Longline fishing is a commercial angling technique that uses a single, extremely long line with thousands of baited hooks attached at regular intervals. This passive method allows the gear to soak in the water column for hours, attracting fish over a wide area. Longlining is practiced globally, utilized by vessels ranging from small-scale coastal operations to large, mechanized fleets. The adaptable design allows fishers to target specific species in different ocean depths worldwide, contributing significantly to the global seafood supply, particularly for high-value species.
Anatomy of the Longline System
The core structure of the longline system is the mainline, a durable material like nylon monofilament that acts as the backbone of the gear. In large-scale fisheries, this mainline can extend for over 100 kilometers, supporting the array of hooks and ancillary equipment. Numerous shorter lines, known as branch lines, snoods, or gangions, are attached to the mainline, each featuring a single baited hook at its end. The type of bait used, often whole finfish like mackerel or squid, is selected based on the feeding preferences of the desired catch. Flotation devices, such as buoys, are placed along the mainline to suspend the gear at the desired depth, while anchors or weights secure bottom lines to the seafloor.
Operational Methods and Target Fisheries
The method of deployment is classified into two main types based on where the line is set in the water column.
Pelagic Longlining
Pelagic longlining, also referred to as drift longlining, targets fast-swimming, highly migratory species that swim in the open ocean, away from the seafloor. This includes species such as swordfish and various tuna species, including bluefin and yellowfin. Pelagic lines are set near the surface, typically suspended between 50 and 300 meters deep, and are allowed to drift freely with the current.
Demersal Longlining
Demersal longlining, or bottom longlining, is designed to catch groundfish that live on or near the ocean floor. Target species include cod, halibut, and Patagonian toothfish. Unlike their pelagic counterparts, these lines are heavily weighted and anchored to the seabed, sometimes in depths exceeding 1,000 meters. The scale of longline operations varies considerably, from smaller vessels to massive factory ships equipped with automated systems for setting and hauling lines.
Bycatch and Ecological Considerations
The non-selective nature of baited hooks means that longline fishing frequently results in the incidental capture of non-target species, a problem known as bycatch. Sea turtles, particularly loggerhead and leatherback species, are commonly caught when they attempt to consume the bait or become entangled in the lines. Seabirds, such as albatrosses, are also vulnerable, often getting hooked and drowning while diving for bait as the line is being deployed. Sharks and marine mammals like dolphins are also susceptible to accidental capture or entanglement, leading to injury or mortality.
Ghost Fishing and Mitigation
A further ecological concern is the issue of lost or abandoned gear, which continues to fish indiscriminately in a phenomenon called “ghost fishing.” It is estimated that approximately 740,000 kilometers of longline mainline and 14 billion hooks are lost into the ocean annually, creating a persistent form of marine debris. Fisheries employ several measures to reduce this unintended impact:
- Using weighted lines to make the bait sink quickly out of the reach of seabirds.
- Setting lines at night.
- Deploying bird-scaring devices known as tori-lines.
- Switching from traditional J-hooks to circle hooks, which reduces sea turtle bycatch mortality by decreasing the likelihood of deep hooking.