I agree and disagree. Oscars released to the wild are doing just fine. Fish don't lose their instinctive abilities when born in a fish tank. Their needs are the same as their ancestors 10 or 20 generations earlier in the wild or we wouldn't see the amazing ecological problem of massive invasive fish species success.
There are special circumstances involved there that help to make an exception to the rule. Most captive bred animals do retain their natural instincts, but instincts aren't everything. Fry are taught to survive on their own from a young age and a captive bred fish that knows when and where the next meal is coming, would likely starve. Not to mention some captive bred fish wouldn't react to larger predatory fish like they would in the wild, especially if they have been king of their domain for a while.
Fish are different from a lot of other animals in the captivity game. Basically it boils down to how any animal is reproduced (eggs vs live birth) and how long it receives care from its parent(s). In the instance of most fish, they are hatched from eggs and cared for by parents for only a few weeks. So yes, it is much easier to get these animals to be introduced into the wild more easily with some preparation, such as giving only live foods or food they have to forage for without giving anything man made, or keeping them with fish that would naturally predate on them. It also helps to get them to a decent adult size before releasing them because as in any aquatic region, size plays a large part of survivability.
So yes, it is possible to reintroduce many captive bred fish species into their natural environment with success if certain preparations are made. Unfortunately, many of those preparations would require tanks many times the "best" size to ensure room for hunting and foraging, and room to keep some predators around to keep them on their defensive toes.
I understand where you were going with the invasive species comment, but it doesn't really apply here unless you are talking about releasing oscars into areas outside of their South American habitats. Invasive species involves introducing an animal into an environment you would never possibly find it in the wild. Invasive species are driven by three major factors. The first is a readily available food source that it can easily out compete other species for. The second is no predators in the new environment where it would have had predators in the old. And the third is the opportunity to reproduce. Snake heads are a great example of this. They were introduced into a lake (I forget where) after an exotic fish seller brought a batch overseas (I believe from Asia) to sell locally. The resulting cute, tiny fish became big and monstrous and because it had not been seen before, people had no idea what they were buying (shame on buyer and seller there). People came home to find entire tanks wiped out by these fish or realized it could not possibly be kept in the tank(s) they had, so they released them into the local lake. Well, male and female met and boom.....babies. There were no animals in that lake that had any interest in eating their new neighbors. Unfortunately, the snakeheads grew up to not have that issue.
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