Vulnerability of melanic Daphnia to brown trout predation

Abstract

The co-existence of melanic Daphnia cf. longispina and facultatively planktivorous brown trout is reported from a clear-water, alpine lake. This co-occurrence is uncommon, presumably due to the vulnerability of pigmented Daphnia to fish predation. Lake Bjornesfjorden (Norway) provided an opportunity to test this assumption. About 20% of the fish caught in gill nets had fed on Daphnia. The trout exerted a marked selection for large-sized Daphnia prey, and a very strong selection for pigmented individuals relative to transparent ones. The persistence of a pigmented Daphnia population probably relies on limited recruitment and a low stock of the predator, and the availability of more favourable benthic prey organisms.