The population dynamics and growth of the freshwater crayfish Austropotamobius pallipes in an aqueduct in Northumbria
Despite the wide distribution and abundance of the freshwater crayfish Austropotcmobius pallipes in Britain, little is known of its population biology. The dynamics and growth of a Northumbrian population of A. pallipes were studied using an intensive-mark- recapture programme. The seasonal timing of moulting and reproduction was found to be predictable and probably related to water temperature. Stomach contents showed crayfish to be omnivorous, with plant material forming the bulk of the diet. Population size, survival and recruitment were estimated from mark- recapture data at frequent intervals. Two principal types of mortality were identified: moult-associated mortality, which was relatively intense and of short (2-3 weeks) duration, and overwinter mortality, which operated at a much lower estimated weekly rate over a period of some 30 weeks. Mortality was shown to be highly variable throughout the year. Population size generally showed a midsummer peak as recruitment into the catchable size classes occurred. Thereafter numbers declined slightly towards the end of the growth season, followed by an overwinter decline of some 50% of total numbers. Longevity was estimated to be at least eleven years. The relationship between size, age and weight was established and males shown to have faster growth rates than females after the size of sexual maturity. Growth increments and moult frequency were shown to differ between the sexes and according to body size. Reproductive females omitted one moult while carrying eggs. Growth rates were depressed by the parasite Thelohania oonteoeani and by the regeneration of a major chela. The fecundity of females and the probability of being reproductive increased with body size. The annual production of the study population was estimated at 196.49 kg ha(^-1) in 1977, 116.80 kg ha(^-1) in 1978 and 87.79 kg ha(^-1) in 1979. Detailed life histories of many marked individuals were collected which support the conclusions drawn at the population level.