First off, I have no advanced degree in biology and fish management. Next, I separate Bass Bay from the BM 'big lake'--due to depths, weeds, and size. I have fished the 'big lake' since it was opened after the reclamation. Then and now. I base my comments on practical experiences averaging around 50 trips there each year. Then 'outstanding', Now 'disappointing'. Are there still decent days on BM? Of course, but there is no comparison of present catches (especially in northerns) in the number and size of these fish.
There may be a mixture of reasons for the decline: CARP are back in numbers, winter kill, mismanagement, the 40-inch rule, the changing make-up of the lake (weeds vs bulls).
Someone made a great comment above, somewhat overlooked, in talking about BM. The beautiful lake scenes, the waterfowl, eagles, owls, and the lack of speedboat trafic all combine to make for some rewarding trips if not in the number or size of fish caught. I personally love fishing weeds, and BM has WEEDS.
I am 'all in' in ending the over-40 rule and going back to the Wisconsin State rules of 2-->26". Frankly, the change back is long over-due.
Good health.
Attached is the largest northern I ever caught there--in Bass Bay. 42" long. Photo taken by the only boat I could find on the Bay. Fish Released alive after photo. Taken in deep water the first year the lake was open.