| dc.description.abstract | 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. | |
