Do planktivorous fish structure the zooplankton communities in New Zealand lakes?

Abstract

We critically evaluate the prevailing view that the species composition of zooplankton communities in New Zealand lakes, and the abundance and 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 conclusion is based first on the seasonal zooplankton dynamics of several New Zealand lakes being similar to those in Danish lakes, in which fish predation has been shown to have major effects. Second, the indigenous planktivorous fish fauna in New Zealand lakes is dominated by larvae of benthic fish with a long breeding season (bullies) and other small potent zooplanktivores such as smelt and larvae of galaxiids. Patterns of diurnal vertical migration of zooplankton in several New Zealand lakes during summer are also consistent with it being a mechanism to escape from predation. Third, the dominant piscivorous fish (brown trout and rainbow trout) are only facultative piscivores and are therefore probably weaker predators on the planktivores than the more obligate piscivores in many north-temperate lakes. Although various other explanations have been offered for the low ratio of zooplankton-to-phytoplankton biomass observed in New Zealand lakes, this phenomenon is equally 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, which was further supported experimentally. We call for further investigation of top-down control of zooplankton in New Zealand lakes. It is important not only for scientific understanding, but potentially also for lake management. © 1997, Copyright Taylor & Francis Group, LLC.