A purse seiner is a fishing vessel designed to encircle entire schools of fish with a large wall of netting, then cinch the bottom closed like a drawstring bag to trap the catch. It’s the most productive method for harvesting schooling fish near the ocean’s surface, responsible for roughly 66% of the world’s tuna catch alone. The name comes from the “pursing” action of pulling a line through rings along the net’s bottom edge, which closes it off and prevents fish from diving to escape.
How Purse Seining Works
The process starts with finding fish. Crews search from a crow’s nest on the vessel, watch for seabirds diving over schools near the surface, or use sonar and echosounders to detect concentrations of fish below. Once a school is located, the actual fishing set unfolds in a specific sequence that typically takes one to two hours from start to finish.
First, a small open boat called a power skiff drops off the stern carrying one end of the net. The main vessel then motors in a wide circle around the fish school, paying out the net over the back of the boat. This encirclement usually takes less than five minutes. When the circle is complete, the skiff passes its end of the net back to crewmembers on the main vessel’s deck. At this point, the net forms a vertical curtain of mesh surrounding the fish, held up by floats along the top and weighted along the bottom with a lead line.
Next comes the pursing. A hydraulic winch pulls in the purse line, which runs through metal rings along the net’s bottom edge. As the line tightens, the rings gather together and the bottom of the net draws shut, forming a sealed bowl beneath the fish. Without this step, the school could simply swim downward and escape under the net.
Once the net is pursed, the crew begins hauling it back aboard. A hydraulic power block, hung from a boom over the side of the vessel, lifts the heavy netting while crewmembers stack it on the back deck. This hauling process takes one to two hours and progressively concentrates the fish into a smaller and smaller pocket of netting called the bunt. When the fish are tightly gathered against the ship’s side, they’re brought aboard by brailing (scooping with a large dip net) or pumped directly into the vessel’s hold.
Throughout the set, the power skiff plays a critical role beyond just carrying the net. It tows the main vessel sideways to keep the large boat from drifting into the net and collapsing it during hauling.
Scale of the Nets and Vessels
On large industrial tuna seiners, the nets can reach 3,000 meters long (nearly two miles) and 300 meters deep, weighing around 6 tons. The vessels themselves are substantial. A study of 72 large-scale tuna purse seiners in the Atlantic found a median length of about 72 meters (236 feet), with fish hold volumes averaging over 1,000 cubic meters per vessel. Collectively, those 72 ships had a combined carrying capacity of roughly 83,000 tons of fish.
Modern purse seiners store their catch in refrigerated wells below deck, keeping the fish chilled or frozen for trips that can last weeks before returning to port. A vessel qualifies as “large-scale” when its fish hold volume exceeds 335 cubic meters, though many industrial seiners far exceed that threshold.
What Purse Seiners Catch
Purse seining targets species that naturally form dense schools near the ocean surface. Tuna is the biggest prize: skipjack, yellowfin, and bigeye tuna are all harvested this way, and purse seines account for two-thirds of all tuna caught globally. That dwarfs every other method, with longlines taking 9% and pole-and-line fishing just 7%.
Beyond tuna, purse seiners harvest a range of smaller pelagic fish including sardines, anchovies, mackerel, and herring. Along the coasts of Oregon and Washington, for example, purse seines are the primary gear used to harvest Pacific sardines. These coastal pelagic species travel in tight schools above the continental shelf, making them well-suited to encirclement.
The Dolphin Problem and Bycatch Solutions
Purse seining’s biggest controversy has been its impact on dolphins, particularly in the eastern tropical Pacific where yellowfin tuna swim beneath dolphin pods. For decades, fishers deliberately set their nets around dolphins to capture the tuna below, killing large numbers of dolphins in the process.
Two key innovations helped reduce dolphin deaths dramatically. The first is the “backdown” procedure: after the net is pursed and roughly two-thirds of it has been hauled aboard, the vessel shifts into reverse. This stretches the remaining net into a long, narrow channel. Dolphins naturally gather at the far end near the surface, and as the vessel continues backing up, the top edge of the net at that far end dips underwater, allowing dolphins to spill out over the top.
The second is the Medina panel, a section of finer mesh sewn into the net at the point where dolphins escape during backdown. Regular netting can snag fins and beaks, but the Medina panel (sometimes called the “dolphin safety panel”) reduces entanglement at the critical exit point. Trained observers from regional fisheries organizations ride along on vessels to monitor whether crews position the panel correctly and execute the backdown properly. These combined measures cut dolphin mortality significantly, though the issue hasn’t disappeared entirely.
How Purse Seine Fisheries Are Regulated
Because purse seining is so efficient, regional fisheries management organizations impose strict controls to prevent overfishing. One common tool is seasonal closures. In the eastern Pacific, the Inter-American Tropical Tuna Commission mandates that each purse seiner observe a multi-month closure period each year. For 2025, vessels must choose between a closure running from late July through early October or one from November into January 2026, with some vessels required to observe extended closures of 13 additional days.
These closures give tuna populations time to recover and reproduce without fishing pressure. Vessels that experience documented force majeure events (equipment failure, severe weather) can apply for a reduced 40-day closure instead, but must have their exemption formally approved. Observer programs, seasonal restrictions on the use of fish aggregating devices (floating objects that attract tuna), and vessel capacity limits all add layers of oversight to what remains the world’s most commercially important fishing method.