Why tuna are skilled hunters
It is not known how deep a yellowfin tuna can swim and still be attracted to a surface-trolled lure. It’s amazing that trollers catch any big yellowfin at all. Typical yellowfin swimming depths showed that the yellowfin remains 180 to 330 feet below the surface for about 80 percent of daylight hours, and they only spend about 6 percent of the entire time within 90 feet of the surface.
Though the tuna do not spend much time near the surface, neither do they venture very often below the thermocline, the level at which the water temperature drops significantly. Instead, the tuna mostly travels and feeds in waters of relatively uniform sea temperatures (68 to 75 degrees). But for reasons not entirely clear, the fish moved up and down throughout the layer often and regularly.
A bait school leaves an odor trail for which the yellowfin could be searching. Odor trails tend to spread out in a plane and stay at one depth. The tuna may be moving up and down in an attempt to find the depth of specific odor trails. Tuna have a well-developed sense of smell, which may partly explain how they can locate FADs and other feeding stations from great distances and navigate directly to them.
Tuna also have magnetite crystals in the bones of their snout. This allows them to sense relatively small changes in the Earth’s magnetic fields. Being able to sense these magnetic fields gives tunas both a compass and a map, since large geologic features like islands and banks have a specific magnetic signature.
Big yellowfins often feed surprisingly close to shore around the Hawaiian Islands. Mackerel scad and bigeye scad, both favorite prey of Pacific yellowfin, are usually found in 20-30 fathoms, depths found within a few hundred feet of shore along the steep ledges of volcanic islands.
Finding yellowfin close to shore is unique to Central Pacific islands. In most places, the water close to a coast contains a lot of plankton and is therefore very turbid. Most tuna species will not enter this type of water. Because the Hawaiian Islands are literally sticking up in the middle of the Pacific, very clear ocean water occurs right up to the coastline, and therefore, yellowfin will move in close to the beaches.
The yellowfin’s aversion to “dirty” water may be partly because they depend on their vision to catch prey. Yellowfin have excellent eyesight. They are apparently able to distinguish the fine features of an object better than most other fishes.
For fishermen, this ability to discern fine detail translates into a need for finesse in rigging baits and lures for tuna. Expert tuna fishermen always hide the hook in the bait and choose the lightest, least visible leaders that still have the strength to bring in a hooked fish.
Sharp eyes and a highly refined sense of smell aren’t the only weapons in a tuna’s arsenal; the fish are also responsive to vibrations. The response to vibrations might explain why yellowfin are attracted by the giant teasers towed by commercial fishermen using the “green stick” technique. The teaser creates some sort of intermittent low-frequency sound that tunas find attractive.
Yellowfin have a residential nature during the summer. Maybe the yellowfin come here to spawn for the same reasons blue marlin appear to. It’s called the “island mass effect.” There is often a higher density of plankton around islands than in the open sea. Larval fish depend on the plankton for food. Higher plankton density is most likely caused by run-off from the islands, which fertilizes the phytoplankton. In addition, local oceanographic eddies tend to form on the down current side of islands and entrap the plankton.