Lake Taupo is located at the centre of the North Island of New Zealand, and is the largest in that country. The drainage basin of the lake is characterized by a mild climate; moderate temperature, light winds, cool summers, relatively warm winters and a fairly even distribution of rainfall throughout the year. The lake is surrounded by gently sloping hills which offer pasture for sheep. About 30 rivers flow into the lake, while the Waikato River is the only outlet. There are two towns, large and small, around the lake. Taupo is the main town which is located at the mouth of the Waikato River. Tall eucalypti, which are characteristic in New Zealand and Australia, are abundant on the lake shore of the town. The lake water is clear and highly transparent, with a maximum transparency of 21 m. The concentration of nitrogen and the nitrogen/phosphorus ratio of the water are remarkably low. This may be mainly attributed to the geology of the catchment area, and is responsible for the oligotrophic level of its productivity. The Koaro and the Toitoi are fish native to the lake. The Koaro, with a flat head and tough but slimy skin, was an important fish resource for the Maori living around the lake, but has become very scarce. On the other hand, artificially stocked rainbow trout is now abundant, and is primarily important for sport-fishing. The common smelt was also introduced to the lake between 1934 and 1940 after the average size of trout declined (2).