One key point of information in this study is that they were raised separately in tanks and fed regularly. In a pond ecosystem containing both species, which do you think will grow fastest? The more agressive species with the biggest mouth. Because of this, the myth isn't a myth. It appears its only a myth in a controlled laboratory setting. Nonetheless, these are interesting findings with applications for those wanting a monoculture.
In a pond, all things are not equal as with a controlled laboratory study.
