Reduction of Zooplankton Standing Stocks by Predaceous <i>Cyclops bicuspidatus thomasi</i> in Marion Lake, British Columbia

Abstract

The standing stock of the copepod Cyclops bicuspidatus thomasi Forbes in Marion Lake, B.C., was observed during the summers of 1966 and 1967. Stage IV copepodids excysted during April and moulted to adults, which produced eggs in May. The new generation developed to the copepodid IV stage by midsummer, when about 85% of the animals moved to the bottom and encysted. The copepodids of stage IV and V and the adults are predators. When each group of the prey of C. b. thomasi was offered separately in the laboratory, these predators fed best on their own nauplii and copepodids, diaptomid nauplii, and rotifers, but ate few diaptomid copepodids and cladocerans. When offered a mixture of prey at the lake, the predators ate nauplii in the same quantities as in the laboratory but, contrary to the laboratory results, very few rotifers. During the summer of 1967 C. b. thomasi copepodids IV and V and adults were estimated to have eaten 31.0% of the C. b. thomasi nauplius standing stock, and 30.2% of the diaptomid nauplius standing stock.