Conserving indigenous crayfish: stock assessment and habitat requirements in the threatened Austropotamobius italicus.

Aims of the study were to assess the conservation status of the threatened indigenous crayfish species Austropotamobius italicus in Central Italy and to investigate the effects of some environmental parameters on its presence and population structure. Surveys were conducted in nine streams harbouring A. italicus (streams WI) and in 10 streams where crayfish populations have gone extinct in at least five years before the study (streams WO). Our analyses showed that the extant populations are healthy; they have balanced sex-ratios and are well structured for their age-class composition, although showing a bias towards adults. We confirmed that A. italicus is k-selected, with a relatively slow growth rate (males: 0.34, females: 0.37) and a long life expectancy (males: 8.2 yr, females: 7.8 yr). Fishing mortality scored high values, denoting the risks of overexploitation, whereas natural mortality was low, possibly due to the scarcity of predators and the absence of parasites and diseases. The loss in heterogeneity of the pristine riverine landscape may be also responsible for the local extinction of this species: a Principal Components Analysis showed that a difference between the streams WI and WO was mainly due to the abundance of allochthonous plant detritus, a source of food for crayfish. Finally, a spatial segregation between age classes was found, with juveniles requiring cobbles as a substrate and adults seemingly avoiding them. We conclude that the complexity of the pristine landscape provides both food and protection to A. italicus, and allows for the maintenance of healthy populations.