Can resident carnivorous fishes slow down introduced alien crayfish spread? Efficacity of 3 fishes species versus 2 crayfish species in experimental design

Crayfish species are a potential food resource for fishes. Spatial expansion and demographic growth of large populations of Procambarus clarkii and Pacifastacus leniusculus during the last decade in French freshwaters raise the problem of relationships with fish populations, especially with carnivorous species as pike (Esox lucius), perch (Perca fluviatilis) and sander (Stizostedion lucioperca). Interactions between these species have been studied in experimental conditions with mesocosms, enclosures in pond and small ponds. In mesocosm pikes of 16 to 46 cm length eat Procambarus throughout the year. Maximum size of ingested crayfish is correlated to pike size. Pikes from 40 to 50 cm length can swallow crayfish adults above 8 cm. Crayfish consumption is regular even if forage fish are present and if each pike can differ in its choice intensity. The level of predation is correlated to temperature, but catches by pike endure along the winter. In pond enclosures pike youngs of the year (YOY) eat successfully crayfish youngs of the year during the first summer while they have a similar growth. Protection by shelters is inefficient, it's the same in presence of forage fish. Comparative approach in natural ponds gives the same result, even with the presence of macrophytes as refuges. Confrontation between pike and Pacifastacus summerlings directly in a pond induces same results, pike reduces the most part of crayfish population. In all cases the few surviving crayfish give a reduced growth and possibly a delayed sexual maturity. Association of perch YOY and Pacifastacus YOY did not affect survival rate of crayfish at the end of the first summer owing to mouth gape too small in perch. But perch summerlings reduce crayfish YOY density, except when the latters are protected by a net. In this case crayfish growth is reduced only by the vision of perch. Occurrence of sander summerlings did not affect the survival rate of Pacifastacus YOY. Occurrence of two years old sander reduce both the survival and growth of crayfish. In conclusion, observations show the pike as the most performant predator independantly of its size, while the predation effectiveness in perch and sander require a sufficient difference in size between fish and crayfish. These results allow to suppose that carnivrous fishes can reduce alien crayfish wild stocks. However the study of a Procambarus population during three years in a pond with numerous predators (pike, perch) shows a population stability over the time. Despite a pike estimated annual consumption above summer biomass of crayfish adults and a removal of the same biomass by summer trapping. Procambarus stock stability must be correlated to the high productivity and to a fast biomass turnover as it is a species with a short life span.