The impact of by-catch on whales and dolphins

The leading factor in the mortality of numerous cetacean species.

The incidental capture (or by-catch) of dolphins and porpoises in fisheries presents one of the most acute threats to cetaceans in many parts of the world, and has been identified as the most serious threat facing the harbour porpoise.

Worldwide attention has been given to the high mortality of dolphins associated with driftnets and certain purse-seine tuna fisheries in the Eastern Tropical Pacific. Other types of fishing gear also endanger cetaceans but have not received as much publicity.

Data from around the world indicate high numbers of dolphins and porpoises are also killed by gill nets and in mid-water trawl fisheries.

Dolphins and porpoises are not the only cetaceans being harmed by fishing gear. Fishing gear can injure and kill large cetaceans as well - even the powerful sperm whales may become entangled in nets and drown, or starve to death if gear becomes wrapped around or embedded in its mouth.

The UN has banned the use of large-scale (defined as greater than 2.5km) driftnets, and the EU has placed a ban on driftnets of any length. These are partial solutions to a problem that is the leading factor in the mortality of numerous cetacean species.

Hector's dolphin calf killed in gillnet. Banks Peninsula, New Zealand. / ©: WWF-Canon / Stephen Dawson
Hector's dolphin calf killed in gillnet. Banks Peninsula, New Zealand.
© WWF-Canon / Stephen Dawson

More information

  • Watch slideshow: Not everything caught in fishing gear makes it to the table.
  • More on bycatch

What are gillnets?

 / ©: Peter Diamond
Gillnet
© Peter Diamond
Gillnets are mesh nets that allow fish to pass their heads and gill coverings through a hole in the mesh and then get stuck on their gill coverings when they try to back out.

When a gillnet is “set” the bottom is weighted to the sea floor by weights and the top is held up by floats creating a wall of net.

These nets are difficult for larger cetaceans to see or detect with echolocation. So they frequently run in to them when swimming or feeding and become tangled in the netting or in the ropes holding the netting.

Gillnets are responsible for the by-catch related deaths of most threatened dolphin and porpoise species.

Fast facts about cetacean by-catch

    • Over 300,000 whales, dolphins, and porpoises are killed each as a result of by-catch. (Based on research in 2003)
    • The World Conservation Union (IUCN) recognizes bycatch as one of the greatest threats to the survival of cetacean populations.

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