Longline fishing is not inherently sustainable or unsustainable. Its environmental impact depends heavily on what type of longlining is used, where it operates, and whether the fleet adopts modern bycatch reduction measures. The method causes far less habitat destruction than bottom trawling, but its bycatch problem, particularly for sharks, sea turtles, and seabirds, remains one of the most serious sustainability concerns in commercial fishing.
How Longline Fishing Works
A longline is exactly what it sounds like: a main fishing line, sometimes stretching for miles, with hundreds or thousands of baited hooks branching off at intervals. There are two main types. Pelagic longlines hang near the ocean surface to catch open-water species like tuna and swordfish. Demersal longlines sit on or near the seafloor to target bottom-dwelling fish like halibut, sablefish, and cod.
This distinction matters for sustainability because each type interacts with different ecosystems and catches different non-target species. Pelagic longlines are notorious for hooking sharks, sea turtles, and seabirds. Demersal longlines interact more with seafloor communities, though their physical impact is comparatively mild.
The Bycatch Problem
Bycatch is the single biggest sustainability challenge for longline fishing. In the western and central Pacific Ocean alone, sharks may represent 10% or more of the total catch on longline vessels. A 2023 study in Science Advances estimated that roughly 287,000 large pelagic sharks were captured as bycatch across eight Pacific shark sanctuaries in 2019. Of those, an estimated 110,000 died either on the line or shortly after release.
Blue sharks and silky sharks bore the brunt, accounting for over 73% of projected shark catches and about 70% of all shark deaths. In the equatorial Atlantic, a separate study found that blue sharks actually made up the largest portion of longline catch by weight at 38%, exceeding even the target species. Bigeye tuna represented just 20.5% of the catch, and swordfish 15.1%. When your bycatch outweighs your target catch, the sustainability math gets difficult.
The mortality rates hit some species harder than others. For silky sharks, bycatch deaths in certain Pacific sanctuaries reached 40% of what scientists estimate those populations can withstand and still remain healthy. In the Federated States of Micronesia and Palau, estimated mortality actually exceeded sustainable levels for silky sharks. Oceanic whitetip sharks and thresher sharks also suffered significant losses, with thousands killed annually as collateral damage.
Juvenile Catches Threaten Future Stocks
Even for the species longlines are designed to catch, sustainability is complicated by the capture of immature fish. In the equatorial Atlantic, the proportion of juvenile yellowfin tuna in longline catches ranged from 12% to over 50% depending on the year. For bigeye tuna, juveniles made up 27% to 36% of the catch. Removing fish before they reproduce undermines a population’s ability to replenish itself, and these percentages suggest that longline gear is not always selective enough to protect young fish.
Global tuna and swordfish production has recently reached some of the highest levels ever recorded, but fishing effort has actually declined over the past decade, likely due to a combination of regulation and the consequences of previous overfishing. That tension, between record harvests and shrinking effort, signals that stocks are under real pressure even as management bodies try to rein things in.
Seafloor Impact Compared to Trawling
One area where longline fishing genuinely outperforms other commercial methods is habitat destruction. Bottom trawling drags heavy nets across the seafloor, stripping away corals, sponges, and other slow-growing organisms that form the foundation of deep-sea ecosystems. Research published in Scientific Reports found that a single bottom trawl causes roughly the same damage as 296 to 1,719 longline deployments, depending on the complexity of the species growing on the seafloor.
The numbers are striking. About 13 trawl passes are enough to remove 90% of organisms living on the seafloor in a given area. Achieving the same level of destruction with longlines would require 4,000 to 23,000 successive deployments. Slow-growing vulnerable species like cold-water corals were still common in areas that had been longlining for more than 20 years. So while demersal longlines do contact the bottom, they are a fundamentally different category of impact than trawling.
Gear Modifications That Reduce Harm
Several proven technologies can make longline fishing significantly less destructive. Tori lines, which are streamers flown from the stern of a vessel to scare birds away from baited hooks as they enter the water, reduced seabird bycatch rates from 0.85 birds per 1,000 hooks down to 0.13 birds per 1,000 hooks in controlled experiments. That is roughly an 85% reduction.
Circle hooks, which are shaped so that fish tend to be hooked in the mouth rather than swallowing the hook, have been shown to reduce sea turtle mortality because turtles can more easily be released alive. Weighted branch lines that sink hooks below the surface faster also reduce the window during which seabirds can grab bait.
When all three recommended mitigation measures (tori lines, weighted lines, and night setting) are used simultaneously, a 2025 study in Biological Conservation estimated that seabird mortality drops by 72% to 93% for threatened albatross and petrel species in the South Atlantic. Even using just two of the three measures reduced mortality by 41% to 86%. The technology exists. The question is whether fleets actually use it.
How Longline Fisheries Are Regulated
Longline fishing for highly migratory species like tuna is governed by Regional Fisheries Management Organizations, or RFMOs, which set catch limits and technical rules for large ocean areas. In the western and central Pacific, the most recent conservation measure set the U.S. longline bigeye tuna catch limit at 6,554 metric tons per year for 2024, nearly doubling the previous limit of 3,554 metric tons. Any overage in one year is deducted from the following year’s allowance.
These quota systems are the primary tool for preventing overfishing of target species, but their effectiveness depends on enforcement and on whether the science behind catch limits is accurate. The doubling of the U.S. bigeye limit, for example, reflects updated stock assessments but also raises questions about whether higher harvests can be sustained long-term. Meanwhile, the same conservation measure removed a loophole that had allowed some vessels to operate outside the quota through agreements with U.S. territories, a sign that regulators are tightening oversight.
For bycatch species, regulation is patchier. Shark sanctuaries in the Pacific were designed to protect shark populations, but the research shows that longline bycatch mortality still occurs at significant levels within those sanctuaries. For silky sharks in some areas, bycatch within sanctuaries exceeded the level scientists consider sustainable. Spatial protections alone are not enough when longline hooks are set across vast stretches of ocean.
So Is It Sustainable?
Longline fishing can be sustainable under the right conditions: well-enforced catch limits based on sound science, mandatory use of bycatch reduction gear, and real-time monitoring of what is actually being caught. Some longline fisheries, particularly well-managed demersal operations targeting species like halibut, already meet high sustainability standards and carry certification from organizations like the Marine Stewardship Council.
But large-scale pelagic longline fisheries operating in the open ocean often fall short. The sheer volume of shark bycatch, the capture of juvenile tuna at rates that can exceed 50%, and the ongoing mortality of threatened seabirds all point to a method that causes serious ecological harm when practiced without strong oversight. The gap between what is possible with best practices and what actually happens on the water remains the central issue. Longline fishing is not sustainable by default, but it is not unsustainable by nature either. The gear is only as responsible as the rules governing it and the willingness to enforce them.