If full research has been done into the possible impacts of introducing a new nonlocal/ endemic species then the out come can be good, provided a back up plan of future control of the introduced species is also considered.
In Australia because people didn't realise what an amazing job our local rainbow fish and blue eyes did in mosquito control they introduced guppies and gambusia into many waterways. The guppies and gambusia breed faster than the natives and eat the native eggs/ fry while also picking at and eating the plants the native fish use to lay their eggs. Goldfish that where either deliberately or accidently let go have become major pests in many waterways and wrecked untold damage to the entire ecosystem thanks to their habit of sucking up mud and not allowing plants to grow. A clever solution that a company came up with is catching these pest goldfish/ carp and turning them into liquid fertilizer for gardeners in a product called "Charlie Carp".
If the two options of mosquito control is introduce some fish or use chemicals such as DDT (yes this is still made, sold and marketed), then I will always pick the fish, at least down the track you can find a use for the fish.