Captive fish bred fish especially if given hand feeding are not used to hunting for food and have a lack of fear of bigger fish.
At the Bribie Research Station, during experiments with jungle perch fingerlings, it was found they did not associate bigger fish with danger.
The researcher actually had to teach the fingerlings o be afraid of bigger fish by using pheromones from the skin of jungle perch which are released when they get injured (ie chewed on).
Even going from a dam to the river below, fish find surviving in the river harder than in the dam. Food is not as plentiful and competition is much fiercer.