Fish are fascinated with floating objects. They use them to mark locations for mating activities. They aggregate in considerable numbers around objects such as drifting flotsam, rafts, jellyfish and floating seaweed.

 

The objects appear to provide a "visual stimulus in an optical void", and offer some protection for juvenile fish from predators. The gathering of juvenile fish, in turn, attracts larger predator fish. A study using sonar in French Polynesia, found large shoals of juvenile bigeye tuna and yellowfin tuna aggregated closest to the devices, 10 to 50m. Further out, 50 to 150m, was a less dense group of larger yellowfin and albacore tuna.

 

Yet further out, to 500m, was a dispersed group of various large adult tuna. The distribution and density of these groups was variable and overlapped. The FADs were also used by other fish, and the aggregations dispersed when it was dark.

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Source: Wikipedia