Everyone knows that choosing the best leader and rigging it properly is an important component to success, especially in clear-water environments. But when you’re talking about yellowfin tuna in offshore territory, using the right leader becomes utterly critical — particularly when the fish are taking static or slow-moving offerings and they have plenty of time to slap their big eyeballs on your bait. This one factor can make the difference between hero and zero on any given day of fishing.
Of all the tuna species, yellowfin have the highest concentration of bright-light photoreceptors (cones) in their eyes, with 406 per 0.1 mm square (“Vision in Tunas and Marlin,” Gunzo Kawamura, Waichiro Nishimura, Soichi Ueda, Nishi, and Tooru Nishi, Kagoshima University Research Center for the South Pacific).
One glance at a yellowfin is all you need to wrap your head around just how much of the fish’s physical evolution has been dedicated to sight. We know how much we humans depend on our vision, and as a matter of sheer body mass, yellowfin dedicate far more to their eyes than we do. This relatively small 45-pound fish has eyeballs about twice the size of the 200-pound man holding it.
Particularly in a fleet of chunkers, leader choice is a make-or-break decision.
There were boats in this fleet that drifted butterfish and squid chunks past countless tuna and never had a strike. Others made limit catches. The one variable responsible: picking the ideal fluorocarbon leader.
The fish pictured here is hooked up on 30# Seaguar fluorocarbon after the anglers experimented with 80#, 60#, and 40#pound leaders without success. They dropped leader size every drift until they started getting bites. As you can see, the thread-like 30# test that finally generated hits isn’t visible at all in this shot.
The same fish from just a few feet away, right before it was gaffed. Note that the hook (which had been completely embedded in the bait) is clearly visible in the corner of the fish’s jaw. But the leader remains virtually invisible in the water. Look closely by the bottom of the fish’s gill plate, however, and you can barely make out the faint image of the leader attaching this 70-pound fish to the angler.
The same fish, post-gaff. Suspended in the air, the leader is far more visible.
This is because the fluorocarbon most closely matches the refractive index (the way light bends) of water as opposed to the refractive index of air. If it were less visible to our eyes above the water’s surface all that would accomplish is making it harder to tie knots or crimp the line — what counts is what the fish see under the water, not what we see above it.
Fishing more or less static chunks or live baits gives the tuna plenty of time to eyeball your offering and the ability to make a go/no-go decision that, in a tuna’s world, is practically slow-motion. Along with using the proper leader, minimizing hardware and connections is important. These anglers chose to use a single ball-bearing swivel (visible just below the rod tip) to connect the 30# mainline to the 30# fluorocarbon leader. Using a uni-to-uni, surgeons, or similar knot to connect directly is a better move in some cases, but since these anglers were using butterfish chunks (which often spin when being retrieved), having a ball-bearing swivel inline was a must.
Leader visibility can become less of an issue when it comes to lures that are being trolled quickly, but only slightly less. While it’s usually possible to get away with heavier tests, using fluoro is still a must.
If you’ve ever fished side by side with other boats using what appears to be the same gear and baits, yet you hit the dock to discover you were thoroughly out-fished, leader choice is a likely reason why.
While choosing the best leader size for yellowfin always requires some experimentation and will be dictated by environmental factors like water clarity, light levels, and just how choosy the fish are on any given day, using high-quality fluorocarbon is a no-brainer.