| Popis: |
An understanding of the population dynamics of crayfish and their trophic position is critical to understanding energy transformation in lakes. In many lakes crayfish dominate the annual production of the benthic biomass, at times reaching mean biomass values of 1000 1400 kg/ha and numerical densities of 15/m2. Density determines the rate of growth of various populations and determines reproductive capacity, age at maturity and life span. For Orconectes virilis, within the lakes we studied, population stability was mostly due to the short lag between a change in population density and an adjustment in fecundity. As a result, these populations appear to be self-regulated through density-dependent control of brood stock size. In Orconectes virilis, annual production was a function of mean annual biomass. Examination of available data on the ratio between other species showed them to fall within the same range as O. virilis. It seems that the evolutionary strategy governing crayfish biomass replacement and energy turnover has remained essentially unchanged. As regards their role in the community, crayfish do not fit the trophic level concept. Attempts to quantify their food habits show them to be polytrophic and a key energy transformer between various trophic links through utilization of all the trophic levels in lakes. Because of the multiple trophic roles of the crayfish, a comparison with the bioenergetics of various freshwater crust?cea is difficult. Crayfish may be most important in lakes with low phytoplankton production. In such lakes detritus and benthic algae, both of which are ingested by the crayfish, are important sources of overall energy flow. Since crayfish also function as major predators on the benthos, they most likely contribute to the stability of lake communities by interacting with a whole subweb of species ranging from large benthic invertebrates and submerged higher aquatics to the periphytic microcommunity. Hence, crayfish act as an important mechanism contributing to the regulation of most production processes and most specifically to the regulation of benthic production available to fish. In low nutrient lakes, because of their biomass dominance and functional role, crayfish greatly surpass the usual role assigned to them from their occurrence in fish diets. Manipulation of the benthic food chain would seem to be the most appropriate strategy to increase the fish production of such lakes. However, a greater appreciation of the complexity of the trophic organization of such food webs is necessary before such modifications can be attempted. 1 A contribution under Dingell-Johnson Project F-30-R, Michigan. 2 Present address: Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada. |
