They are large rafts that drift thousands of kilometers across the ocean surface, moving with the currents in an otherwise featureless marine environment. Tracked by satellites, the rudimentary floats—which may also be outfitted with long, submerged tails of netting—are used to attract schools of fish that can be scooped up by industrial tuna fishing vessels.
Known as drifting Fish Aggregating Devices (dFADs), they were introduced into commercial fisheries in the 1990s to produce “dolphin-safe” tuna, but have themselves become so abundant that they pose their own environmental risks.
Now, a comprehensive study by Dalhousie researchers details their impacts on a global scale for the first time, surprising even the researchers quantifying their numbers.
“We found that these devices have drifted through at least 37% of the global ocean—an area as large as all inhabited continents combined. Yet most people have never heard of dFADs because tuna fishing occurs mostly in the tropics,” lead author Laurenne Schiller, a postdoctoral research fellow at Dalhousie and Carleton, says of the study published in Science Advances.
“Our results demonstrate that the cumulative environmental footprint of dFADs reaches far beyond tuna fishing grounds and remains inadequately mitigated at the global scale.”
Growing concern
The researchers, including those from the Manta Trust, estimate that 1.4 million devices were released between 2007 and 2021 and were used to help catch nearly one-third of the world’s tuna. Tuna harvesters in eastern Canada don’t tend to use dFADs, which are more often deployed in the Pacific.
The study also shows that lost devices—made from natural or synthetic materials—have washed ashore in more than 100 coastal countries, contributing to marine pollution, ensnaring other species and damaging coral reefs. More than 90% of some tuna species are not yet mature when caught because smaller fish are more likely to seek shelter under the raft.
The authors also make clear that scientists, regulators and the fishing industry have been expressing concern about the growing use of dFADs for more than 20 years after fishing practices changed from targeting free-swimming tuna schools to catching fish associated with floating objects.
Commercial use of the devices was inspired by the observation that fish seeking a protective cove tend to congregate around floating natural flotsam, such as logs or other debris. They have since become a key piece of gear for different tuna fisheries, including yellowfin, bigeye and skipjack, the latter being the main source for canned tuna products globally.
Drifting devices often get stranded in regions far away from tuna-fishing grounds. This one found on a beach in Texas was likely deployed off the coast of West Africa. © Jace Tunnell
Monitoring use, retrieving gear
When outfitted with a global positioning system (GPS) transponder, a dFAD raft can be deployed from a vessel and then located weeks to months later after drifting with prevailing currents and aggregating fish.
“Because the ocean is all connected, there are no boundaries for dFADs in their global travels, and their impacts are often felt far from where they were initially deployed. We need to consider how many devices can be in the water at once and make sure they all get
