Sure a lake can be over fished, I do think they monitor Beaver pretty carefully and that will help. For myself I need just 1 20' fish to feed me and my wife typically by the end of May I have enough in vacuum bags to get me through the summer dull drums so I only keep 1 freshy or two at most per week the rest get to go back, we eat walleye on average twice a week so the fresh one plus a froze one is perfect for us. To me its about self regulating only what you really need to same goes for crappie unless I have a fish fry coming up.
I found it takes 4 crappie for me and my wife and we eat it about once a week to once every two weeks so I build up stock in the vacuum bags and then only keep what I need for a meal for fresh. I cannot begrudge anyone for keeping any fish to eat that's why God put them here, but if you are going to keep some in the freezer use a vacuum bag and only keep what you need for your family plus some for when its slow in that way I believe we would insure the continued sustainability of the fish.
What I do see as potential problems to the fish are the limb, jug and trot lines in the lake. They are indiscriminate killers! I cannot tell you how many times I have seen the lines floating unattended during the day which is ILLEGAL and seen ( J ) style hooks on them which are great gut-hookers which kills sub-legal fish. I wish AGFC would get on these guys and also change the law to require circle hooks on them. Same for Limb and Trot lines, make them use circle hooks. If you are going to do this type of fishing that's fine but use hooks that are more friendly on sub legal fish. My Jugs are all set with Circle Hooks and its common to catch undersized fish and they are not gut hooked because of the Circle and can be released.