which may also be related to the low current velocity
of late spring and summer that provides favourable conditions for a
fast upstream colonisation. Coexistence of signal and red swamp crayfish
was detected in a large river sector. Signal crayfish strongly dominates in
the upper and intermediate sectors which are the colder part of the river
and in the lower sectors, species abundances are not significantly different.
Winter temperature in the higher altitude reaches seems to moderate
the competitive abilities of P. clarkii
