The pupils have been conducting an ongoing study of the water quality in the lagoon, and last week looked at what aquatic life, if any, was in the water.
Department of Conservation staff placed nets and traps in the lagoon in an attempt to catch native eels and trout.
When the nets and traps were hauled in, they contained only eight native bullies.
However, local fisherman Danyon Stenton caught a rainbow trout in the lagoon while the pupils were checking the nets, and they were delighted to see it.
Mr Stenton said many eels also lived in the lagoon.
Tahuna Normal Intermediate teacher Roddy Scoles said he was not sure why the nets and traps had not caught any fish or eels, but was not deterred.
The nets would be reset, this time with bait, in a bid to catalogue the catches, he said.
The pupils will join the local community later this year in planting native trees around the lagoon, in the hope of improving the habitat, he said.
