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Fish Aggregation Devices (FADs)
Over the past 20 years, industrial tuna fleets, targeting tropical tuna species have developed a technique that takes advantage of tunas’ propensity to form large aggregations around floating objects. Historically, when fishers found logs or other terrestrial debris floating in the open ocean they would set their nets around them to catch the tuna aggregated below. Over time, to increase their efficiency, they began building and deploying rafts, to which a tracking beacon was attached. These rafts, or fish aggregating devices (FADs), are left to drift for several months and are revisited by the fishing vessels to capture aggregated schools, using large purse-seine nets.
This fishing strategy has become increasingly important and now accounts for about 40% of the annual global tuna catch (1). Generally FADs are constructed using bamboo, foam floats and netting. The netting provides strength to the surface structure but is also used to increase the FADs drag, as it hangs like a curtain below the raft. In this way the FAD follows ocean currents and is not blown off course by surface winds. It is also believed that the sub-surface netting attracts and provides shelter for small fishes.
Unfortunately, tuna are not the only fish that tend to aggregate around FADs. In fact, such aggregations can contain up to 40 species, including two species of pelagic sharks. The Silky Shark (Carcharhinus falciformis) and the Oceanic Whitetip Shark (C. longimanus) both share the aggregative trait, however, Silky Sharks are far more common and constitute up to 90% of the elasmobranchs caught in this pelagic fishery (2). Concern over the incidental capture of these sharks has been the subject of several programmes aimed at improving measures to reduce their capture. A recent study showed that the incidental capture of Silky Sharks was not the only threat to this species. Most Silky Sharks that aggregate around FADs are juveniles, with an average size of around one metre. Dive surveys at FADs in the Indian Ocean conducted between 2010 and 2012 revealed that these juvenile sharks also regularly become entangled in the netting that hangs below the FADs. As FADs are only visited sporadically, and in the Indian Ocean, seldom retrieved, sharks entangled in this way are very rarely observed.
➤ Made European Project Website
1. Miyake MP, et al. 2010. Recent developments in tuna industry: stocks, fisheries, management, processing, trade and markets. Rome, Italy: FAO.
2. Gilman EL. 2011. Bycatch governance and best practice mitigation technology in global tuna fisheries. Mar Policy 35: 590-609.
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