Have found that I use the ps 30 more for this kind of fishing....hybrids, striper, etc. Just turned on the 21 to get some video after getting a ground wire run down the shaft of my trolling motor. Had some interference that looked like rays coming out. Haven't used a 31 before. Have used the 21 on a brushpile with fish on it, and it worked well. Just different styles of fishing requiring different transducers.
Idk if either the 21 or 31 would be beneficial with these fast moving fish. Haven't had to chase them around much, they seem to like my boat.