Purse seining is a commercial fishing method that uses a large wall of netting to encircle an entire school of fish, then cinches the bottom closed like a drawstring bag to trap them. It is one of the most efficient ways to harvest schooling species like tuna, sardines, and mackerel, and accounts for a significant share of the world’s commercial fish catch.
How a Purse Seine Works
The process starts with locating a school of fish near the ocean surface, typically using sonar, spotter planes, or sometimes helicopters on industrial vessels. Once a school is identified, a small auxiliary boat called a skiff takes one end of the net and motors around the school in a wide circle while the main vessel pays out the netting. The net forms a vertical curtain in the water, with floats along the top edge keeping it at the surface and a weighted lead line pulling the bottom edge down.
The critical step is “pursing.” A steel wire or heavy rope runs through a series of metal rings hanging along the bottom of the net. Once the circle is complete, a powerful winch on the main vessel hauls in this purse line, drawing the bottom of the net closed like a coin purse. This prevents fish from escaping downward, which is their natural instinct when surrounded. With the bottom sealed and the floats holding the top at the surface, the fish are contained in a bowl of netting. The crew then either hauls the net aboard or brings it alongside the vessel and uses a brailer (a large scoop) or fish pump to transfer the catch into the hold.
Net Components and Vessel Equipment
A purse seine net can stretch over a kilometer long and reach depths of 200 meters or more, depending on the fishery. The three structural elements are the float line along the top, the lead line along the bottom, and the purse rings and purse line that allow the bottom to close. The netting itself is typically made of synthetic mesh sized to the target species.
Industrial purse seiners carry specialized gear: a hydraulic power block mounted on a boom to haul in the heavy net, a purse seine winch for the purse line, derricks for maneuvering gear, and the skiff that sets the net. Some large tropical tuna seiners also carry helicopters for spotting schools across a wider area. The vessels range from small coastal boats targeting sardines to ocean-going ships over 80 meters long that can store thousands of tons of tuna.
What Species Are Caught This Way
Purse seining targets fish that form dense schools near the surface. The biggest global fishery using this method is tropical tuna, including skipjack, yellowfin, and bigeye. In other regions, the gear targets southern bluefin tuna, sardines, anchovies, mackerel, herring, and menhaden. The common thread is schooling behavior: the method works because these species naturally aggregate in tight groups, making it possible to surround them in a single set.
FAD Fishing vs. Free-School Fishing
In tropical tuna fisheries, purse seiners use two distinct strategies. Free-school fishing means locating and encircling a naturally occurring school of tuna swimming in open water. Fish aggregating device (FAD) fishing involves deploying floating objects, often equipped with GPS and sonar buoys, that attract tuna and other marine life over days or weeks. The vessel returns later to set its net around the aggregation beneath the FAD.
FAD sets have a higher success rate, around 96%, compared to roughly 81% for free-school sets. But FADs come with trade-offs. They use more fuel per ton of fish caught (about 544 liters per ton versus 439 liters per ton for free-school sets), giving them a larger carbon footprint. FADs also tend to aggregate a wider mix of species and sizes, including juvenile tuna and non-target fish, which raises sustainability concerns. Free-school sets generally produce a cleaner catch of the intended species.
Bycatch and Environmental Impact
Purse seining has a complicated environmental reputation. On one hand, when targeting a visible, well-defined school, the method can be highly selective, capturing mostly the intended species with relatively little bycatch. The net doesn’t drag along the seafloor, so it causes no habitat damage to reefs or the seabed.
On the other hand, bycatch depends heavily on how and where the gear is used. FAD-associated sets tend to catch more non-target species than free-school sets. The most high-profile bycatch issue historically involved dolphins in the eastern tropical Pacific tuna fishery, where tuna schools swim beneath dolphin pods. That led to major regulatory changes and the “dolphin-safe” labeling movement. Observer data from the Canary Current region of the Atlantic found that purse seiners caused negligible mortality to sea turtles, with only loggerhead and leatherback turtles bycaught at very low rates. The impact varies significantly by ocean region, target species, and whether FADs are used.
How the Catch Is Handled on Board
For tuna especially, what happens after the net closes determines the quality and value of the fish. Tuna arrive on deck with high internal body temperatures and begin deteriorating quickly from bacterial growth, enzyme activity, and oxidation. The goal is to chill each fish to about 30°F within eight hours of capture using refrigerated seawater, then freeze it to a core temperature of around 10°F within 24 to 36 hours.
Fish are packed loosely in refrigerated wells to allow seawater or brine to circulate evenly. Packing density matters: U.S. fleets typically fill wells to 70% to 85% capacity, while some Asian and European fleets use lower densities to avoid crushing fish at the bottom under their own weight. Once frozen solid, the wells are drained and the fish are stored dry at around 0°F. Temperature stability during storage is critical, so wells are insulated and monitored. The last fish brailed aboard from any set get priority for chilling, since they’ve been stressed and exposed to heat the longest.
How Purse Seining Is Regulated
Because purse seining can harvest enormous volumes in a single trip, it is tightly managed through international agreements and national regulations. In the western and central Pacific, the largest tuna purse seine fishery in the world, the U.S. limits its fleet to 1,828 fishing days per calendar year across the region between 20°N and 20°S latitude. This cap was established under the Convention on the Conservation and Management of Highly Migratory Fish Stocks and is enforced by NOAA Fisheries.
Other regional fisheries management organizations set similar effort and catch limits in the Indian Ocean, eastern Pacific, and Atlantic. Common regulatory tools include seasonal closures, limits on the number of FADs each vessel can deploy, mandatory onboard observer coverage, and vessel monitoring systems that track fleet movements by satellite. These measures aim to prevent overfishing of target stocks while reducing bycatch of juvenile tuna and non-target species.