A longline is a commercial fishing setup consisting of a single mainline, sometimes stretching dozens of miles, with hundreds or even thousands of baited hooks attached along its length. It is one of the most widely used methods for catching open-ocean and deep-water fish worldwide. The term also appears in medicine (a type of catheter for newborns) and fashion (a garment cut longer than standard), but fishing is by far the most common context.
How a Longline Works
The basic design is simple. A heavy mainline runs through the water, and shorter branch lines called gangions or snoods hang off it at regular intervals, each tipped with a baited hook. Buoy lines at each end hold the gear in position and are marked with flags (called high flyers) so the crew can find them again. Once deployed, a longline set can carry up to a thousand baited hooks and soak anywhere from a few hours to several days before being hauled back in.
In the Atlantic pelagic longline fishery, regulations cap the mainline at about 37 miles (roughly 60 kilometers), with no more than 34 miles of that length carrying active hooks. That gives some sense of the scale: a single set can cover a stretch of ocean longer than a marathon course.
Pelagic vs. Demersal Longlines
There are two main categories, defined by where in the water column the hooks sit. Pelagic longlines suspend hooks near the surface to target fast, open-ocean species like tuna and swordfish. The mainline floats at a set depth using a system of buoys and weighted dropper lines. Demersal (or bottom) longlines, by contrast, are weighted so the mainline rests on or just above the seafloor. These target groundfish like halibut, sablefish, and cod. The mainline sits against the bottom, with buoy lines running up to the surface on either end.
The choice between pelagic and demersal gear depends entirely on what the fishing vessel is after. Pelagic longlines dominate the global tuna and swordfish trade, while demersal longlines are a staple of cold-water fisheries in places like Alaska and the North Atlantic.
Environmental Concerns and Bycatch
Longline fishing’s biggest environmental problem is bycatch: the accidental capture of non-target species. Because hundreds of hooks sit in the water for hours, sea turtles, seabirds, sharks, and marine mammals can all take the bait. Pelagic longlines are especially prone to hooking seabirds (particularly albatrosses and petrels) that dive for the bait as it’s being set, and sea turtles that swallow hooks meant for tuna.
Several gear modifications have proven effective at reducing this damage. Circle hooks, which are rounded rather than J-shaped, cut loggerhead sea turtle bycatch rates by as much as 86% in experimental fisheries and reduced the proportion of turtles hooked deep in the throat by 47%. For loggerheads that were still caught, the likelihood of dying after release was cut roughly in half compared to traditional J-hooks. The picture is more mixed for leatherback turtles, where circle hooks reduced catch rates significantly but didn’t change post-hooking survival much.
For seabirds, the most important tool is the tori line: a streamer line towed behind the vessel during setting that scares birds away from the sinking bait. In controlled experiments, tori lines reduced seabird bycatch from 0.85 birds per 1,000 hooks to just 0.13 per 1,000 hooks. Weighting the branch lines so baited hooks sink faster also helps, pulling them below diving depth before birds can reach them. Placing a 65-gram weight close to the hook nearly doubled the sink rate compared to standard setups, pushing bait past the roughly 4.5-meter maximum diving depth of most albatrosses. Night setting, which takes advantage of reduced seabird activity after dark, rounds out the three strategies with the strongest scientific support.
Sustainability and Certification
Not all longline fisheries operate the same way, and several have earned sustainability certification from the Marine Stewardship Council (MSC). The MSC evaluates longline fisheries on several criteria: whether target stocks are being harvested sustainably, whether the fishery has effective management strategies with catch or effort limits, and how well it minimizes impacts on the broader ecosystem. Certified fisheries must also adopt a fins-naturally-attached policy for any sharks they retain, take measures to reduce lost or abandoned gear (ghost gear), and demonstrate that they are managing impacts on endangered, threatened, and protected species against objective conservation benchmarks.
For consumers, MSC-certified longline-caught fish represents one way to choose seafood harvested under stricter oversight, though certification standards are periodically revised and debated within the conservation community.
Other Meanings of “Longline”
Neonatal Medicine
In hospitals, a longline is a thin, flexible catheter threaded through a vein in a newborn baby’s arm or leg into a larger central vein. It serves the same purpose as an adult PICC line: delivering nutrition and medications that would irritate smaller veins. The higher blood flow in larger veins dilutes the solutions and reduces the risk of tissue damage. A longline can stay in place for several weeks, which is a major advantage for premature or critically ill infants who would otherwise need repeated needle sticks. Risks include bloodstream infections, which occur at a rate of roughly 6 to 8 episodes per 1,000 patient-days in neonatal intensive care units, with the smallest and most premature infants facing the highest risk. Correct tip placement is essential to avoid fluid leaking into the chest, abdomen, or tissue around the heart.
Fashion and Apparel
In clothing, “longline” simply describes a garment cut longer than its standard version. A longline bra extends several inches below the bust to cover the lower ribcage, distributing support across a wider area. Longline shirts, hoodies, and jackets follow the same principle, with hemlines that fall well past the waist. The extended coverage in longline bras can offer additional support for the torso, though the effect is more about distributing pressure than correcting posture in any clinical sense.