Longline fishing is a widely used commercial method that involves deploying a very long main fishing line, sometimes extending for miles, with numerous baited hooks attached at regular intervals. This technique is employed globally to catch a variety of marine species. It is a significant global commercial fishing method, targeting fish that range from surface dwellers to those found deep on the ocean floor.
How Longline Fishing Works
Longline fishing uses a main line, often heavy nylon monofilament, stretching up to 100 kilometers (approximately 60 miles). Shorter lines, called snoods, are attached to this main line at consistent intervals, each with a baited hook. The bait used, such as squid, mackerel, or sardines, is selected based on the target species.
The longline is deployed from a fishing vessel, often using machinery to set it into the water. Depending on the target species, the line can be set to drift near the surface or anchored to the seafloor using weights. Floats and marker buoys are spaced along the line to indicate its position and depth. After a “soak” period, which can last from hours to days, the longline is retrieved, with fishermen unhooking the catch.
Variations in Longline Methods
Longline fishing adapts to different marine environments and target species through distinct variations. The primary classifications are pelagic and demersal longlining, each designed for specific depths and fish behaviors.
Pelagic Longlining
Pelagic longlining targets species that inhabit the open ocean’s water column, such as tuna and swordfish. These lines are set near the surface or at depths between 50 to 300 meters (approximately 160 to 1,000 feet), with buoys maintaining their position in the water. Pelagic lines can extend for 50 to 100 kilometers (approximately 30 to 60 miles).
Demersal Longlining
Demersal longlining, also referred to as bottom longlining, is used for groundfish that live on or near the seafloor, including species like cod, halibut, and snapper. These lines are weighted to ensure they sink to the ocean floor and are generally anchored in place. Demersal longlines are often shorter than their pelagic counterparts, ranging from a few hundred meters to 50 kilometers in length, and can be set in depths exceeding 1,000 meters (3,600 feet).
Environmental Concerns
Longline fishing, despite its selective nature compared to some other methods, presents several environmental challenges. A primary concern is bycatch, the unintended capture of non-target marine life. This can include seabirds, sea turtles, sharks, and marine mammals, many of which are protected or endangered species. For instance, global longline fisheries were estimated to kill between 160,000 and 320,000 seabirds annually around 2003.
Another problem is “ghost fishing,” where lost or abandoned longline gear continues to indiscriminately catch marine organisms. These lines may be severed by other vessels or strong currents, becoming derelict and posing a long-term threat to marine life until they degrade. While less pronounced than with some other fishing methods, demersal longlining can also cause minor habitat impact, particularly from weighted lines or anchors contacting the seafloor, potentially disturbing sensitive benthic communities like corals.
Minimizing Environmental Impact
Efforts to reduce the environmental impact of longline fishing focus on various mitigation techniques and regulatory measures. To lessen seabird bycatch, bird-scaring lines (tori lines) are deployed from vessels. These lines feature brightly colored streamers that deter seabirds from accessing baited hooks as they are set, significantly reducing entanglement and drowning.
Another effective strategy involves weighting the lines to ensure hooks sink rapidly beyond the diving range of seabirds. For pelagic longlines, this can involve adding weights like 40 grams within 0.5 meters of the hook, aiming for a sink rate of 0.3 meters per second. The use of circle hooks, which have a distinct curved shape, helps reduce sea turtle bycatch by minimizing deep hooking, as these hooks are more likely to catch in the corner of a turtle’s mouth, allowing for safer release. Setting lines at night or avoiding specific fishing seasons and areas can also reduce interactions with non-target species. Research also explores biodegradable fishing line materials to reduce ghost fishing. International agreements and regional fisheries management organizations implement quotas and manage protected areas to promote sustainable longline practices.