Effects of fish on the microdistribution of different larval size groups of Onychogomphus uncatus (Odonata: Gomphidae)

Four types of field experiments were carried out in order to study the effects of fish predation on different size groups of burrowing larval dragonflies Onychogomphus uncatus (CHARP.). (1) The effects of the bottom feeding fish Barbatula (Nemacheilus) barbatula (L.) on three size groups of O. uncatus in three types of substrate: sand, gravel and stones, were recorded using field enclosure cages. The densities of small sized larvae inhabiting cages with stones and gravel were reduced in the presence of fish compared with fish-free controls. No effects due to fish presence were found in larger larvae. (2) The colonisation of sand, gravel and stones in the field by different size groups of O. uncatus was studied using frames filled with substrates. Substrate as well as size specific effects were found. The low density of small larvae in coarse substrates is interpreted mainly as a direct effect of predation by B. barbatula which exclusively colonised these substrates. (3) To test the effect of exclusion of all fish on colonisation of O. uncatus, frames filled with stones were exposed in a running water and half of these were protected by cages. The densities of very small and medium sized larvae in the protected substrates were higher than in those without cages. (4) In cages with a wide mesh size, which allowed a free exchange of dragonfly larvae and B. barbatula but prevented predation by large fish, the density of small larvae of O. uncatus was reduced. B. barbatula were found exclusively inside the cages. It is suggested that predation by B. barbatula using the cages as a shelter against predation by large fish was responsible for this reduction.