Last updated 25 September 2020

By-catch refers to all the unintended marine life caught by commercial fishing vessels. Marine animals often become trapped in the various nets used in large-scale fishing operations.

A dolphin caught in a fishing net. Credit: Greenpeace.

The marine animals worst affected by this are those who are otherwise long-living, and who slow breed. Dolphins, whales, seals, turtles and albatrosses are among the species impacted.

38 million tonnes of by-catch are killed by the fishing industry annually. 40% of all marine life caught by fishing vessels each year is accounted for by by-catch. 

How does this happen?

Fishing nets do not discriminate. Take, for example, tuna and salmon fishing, which commonly utilise purse seine nets; these nets can stretch as long as 1.5km and often over 150 metres deep. Unsurprisingly, many different species of fish get caught in these nets. 

Likewise, when longline fishing methods are used to capture fish such as tuna, cod and swordfish, many non-targeted species are caught and killed. 

Longlines can reach across 130 kilometres long and can carry up to 40,000 hooks along it. 

These hooks capture whichever sea animals take the bait. By-catch species for longline fishing include sea birds (who dive under the water to eat the bait), like albatrosses, as well as sharks, in high numbers.

15 of the 22 species of albatross are facing extinction, and the worst threat they face is longline fisheries.

Does by-catch only occur in large-scale fishing operations?

These fishing methods exist in order to catch as many fish as possible to keep up with the world's ever-growing appetite for seafood. More fish are killed for food every year than any other animal on the planet. 

Although by-catch occurs more commonly and in larger quantities in commercial fishing operations, even in line fishing (which is practised both commercially and recreationally) it can occur. There is no way to control what  species of animals are taking the bait on the end of a hook.

Catch and release fishing is not only painful and incredibly traumatic for fish but can also be a cause of death. This is due to the fact that after being hooked many fish are no longer able to feed or cannot feed as well as previously.