Purse Seine Net Product

Overview

A purse seine net is a large encircling fishing gear designed to surround schools of pelagic fish (tuna, mackerel, herring, sardine, anchovy) in open water. The net is a rectangular curtain of mesh, typically 50–200 m long and 40–60 m deep, with a floatline (cork or plastic floats) along the upper edge and a weighted leadline (lead shot) along the lower edge. The critical innovation is the Purse Rings—stainless steel rings spaced every 5–10 m along the bottom edge—through which a single continuous Purse Line is threaded. When the Purse Line is hauled tight, it cinches all the rings upward, drawing the leadline and net bottom closed like a drawstring pouch, trapping fish inside.

The Net Panels are constructed from machine-woven nylon or polyethylene twine in a square or diamond knot pattern. The Floatline Assembly uses cork discs or plastic spheres spaced every 0.5 m on a 18–24 mm braided polypropylene rope, providing 8–12 kg/m distributed buoyancy. The Leadline Assembly employs lead shot packets (150–250 g each) spaced similarly for rapid sinking. The Anchor Bridles are rigged from the net corners to the vessel's davits and a tender skiff, stabilizing the net during the encirclement maneuver. All connections use Hanging Lines—short nylon segments that attach the net shoulder and foot ropes to the floatline and leadline at 2 m intervals, distributing strain evenly.

How It Works

Set and encirclement: The vessel approaches a fish school and deploys the net from the stern or net drum, laying out the net in a large circle around the target school. A fast tender skiff (also called a net skiff) trails the far end of the net and circles back, anchoring one end while the vessel completes the encirclement with the other end. Floats keep the upper edge at the surface; leads sink the lower edge to intercept fish attempting to dive. The entire operation takes 15–30 minutes depending on net size and school behavior.

Pursing phase: Once the circle is closed, crew haul on the Purse Line using a powered winch or Power Block. As the purse line is drawn upward, the Purse Rings slide up the line, and the net bottom cinches shut like a bourse (hence "purse seine"). Fish inside the net cannot escape below or dive; they are contained in a shrinking ball of net mesh. The Leadline Assembly collects at the surface as the purse completes.

Brailing phase: Crew then use a large dip net or scoop (called a "brail net") to transfer fish from the purse to the vessel's hold or live-fish tank. On modern industrial purches, a vacuum fish pump (with Wash Pump-like principle) can directly suction fish from the net into refrigerated holds. Alternatively, the entire purse bag is hauled aboard using a Power Block or net winch.

Recovery and storage: Once emptied, the net is retrieved, coiled, inspected for tears, and stored in a net drum or on specialized net racks. Ropes and floats are checked for damage; repaired using the Auxiliary Tackle spares.

Typical Purse Seine Operation (Tuna Fishery Example)

• Vessel size: 30–90 m
• Crew: 8–15 personnel (skipper, navigator, net master, 3–4 net crew, engineer, deck crew, hold workers)
• Net size: 100–150 m length, 50 m depth
• Expected catch: 20–200 tonnes per successful set (school-dependent)
• Set duration: 20–40 minutes encirclement + 10–20 minutes purse + brailing
• Daily sets: 1–4 per day depending on school availability and hold capacity
• Power requirements: 60–80 hp main engine, 15–25 hp auxiliary for net winch

Design Variations

Tuna purse seine: Larger nets (150–200 m), designed for larger schools in open ocean; typically uses Purse Rings every 5 m and heavier Leadline Assembly (20–25 kg/m) to reach deep schools (50+ m depth).

Small pelagic (sardine/anchovy): Smaller net (50–80 m), shallower depth (30–40 m), finer mesh (0.5–1.5 cm) to retain small fish, lighter lead (12–15 kg/m) for rapid deployment and quick haul cycles.

Live fish retention purse: Features live-bait tank integration and slower haul speeds to minimize fish damage; higher Floatline Assembly buoyancy and specialized Hanging Lines to prevent fish injury during purse closure.

Maintenance

Mesh damage is inspected daily and patched using the Repair Mesh and standard fishing net needle and twine. Cork floats are tarred annually to prevent waterlogging; salt-saturated corks are replaced with Float Replacement units from onboard spares. Purse Line is visually checked for chafe and wear; sections with >20% surface degradation are cut out and spliced using the Splice Kit Backup. Lead shot packets are checked for rust and leakage; torn bags are replaced with fresh lead from the spares locker.

Environmental and Regulatory Notes

Purse seining is a highly selective gear when used on school fisheries (tuna, mackerel), but can incur bycatch of dolphins, sea turtles, and juvenile fish if not properly managed. Many jurisdictions mandate observer placement or electronic monitoring on purse seiners. Mesh size regulations ensure immature fish escape (typically 1.5–2 cm minimum knot-to-knot). Closure of sensitive spawning grounds during breeding season protects stock recruitment. International agreements (IATTC, WCPFC) regulate purse seine fleets in shared tuna stocks.

Historical Context

Purse seining emerged in the early 20th century as a mechanized alternative to hand-drawn seines. The invention of the power block (1950s, Power Block) revolutionized hauling speed and efficiency, transforming small-scale netting into industrial-scale fisheries. Modern purse seiners now land millions of tonnes of small pelagic fish annually (sardine, anchovy, mackerel) for both direct human consumption and reduction to fishmeal and fish oil.