The Lake Okaro ecosystem 2. Production of the chironomidPolypedilum pavidusand its role as food for two fish species

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Abstract
Lake Okaro, a nutrient‐rich lake on the central volcanic plateau of the North Island, New Zealand, supported a maximum littoral population of the chironomid Polypedilum pavidus Hutton of 309 000 larvae m‐2 in 1983/84. This population depended for food on the shoreward drift of cyanobacteria (blue‐green algae). The larvae were an important food source for the benthic bully Gobiomorphus cotidianus McDowall from January to June. From June to December the cladoceran Ceriodaphnia dubia Richard was prominent in the diet when the fish moved towards the open water of the lake. The bully removed up to 128% of chironomid production only when population density of the larvae was low. At other times fish predation had little impact on the larvae. Most larvae were eaten by bullies during the day. Bullies caught at night were on average larger and preferred to eat small individuals of their own species. The smelt Retropinna retropinna (Richardson) played a minor predatory role in the lake. It ate P. pavidus larvae at all times but favoured C. dubia in winter and small bullies in autumn. In Lake Okaro cyanobacterial food supply was of overriding importance to the chironomid population but fish predation was a minor influence. Both fish species were opportunistic feeders switching seasonally to the most readily available prey.