Population structure of the freshwater crayfish, Euastacus spinifer "(Decapoda: Parastacidae)", from the Sydney Region, Australia

The structures of Euastacus spinifer populations in two pools of the Loddon River (south of Sydney, New South Wales) are described. No significant differences in catchabilities have been detected between the sexes, related to size classes, or between precocious (very small, sexually mature males) and normal males. Sex ratios in both populations show similar long-term trends, although short-term fluctuations are evident. Normal males comprise about 50% of populations up to 25-30mm CL, but above this size the male percentage declines well below 50%. Substantial rises and declines in frequencies of precocious males are noted, although few survive above 30mm CL; however, generally there is a surplus of males in the Loddon populations. Frequency distributions are skewed to the smallest size classes, with pulses of recruitment detected in spring and summer; numbers of mature adults captured are low, females (3%) and males (6%) from Pool 3. It is suggested that the dominance of smaller size groups at Loddon River sites may be related to lowered predation, especially by eels. Possible origins of the precocious males are discussed and two hypotheses advanced to explain their observed abundances relative to females; however, the available data do not permit a decision between almost total mortality over a confined size range (25-30mm CL) or recent selective recruitment of precocious males to the study populations.