vertical das larvas de Chaoborus brasiliensis Chaoborus brasiliensis Chaoborus brasiliensis Chaoborus brasiliensis (Theobald, 1901) (Diptera,em um reservatório tropical: Chaoboridae) em um reservatório tropical: Chaoboridae) em um reservatório tropical: Chaoboridae) em um reservatório tropical: lago

Vertical migration of Chaoborus brasiliensis (Diptera, Chaoboridae) larvae in Nado Lagoon, a tropical reservoir in Belo Horizonte, state of Minas Gerais, Brazil. Many investigations have shown that Chaoborus larvae develop diel vertical migration (DVM) to avoid fish predation and that this migration's amplitude is often associated with the instar stage. In this study, the Chaoborus brasiliensis vertical distribution was monitored in a shallow reservoir (Nado Lagoon, Belo Horizonte, Brazil), during the diel cycle, from October 1999 to September 2000. The mean depth in which each stage was found was estimated day and night and compared to their migration amplitude. No association was found between instar phase and DVM amplitude. Nevertheless, a clear association was found between the dissolved oxygen critical concentration and the amplitude of Chaoborus DVM. 1.0 mg.1(-1) was considered the concentration below which fish cannot survive. This finding gives support to the hypothesis that vertical migration is a defense mechanism against predation, since the larvae use the anoxic layer as a refuge against fish predation.

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