thika, good question. Much depends on your target species and knowing their preferred temperature range. The first thing is to see if and where the thermocline is. For some fish, this is like a barrier and they will not go through it, for others, it is their main resting area and they only leave it to feed. In the Cape, longfin generally will sit above the line whilst yellows (depending on the variance between surface and thermocline) will be in the mixing area. For marlin, blues will be around the bait - so find that and you will find the fish. Use the same rule of thumb, are they feeding on skippies, yellows, dorries etc. In turn, what are they feeding on - baitfish, squid, flying fish etc. Each of these has a preferred temp range (and this can differ per area). Find the right temp, you will find the right bait and hopefully the billfish will be close. For blacks, I ALWAYS used to run a big livie on a deep downrigger and stick it right on the top mixing layer of the thermocline. That used to pick up over 70% of our fish.