Purse seine fishing is a commercial method that uses a large wall of netting to encircle an entire school of fish, then cinches the bottom closed like a drawstring purse to trap them. It is one of the most productive fishing techniques in the world, responsible for roughly two-thirds of the global tuna catch and significant shares of sardine, anchovy, and mackerel harvests.
How a Purse Seine Works
The process starts with finding a dense school of fish, usually near the ocean’s surface. Once a school is spotted, the fishing vessel deploys a small open boat called a skiff. The skiff holds one end of the net and motors away from the main vessel, towing the net in a wide circle around the fish. The net hangs vertically in the water like a curtain, with floats along the top edge and a weighted lead line along the bottom.
After the circle is complete, the crew pulls a cable threaded through rings along the bottom of the net. This is the purse line, and drawing it tight is called “pursing.” It gathers the bottom edge of the net together the same way pulling a drawstring closes a bag, preventing fish from diving down and escaping. At this point the fish are fully enclosed in a bowl of netting. The skiff then repositions alongside the main vessel, keeping tension on the net so it doesn’t collapse around the hull as the crew begins hauling.
To bring the net back on board, crews use either a hydraulic power block or a large drum. A power block hangs from a boom arm and pulls the net up mechanically while crew members stack it on the back deck. A drum works more like a giant spool, winding the net around itself. The catch is either hauled aboard or brailed (scooped) from the net while it’s still alongside the vessel.
Vessels and Scale
Purse seine operations range from small coastal boats targeting sardines in a local bay to industrial vessels working the open Pacific. Industrial tuna purse seiners are typically longer than 25 meters (about 80 feet), and many are far larger, with refrigerated holds exceeding 1,000 cubic meters of storage. These vessels can stay at sea for weeks, freezing their catch on board. Smaller coastal seiners, like those operating in California’s Monterey Bay, work much closer to shore and make shorter trips targeting species like squid, anchovies, and sardines.
What Purse Seines Catch
Purse seining targets schooling species that swim together in large, dense groups near the surface. The most commercially important targets are skipjack, yellowfin, and bigeye tuna. In 2022, purse seine vessels landed approximately 67.5% of the world’s tropical tuna harvest. Beyond tuna, the method is widely used for anchovies, sardines, mackerel, herring, and squid.
The technique works because these species naturally aggregate in tight schools, sometimes numbering in the millions of individual fish. A single set of the net can capture an enormous volume in minutes, making purse seining highly efficient compared to methods like hook-and-line fishing that catch one fish at a time.
Fish Aggregating Devices
In open-ocean tuna fishing, crews often use fish aggregating devices, or FADs, to improve their odds. A FAD is a floating object, sometimes as simple as a raft of logs or bamboo, sometimes a purpose-built buoy equipped with sonar and GPS. Fish are naturally attracted to floating debris, and over days or weeks a FAD accumulates a large aggregation of tuna and other species underneath it. The purse seiner then sets its net around the FAD.
FADs dramatically increase catch rates, but they also create environmental problems. Because FADs attract many species indiscriminately, sets around them tend to catch more non-target animals than “free school” sets where the crew encircles a naturally occurring tuna school. Research in the Indian Ocean found an average of five vertebrates entangled per drifting FAD at any given time, including sea turtles, sharks, and porpoises. One study estimated that up to 960,000 silky sharks were entangled and killed by drifting FADs in the Indian Ocean each year, and post-release mortality for silky sharks pulled from purse seine nets exceeds 80%.
The Dolphin Problem and Its Solution
In the eastern Pacific, yellowfin tuna frequently swim beneath pods of dolphins. During the 1960s and 1970s, purse seine fleets routinely set their nets around dolphin pods to capture the tuna below, killing hundreds of thousands of dolphins every year. Public outrage over this practice led to one of the most successful bycatch reforms in fishing history.
Crews developed a technique called the backdown maneuver: after pursing the net, the vessel reverses, pulling the far edge of the net below the waterline so dolphins can swim out over the top. Nets were also redesigned with a section of finer mesh called a Medina panel, placed at the point where dolphins are most likely to escape, reducing the chance of entanglement. Floodlights help crews spot dolphins at night, and trained swimmers enter the net to guide animals out. These combined measures dropped dolphin deaths from hundreds of thousands per year to fewer than 1,000 annually since the 1990s.
How Purse Seining Is Regulated
Because tuna and other target species migrate across international boundaries, purse seine fishing is managed largely through regional fisheries management organizations, or RFMOs. These international bodies set rules for specific ocean regions. The Western and Central Pacific Fisheries Commission, for example, limits the total number of days U.S. purse seine vessels can fish in tropical waters each year, capping effort at 1,828 fishing days annually for the fleet. Similar commissions govern tuna fishing in the Indian Ocean, the eastern Pacific, and the Atlantic.
Common regulatory tools include seasonal closures (particularly around FADs), limits on the number and type of FADs a vessel can deploy, mandatory observer programs that place independent monitors on boats to record bycatch, and total allowable catch limits for specific species. Some fisheries have earned sustainability certifications by meeting standards for healthy target stocks, low bycatch, and effective management, though environmental groups continue to debate whether current regulations go far enough, particularly regarding FAD use and shark bycatch.
Purse Seining Compared to Other Methods
Purse seining’s main advantage is efficiency. It can harvest large volumes of a single species quickly, which keeps fuel costs per ton of fish relatively low. When targeting a free school of a healthy, abundant species, it can be one of the more selective commercial methods available, since the crew chooses which school to encircle.
Longlining, by contrast, uses miles of baited hooks and catches fish one at a time. It tends to have higher interaction rates with seabirds, sea turtles, and sharks per unit of target catch. Trawling drags nets along the seafloor or through midwater, often capturing a wide mix of species and, in the case of bottom trawling, damaging seabed habitats. Purse seines don’t contact the seafloor and can avoid mixed-species catches when used on free schools. The environmental trade-off shifts when FADs enter the picture, since FAD-associated sets significantly increase the variety and volume of non-target species caught.