Tuna do associate with FADs (Fish Aggregation Devices) and have particular behaviors based on the time of day, water temperature and fish species.
Ahi (yellowfin tuna) show a stronger attraction and behavior relationship to FADs than aku or marlin. The ahi that are associated with a FAD are almost never in the top 10 meters (33 feet) of the ocean. During the day, they are at depths from 70 to 110 meters (230-360 feet) and often near the FAD. Late in the afternoon or early evening, they go out away from the FAD, possibly to feed. They also come up to the warmer surface waters to depths of around 40 to 70 meters (130-230 feet). The bigeye tuna follow this same pattern but in even deeper waters.
The temperature range of the depths is closely related to the thermoclines, which are boundaries made by distinct changes in the water temperature. Ahi spend most of the day near the 72-degree layer and move up into the mixed thermocline around 84 degrees at night. The bigeye are found in the colder 62-degree, deeper water during the day and come up to the mixed thermocline layer at night.
The depth/time of day association phenomena could be related to feeding behavior. The increase in activity of ahi occurring just before sunset, and the swimming patterns away from the FAD in the shallower water, could be a foraging activity for food.
Another interesting behavior noted in ahi was that they tend to "hang out" on the up-current side of a FAD. Often when ahi approached a FAD, it was at a slower speed and from the up-current side. By the way, the average swimming speed of an ahi is 1.5 to 4.5 knots and 1.5 to 2.5 knots for aku. Faster speeds were recorded at times of stress, such as the arrival of a school of porpoises. Overall, ahi associated with a FAD tend to swim closer to the surface during the day than those in the open ocean.
Now a few words on the aku (skipjack tuna). One thing is for sure: aku are found in much shallower waters during the day and make deep dives at night. They spend nearly half their day in the top 20 meters (66 feet) of the ocean. However, there is no conclusion on their association pattern to a FAD.
Most tuna, during their nightly excursions from a particular FAD, were found to travel in a circular direction off the FAD on a track ranging from a few miles to over 30 miles. The distance of a track from the FAD ranged from four-and-a-half to seven-plus miles away. This is an average range of 5.5 nautical miles. Therefore, no two FADs should be closer than 11 nautical miles to each other to reduce competition between the FADs. Some FADs are closer to each other than they should be.
The FADs were put out as fishing aids for any and all fishermen to use. They do not belong to any group of fishermen, nor are they restricted to the fishermen of the community they are set outside of. FADs work great when the fish run comes through, but they do not guarantee there will be fish all the time.