Abstract

The importance of juvenile crayfish density and invertebrate predation to the survival of young-of-the-year Orconectes virilis was determined for two small lakes in Michigan during 1973. Weekly sampling established the relative numbers of predatory macroinvertebrates and young crayfish in the lakes. Young-of-the-year crayfish and Notonectids dominated the benthic populations in West Lost Lake while Anisopteran nymphs of the genera Aeshna and Ladona were the most abundant macroinvertebrates in North Twin Lake. In field enclosure experiments on density effects, young crayfish were stocked at densities of 10.8/m2, 43/m2, and 172/m2 with no predators. Over a 2-week period the mean percent survival was significantly greater at the low density (74.1±18.14) but there was no significant difference at medium and high densities (45.9±14.85 and 52.4±12.61). These results are attributed to limited food availability. Predation on young crayfish at medium density (43/m2) was studied by introducing Aeshnid nymphs or adult crayfish or Notonectids into the field enclosures. The mean percent survival of crayfish with a carpace length of 4-6mm was significantly reduced in a week's time (P<0.05) to 27.2±13.68 with three Aeshnids/m2 and to 10.5±5.61 with six Aeshnids/m2. Predation by Aeshnids was not effective on larger crayfish. Predation by adult Orconectes virilis and Notonectids was not effective on either size of young-of-the-year crayfish.

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