From Creatures Wikia
Stickletrout are aquatic fish critters that live in the small pond at the bottom right corner of the Norn Terrarium. They eat insect critters such as Dragonfly nymphs, but are often subject to extinction from over-hunting by the Kingfisher and not enough to eat. This can be easily solved by putting some Balloon Bugs or the Fish Food agent in the pond with them.
The stickletrout can change gender if it is required for the species' survival. If there are no females left, a male will change into a female, and vice versa.