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At first, the Kaikorai Valley College science and biology teacher thought she was imagining it, but she soon realised it was a baby rainbow trout trying to make a great escape from the classroom fish tank.
''I discovered the hard way that the fish get really excited sometimes and they will chase each other, and sometimes it just seems they think it's a better option to be on the other side of the glass.''
After finding one or two on the floor, she decided to put a lid on the tank.
''With the lid in place, I have heard the occasional plonk, but none have escaped since.''
Dr Tyrrell said she has more than 40 trout in the tank, grown from eggs from Macraes Trout Hatchery. They were given to the school by Fish & Game as part of its schools project. This is the fifth year the school has participated.
She said the project gave pupils an opportunity to see the transformation from an egg to a small fish.
''Fish go through a metamorphosis, like a butterfly does. There's different life stages and the students get to see that - it's really valuable.''
Pupil Brooklyn Taylor (13) said the trout had become classroom pets for her and fellow pupils. They had named most of the fish after themselves, with names such as Michael Junior or Sally Junior.
''But I named mine Fluffy.''
She said it was possible to identify some of the fish based on their size or markings but in reality it was virtually impossible to tell them apart.
Dr Tyrrell said the fish were now about 5 weeks old and about 4cm long. Within three years, they would be more than half a metre long.
''There's a limit on how long we can keep them in the tank,'' she said.
Next week, the fish will be released into Dunedin's Southern Reservoir on Mount Grand, above Kaikorai Valley.