They`re not much to look at. They`re gray and little, with tiny spots on their tails,and not many folks would want them for their aquarium. But the native
mosquito fish, Gambusia holbrooki, is one of nature`s front line critters in the
ongoing battle against mosquitoes. Or are they? The mosquito fish is a small aggressive fresh water fish that is a "Livebearer," family Poeciliidae. They are less than 2" long and are interesting in that the females are easily distinguished from the males by body shape. The female mosquito fish is larger and stouter than the male and has a black patch on her abdomen. The male is a smaller, slender fish that has a highly modified anal fin that is used in mating.
Mosquito fish are live bearers. The female delivers around a half-dozen tiny fish that are immediately ready to live up to their name. They hide from predators in submerged aquatic vegetation, dine off of algae and whatever else is smaller than they are, and grow quickly.
Mosquito fish are hardy. They can take the heat and some cold. I`ve seen adults in a semi-frozen shallow pond, thaw out, and swim away when the sun came out. They can live in brackish water. They just need a reliable source of food - larvae, zooplankton, algae, detritus, and smaller fish. And there`s the problem. They have been known to prey on the eggs, larvae and juveniles of the sunfishes and the largemouth bass, which in turn, will prey on them when they grow larger.
I have mosquito fish in the stock tanks at my farm and they ensure that no mosquitoes breed in the standing water. Many years ago in south Florida, I had a fish pond with goldfish. Raccoons made short work of the goldfish and when I discovered mosquito larvae in the pond, I netted a few mosquito fish from a ditch and stocked the pond. The end result was a thriving population of mosquito fish and no mosquitoes. It was a no-brainer to put them in my stock tanks up here after I noticed mosquito larvae in them. I understand that mosquito fish are also used in rice paddies. They work for me, but some biologists feel that mosquito fish pay more attention to juvenile fish than to mosquito larvae.
Several years ago, I became interested in their behavior. I put 6-8 fish in two 10 gallon tanks, without filters or aeration. The results were interesting. If there were more than one female in the tank, the smaller females disappeared over a short time, leaving the largest female. The population of males generally remained the same and the males spent a lot of time chasing each other, and the female, around the tank
After observing this, I wondered what would happen with the young. Nothing happened. The big female co-existed with her daughters and sons, but would go after a strange female if introduced. She was very territorial with regard to strange females but not toward her brood. I don`t know if this behavior persists in the wild, but in a 10 gallon tank, there is only room for a big mama mosquito fish, her daughters, and whatever males might be around. I never allowed the fish to overpopulate the tank, so with crowding, things might change.
These little fish are amazing in their predation of mosquito larvae in an enclosed environment. They can take out a large population of larvae in no time. They are like tigers in a goat flock.
The mouths of mosquito fish are specialized in that they tilt upward to make it easier to feed off of the surface of the water, where mosquito larvae hang out. When threatened, mosquito larvae will dive for the bottom, but they are rarely fast enough to outrun the speedy little mosquito fish. Overall, I`d say we folks in Florida owe a lot to these little freshwater fish that live in all types of freshwater habitats and niches and are always on task. Some biologists, though, might dispute that claim. These are voracious little fish and anything smaller than the Eastern mosquito fish is fair game, which in some instances might not be good for other native aquatic populations. | 
