some lakes it varies.
The 15" minimum isn't a very good idea- it makes only SMALLER bass. When the DOW put it on purdy mesa, bass size dropped and thus bluegill numbers increased and the previous trophy bass and bluegill fishery was destroyed. Then, around 2000 I believe, they put in a 12-15" must release slot with only 1 over the slot, and 4 below I believe. The bass size shot up immediately, and the fishery was getting excellent for largemouths of up to and even over 5 pounds again. The place sadly was drained in 2005...but it was a good example of how the 15" minimum fails and how a slot succeeds. Some lakes have an 18" minimum for bass, or at least used to. (note, the above post was about largemouth bass).