Observations on the yearly life cycle of Astacus astacus in a small lake in Sweden
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Abstract
The noble crayfish, Astacus astacus, was investigated in a small lake (Gotland, Sweden) of two hectares having a maximum depth of 11 m. Twelve different physico-chemical water parameters were measured, viz temperature, conducti-vity, colour, alkalinity, calcium, magnesium, chloride, sulphate, iron, copper, aluminium and pH. In addition nitrogen and phosphorus were analysed. Sampling of animals was carried out with baited traps at different times of the year during a six year period but with a focus on 1995-1998. The main objectives were to investigate the seasonal cycle of the physiological status. The vertical distribution was studied as well. The development of gonads, internal and external eggs, egg development, mating and hatching were noted as well as disease frequency. Females were active from the end of June or beginning of July until November. Males (and females without external eggs) were active all the year round. The sex ratio of the trapped crayfish (M:F) from December to June was 5:16, while it was close to one from July until November. Immature eggs were found in the gonads of the females all the year round. In July most females started to develop their eggs which gradually became yellow and finally brown-green. 58-80% of the females developed cement glands and thus prepared to mate. On most occasions the majority of the animals was found in the warmest temperature interval. However, in late autumn when the thermocline disappeared they became fairly evenly distributed from shallow waters down to the bottom. The temperature range of occurrence was 0-24 °C. There was no significant relative difference between length groups, nor any real difference between the sexes at various depths during the part of the year when the females were active. On average the length of females were shorter compared to males.
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Ackefors HEG. (1999). Observations on the yearly life cycle of Astacus astacus in a small lake in Sweden. Freshwater Crayfish 12(1):413-429. doi:
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