Winter mortality and growth of juvenile noble crayfish (Astacus astacus) under realistic environmental conditions – Implications for conservation management

Noble crayfish (Astacus astacus) are endangered in many parts of Europe. Conservation efforts, including stocking measures, are widely conducted to safeguard the species and associated ecosystem functions. Due to limited availabilities of adult crayfish, juvenile noble crayfish are frequently released into lakes and rivers during autumn, usually without information on their fate and growth during the following winter. To investigate size- and density dependence of winter mortality and growth of newly stocked noble crayfish we conducted a mesocosm experiment in a small artificial lake in northern Germany. The experiment was carried out at water temperatures between 0.7 °C and 9.1 °C from mid-December until mid-March and simulated stocking densities of 20, 60, and 120 crayfish per square meter with unlimited food availability. Winter mortality rates were generally low and strongly size dependent. Specifically, a winter mortality rate of 9.2% was observed and crayfish with a total length (TL) above 34 mm all survived. The relative risk of mortality in dependence of TL increased by 41–94% (95% CI) with every decreasing centimetre of TL at the onset of winter. Increasing density had no effect on winter survival, but tended to reduce growth. Our work indicates that stocking measures of noble crayfish for conservation reasons can decrease costs and increase prospects of success if juveniles larger than 30 mm TL are used as stocking material.