There has been gill studies done and it seems it comes down to the maturation process. Once gills start spawning they grow at a much slower rate. Take a bunch of 8'' spawning gills and you give a chance for smaller ones to start spawning, catch them, and then even smaller ones can spawn(no large ones to chase them off) and soon you have 5'' gills spawning and not growing very fast. They aren't stunted, they just reached spawning size. Arrowhead is a good example. Years ago, there were loads of 7'' gills on the beds. Meat hogs took 'em by the bucketloads and now they run about 5'' on the beds. That's why some lakes have the 8'' min size on the gills. They are trying to increase the average gill size by forcing anglers to let them reach a respectable spawning size first!