Effects of selected biotic and abiotic factors on growth and survival of young crayfish, Orconectes virilis (Hagen) / by Sabine Maxwell
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The growth and survival of Orconectes virilis young was studied in 50 x 50 x 50 cm lake enclosures during the summer seasons of 1986 and 1987. The effects of density (12/m2, 40/m2, 160/m2), substrate (mud, sand, no-substrate control), cover type (leaf-litter, muskgrass, plastic plant, no-cover control), treatment (control, food, fertilizer, 30 and 100 cm depth, and presence of adult crayfish were investigated. When first placed in the enclosures, the young crayfish were 4,0 - 4.5 mm in carapace length (tip of the rostrum to the posteriomedia1 edge of the carapace). Experiments lasted two weeks, after which the surviving crayfish were counted and measured. In 1987, crayfish were returned to their cages after being measured, to permit sex determination at the end of the summer season and to provide further information on growth and survival. Survival of crayfish was relatively high (75 - 100 %). It was reduced by the presence of adults (50 %) and was extremely low in cages lacking substrate and cover (6 - 15 %). It was not significantly influenced by any of the other factor combinations. Mean carapace length after the two weeks experimental period ranged from 4.9 to 8.8 mm. Growth was poorest in cages lacking substrate and cover (4.9 - 6.4 mm) and in cages containing adult males (6.8 mm). Growth was inversely related to density, even though no difference existed between the low and the very low density after the first two weeks. Growth was also enhanced by cover and a mud substrate. At the end of the summer season crayfish size was distinctly different at all three densities (16.1, 12.2 and 11.0 mm carapace length, respectively). Crayfish were also significantly larger in the fertilized cages (10.8 and 12.7 mm carapace length, respectively). Differences in growth are ascribed to food availability and feeding opportunity. Sex ratio of crayfish that survived until the end of the summer, was 1 1 . Mortality of juvenile crayfish studied in Powell's lakes is mainly ascribed to intraspecific competition, moulting and oxygen deficiency. The extent of intraspecific competition is related to the size and quality of the nursery habitat.