Ed - I used the 20" length for the HSB because of the open width of the mouth size of this sized fish is about 2" wide. Nine inch long, average, body condition, yellow perch are around 2" body height. If it will fit into a predator's mouth they can theoretically eat it. Perch 8" long are about 1.7" body height this corresponds to mouth size of HSB around 18" to 19" long. As your fish grow, measure body lenght and mouth width and you will get a feel for what size fish they can eat.
Just because HSB can get a perch into their stretched open mouth does not mean that they will always eat prey this size. Often smaller more abundant sized prey will be eaten. Numerous things will affect this behavior. But at least you will know the upper limit of prey size for your HSB. Keeping the HSB in the smaller size range will cause more predatory pressure on the smaller sized prey.
