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

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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 3 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. O. 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 production 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.

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