Bycatch and Ghost Nets

What Is Bycatch and How Much Is There?

Bycatch encompasses several distinct phenomena: the capture of non-target fish species that are then discarded; the incidental entanglement or hooking of megafauna such as sea turtles, marine mammals, and seabirds; and the capture of target species that are nevertheless discarded because they are undersized, the wrong sex, or caught when a quota has been filled. The global scale of discards was systematically assessed by Kelleher (2005) in an FAO Fisheries Technical Paper, which estimated that global discards from marine fisheries averaged 7.3 million tonnes per year over the period 1992–2001, representing approximately 8% of total global catch [1]. This figure, while large, actually represented an improvement over earlier estimates of 27 million tonnes from the 1990s, largely reflecting reductions in particularly discard-intensive fisheries such as shrimp trawling in the Gulf of Mexico.

Shrimp trawling remains among the most bycatch-intensive fishing methods in the world. In tropical shrimp fisheries, the ratio of bycatch to target shrimp can exceed 10:1 by weight. This means that for every kilogram of shrimp landed, more than 10 kilograms of juvenile fish, invertebrates, sea turtles, and other organisms may be captured and discarded, typically dead. The economic waste is staggering; the ecological toll — particularly on juvenile fish of commercially important species — is self-defeating, removing fish before they have the opportunity to contribute to spawning stock biomass or to future harvests.

Sea Turtles and Longline Fisheries

Of all bycatch interactions, the capture of sea turtles in pelagic longline fisheries has attracted the most sustained scientific and conservation attention. All seven species of sea turtle are listed as threatened or endangered by the IUCN, and longline fishing is among the most significant human-caused threats to several species. Lewison et al. (2004) conducted the first global assessment of sea turtle longline bycatch, estimating that pelagic longlines capture approximately 4.4 million hooks per year, resulting in the incidental capture of more than 250,000 loggerhead sea turtles (*Caretta caretta*) and more than 60,000 leatherback sea turtles (*Dermochelys coriacea*) annually [2]. The leatherback figure is particularly alarming given the species' critically low population sizes in the Atlantic and Pacific.

Longlines are extraordinarily effective at capturing turtles because both turtles and target pelagic fish species such as tuna and swordfish occupy similar water depths and are attracted to the same prey. Turtles bite hooks baited with squid or fish, swallow them, and frequently die from the internal injuries sustained or from drowning when unable to reach the surface to breathe. Circle hooks, which curve back toward the shank and are less easily swallowed, have been shown to reduce turtle bycatch by 60–90% in some longline fisheries with minimal impact on target catch — a rare technological solution that is both effective and economically neutral.

Megafauna Bycatch Hotspots

The global distribution of bycatch impacts on marine megafauna is not uniform. Lewison et al. (2014) used a spatially explicit modelling approach to map bycatch hotspots for marine mammals, seabirds, and sea turtles simultaneously, revealing that certain ocean regions — particularly the northwest Pacific, the southwest Atlantic, and portions of the Indian Ocean — represent cumulative megafauna hotspots where multiple gear types interact with multiple threatened taxa [3]. These hotspots often coincide with areas of high fishing intensity rather than simply high biodiversity, meaning that management interventions in a relatively small number of regions could produce disproportionate conservation gains.

Marine mammals are particularly vulnerable to gillnet fisheries. Small cetaceans — dolphins, porpoises, and small whales — swim into drift gillnets at speed, become entangled, and drown within minutes. The vaquita (*Phocoena sinus*), a small porpoise endemic to the northern Gulf of California, has been driven to the edge of extinction almost entirely by gillnet bycatch: fewer than 20 individuals are estimated to remain. Numerous dolphin populations in the Mediterranean, Baltic, and Black Seas face analogous pressures from small-scale coastal gillnet fisheries that operate below the threshold of regulatory scrutiny. Seabirds, particularly albatrosses and petrels, are killed by the tens of thousands annually on longline hooks set for tuna, toothfish, and other species across the Southern Ocean.

Ghost Nets: ALDFG and the Ocean's Silent Killers

Fishing gear does not become inert when it enters the ocean. Lost, abandoned, or discarded fishing gear — collectively termed ALDFG — continues to entangle and kill marine organisms for months, years, or even decades after it is separated from any vessel. MacFadyen et al. (2009), in an FAO/UNEP technical report that remains the definitive assessment of the ALDFG problem, documented that approximately 640,000 tonnes of fishing gear enters the ocean each year, constituting roughly 10% of all marine plastic pollution [4]. Gillnets and fish traps were identified as the gear types most likely to continue 'ghost fishing' after loss, given their design — which is optimised to capture any organism that swims into them.

The mechanisms by which ALDFG continues to fish are well-documented. Nets drifting in the water column entangle fish, which attract scavengers, which also become entangled, creating a cycle of mortality termed the 'ghost fishing cascade.' As nets accumulate carcasses they become more effective fishing devices — heavier, with more olfactory attractants — until they sink to the seabed, where they smother benthic habitats and continue capturing bottom-dwelling species. In shallow coastal environments, particularly around coral reefs, ghost nets are particularly destructive: they physically damage coral structures while simultaneously killing the reef fish that inhabit them. MacFadyen et al. (2009) noted that some ghost net surveys in the northwest Hawaiian Islands recovered nets containing hundreds of entangled seabirds and sea turtles from single net masses [4].

Which Gear Types Generate the Most Bycatch?

Bycatch rates and composition vary substantially by gear type. Pelagic longlines, which deploy thousands of baited hooks on mainlines extending up to 130 kilometres, interact with turtles, sharks, billfish, seabirds, and marine mammals. Drift gillnets — large walls of monofilament netting suspended in the water column — are among the least selective gear types ever deployed; some drift gillnets set for tuna in the Mediterranean and Indian Oceans historically captured more non-target species by weight than target species. Bottom trawls capture benthic organisms indiscriminately, including juvenile commercial fish, invertebrates, and — in coastal fisheries — sea turtles. Purse seine fisheries for tuna, particularly in the eastern tropical Pacific, were historically associated with massive dolphin mortality because yellowfin tuna frequently school beneath herds of spotted and spinner dolphins; the introduction of backdown procedures and the 'dolphin-safe' tuna certification have substantially reduced but not eliminated this mortality.

Kroodsma et al. (2018) demonstrated that the global fishing fleet covers more than 200 million square kilometres of ocean annually, with longlining alone accounting for nearly a third of that spatial footprint [5]. At this scale, even low bycatch rates per unit of fishing effort translate to enormous absolute mortality numbers when aggregated across the entire fleet operating over a full year.

Technological and Regulatory Solutions

Several technology-based mitigation measures have demonstrated effectiveness. Turtle Excluder Devices (TEDs), mechanical grids installed in the throat of shrimp trawls, allow sea turtles to escape through a trapdoor while retaining shrimp, and are now mandatory in shrimp trawl fisheries in the United States, Australia, and several other countries. Circle hooks and mackerel-type bait replace J-hooks and squid in some longline fisheries with significant turtle bycatch reduction. Tori lines — streamers deployed above longline sets — scare albatrosses away from hooks during the critical window between deployment and submergence, and international agreements under the Agreement on the Conservation of Albatrosses and Petrels (ACAP) promote their adoption. Acoustic deterrents (pingers) installed on gillnets have shown some effectiveness in reducing porpoise bycatch in Baltic Sea cod gillnet fisheries, though their effectiveness varies by species and context.

On the ALDFG side, several nations have established gear marking requirements — mandatory tagging of nets and pots with vessel identification — that allow lost gear to be traced to its owner and create financial accountability for loss. Norway, the United Kingdom, and Australia operate ghost gear retrieval programmes in which commercial divers and decommissioned fishing vessels are contracted to locate and remove lost gear from priority areas. The Global Ghost Gear Initiative, a multi-stakeholder programme involving fishing industry, government, and NGO partners, coordinates international action on ALDFG and has facilitated the retrieval of thousands of tonnes of ghost gear from ocean environments worldwide. Yet the scale of the problem dwarfs current retrieval capacity.

The Economics of Bycatch Waste

Beyond its ecological dimensions, bycatch represents an enormous economic waste. Kelleher (2005) estimated that global discards in the early 2000s had a landed value that, if retained, would represent billions of dollars of foregone revenue [1]. In many developing-country fisheries, where artisanal fishers already operate on thin margins, discards represent food security waste as well as economic loss: fish discarded dead at sea could have fed communities. Some fisheries economists have argued that policies mandating the landing obligation — requiring that all caught fish be landed rather than discarded — create incentives to use more selective gear from the outset, because fishers must count all catch against quotas. The EU introduced a phased landing obligation beginning in 2015, with mixed results; compliance has been uneven, and some observers fear the policy has in some cases led to increased high-grading rather than reduced bycatch.

The path toward reducing bycatch and ALDFG requires action across multiple fronts simultaneously: technology adoption, regulatory enforcement, consumer pressure through ecolabelling, international cooperation to manage high-seas gear types, and investment in gear retrieval infrastructure. The science is clear about both the scale of the problem and the efficacy of available solutions. The deficit, as in so many fisheries challenges, is one of implementation rather than knowledge.

"Incidental catch of non-target vertebrates has been proposed as a serious conservation issue... our analysis suggests [longline] bycatch may jeopardise population persistence for loggerhead and leatherback turtles." — Lewison et al., Ecology Letters, 2004 [2]