You can thank the oil companies for this world-class tuna-on-fly fishery. The tuna off Louisiana relate heavily to the offshore oil platforms. The rigs hold bait, and the bait draws in the tuna. We spend most of our time chasing yellowfin in at least 1,300 feet of water and, most often, in 1,500 to 3,500 feet. If conditions are right (temp, clarity, current), the tuna usually hang one-fourth of a mile to a half-mile up-current of the rigs. Look for surface activity; if you don't see any, do a sonar sweep to determine if the fish are holding deeper. If you mark fish in the upper 300 feet of the water column, you can catch them on flies. Yellowfin will not hesitate to come up from 300 feet down and crush a fly on the surface, and it's something to see.