Bycatch is the capture of non-target animals during commercial fishing. It includes fish, sea turtles, dolphins, whales, sharks, seabirds, corals, and sponges that end up in nets or on hooks meant for other species. By one widely cited estimate, at least 38.5 million tonnes of marine life each year, roughly 40% of everything pulled from the ocean, qualifies as bycatch. Some of that catch gets sold at lower value. Much of it is thrown back, often dead or dying.
Target Catch, Bycatch, and Discards
The simplest way to think about bycatch: it is catch that is either unused or unmanaged. A shrimp trawler dragging a net along the seafloor will haul up juvenile fish, rays, crabs, and sometimes sea turtles alongside the shrimp it wants. Some of those non-target animals have commercial value and can be sold, though typically at far lower prices. In U.S. fisheries, discarded species average about $0.45 per pound at landing compared to $1.86 per pound for target species.
The portion of bycatch that gets thrown overboard is called “discards.” Fish and other animals may be discarded because they’re the wrong species, the wrong size, or because regulations prohibit keeping them. Most discarded animals don’t survive the process of being caught, hauled to the surface, sorted on deck, and tossed back.
Which Fishing Methods Cause the Most Bycatch
Not all fishing gear is equally destructive. Trawls, the large funnel-shaped nets dragged behind boats, produce the highest bycatch rates by a wide margin. Bottom trawls are particularly damaging because they scrape the seafloor, catching everything in their path and disturbing habitat like coral beds and sponge fields. The 40% global bycatch estimate draws heavily from trawl fishery data.
Longlines, which use a main line strung with hundreds or thousands of baited hooks, produce less bycatch than trawls but still hook non-target species. Sea turtles, sharks, and seabirds are especially vulnerable to longlines. Gillnets, large curtains of nearly invisible mesh hung in the water column, entangle marine mammals and sea turtles along with target fish.
Pots and traps consistently show the lowest bycatch rates among commercial gear types. They also cause the least habitat damage. The tradeoff is that pots catch smaller volumes, which makes switching gear types a complicated economic decision for fishers.
Why Bycatch Matters for Ocean Ecosystems
When millions of tonnes of marine life are removed from the ocean each year without management, the ripple effects are significant. Bycatch slows the rebuilding of overfished stocks because juvenile fish that would otherwise grow to reproduce are killed before they can. For species already under pressure, like certain populations of whales, sea turtles, and dolphins, even small amounts of bycatch can impede population recovery.
The effects also move through food webs. Removing large quantities of prey species changes what’s available for predators to eat, which can shift the balance of entire ecosystems. When trawls destroy corals and sponges on the seafloor, they eliminate habitat that other species depend on for shelter, feeding, and breeding. These habitats can take decades or longer to recover.
The Economic Cost
Bycatch represents a significant financial loss. In U.S. fisheries alone, the combined value of species discarded for regulatory and economic reasons has been estimated at $4.2 billion in missed opportunity. Even when discarded fish are low-value individually, the volumes are enormous. In 2014, U.S. economic discards totaled roughly 126 million pounds, worth an estimated $57 million to fishers at landing. That same volume, if it had moved through processing and into restaurants, could have generated between $355 million and nearly $2 billion in revenue.
The economics are counterintuitive, though. Simply eliminating all discards wouldn’t automatically boost profits. If fishers had to land everything they caught, the shift in species mix could actually reduce the overall landed value because lower-priced species would displace higher-priced target catch. One analysis found that a hypothetical 100% discard reduction could mean a net loss of $150 million in landed value. The real gains come from more selective fishing that avoids catching unwanted species in the first place.
Devices and Techniques That Reduce Bycatch
The most proven bycatch reduction technology is the turtle excluder device, or TED, used in shrimp trawl fisheries. A TED is a grid of metal bars fitted inside the neck of a trawl net. Shrimp pass through the bars into the back of the net, while sea turtles and other large animals hit the grid and exit through a flap. Current TED designs are 97% effective at excluding turtles from shrimp trawls.
Other approaches target different species. Bird-scaring lines, also called streamer lines, are towed behind longline vessels to keep seabirds away from baited hooks as they enter the water. Circle hooks, which are shaped to snag the corner of a fish’s mouth rather than being swallowed, reduce sea turtle deaths on longlines because turtles can be released alive more easily. Modified net mesh sizes and escape panels allow juvenile fish to swim free before the net is hauled up.
Newer technologies are pushing further. Cameras mounted inside trawl nets can record how animals interact with bycatch reduction devices, letting researchers see exactly what works and what doesn’t. Artificial intelligence and computer vision tools are being developed to process that footage automatically, which could speed up the testing cycle for new excluder designs. Real-time monitoring tools may eventually give fishers immediate feedback on what’s entering their nets, allowing them to adjust on the fly.
How Bycatch Is Regulated
In the United States, three major federal laws govern bycatch reduction. The Magnuson-Stevens Act covers fishery management broadly and requires minimizing bycatch to the extent practicable. The Endangered Species Act and the Marine Mammal Protection Act add layers of protection for vulnerable species that are frequently caught as bycatch.
Federal priorities include expanding observer programs and electronic monitoring on fishing vessels, identifying ocean areas with high bycatch of deep-sea corals and sponges and closing them to damaging gear types, and promoting more selective fishing equipment. Regulators also track how bycatch of prey species indirectly affects protected animals like large whales by reducing their food supply.
Internationally, bycatch regulation varies widely. Some countries have discard bans that require fishers to land everything they catch, which creates incentives to fish more selectively. Others have little monitoring or enforcement, meaning bycatch goes largely unreported. The global estimate of 38.5 million tonnes per year is considered conservative because data from many fisheries, particularly in developing nations, is incomplete.