MOUTH GAPE, FOOD SIZE, AND DIET OF THE COMMON SMELT RETROPINNA-RETROPINNA (RICHARDSON) IN THE WAIKATO RIVER SYSTEM, NORTH-ISLAND, NEW-ZEALAND

The feeding ecology of the common smelt, Retropinna retropinna, was de termined at several locations on the Waikato River system. The mean si ze of the dominant prey was found to increase as smelt mouth gape incr eased. Smelt smaller than 40 mm total length fed mostly on zooplankton . Chironomid larvae, pupae, and adults were the dominant foods of larg er fish from most sites, although mysids and amphipods were the major prey in Lake Waahi and the Waikato River estuary. Algae were the domin ant food in many smelt but ingestion was probably incidental. Seasonal and annual differences in dominant foods were minor. Common smelt an generalists, capable of feeding on the majority of smaller organisms p resent in their different environments; however, they may grow faster and achieve a larger adult size when food of optimal size and nutritiv e value is present.