Population Dynamics and Productivity of the Crayfish, Orconectes virilis, in a Marl Lake

Abstract

The population dynamics of the crayfish, Orconectes virilis, were studied to define the role of the crayfish as a consumer in a marl lake ecosystem. Insight was gained into the factors controlling its production. There was year-class fluctuation in the age structure of the crayfish population. Males had a greater growth rate than females. After age-I, mortality rates for females were greater than for males. Both sexes matured after a molt in July, at age-I; mating followed and eggs were laid the following spring. Reproductive capacity was 58% of the potential capacity. Two-year-old females produced most of the eggs (92.5%). The estimated reproductive rate for 1963 was low (0.78 per generation) due to poor survival of the 1963 year class. The maximum life span for both sexes was three years. Newly hatched young left the females and remained in shallow water. Adult females then molted and migrated to a depth of 7.6 m, where most of them remained all summer. A migration of males to deep water followed. 0. virilis acts as a herbivore in marl lakes, feeding chiefly on the algae and the aufwuchs with marl incrustations, though it is also a scavenger on animal materials. The peak standing crop of crayfish (119.5 kg/ha) occurred in the spring of 1963. The annual net production of crayfish was 205.3 kg/ha, about 8.9 times greater than an estimate of the productio-n of all other bottom invertebrates in West Lost Lake (23.0 kg/ha). The total net production of 310.8 kg was 2.33 times the average standing crop during the summer of 1963. This crayfish has a lower daily numerical turnover rate (2.0%) than the amphipod Hyalella (2.9%) and the cladoceran Daphnia (25.0%). Population size is probably regulated by cannibalism and natural mortality at molting.