A skip jack tuna, known as "Aku" to all Polynesian anglers and most "Haole" fishermen who have ever been to Hawaii or anywhere in the South Pacific, is the ultimate live bait for any angler chasing blue or black marlin in any of the world’s oceans.
An aku is the best of all live baits for a decent sized marlin. A Marlin will eat any species of tuna, dead or alive! Since a marlin can easily eat a tuna weighing one tenth the weight of the marlin, it is hard to find an aku that is too big to be used as live bait.
We once caught and released a black marlin that might have made 500 pounds. It ate a yellowfin tuna that we caught, rigged as a live bait, weighed it quickly, and put it back out as a live bait. That bait weighed 35 pounds!
We could not use an outrigger clip and my deckhand had to function as an outrigger, for a short time, until we got the bite!
I will use any species of tuna as a live bait but skip jack is the best! Captain Bobby Brown had told me that years ago, but I was slow to become a believer. I would use the first bait size tuna I could catch as a live bait.
I now think it has to do with what I call the "reflectivity" of this particular little tuna. When a strong ray of sunlight hits a live skip jack it makes a powerful FLASH of light off the side of the little tuna’s body. Several times I have started to yell down "Marlin" to alert my crew because the tuna lit up so much water it looked like a lit up marlin.
There is NO doubt in my mind a marlin can spot a live aku from farther away than any other tuna species which makes it a great bait.
When you have found, and can catch, any kind of small tuna your odds of catching marlin just went WAY UP! The very best species of tuna for a live bait is a skip jack tuna or Aku.
When an aku dies, it loses its reflective power and is no better as a bait than any other small tuna of whatever species.
I would use any freshly caught species of tuna as a dead bait and be confident a marlin would snap it, up but a lively Aku is the very best bait you can ever get when you are looking for a big marlin.
Unfortunately, An aku that has been dead more than a few hours,quickly becomes a terrible bait, unless it has been kept in a salt water brine since shortly after it was caught. An old, dead aku quickly becomes very soft and fragile and will be easily torn to bits and pulled off the hook when a marlin does bite. Old dead tuna of other species can still b great baits for about as long as the crew can stand to touch them and rig them!