Biomagnification of contaminants through food web/ Role of plankton (phyto- and zoo-) in proximate nutrient content of cultivated fish/ Influence of ambient ecosystem on fish behavior (physiology)/Prevention of outbreak of disease through probiotics/ Role of feed quality of fecundity and egg viability etc are to name a few of the topics in the vastness of the concerned discipline that can be dealt with. Of course any other subject can be chosen as long as it remains personally, temporally and economically feasible.
How hermaphroditism impacts Maximum Sustainable Yield calculations. I.e. what if your MSY allows you to fish out all of the males, because they are larger ?