Bycatch, also called by-catch, is a fish or other sea creature that is caught accidentally when fishing for certain types of fish or sizes. This happens when the wrong species, the wrong sex, or young or small fish of the target species are caught. The term "bycatch" is sometimes used for animals caught unintentionally in other types of hunting or collecting. Freshwater fish that are caught but not wanted are called rough fish in the United States or coarse fish in the United Kingdom.

In 1997, the Organisation for Economic Co-operation and Development (OECD) defined bycatch as "the total number of fish that die from fishing, excluding those that are kept intentionally as the main goal of fishing." Bycatch causes fish populations to decrease and contributes to overfishing because it includes animals caught unintentionally.

Between 1990 and 1999, the average number of seals, sea lions, and whales accidentally caught each year in the United States was about 6,215, with a possible error of 448.

Bycatch problems began in the 1960s when dolphins were accidentally caught in tuna fishing nets.

The word "bycatch" is used in at least four different ways in fishing.

The term "deliberate bycatch" refers to animals caught accidentally that are later sold illegally in some parts of the world.

Scientists use tools to estimate how many animals can be safely removed from a population without harming it. These tools include "potential biological removal" (PBR) and "sustainable anthropogenic mortality in stochastic environments" (SAMSE), which considers unpredictable factors to set limits on bycatch and other human-caused deaths of wildlife.

Activities that produce bycatch

Bycatch is common in any area where fishing occurs. Unintentional catch is not limited to fish alone; dolphins, sea turtles, and seabirds are also caught accidentally.

Bycatch most often happens when using gillnetting, longlines, or bottom trawling. Longlines with baited hooks can be many kilometers long. These fishing methods, along with gillnets and bottom trawls, can catch nearly everything in their path. Thousands of kilometers of nets and lines are placed in the world’s oceans every day. This modern fishing equipment is strong and hard to see, making it effective at catching fish and accidentally trapping other sea life.

Hook-and-line fishing might help reduce bycatch because non-target animals can be released back into the ocean quickly.

Because recreational fishing is popular worldwide, a local study in the United States in 2013 suggested that discards could be a major, unmonitored cause of fish deaths.

The highest rates of unintentional catch of non-target species are linked to tropical shrimp trawling. In 1997, the Food and Agriculture Organization of the United Nations (FAO) recorded bycatch and discard levels from shrimp fisheries globally. They found discard rates (bycatch to shrimp catch ratios) as high as 20:1, with an average of 5.7:1 worldwide.

Shrimp trawl fisheries catch 2% of the world’s total fish weight but produce more than one-third of the world’s total bycatch. In the United States, shrimp trawlers have bycatch ratios between 3:1 (3 bycatch:1 shrimp) and 15:1 (15 bycatch:1 shrimp).

Trawl nets, especially shrimp trawls, are known to harm cetacean and finfish species. When bycatch is discarded (returned to the sea), it is often already dead or dying.

Tropical shrimp trawlers often travel for several months without returning to port. A typical haul lasts four hours, after which the net is pulled in. Before pulling the net on board, it is washed by moving quickly in a zigzag pattern. The contents are then dumped on the deck and sorted. An average of 5.7:1 means that for every kilogram of shrimp, 5.7 kilograms of bycatch are caught. In tropical inshore waters, bycatch usually includes small fish. Shrimp are frozen and stored on board, while bycatch is discarded.

Recent sampling in the South Atlantic rock shrimp fishery found 166 species of finfish, 37 crustacean species, and 29 other invertebrate species among the bycatch. Another study over two years found that rock shrimp made up only 10% of the total catch weight. Other species, such as iridescent swimming crab, dusky flounder, inshore lizardfish, spot, brown shrimp, and longspine swimming crabs, made up the rest.

Even with the use of bycatch reduction devices, the shrimp fishery in the Gulf of Mexico catches about 25–45 million red snapper each year as bycatch, nearly half the amount taken in recreational and commercial snapper fisheries.

The term "bycatch" is also used in other situations besides fishing. Examples include insect collecting with pitfall traps or flight interception traps for financial, control, or scientific purposes (where bycatch may include small vertebrates or untargeted insects) and controlling introduced vertebrates that have become pests, such as muskrats in Europe (where bycatch in traps may include European minks or waterfowl).

Victims

Longlines, trawls, and purse seine nets are main causes of danger for at least fifteen shark species. Bycatch can also harm the reproduction of these populations because young sharks are often caught unintentionally.

Oceanic sharks and rays have seen their populations drop by more than 70% between 1970 and the 2020s, mostly due to bycatch.

Cetaceans, such as dolphins, porpoises, and whales, can be seriously harmed by getting tangled in fishing nets or lines, or by being caught directly on hooks or in trawl nets. Cetacean bycatch is happening more often and with greater intensity. In some fisheries, cetaceans are caught as bycatch but then kept because they are valuable as food or bait. In this way, cetaceans can become targets of fishing activities.

An example of bycatch is dolphins caught in tuna nets. Dolphins are mammals and do not have gills, so they can drown if they are stuck in nets underwater. This issue has led to the growth of the ecolabelling industry, where fish producers add labels like "dolphin friendly" on packaging to assure buyers. However, "dolphin friendly" does not mean dolphins were not killed during the production of a tuna product, but that the fishing fleet did not specifically target a group of dolphins feeding together and instead used other methods to find tuna. The bycatch of the Caspian seal may be one of the largest entanglements of pinnipeds (seals and sea lions) in the world.

Of the 22 albatross species listed by IUCN on their Red List, 15 are threatened with extinction, six are Near Threatened, and only one is of Least Concern. Two species, the Tristan albatross and the waved albatross, are Critically Endangered. A major threat is commercial longline fishing because albatrosses and other seabirds that eat offal (waste from fishing) are attracted to the bait on fishing lines. Once hooked, they can drown. About 100,000 albatrosses are killed this way each year. Unregulated pirate fisheries make the problem worse.

A research study looked at the impact of illegal longline fishing vessels on albatrosses using environmental criminology as a framework. The study found that illegal longline fishing is most common in areas where illegally-caught fish species are found, and the risk of albatross bycatch is much higher in these areas. These findings show that illegal longline fishing is a serious threat to seabird survival.

Sea turtles, already critically endangered, have been killed in large numbers in shrimp trawl nets. Estimates show thousands of Kemp’s ridley, loggerhead, green, and leatherback sea turtles are caught in shrimp trawl fisheries in the Gulf of Mexico and the US Atlantic each year. The speed and length of the trawl method matter because, "for a tow lasting less than 10 minutes, the death rate for sea turtles is less than one percent, but for tows lasting more than 60 minutes, the death rate rises rapidly to fifty to one hundred percent."

Some sea turtles can escape from trawls. In the Gulf of Mexico, Kemp’s ridley turtles had the most interactions, followed by loggerhead, green, and leatherback turtles. In the US Atlantic, loggerhead turtles had the most interactions, followed by Kemp’s ridley, leatherback, and green turtles.

Mitigation

Concern about bycatch has led fishers and scientists to find ways to reduce unwanted catch. There are two main methods.

One method is to stop fishing in areas where bycatch is too high. These closures can be permanent, seasonal, or temporary, depending on the situation. Temporary closures are often used in bottom trawl fisheries when small fish or non-target species are caught unexpectedly. In some cases, fishers must move to different areas when bycatch problems occur.

The second method involves using different types of fishing gear. A simple change is using nets with larger mesh sizes, which allow smaller fish to escape. This often requires replacing old gear. In some cases, gear can be modified. Bycatch reduction devices (BRDs) and the Nordmore grate are examples of net modifications that help fish escape from shrimp nets.

BRDs allow many commercial fish species to escape. The US government approved BRDs that reduced finfish bycatch by 30%. In the South Atlantic, BRDs helped reduce bycatch of Spanish mackerel and weakfish by 40%. However, recent studies suggest BRDs may not work as well as expected. In a Florida rock shrimp fishery, BRDs failed to keep out 166 fish species, 37 crustacean species, and 29 other invertebrate species.

A device called SharkGuard, which uses pulsed electric fields, was reported in a 2022 study to reduce blue shark bycatch by 91% and stingray bycatch by 71% in a French longline tuna fishery in the Mediterranean.

In 1978, the National Marine Fisheries Service (NMFS) began developing turtle excluder devices (TEDs). TEDs use a grid to direct turtles and large animals out of trawl nets through an opening above the grid. US shrimp trawlers and foreign fleets that sell shrimp in the US are required to use TEDs. Not all countries enforce this rule.

When used, TEDs have generally reduced sea turtle bycatch. However, they are not fully effective, and some turtles still get caught. NMFS certifies TED designs if they are 97% effective. In heavily fished areas, turtles may pass through TEDs multiple times. Recent studies show that up to 20% of turtles may be recaptured, though it is unclear how many survive the escape process.

The ability of trawl nets to select fish sizes depends on the net openings, especially in the "cod end." Larger openings help small fish escape. Improving gear to increase selectivity and reduce harm is called "conservation engineering."

Longline fishing is controversial in some areas due to bycatch. Some fisheries have successfully used methods to reduce bycatch, including:

• weights to sink fishing lines quickly
• streamer lines to scare birds away from baited hooks
• setting lines only at night with minimal ship lighting
• limiting fishing seasons to the southern winter
• avoiding discharging waste while setting lines

Despite these changes, gear modifications do not always stop bycatch. In March 2006, the Hawaii longline swordfish fishing season was closed due to high loggerhead sea turtle bycatch, even though modified circle hooks were used.

Seabirds get tangled in longlines when they gather around fishing vessels, which can lead to drowning as they try to reach bait. Streamer lines, made of bright polyester rope, have been used to reduce seabird deaths. These lines flap in the water and scare birds away from hooks. In Alaska, streamer lines reduced seabird deaths by about 70% in groundfish longline fisheries.

Retention

Some fisheries keep bycatch instead of throwing it back into the ocean. Sometimes bycatch is sorted and sold as food, especially in Asia, Africa, and Latin America, where labor is less expensive. It can also be sold in frozen bags labeled "assorted seafood" or "seafood medley" at lower prices. Bycatch can be turned into fish hydrolysate (ground fish parts) for use as a soil additive in organic farming or as an ingredient in fish meal. In Southeast Asia, bycatch is sometimes used to make fish sauce. Bycatch is often cleaned, shelled, ground, and mixed into fish paste or shaped into fish cakes (surimi) and sold either fresh for local use or frozen for export. This is common in Asia or among Asian fisheries. Sometimes bycatch is sold to fish farms to feed farmed fish, particularly in Asia. Norway has a "no discards" policy to reduce bycatch. This means fishermen must keep all the fish they catch. This policy has helped encourage research on bycatch, which has helped encourage changes in how fishers behave and reduce waste of life.