The Best Bait of All
There's no better live bait than a small tuna. Almost every large school of small tuna will have a marlin hanging around the periphery; ones that don't will have one sooner or later, depending on how many marlin frequent the area.
 
Any fixed point of reference, either anchored to the bottom like a permanent buoy or drifting in the open ocean, will eventually hold a school of tuna and the marlin that prey on them. Even an underwater bathymetric contour, where a ledge 100 fathoms down projects into the oceanic current, can hold tuna. One highly productive ledge off Jupiter, Florida, starts in only 30 fathoms and stays loaded with both bonito/false albacore and blue marlin.
 
Oil rigs in the deep waters of the Gulf of Mexico act like marlin magnets, just as fish aggregating devices (FADS) or offshore weather buoys do in other parts of the world. This is mainly due to the tendency for small tuna, as well as dolphin and jacks, to gather around any large floating or fixed point of reference in the three-dimensional vastness of the open sea.
 
To catch your small tunas, use small bonito jigs. They are sold in every saltwater tackle shop in the world. Tie two of the small jigs directly to 50-pound mono, and using identical rods and reels with the same size and brand of line on them, mark both lines with a pen at the exact same distance — roughly 50 to 70 yards back. If you let out each lure to those marks every time, it will keep the jigs from tangling when you have to troll in crazy curves and loops to keep the lures in or near the tuna.
 
This technique makes a lot of money every summer for top crews fishing the Texas and     Louisiana oil rigs.

Blue Runners
When I was a kid fishing out of Hillsboro Inlet at the northern end of Pompano Beach, blue runners, known as "hard tails" in the Gulf of Mexico, represented the most common live bait used for kingfish, amberjacks, barracuda, grouper and sailfish. In the late 1950s and throughout the 1960s, we used them way more frequently than today's bait of choice, goggle-eyes. I haven't used blue runners often when targeting blue marlin, but I rate them highly for live-baiting around oil rigs in the northern Gulf of Mexico.
 
On the plus side, you can almost always locate and catch some — they are often way easier to catch than the small tunas you would prefer. With the almost universal availability and usage of kites by even the smallest center-console fishing craft, blue runners might even become a favorite bait — especially when bridle-rigged through the flesh and muscle of the back and suspended so they dangle halfway in the water. This makes them kick and struggle constantly, emitting vibrations that attract all the big marine predators including tuna and marlin.
 
Capt. Skip Smith enjoyed considerable success using blue runners and won a number of tournaments and some sizable sums of money in his early days on Jerry Dunaway's Hooker. He even tells a story of raising a blue marlin near a rig that did not bite his bait.
 
"I had an aku (skipjack tuna or oceanic bonito) and a little ahi (small yellowfin or blackfin tuna) as live baits," he said. "Some Texas guy on a big old Striker boat couldn't get a good live bait. He was drifting past the rig with a damned old blue runner when I raised a really nice fish, a sure moneymaker. It never bit either of my tunas, but a few minutes later the Texan got a bite from a 600-pounder that won the jackpot. His blue runner beat my two beautiful tuna — which once again proved that it's better to be lucky than good."
 
In Australia and New Zealand some other species of small jacks, (known Down Under as trevally) are commonly used as live bait for blue and black marlin and the biggest striped marlin on the planet.
 
If you know, or even suspect, the exact location of a large marlin, nothing beats a live bait — besides exceptional luck!