Indirect effects

One of the problems with all types of fishing gear is that it always has an incidental or by-catch of non-target species. This may include other commercial and noncommercial fish, seabirds and marine mammals.

Drift or gill nets are of particular concern in this respect. Modern drift nets may extend for several kilometres and are made from monofilament nylon. This makes them virtually invisible to animals using sonar. High seas drift netting was banned in December 1992 by a United Nations resolution, when it was discovered just how huge the incidental catch was. Observers on boats from the Japanese squid fleet counted the animals caught and these figures were extrapolated to give a total for the whole fleet during the whole season, in 1989 and 1990. The totals for 1989 are shown in Table 10.7. When it is considered that Taiwan and Korea run equally large squid fleets, the total effect on the populations of some of the less common by-catch species such as albatross must be very significant.

Drift nets continue to be used, albeit on a much smaller scale, and have been nicknamed 'walls of death'. When drift nets are lost, they may continue to kill many animals by 'ghost fishing'. This is the term used for the vicious cycle whereby animals are caught in the net which eventually sinks under the weight of catch. The dead animals then rot away or are eaten and the net rises up into the water to start fishing all over again. The nylon net itself will not rot for many years.

Tuna fisheries have been a particular problem for dolphins especially in the eastern tropical Pacific. Estimates suggest that at least 6 million dolphins have

Table 10.7 Estimated catch and main incidental catch of the Japanese squid fishing fleet in 1989.

Type of animal

Numbers caught (thousands)

Squid (target species)

70 000

Other fish:


32 000



Blue sharks

1 200

Skipjack tuna





Shearwaters (various spp.)





Northern fur seals





been killed there in the past 30 years. Dolphins follow schools of yellowfin tuna and fishermen use boats, helicopters and light aircraft to spot the dolphins. The fishermen then herd the tuna using small speedboats and the main fishing boat encircles and catches them using large seine nets (see Section 9.1.3). Unfortunately many dolphins are also caught in the nets and drown. New nets have now been developed which have a fine-mesh area called the Medina panel. As the net tightens, the dolphins can jump over the net rim because the panel makes it visible to them. Sometimes divers and small boats are used to help the dolphins out.

As a result of public pressure, tuna fishing using drift nets has now mostly stopped in the eastern tropical Pacific and some fishing fleets are using so-called 'dolphin friendly' fishing methods. These include longlines and purse seining of tuna schools not associated with dolphins. Longlines are also used for catching other species such as squid and have proved a danger to some seabirds, especially albatrosses which are attracted to the lines and swallow the baited hooks.

Turtles are vulnerable to accidental capture in a variety of nets and trawls. Shrimp trawls in the Gulf of Mexico kill many Kemp's ridley turtles, an endangered species, each year. The Fisheries Service there invented a turtle exclusion device which, when attached to the trawl, allowed any turtles caught to escape through a special trap door. However, the device was fiercely rejected by the shrimp fishermen and is little used.

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