Annual variability in reproduction of the white-clawed crayfish (Austropotamobius pallipes) : implications for survival

The white-clawed crayfish is generally considered as an endangered species and it is necessary to estimate the functioning quality in residual population patches, particularly reproduction. For six consecutive years, a population was studied in a forest brook in Normandy (France). The size, state of maturity, density and egg number in mature females as well juvenile density were measured. The proportion of mature females varied between 16% and 31% among all 1078 females (>1+ year old). The number of both ovocytes and eggs laid are correlated with the size of females and the majority of the mature females expel all ripe ovocytes. In June, the potential hatchlings number remains correlated with the female size and this relationship evolves from year to year. The females are more numerous downstream associated with greater structural complexity of the substrate: at laying, the potential egg stock is on average 45.8 m−2 (upstream: 22.2 m−2) and at hatching 15.5 m−2 (upstream: 5.8 m−2). The losses are due above all to reduction of berried females and the smaller females tend to lose eggs more easily than the larger ones. The survival of juveniles during the first summer of independence is greater upstream. Nevertheless, it corresponds to a stable rate of 12% from potential eggs. Some results show a regulation process with an optimum density. The clogging of substrate can reduce greatly juvenile survival and in some years the upstream site can be considered as an ecological trap. The downstream site seems to be the most suitable habitat for conservation purposes, in spite of higher individual egg load upstream. For the future of the species, population functioning must be studied at the scale of the whole population patch. Meanwhile the upstream biotic capacity may be ameliorated by an increase in number of shelters.