DO PLANKTIVOROUS FISH STRUCTURE THE ZOOPLANKTON COMMUNITIES IN NEW-ZEALAND LAKES

We critically evaluate the prevailing view that the species compositio n of zooplankton communities in New Zealand lakes, and the abundance a nd size distribution of the zooplankton, are more commonly controlled by food and temperature than by predation. We conclude that predation may play a far more important role than was hitherto thought. This con clusion is based first on the seasonal zooplankton dynamics of several New Zealand lakes being similar to those in Danish lakes, in which fi sh predation has been shown to have major effects. Second, the indigen ous planktivorous fish fauna in New Zealand lakes is dominated by larv ae of benthic fish with a long breeding season (bullies) and other sma ll potent zooplanktivores such as smelt and larvae of galaxiids. Patte rns of diurnal vertical migration of zooplankton in several New Zealan d lakes during summer are also consistent with it being a mechanism to escape from predation. Third, the dominant piscivorous fish (brown tr out and rainbow trout) are only facultative piscivores and are therefo re probably weaker predators on the planktivores than the more obligat e piscivores in many north-temperate lakes. Although various other exp lanations have been offered for the low ratio of zooplankton-to-phytop lankton biomass observed in New Zealand lakes, this phenomenon is equa lly consistent with high rates of predation. Re-analyses of data from earlier investigations in Tomahawk Lagoon No. 2, provide evidence for a periodically high predation pressure on zooplankton during summer, w hich was further supported experimentally. We call for further investi gation of top-down control of zooplankton in New Zealand lakes. It is important not only for scientific understanding, but potentially also for lake management.