Back in the late eighties or early nineties when I first started fishing for them on Cape Cod they had to be 40 inches to keep and as the years went along the size went down into the twenties. They built up populations relatively fast when the size limit was 40 inches. Like everything else though things may be more complicated than they seem...the oceans are radically changing, bait populations changing etc. so maybe slot limits may not answer the problem.