Bycatch is the portion of a fishing catch that isn’t the species the fishers were trying to catch. It includes everything from undersized fish and non-target species to sea turtles, dolphins, seabirds, and sharks that get swept up in commercial fishing gear. Some of this unintended catch gets kept and sold, but a large share is simply thrown back into the ocean, often dead or dying.
Why the Definition Varies
Bycatch sounds like a simple concept, but its exact meaning depends on who’s using the term. In American fisheries law, bycatch generally refers to fish and marine animals that are caught and then discarded. In other parts of the world, it can mean any non-target species hauled in, whether it’s sold at market or thrown overboard. The FAO, the United Nations body that oversees global food and agriculture, treats bycatch as a broad umbrella covering both “incidental catch” (non-target species that fishers keep and sell) and “discards” (everything that goes back over the side).
The distinction matters because the environmental consequences are different. A non-target fish species that gets sold at market at least enters the food supply. Discards, on the other hand, represent pure waste. When discussions focus on bycatch as a conservation problem, they’re usually talking about discards and the killing of vulnerable species like marine mammals and sea turtles.
How Much Bycatch Happens Globally
The most widely cited global estimate comes from the FAO, which calculated that commercial fisheries discard an average of 27 million metric tons of fish per year, with a range of roughly 18 to 40 million tons. That figure is from the mid-1990s, and while fishing practices have changed since then, the sheer scale gives a sense of the problem. In U.S. waters alone, bycatch totals more than 1 million metric tons annually.
The economic toll is enormous. A study published in Marine Policy estimated that bycatch-related discards cost U.S. fisheries about $427 million per year in lost revenue at the dock. When you trace those losses through the broader seafood economy, including processing, distribution, and retail, the figure balloons to roughly $4.2 billion in potential sales, $1.5 billion in income, and around 64,000 jobs. Fisheries also face premature seasonal closures when they hit bycatch limits too quickly, costing individual fisheries anywhere from $34 million to $453 million a year in foregone harvest.
Which Fishing Methods Cause the Most
Not all fishing gear creates bycatch equally. Bottom trawling, where large weighted nets are dragged along the ocean floor, consistently produces the highest discard rates of any commercial method. These nets are indiscriminate by design: they scoop up nearly everything in their path, and sorting happens only after the haul is on deck.
Gillnets, the large mesh walls of netting set vertically in the water, are more selective for fish size but still catch a wide range of non-target species. Globally, gillnet fisheries discard an estimated 800 million kilograms of marine life each year, with an overall discard rate around 10%. That average hides significant variation. In one Brazilian monkfish gillnet fishery, for every target fish caught, roughly one non-target animal was discarded, a 50% discard rate. A nearby fishery targeting a different species discarded only 0.27 non-target animals per target fish caught. The differences come down to target species, water depth, season, and where the nets are set.
Gillnets are also a particular threat to large marine animals. Studies have documented bycatch of sea turtles, dolphins, whales, seabirds, and sharks in gillnet fisheries. Bottom gillnets in southern Brazil discarded animals from 77 different species groups, with about a third of discarded weight made up of sharks and rays.
Longlines, which trail thousands of baited hooks behind a vessel, are notorious for hooking seabirds (especially albatrosses) and sea turtles that go after the bait. Purse seine nets, used heavily in tuna fisheries, can encircle entire schools of dolphins that swim above tuna.
Which Species Are Most Affected
Bycatch threatens a remarkably broad range of marine life. Sea turtles get entangled in gillnets and hooked on longlines. Marine mammals, from small dolphins to large whales, become trapped in nets or entangled in the vertical lines connecting lobster and crab traps to surface buoys. Seabirds dive for bait on longlines and drown. Sharks and rays are caught in nearly every type of commercial gear, and many species are now threatened partly because of bycatch mortality.
Fish species suffer too. Juvenile fish of commercially valuable species are often caught before they’ve had a chance to reproduce, undermining the long-term health of the very populations fishers depend on. Non-commercial fish species can see population declines with little public attention because they have no market value and no dedicated monitoring.
Technologies That Reduce Bycatch
Several devices and techniques have been developed to let non-target animals escape or avoid fishing gear altogether. Turtle excluder devices (TEDs) are metal grates fitted inside shrimp trawl nets that allow turtles to exit through a flap while shrimp pass through to the collection bag. Similar excluder devices exist for seals and sea lions. In pot and trap fisheries that risk entangling whales, some operations now use weakened rope for the line running from the trap to the surface buoy, designed so that a whale can break free.
Sensory deterrents are a growing area of innovation. Acoustic pingers attached to gillnets emit sounds that warn dolphins and porpoises away. Researchers have also tested predator sounds, such as recorded killer whale calls, and found they can deter multiple shark species from approaching fishing gear. LED lights attached to gillnets are one of the most promising tools available: they are the only technology so far shown to significantly reduce bycatch across all four major groups of vulnerable marine megafauna (sea turtles, marine mammals, seabirds, and sharks/rays).
No single solution works everywhere. The effectiveness of each deterrent depends on the species involved, the type of fishery, water conditions, and local ecology. What dramatically cuts turtle bycatch in one region may do nothing for seabird bycatch in another.
How Bycatch Is Regulated
In the United States, the primary law governing bycatch is the Magnuson-Stevens Fishery Conservation and Management Act, which includes bycatch reduction as one of its national standards for fishery management. Every federally managed fishery must have a plan that addresses bycatch, including measurable criteria for tracking it. The Marine Mammal Protection Act adds another layer, requiring programs to minimize the deaths and serious injuries of marine mammals in commercial fishing gear, including formal “take reduction plans” for the most at-risk populations.
Internationally, the picture is patchier. U.S. law requires NOAA Fisheries to report to Congress every two years on foreign nations identified for excessive bycatch of protected species, illegal fishing, or unregulated shark harvesting. The Port State Measures Agreement aims to keep illegally caught fish out of ports and markets. But enforcement on the high seas remains difficult, and many of the world’s most bycatch-heavy fisheries operate in waters with limited oversight.
Bycatch reduction ultimately depends on a combination of better gear, smarter regulations, and economic incentives that make it worthwhile for fishers to avoid non-target species rather than simply absorb the cost of discarding them.