Norm,
Most of the commercial fish raised in Thailand are imported, including the Pacu, Talipia, various carp and Barramundi and a lot of them have already established in the streams and rivers. I am conscientious about controlling the species that I have within my own pond and have a seven foot levy surrounding it, with no spillway. Also none of the species in my pond have been imported by me yet...but if I ever find a source for LMB I will be adding them to the pond. Most of the exotics in the USA are already well established in Thai waters. The worst fear of Americans - the walking catfish and the dreaded snakehead are native, with one of the local species the Giant Snakehead growing up to 45 pounds:
http://www.anglingthailand.com/gallery/gallery.htm I have both the walking catfish and snakehead (though not the giant) in my pond, and honestly I have not noticed their impact. Fish in the tropics are very adaptable, most are prolific breeders and many protect their young up to two months. I'm sure that in the northern climates that fish would be more niche sensitive, but in the tropics its dog it dog!
Having said that I have noticed a couple of the smaller fish species that were established in my original pond (expanded from 1 acre to 5) have disappeared, particularly gouramis and a couple of other small fishes most likely from the Peacock Bass, but these fish stay isolated in a bowl, similar to the flathead minnows and shiners in many of the stateside ponds. The flood plain where I live is enormous as I live within an hours drive of the Mekong River, unlike the Missisipi where the flood plain has been greatly reduced by levies the flood plain here is left open as it brings the waters for the staple crop of rice. I really think it would be difficult for any fish species to totally eliminate another.
Norm a good question and seriously glad you asked. The answer is yes I do exercise a conscientious approach when stocking.
