Dynamics of an Isolated Population of the Crayfish Astacus astacus Linné

Abstract

A population of crayfish isolated in S artificial ponds at Rogle (Skane, southern Sweden) was studied by manual sampling, electrical fishing and catching with various kinds of tackle. The density of the catchable crayfish population in a pond of 25,000 m2 by means of the capture-recapture method was calculated to be 12,400 individuals. An abnormal sex ratio (65.3% males and 34.7% females) appeared to be a result of intraspecific competition. Further sampling by electrical fishing made a rough estimation of the total population possible: in a pond of 25,000 m2 there were about 50,000 individuals (23.9% juveniles, 41.9% males and 34.2% females), or 2 crayfish per m2. About 75% of the populations consisted of individuals less than 3 years old. Crayfish growth was very slow in the ponds compared with most other Swedish populations, because of the high density of the unexploited population. The dense population, through feeding, retarded vegetation growth. Submerged plants were almost absent and the littoral vegetation was heavily grazed, especially in Aug. when cray-fishes were grazing on land as much as 0.5 m from the water''s edge. After the population had been exterminated by crayfish plague (Aphanomyces astaci Schikora) there was a marked development of the aquatic macrophytic vegetation and certain sedentary invertebrate species.