How Fish Aggregating Devices Reshape Tuna Fishing

There is an old observation among fishers that tuna gather beneath anything floating. A drifting log, a mat of seaweed, a piece of wreckage: whatever the object, small fish shelter under it, larger fish come to feed on them, and tuna arrive in turn. Nobody fully understands why the behaviour exists, but people have exploited it for centuries. What has changed in the past three decades is scale and technology. Fishing fleets no longer wait to find a floating log. They manufacture logs by the tens of thousands, fit them with satellite buoys and sonar, and scatter them across the open ocean, then return when the instruments say enough fish have gathered. Fish Aggregating Devices have transformed tuna fishing from a search problem into a monitoring problem, and in doing so they have become one of the most consequential and most contested technologies in commercial fishing. Here is how they work and what they have changed, step by step.

 

1. FADs Create Artificial Gathering Points

 

A Fish Aggregating Device is a floating object built to exploit the natural tendency of tuna and other pelagic species to associate with things drifting at the surface. In its simplest form it is a raft of bamboo, cork, or plastic floats with a submerged appendage hanging beneath it, historically old netting and increasingly rope or biodegradable material.

Two types exist and the distinction matters. Anchored FADs are moored to the seabed, usually in coastal waters, and are widely used by small-scale and artisanal fishers, particularly in Pacific island states where they can bring fish within reach of small boats and reduce fuel costs. Drifting FADs, which are the subject of most controversy, float freely on the open ocean and are used almost entirely by industrial purse-seine fleets. It typically takes a month or two for a new drifting FAD to accumulate a substantial aggregation of skipjack, so the device is planted and left to work.

 

2. Fishers Can Find Tuna More Efficiency

 

The economic logic is straightforward. Searching an ocean for free-swimming tuna schools is slow, fuel-hungry, and uncertain. Deploying hundreds of FADs and monitoring them remotely converts that search into a scheduling exercise, where a vessel sails to the device the data says is most productive.

The productivity gains are large and well documented. Research indicates FAD fishing is roughly 1.35 to 3 times more productive than free-school fishing for yellowfin and bigeye, and 2 to 7 times more productive for skipjack. The difference in reliability is starker still: for the European purse-seine fleet over one period, around 96 percent of FAD sets in the Atlantic and 94 percent in the Indian Ocean caught at least a tonne of tuna, against 80 percent and just 58 percent respectively for sets on free-swimming schools. A null set, where the net is deployed and comes up empty, is enormously expensive, and FADs nearly eliminate them. This efficiency is also why FAD-caught skipjack is cheap enough to fill the world's canned tuna market.

 

3. Satellite Buoys Add Technology

 

Modern drifting FADs carry solar-powered satellite buoys with GPS, and most now include an echosounder. The buoy reports the device's position continuously, and the sonar estimates how much fish biomass is beneath it, in some systems distinguishing between species and depths.

This is the change that transformed the technology from a passive attractor into an active remote sensing network. A fleet operating a thousand instrumented FADs effectively runs a distributed array of ocean sensors reporting back by satellite, allowing a vessel to rank every device by expected catch and steam only to the best. It also created an unusual property rights situation, since the buoys are privately owned and their data proprietary, which means the most detailed information about tuna aggregation across large parts of the ocean sits with fishing companies rather than with the bodies that manage the stocks.

 

4. Catches Can Become Larger

 

The instrumentation measurably increases catch. Research in the Indian Ocean found that echosounder-equipped FADs raised the average catch per set by roughly two to two and a half tonnes, about ten percent, compared with FADs carrying only position buoys.

That figure is worth dwelling on, because it illustrates something important about fisheries management. Catch limits are typically set in tonnes, but fishing capacity is not fixed, and a technological improvement of this kind increases the effective fishing power of a fleet without adding a single vessel. Managers refer to this as technological creep, the steady erosion of the relationship between nominal fishing effort and actual fishing mortality. If a quota assumes a certain catch rate per vessel-day and the technology quietly makes each vessel-day more productive, the stock experiences more pressure than the paperwork suggests.

 

5. Juvenile Tuna Can Be a Concern

 

FAD aggregations are not made up only of the target species. Skipjack dominates, accounting for around seventy percent of the catch by weight from drifting FAD sets, but juvenile bigeye and yellowfin tuna associate with floating objects too and are caught alongside it.

This is the most consequential biological concern, because bigeye and yellowfin are far more valuable and far more vulnerable than skipjack. Skipjack is fast-growing and highly productive, and can sustain heavy fishing. Bigeye grows slowly, matures late, and cannot. Catching immature bigeye before they have spawned removes them from the population entirely, altering the age structure of the stock and reducing its future reproductive output. The WCPFC assessed that juvenile bigeye taken on FADs in the Western Pacific rose from essentially nothing in the 1960s to close to 80,000 tonnes a year by 2010. It is a textbook growth overfishing problem: a fishery targeting an abundant species inflicts its heaviest damage on a different, scarcer one.

 

6. Bycatch Risks Increase

 

Sharks, rays, turtles, and other non-target species also aggregate around FADs and are caught when the purse seine closes. Studies consistently find higher bycatch rates on FAD sets than on free-school sets, and the effect on pelagic sharks in particular has been linked to rising extinction risk for several species.

An honest comparison requires a caveat that is often omitted. FAD sets have more bycatch per set than free-school sets, but they also catch more tuna per set, and in the Eastern Pacific the main alternative historically involved setting nets on dolphins, which carried its own severe consequences. The question is therefore not simply whether FADs cause bycatch, which they plainly do, but how the total ecological cost of a given tonnage of tuna compares across methods. That framing matters because it explains why the policy debate has generally landed on regulating FADs rather than banning them outright.

 

7. Lost FADs Can Keep Fishing

 

FADs are frequently lost, abandoned, or simply allowed to drift away once they leave a fleet's area of interest, and the scale of this is only now becoming clear. A 2025 study in Science Advances estimated that 1.41 million drifting FAD buoys were released between 2007 and 2021, drifting across at least 134 million square kilometres, equivalent to 37 percent of the ocean surface, and that lost devices have stranded in 104 maritime jurisdictions.

The consequences are twofold. Devices built with netting continue to entangle sharks, turtles, and other animals long after anyone is watching, which is ghost fishing in miniature and repeated on an enormous scale. Beached devices damage coral reefs and litter coastlines, and the burden falls disproportionately on small island states that did not deploy them. More than 1,300 beaching events were recorded in the Western and Central Pacific over 2016 and 2017 alone, concentrated in Papua New Guinea, the Solomon Islands, and Tuvalu, driven simply by where prevailing currents carry the devices. There is also a subtler worry that deploying tens of thousands of artificial floating objects may alter tuna behaviour itself, potentially acting as an ecological trap that changes feeding and migration patterns in ways not yet understood.

 

8. RFMOs Are Tightening Management

 

The four tropical tuna Regional Fisheries Management Organisations, covering the Pacific, Eastern Pacific, Indian, and Atlantic oceans, have progressively brought FADs under regulation. Measures now include limits on how many active FADs a vessel may deploy, seasonal closures on FAD fishing, mandatory reporting of deployments and buoy data, requirements for non-entangling designs that replace netting with rope, encouragement or requirement of biodegradable materials, and emerging recovery and retrieval obligations.

Progress has been real but uneven, and the same institutional constraints described in any account of RFMO decision-making apply here. Consensus requirements slow the adoption of stricter limits, member states disagree over whether FAD caps disadvantage particular fleets, and reporting obligations depend on data that fishing companies own. Independent assessments of thirty years of FAD regulation have concluded that although meaningful progress has been made on data quality, entanglement, and pollution, concerns persist over unregulated deployments, bycatch levels, and industry accountability. The transition to biodegradable designs, in particular, has proved slower in practice than in commitment.

 

The Real Question

 

It is tempting to treat FADs as straightforwardly harmful, and much campaigning has taken that line, but the trade-off is genuinely difficult. Banning drifting FADs would not eliminate tuna fishing; it would push fleets back toward free-school sets, which means more searching, more fuel burned, more sets made per tonne landed, and in some regions a return to methods with their own serious problems. It would also raise the price of canned tuna, which is among the cheapest animal protein available globally. Against that, FADs demonstrably increase juvenile bigeye mortality, elevate shark and turtle bycatch, and scatter plastic across a third of the ocean surface.

The productive framing, and the one the management bodies have largely adopted, is that FADs are a tool whose harms are mostly design and volume problems rather than intrinsic ones. Non-entangling construction removes most of the entanglement risk. Biodegradable materials remove most of the pollution risk. Caps on deployment numbers address the volume. Recovery obligations address abandonment. Better data addresses everything else. None of that reduces juvenile bigeye catch, which remains the hardest problem and probably requires seasonal closures or direct limits on FAD sets rather than better FADs. What is clear is that a technology which now underpins a majority of the world's tuna supply cannot simply be legislated away, and that the interesting work lies in making it less damaging rather than in arguing about whether it should exist.

 

Did You Know?

 

More than sixty percent of the world's tuna is caught by purse-seine vessels, and around 37 percent of tropical tuna purse-seine sets are made on floating objects, which is why the way these fleets use FADs is such a central issue in global tuna management. The numbers behind that are striking: somewhere between 81,000 and 121,000 FADs are deployed worldwide each year, over 100,000 are drifting or anchored in the ocean at any given moment, and roughly 40 percent of the world's skipjack catch, the fish in most cans of tuna, comes from fishing on them. A technology that began as fishers noticing that tuna like to gather under logs has become an instrumented, satellite-tracked infrastructure spanning more than a third of the ocean surface.

 

Note: This article reflects the state of FAD use and regulation as of mid-2026, drawing on sources including studies published in Science Advances, Fish and Fisheries, and the ICES Journal of Marine Science, together with the International Seafood Sustainability Foundation, the Marine Stewardship Council, and the tropical tuna RFMOs. Deployment estimates vary considerably between sources and ocean basins.