The way my skippers used to work the machine was if in water shallower than about 200m which is approx the depth of the shelf water the machine would be set to record the bottom to mark structures and bait , although in the area we fished you could get a good feel for where the structure was with a bit of experience (using shore marks)

if operating off the drop the sounder would be set to 150m max and usually 100 as it was considered if bait or fish were marked deeper than this the odds of raising them with trolled lures was minimal

i would say that the machine wasn't a great deal of help in blue marlin fishing in our area, mainly because of the scarcity of bait you tended to be looking for individual lone fish - the odds of seeing those on the machine were pretty poor (although I have seen blue marlin on the screen when fishing in Kona) and the typical blue marlin bite would be a fish in the lures without warning of any kind, normally when you least expected it ... bait and tuna would show quite well - it is quite a trick to drive the boat with one eye on the spread, one eye looking for sign in front and around the boat and one eye on the screen - on the wahoo grounds (50 to 120m) being able to see bait and structure on the machine was very worthwhile , good looking areas could be identified and worked hard

the last machine we used was a dual frequency 200/50 my skipper used to insist that the gain be fine tuned in an area where there was no bait - he'd turn up the gain till interference started appearing on the screen and then turn it down just a little - he advised to resist the temptation to turn the gain on till you started to see something cos you'd end up seeing loads of imaginary bait and whenever looking for bait he'd use 200

i don't claim to any expertise whatsoever in fishing by the machine so i definitely want to know more about it myself

cheers dustin