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 porpoise. 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: the 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.
Sub-adult yellowfin tuna tend to spend daylight hours around the FADs but left them at night, venturing as far as eight kilometers away, only to return at dawn.
Small bigeye tuna spend more time closer to the surface around FADs than when they are in free schools away from the FADs.
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 over seven miles away, with an average range of five-and-a-half nautical miles.
Therefore, no two FADs should be closer than 11 nautical miles to each other to reduce competition between them. Some FADs are closer to each other than they should be, but it is an important consideration to site placement.
Tuna return often to the same FAD but rarely to other nearby FADs. They normally stayed near a FAD for less than one hour, but sometimes more than ten hours, and they would arrive at the FAD and leave it at the same time each day (or often, once in the day and once at night). Some fish even returned to the same FAD after absences of nearly six months, but the long-term returnees were not as faithful to their original FADs.
Often, two tuna tagged at the same time would stay together for weeks, as evidenced by their leaving and returning to the same FAD or FADs at the same time. This strongly suggests not only that tuna remain in the same school for extended periods, but that they use the FADs as landmarks or points of reference over extended periods.
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.