SELECTIVE FORAGING BY THE CRAYFISH ORCONECTES-VIRILIS AND ITS IMPACT ON MACROINVERTEBRATES

SUMMARY. 1. The impact of crayfish predation on the abundance of macroinvertebrates was examined under semi‐natural conditions. Female (Experiment 1) or male (Experiment 2) crayfish (Orconectes virilis) were held for 5 weeks in twelve small pools (4.67 m2 surface area) at biomasses of 0. 5, 10 or 18 g m−2 (live weight). The pools were stocked with known densities of macroinvertebrates. 2. Crayfish significantly affected the abundance of macroinvertebrates in the pools. Differences in the effects of crayfish on macroinvertebrates were related to crayfish sex, the presence of age‐0 crayfish, and the species of macroinvertebrate. 3. The abundance of snails (Stagnicola elodes and Physa gyrina) was greatly reduced, in comparison with controls, by biomass of female crayfish ≥10 g m−2 and by biomasses of male crayfish ≥5 g m−2. The total density of non‐molluscan invertebrates was inversely correlated with the biomass of female crayfish but the total biomass of non‐molluscan invertebrates did not differ between treatments. This is consistent with our observation that small invertebrates (<2 mg wet weight) were less numerous, and large amphipods (32–64 mg) were more numerous, in pools stocked with female crayfish. In contrast, male crayfish had little apparent effect on the abundance of non‐molluscan invertebrates. 4. Age‐0 crayfish hatched at the end of Experiment 1 and were present in each pool at the start of Experiment 2. Surprisingly, male crayfish preyed little on age‐0 crayfish. At the end of Experiment 2, the densities of age‐0 crayfish varied between six and 116 individuals m−2 and there was a strong inverse correlation between the mean biomass and density of age‐0 crayfish recovered from the pools. This suggests age‐0 crayfish were food limited in the pools and may explain the dominance of oligochaetes (which largely escape predation by burrowing) in the invertebrate community at the end of Experiment 2. 5. These results indicate that even relatively low densities of crayfish could greatly affect the abundance of macroinvertebrates in lakes. The introduction of crayfish into lakes (most lakes in Alberta currently have no crayfish) could substantially affect abundance and species composition of the macroinvertebrate community and, ultimately, the fish populations. Copyright © 1990, Wiley Blackwell. All rights reserved