What Is Bycatch and How Does It Affect Marine Life?

Turtles, sharks, marine mammals, and birds are all at risk of becoming bycatch.

A person holding up a fishing net with fish attached.

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Bycatch is a term used in the fishing industry for animals unintentionally caught while fishers target other marine species. Bycatch includes both the animals that are caught and released and the animals that are accidentally killed through fishing operations.

While fish, marine mammals, and seabirds can all be caught as bycatch, certain marine animals are far more likely to end up in fishing nets by mistake. A variety of regulations are used today to reduce the amount of bycatch captured while fishing, but certain marine animals still end up as bycatch at dangerous rates.

How Bycatch Affects Marine Animals

While any marine animal can be mistakenly caught as bycatch, certain species are more susceptible to becoming bycatch based on where they live, what they eat, and their ability to escape from nets.

Marine Mammals

Marine mammals populations are among those most affected by bycatch. In fact, research suggests that bycatch is far more deadly to marine mammals than any other human activity.

Since marine mammals need to breathe air at the surface, they are particularly susceptible to drowning in fishing nets. Marine mammals can also become bycatch because of their associations with species being targeted by fishers.

For example, certain dolphin species in the Eastern Tropical Pacific Ocean tend to swim above schools of yellowfin tuna. To increase their chances of catching yellowfin, fishers will set their nets around the dolphins. Unsurprisingly, fishing methods that intentionally seek out marine mammals substantially increase the number of mammals mistakenly caught as bycatch.

At a population level, marine mammals are also particularly sensitive to bycatch because of how long it takes for populations to rebuild. Like humans, marine mammals can live a long time but only produce a few offspring a year. If too many marine mammals are killed by fishing operations, populations may be unable to reproduce fast enough to keep pace with these losses.

Turtles

Bycatch is considered to be one of the greatest threats to sea turtle populations around the world.

Turtles are susceptible to ending up as bycatch for many of the same reasons as marine mammals. Like marine mammals, sea turtles must reach the surface to breathe. Sadly, the need to breathe air makes sea turtles susceptible to drowning in nets.

While sea turtles are also caught as bycatch by longlines, research shows sea turtles are killed far more often by nets and trawls.

Seabirds

Seabirds are also at risk of being unintentionally caught in fishing gear. Many seabirds are attracted to fishing vessels by the presence of fish; to them, a fishing vessel can look like a great place to get an easy meal. Unfortunately, these interactions can be deadly.

Seabirds are particularly at risk of becoming bycatch from the use of long lines. In the process of adding bait to the hooks of a longline, birds get caught on the hooks and then dragged underwater when the line is set, ultimately causing the birds to drown. Albatross, cormorants, loons, puffins, and gulls are all seabirds susceptible to becoming bycatch.

Seabirds flocking to a net full of fish as it's being pulled out of the water.
To seabirds, fishing vessels look like an opportunity for an easy meal.

Jeff J Mitchell / Getty Images

Bycatch Prevention

Managing the impacts of bycatch is particularly challenging due to a lack of data and high levels of uncertainty.

Most information on bycatch comes from fisheries observers. However, the frequency of bycatch captured in observer data inevitably underestimates bycatch's true impact because observers can only account for animals captured as bycatch that make it to the surface.

Presumably, additional animals are caught by fishing gear but escape before reaching the surface. These escapees go undetected by fisheries observers, yet contribute to bycatch's toll on marine animals.

Fishing Gear

Many fisheries have mandated fishing operations and use specialized fishing gear that is known to reduce rates of bycatch. For example, U.S. regulations require the use of "turtle excluder devices", or TEDs, by fishers using trawl nets to pursue shrimp and summer flounder. Other regulations, like California's Drift Gill Net Transition Program, incentivize the use of safer equipment,

Fishing Locations

Fishery managers can also reduce the likelihood of fishers setting nets in areas full of susceptible marine animals by restricting fishing operations in certain locations. Depending on the circumstances, access to certain fishing locations can be permanent, like with certain marine protected areas, or temporarily instated when a certain level of bycatch is reached in a given fishing season.

Timing

Fisheries can also be managed to only operate during certain times of the year to avoid periods when non-target species are abundant. For example, U.S. fisheries managers have mandated seasonal closures of the swordfish fishery to reduce sea turtle bycatch.

Similarly, efforts are underway to reduce seabird bycatch by requiring fishers to set longlines at night, reducing the chance seabirds will be around to interact with the fishing gear.