Tuna behavior around FADs, Part I
The medium-sized tuna have started to show up at the north side buoys off Molokai and Maui, just in time for the holidays. Weather permitting, the predawn boats are coming back with some nice fish in the 50- to 80-pound range.
Tuna do associate with FADs, or 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 ten meters (33 feet) of the ocean.
During the day, they are at depths from 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 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.