Fish Predation, Interspecific Predation, and the Distribution of Two Chaoborus Species

In sampling 22 lakes in the Upper Peninsula of Michigan, I found that 2 species of Chaoborus, C. punctipennis and C. americanus, never co—occurred (von Ende 1975). Chaoborus punctipennis was found only in lakes with fish whereas C. americanus occurred along in stained, bog lakes without fish. Third and 4th instar C. americanus lack extensive diel, vertical migratory behavior and are found near the surface waters in stained bog lakes. Adults of this species emerge in the middle of May. Third and 4th instar C. punctipennis exhibit diel, vertical migration. They are benthic during the day. This species emerges at the end of June. In situ rearing experiments indicate the Chaoborus americanus larvae can survive in lakes with fish, when isolated from the fish. Fish (Umbra limi) added to a lake with C. americanus eliminated this Chaoborus species from the lake. It is concluded that the absence of C. americanus from lakes with C. punctipennis is due to fish predation on the older larvae of C. americanus. Chaoborus punctipennis adults are able to disperse to lakes from which their larvae are absent. In situ experiments indicate C. punctipennis can survive on the zooplankton in a stained, fishless bog lake, but is subject to heavy predation by 3rd and 4th instar C. americanus larvae. It is concluded that because of early recruitment by C. americanus, as well as its lack of extensive vertical migration, this species excludes C. punctipennis from stained, fishless bog lakes.