A tiny terror of the shallows. This is a fish that should never have been brought here. Gambusia rarely grow longer than a thumb. The body is chunky, with a flattened head and an upturned mouth. Colour ranges from nondescript olive-green to silvery-brown. Females are larger and rounder than males. Males possess a long, modified anal fin called a gonopodium. It is used to mate with females in a rapid, almost violent process. The fish does not ask permission. They are aggressive little bullies. Known as mosquitofish, they were introduced to New Zealand in the 1930s to eat mosquito larvae. But they do not stop at mosquitoes. They eat anything that fits in their mouths. Insect larvae, small crustaceans, the eggs and young of native fish are all on the menu. A single gambusia will attack a fish twice its size. It nips at fins and chases rivals out of the shallows. It fights above its weight class. This is a textbook example of a biological solution becoming a biological disaster. Someone thought they would help control mosquitoes. Instead, the fish spread through waterways. They outcompeted native fish and turned the shallows into a battleground. They are one of the most widespread and damaging introduced fish in New Zealand. Good intentions. Bad outcomes. It is a familiar story. To catch a gambusia is to catch a mistake. It is a tiny, aggressive, impossible-to-eradicate reminder that good intentions do not always lead to good results. This is the fish of the roadside ditch. It is seen darting through the shallows, nipping at everything that moves. It is a tiny, relentless engine of ecological damage. The ditch is full of them. Thousands. Each one is a small green torpedo of destruction. The native fish are gone. The gambusia own the shallows. And they will not leave. They never do.