Welcome to posting. You are asking a difficult multifaceted question and we don't have a lot of info. Best advice is to write down everything you see and know and then talk to a fishery scientist with knowledge of northern ponds/lakes (call Bob Lusk at Pond Boss). Send them your notes and arrange for an electrofishing survey. In the meantime, learn to do a seine survey and write down and take pics of results. Do the same with catch results.

If the fish are skinny/underweight, then they need more food. This is a common problem. You can increase forage amounts or decrease the number of predators or a combination of both. Lots of small skinny NP and or LMB or both indicates to many predators for the forage base.