1.Lake Ngaroto has a diverse fauna of native and introduced fish. It is largest of the Waipa lakes, is hypertrophic (i.e., has very high concentrations of plant nutrients), and is highly productive for this reason.
2.In an exhaustive fish survey that used gill nets, fyke netx, and beach seining, 4,317 fish of nine species were caught. The catch included five species of introduced fish. Over 70% of the catch was brown bullhead catfish; rudd, goldfish, a single mosquitofish, and a single koi carp were also caught. In summer, mosquitofish numbers a likely to be very high.
3.Four species of native fish were caught, and of these, shortfinned eels wer the most numerous. Common bullies, a few longfinned eels, and a single common smelt were caught, thought common smelt and common bullies are expected to be much more numerous in summer.
4.The migratory species in the lake are the eels and common smelt. Eels are always migratory, as they spawn in the tropical ocean. Common smelt may be wither migratory or lake-resident, and the single individual caught in this survey had vertebral and gill raker counts diagnostic of a migratory fish. As smelt migrate upstream from the ocean in spring and summer, but are generally absent from freshwaters in winter, the low abundance in August and October 2001 is not surprising.
5.Eels are string migrators, and can climb rock faces and wriggle through small cracks and crevices during their migration. Common smelt migrate by swimming, and high velocities and free-falling water barriers can prevent their upstream passage.
6.We recommend that to all upstream passage of swimming species such as common smelt the rebuilt weir has zones with mean water column velocities no greater than 0.5 m s⁻¹, and preferably 0.3-0.4 m s⁻¹. The length of the downstream slop of the weir should be no more than 4 m, and the water depth over the weir should be at least 5 cm during the time of principal upstream migration (August to December).
