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A fish aggregating (or aggregation) device (FAD) is a man-made object used to attract ocean going pelagic fish such as marlin, tuna and mahi-mahi (dolphin fish). They usually consist of buoys or floats tethered to the ocean floor with concrete blocks. Over 300 species of fish aggregate around FADs in the open ocean around the world; the reasons behind aggregation are numerous and vary for each particular fish species. Fish tend to move around the FADs in orbits of differing dimensions, as opposed to remaining stationary underneath the buoys. They are deployed for use by both recreational and commercial fisheries and there are several types; natural drifting FADs (such as logs) and man-made FADs which can either be drifting or moored so that they float at the surface or lie subsurface. Subsurface FADs last longer due to less wear and tear from surface tension, but have the disadvantage of being harder to locate.
Drifting FADs are used in the Atlantic, Pacific and Indian ocean purse seine fisheries; over 1 million tons of tuna and over 100,000 tons of by-catch are caught in the vicinity of FADs each year. Many of the FADs deployed by the vessel contain sonar equipment and GPS receivers. A vessel can remotely contact a FAD, via satellite and look at the sonar readings to determine the size of a school of fish under the FAD. These are often referred to as "smart FADs". The widespread use of FADs has shifted the pattern of fishery exploitation of tunas over the past 20 years, with more smaller tunas being caught now than previously when pelagic purse seiners predominantly targeted free-swimming schools of tuna. Their use has also caused concern as by-catch of this fishery includes several species of pelagic sharks belonging to declining populations.
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