A seine net is a large fishing net designed to encircle a school of fish, trapping them in a wall of netting that closes around them like a curtain. It’s one of the oldest and most widely used fishing methods in the world, employed by everyone from small-scale fishers working from shore to industrial fleets targeting tuna across open ocean. The basic principle hasn’t changed much since ancient times: surround the fish, then draw the net tight.
How a Seine Net Works
Every seine net has the same core components. The top edge is attached to a rope called the headline or floatline, strung with floats that keep the upper edge at or near the surface. The bottom edge connects to a footrope or leadline, weighted with lead or other sinkers that pull the lower edge down and spread the net vertically through the water column. Between these two edges hangs a curtain of netting, sometimes hundreds of meters long.
Most seine nets include a reinforced section called the bunt, which is where the fish end up concentrated as the net is hauled in. Some designs place a bag at one end or in the center of the net to hold the catch. The bunt takes the most strain during retrieval, so it’s built with heavier, more durable netting than the rest.
The fishing technique itself is straightforward. A boat pays out the net in a wide arc around a school of fish, then closes the circle. Depending on the type of seine, the net is either pulled to shore or cinched shut from the bottom to prevent fish from escaping downward.
Beach Seines vs. Purse Seines
The two main types of seine nets differ dramatically in scale and setting.
A beach seine is the simpler, older version. A small boat sets the net in a sweeping arc close to shore, and then crews on the beach haul it in using long ropes attached to each end. This method works in shallow water and targets whatever schooling fish are near the coast. It requires minimal equipment and has been used for thousands of years. Archaeological and historical evidence from the Mediterranean traces beach seining back to prehistoric times, with ancient Greek and Roman texts describing nets called “sagena” that were essentially straight-haul seines pulled to shore.
A purse seine is the industrial-scale version. These nets can stretch over 2,000 meters (about 6,500 feet) long and 200 meters (650 feet) deep. The defining feature is a drawstring-like cable running through rings along the bottom edge. Once the net encircles a school of fish, the crew “purses” the bottom shut by pulling this cable tight, creating a closed bowl shape that prevents fish from diving beneath the net. This is what separates a purse seine from other types: it seals the bottom.
What Seine Nets Catch
Seine nets primarily target schooling fish, species that swim together in dense groups near the surface or in mid-water. The most common targets include sardines, mackerel, herring, anchovies, and various tuna species. Purse seines along the U.S. West Coast also target market squid and schooling salmon like chum, pink, and coho. In tropical waters, large purse seiners focus heavily on skipjack, yellowfin, and bigeye tuna.
The method works best on pelagic species, fish that live in the open water column rather than near the bottom. Because these fish tend to school in large, visible groups (sometimes detectable by sonar, circling birds, or surface disturbance), they’re well suited to encirclement.
Modern Net Materials
Traditional seine nets were made from cotton, hemp, or other natural fibers that rotted quickly and required constant maintenance. The shift to synthetic materials in the mid-twentieth century transformed the industry. Modern seine nets are built primarily from nylon (polyamide), polyethylene, and polypropylene.
Nylon is the most common choice for the netting itself because it combines high strength with elasticity and good chemical stability in saltwater. It stretches slightly under load, which helps absorb the shock of a large catch pushing against the mesh. Polyethylene costs less and resists chemicals well but isn’t as strong. Polypropylene is lighter than both, which helps with buoyancy, but degrades faster under UV exposure. The structural ropes and edges of the net often use braided polyethylene or polyamide fibers for maximum toughness.
Modern nets also come in knotted and knotless varieties. Knotless mesh is more flexible and causes less damage to fish, which matters for species sold fresh. Knotted mesh offers more stability and abrasion resistance, making it better suited for rough conditions or rocky bottoms.
Environmental Concerns
Seine nets are generally considered one of the more selective fishing methods because they target visible schools of a known species. Industrial purse seine data from the Indian Ocean shows non-target species make up an average of just 1.12% of total catch by weight. That’s low compared to methods like bottom trawling, which can produce bycatch rates many times higher.
The bigger environmental issue involves fish aggregating devices, or FADs. These are floating objects (often purpose-built rafts with sonar buoys) deployed to attract fish. Schools of tuna and other species naturally congregate beneath floating debris, and purse seiners set their nets around FADs to guarantee a catch. The problem is that FADs attract more than just the target species. Research in the Indian Ocean found turtles, sharks, and porpoises entangled in drifting FADs. One estimate put silky shark deaths from FAD entanglement at up to 960,000 per year in the Indian Ocean alone.
FAD-free fishing, where the vessel locates a free-swimming school rather than relying on an aggregating device, produces significantly less bycatch. Fisheries certified as sustainable have adopted several modifications to reduce their impact: using smaller or shallower nets, building FADs from biodegradable and non-entangling materials, installing secondary chutes on vessels to release sharks alive, and participating in tag-and-release programs for non-target species caught accidentally.
Small-Scale vs. Industrial Seining
The gap between a village beach seine and an industrial purse seiner is enormous. A beach seine might be 100 meters long and operated by a handful of people pulling ropes by hand. It catches enough fish to supply a local market. An industrial purse seiner carries a net over two kilometers long, uses hydraulic winches and power blocks to haul it in, and can pull aboard hundreds of tons of fish in a single set. These vessels often carry helicopters or drones to spot schools from the air, and they work alongside supply ships that transport the catch to port while the seiner keeps fishing.
Despite the scale difference, the underlying mechanics are identical. You set a wall of netting around the fish, close it off, and bring it in. That simplicity is why seine netting has persisted from ancient Mediterranean shores to modern industrial oceans spanning thousands of miles.