2/3 of all sharks that I've ever had a run in with have hit the middle line, usually set around 25/30 ft down and approx 100 ft from the boat. I have never hooked a thresher, but I believe this depth is the most effective for the type of chum slicks deployed by sharkers in this area. On balance, the thermocline MAY be the key. In the early season the fish may be above it (warmer water), in the fall they are probably below it (warmer). Good Topic.