Fishing may have various negative effects on the environment: effluent and waste from fish farms may damage wild fish, seals, and shellfish. Fish farmers use tiny quantities of highly toxic chemicals to kill lice: one overdose could be devastating. So-called by-catches, or the incidental taking of non-commercial species in drift nets, trawling operations and long line fishing is responsible for the death of large marine animals and one factor in the threatened extinction of some species. Some fishing techniques, like the drift nets, yield not only tons of fish but kill millions of birds, whales and seals and catch millions of fish not intended. Small net holes often capture juvenile fish who never have a chance to reproduce. Some forms of equipment destroy natural habitats, for example bottom trawling may destroy natural reefs. Other destructive techniques are illegal dynamite and cyanide fishing.
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