Understanding population structure and dynamics in threatened crayfish

This study aimed to improve our knowledge of the population status and dynamics of Austropotamobius pallipes, by applying length-based fish stock assessment approaches based on the analysis of polymodal size-frequency distributions and mark-recapture datasets. Analysis of nearly 8000 crayfish collected from May to October of the years 2001-2004 showed a well-structured population and a low growth rate, the latter showing evident seasonality in both sexes (with high value of amplitude and winter point). Moreover, sexual differences were found, because females have higher growth rates than males. This analysis also showed that A. pallipes is a long-lived species, with an expected lifetime of 10 years, and a mortality rate affected by illegal harvesting. The results obtained in this study by length-frequency analysis are the same as those obtained by approaches based on recapture. Statistical evidences showed that the growth rate is affected by latitude, the former increasing with a decrease of the latter, probably as a result of the associated different environmental and climatic conditions. Dynamics studies are useful for planning monitoring, and could have a role in introduction programmes and control of reintroduced individuals during any restocking project. Considerations of population dynamics can be used to derive generalizations on the stress and exploitation grade of a stock, based on the concept that dynamics are strictly related to biological processes and to the adaptation of a species to its habitat. Through a survey of a body size, it is possible to avoid the use of invasive techniques for ageing crayfish and recapture methods that can cause a certain degree of stress. We discuss and underline the importance of using polymodal frequency analysis in order to determine important parameters in population structure and dynamics and further population properties of great usefulness for the monitoring and preservation of threatened crayfish.