What the record shows
Roughly 30–40 m can hold reef-associated species, while 50–70 m is often relevant to pelagic targets such as tuna. Bait, birds and current can be stronger clues than the number on the depth sounder.
What it means for your trip
Moving between water and testing layers takes time. The 9-hour format leaves more of the day for those decisions without pretending any one depth guarantees fish.