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. Millions of fish may be killed because of being caught by lost or abandoned fishing equipment. Some forms of equipment destroy natural habitats, for example bottom trawling may destroy natural reefs. Other destructive techniques are illegal dynamite and sodium cyanide fishing that kill or stun reef fish so that they can be caught quickly in large quantities (the latter killing coral reefs and few months later also the captured goldfish).
In purse-seine fishing, schools of dolphins are first spotted, indicating to fishermen that schools of tuna are most likely swimming beneath them. The dolphin schools are then chased by small high-speed boats or even helicopters that accompany the fishing boats. When the dolphins begin to tire, the fishermen encircle the school with huge nylon nets that are up to one mile long and 325 feet deep. When both the dolphins and the tuna have been completely surrounded, the bottom of the net is pulled closed, much like a drawstring purse, hence the name purse-seining. Purse-seining has proven to be an extremely effective method of catching fish. Entire schools of tuna are able to be scooped up without a single fish escaping. Unfortunately, many dolphins are also killed in the process, as they become entangled in the nets and drown, or are crushed as the nets are pursed and hauled in.
Every year, approximately 20,000 dolphins are believed to die in purse-seine nets. The method of fishing is especially predominant in the Eastern Tropical Pacific Ocean, a region which extends from southern California to northern Chile.The primary species of dolphins that are affected include spotted, spinner and common dolphins. Purse-seining is believed to have dramatically reduced populations of these species.
The use of long lines, that spare the dolphin but kill wandering albatrosses, increases alarmingly. In 1955, Japanese ships catching tuna cast only 20,000 hooks a year into the waters off New Zealand. By 1987 this had risen to 100 million hooks annually. Several 1995 studies showed that the boats around New Zealand alone killed an average of five albatrosses a day. In 1995, one black-browed albatross is killed for every 100 hooks deployed on long lines used for tuna fishing off Uruguay. The long lines were also blamed for a 50 per cent decline in southern giant petrels at Heard Island in the southern extremities of the Indian Ocean.
According to a 1995 report, the groupers, humphead wrasse and other fish taken from coral reefs in the southwestern Pacific Ocean and held in restaurant aquariums were commonly captured by divers who squirt sodium cyanide at them. The chemical – used to execute criminals – merely stuns the fish and is not toxic to people in the dose commonly used for fishing. But scientists have determined that the dose is more than enough to kill the sensitive corals that build the rich reef habitat. According to a 1997 report, hundreds of tons of cyanide was being pumped each year into coral reefs in Southeast Asia, and the annual volume of reef fish caught live in Southeast Asia and the Western Pacific and sold to seafood restaurants in the region was between 11,000 tons and 16,000 tons, worth at least several hundred million dollars.