Environmental Problems, Their Causes, and Sustainability 1

FishShellfishDemersal(mostly bottom dwelling)Pelagic(surface dwelling)CrustaceansMollusksHakeSardine Anchovy KrillOysterClamHerringShrimpHaddockMackerelLobsterOctopusCodTunaCrabSquidFigure 14-23 Some major types of commercially harvested marine fish and shellfish.endangered and threatened sea turtles. Only the largefish are kept. Most of the fish and other aquaticspecies—called bycatch—are thrown back into theocean dead or dying.Another method, purse-seine fishing, involvescatching surface-dwelling species such as tuna, mackerel,anchovies, and herring, which tend to feed inschools near the surface or in shallow areas. After locatinga school the fishing vessel surrounds it with alarge net called a purse seine. Then they close the netlike a drawstring purse to trap the fish. Nets used tocapture yellowfin tuna in the eastern tropical PacificOcean have killed large numbers of dolphins thatswim on the surface above schools of tuna.Fishing vessels also use longlining. It involvesputting out lines up to 130 kilometers (80 miles) long,hung with thousands of baited hooks. The depth of thelines can be adjusted to catch open-ocean fish speciessuch as swordfish, tuna, and sharks or bottom fishessuch as halibut and cod. Longlines also hook endangeredsea turtles, sea-feeding albatross birds, and pilotwhales and dolphins.With drift-net fishing, fish are caught by huge driftingnets that can hang as much as 15 meters (50 feet)below the surface and be up to 64 kilometers (40 miles)long. This method can lead to overfishing of the desiredspecies and may trap and kill large quantities ofunwanted fish and marine mammals (such as dolphins,porpoises, and seals), marine turtles, andseabirds. Since 1992, a UN ban on the use of drift netslonger than 2.5 kilometers (1.6 miles) in internationalwaters has sharply reduced use of this technique. Butlonger nets continue to be used because compliance isvoluntary and it is difficult to monitor fishing fleetsover vast ocean areas. Also, the decrease in drift netshas led to increased use of longlines, which often havesimilar effects on marine wildlife.Figure 14-25 shows the effects of these now commonefforts to increase the seafood harvest. After increasingfourfold between 1960 and 1982, the annualcommercial fish catch (marine plus freshwater harvestbut excluding aquaculture) has declined and leveledoff (Figure 14-25, left). After doubling between 1950and 1956, the per capita catch leveled off until 1980and since then has been declining (Figure 14-25, right)and may continue to decline because of overfishing,pollution, habitat loss, and population growth.Connections: How Are Overfishing andHabitat Degradation Affecting FishHarvests? Dropping YieldsAbout three-fourths of the world’s commerciallyvaluable marine fish species are overfished or fishedat their biological limit.Fish are renewable resources as long as the annual harvestleaves enough breeding stock to renew the speciesfor the next year. Overfishing is the taking of so manyfish that too little breeding stock is left to maintainnumbers.Prolonged overfishing leads to commercial extinction,when the population of a species declines to thepoint at which it is no longer profitable to hunt forthem. Fishing fleets then move to a new species or a298 CHAPTER 14 Food and Soil Resources