They are probably dying off because they are sick, or are less healthy and stressed by poor conditions. You just have to figure out what is wrong. Did you notice that the Perlmutts came out and ate normally?
It is unlikely that this is a murder mystery, juveniles are not that violent. There is no particular reason for the Haras to selectively bully the Perlmutt. Fighting does not happen for no reason, larger fish do not pick on smaller fish just to be mean, the smaller fish are of no threat, fish are not like humans. Most fish may pick on sickly fish that might be near death so they can eat them, something humans hopefully do not do.