Bycatch usually refers to fish that are caught by mistake (see also seabird bycatch). It consists of anything other than the intended catch and is either unwanted or commercially unusable. Fishing boats throw bycatch back into the sea. Bycatch can include turtles, dolphins and seabirds that are also caught unintentionally. In many cases, the animals are dead or dying when they are discarded. On average, 27 million tons of unwanted fish is thrown back every year. A quarter of all fish pulled from the sea never makes it to market.

In the last decade, tuna fishing has often made headlines because of the dolphins killed in purse seine nets. While dolphins are attractive animals that capture our imagination, and it’s important to preserve them, tuna bycatch is a much more significant problem for other species. Over a million sharks die each year because of tuna fisheries, hundreds of thousands of wahoo, dorado, thousands of marlin and many mola mola meet the same fate. The resulting reduction in numbers of such major predators has a huge environmental impact that’s often overlooked. Scientists are only just starting to assess the scale of the problem.

Bycatch is also a major factor in the decline of turtle populations. Turtles get caught up in nets and they may be attracted to chemical light sticks attached to longline hooks because they look like jellyfish. This is a particular threat to leatherback turtles in the Pacific.

Shrimp fisheries have the world's highest bycatch rate. About 5kg of marine life is killed for each kilo of shrimp harvested. An estimated 11 million tons of finfish is discarded each year in shrimp trawl fisheries and the nets used to dredge the sea floor can be highly damaging to a number of marine habitats.

People are now starting to look at ways of farming fish instead of catching them, but this also has its drawbacks. Although the farmed shrimp industry creates less bycatch, it has been responsible for massive mangrove destruction in East Asia (see mangroves).