Fig. 1

The left of the diagram (under letter ‘a’) shows a typical commercial FAD (grey oval) with a single receiver directly underneath it to collect presence/absence and, if a transmitter is equipped with a pressure sensor, depth data, which represents the limits of previous epipelagic open-ocean passive acoustic telemetry applications. The center of the diagram (under letter ‘b’) represents the moored sub-surface FAD used in the development of the VAR, with three receivers aligned down the taut mooring line, which can be used to calculate transmitter depth (in the absence of a pressure sensor) and distance from the array. The right of the diagram (under letter ‘c’) shows the same array suspended beneath a free-drifting FAD buoy. The grey oval at the surface represents the FAD float (Zunibal Zunfloat, 180 cm diameter, 150 kg of flotation). A 200 m length of Samson 6 mm AmSteel Dyneema line was tied through the holes in the float, and the receivers in the mounting cups were clipped to this line at 15 m, 100 m, and 200 m. A 3 kg steel shackle was used as the bottom weight next to the deepest receiver. Diagram is not to scale