The ‘mosquito fish‘ or Gambusia does get a lot of bad publicity because it is eating tadpole and threatened larvae. I feel it is important to realize that it is OUR environmental impacts on the waterways that has pushed on these populations, not really the Gambusia.
Trout will eat anything that moves and wiggles without discrimination. All bugs, tadpoles, worms, larvae, etc. And they eat a lot, every day. Trout are often raised in farms and released into rivers for anglers to catch on weekends. Trout is of course native.
So who do we blame? Probably not the fish.
Though I think that the bacterial solution would work safely on a very small scale, my inner voice automatically winces at the strategy. In general, my favored response to an abundance of one form of life is to balance it with an opposing force of life, or predator. Increasing life rather than subtracting it. Those mosquitos are a great source of food and their bodies could be converted to more life by another animal.
“You don’t have a snail abundance, you have a duck deficiency”
-I forgot who’s quote this is
If you have a pond in your yard, I really, really doubt the little Gambusia will make it out to a river or stream. All the drains lead out to the bay in one way or another and those guys would die on the way or die when they made it.
If you really want to avoid this fish for personal reasons, I think another non-native species is worth looking at, the carp. There are many species of carp, but most all are omnivores just like the Gambusia. The advantage of carp over gambusia is they they also fill other functions and provide a yield (food).
The black carp for example will primarily feed on mollusks so if you water snail population is booming those guys can help balance. The grass carp (banned in California) east exclusively vegetation.
Carp can get very big, like this common carp at left named Jack. They can be very affectionate and make wonderful companions.
All Joking aside…
A couple goldfish (a type of carp) from the pet store will eat mosquito larvae. The trick is that you want an ommivore not a carnivore. If you put a carnivore in your pond, you now have a pet that needs regular feeding because they will quickly exhaust all the food in their space. Carnivores eat high on the trophic order and so are expensive in a resources sense.
Omnivores will switch to vegetation if the protein sources are gone, so in general you don’t have to feed these fish in a diverse pond setting. This is one of the reasons Gambusia or small goldfish would be good.
Herbivorous fish are completely banned in California and you will open up a whole new can of worms(hee hee) bringing those here.
Hope it helps!