Overfishing of one of the ocean’ s top predators has triggered a lethal chain reaction that threatens to decimate populations of bay scallops, oysters, and clams, warns a new study, e Plunging populations of11 large shark species have caused a spike in the numbers of their prey, including smaller sharks, rays and skates．These creatures in turn are wiping out other marine organisms and may be destroying sea grass habitat, which serves as a nursery for many species．
　　 Sharks have had a rough few decades．Demand for their fins and meat has resulted in increasin exploitation, and the creatures are often accidentally captured by swordfish and tuna fishers, As man as 73 million sharks are killed annually, and past studies have indicated many populations have been ct by half．Nonetheless, scientists have had difficulty gauging the decline in several shark species over the years because of a lack of data．The effect of this decline on ocean ecosystems has also proven hard t measure．
　　 To get a better grip on the problem’, marine biologist Julia Baum of Dalhousie University in Halifax Canada, and colleagues trawled 35 years of marine surveys and fisheries data to document declines i large sharks off the eastern seaboard of the U．S．The numbers, reported today in Science, are stark The blacktip shark population has plummeted 93% since 1972, with similar declines for tiger, bull, an smooth hammerhead sharks．
　　 Simultaneously, the abundance of 12 of the 14 species that these sharks prey on--including the litt] skate, the chain catshark, and the cownose ray--increased dramatically．The latter has seen its numbers swell ten-fold．The jump in ray numbers is especially troubling, notes co-author and marine biologist Charles Peterson of the University of North Carolina at Chapel Hill, because the fish shred the sea grass that houses crabs and clams．In fact, he notes, by 2004 ray predation had shut down Nort Carolina’s century-old bay scallop fishery．
　　 The 11 other shark prey species whose populations rose over the 35-year period could be havin similar impacts to that of the cownose ray, says Baum．
　　 The result is profound because the high diversity of fishes in the northwest Atlantic has bee thought to minimize catastrophic changes in the food web, says marine scientist Kenneth Frank of the Bedford Institute of Oceanography in Dartmouth, Canada．Sharks are only part of the puzzle, however Frank warns．People’s appetites for oysters, clams, and scallops are likely having a more direct impact on these populations, he says．[414 words]
　　 1．The overfishing of sharks may result in______
　　 A．the abundance of sea grass habitat
　　 B．the ruin of nursery for many marine species
　　 C．the obvious increase of most marine species
　　 D．the huge decline of the populations of their prey
　　 2．What contributes most to the overfishing of sharks is______
　　 A．the sharks accidentally captured by tuna fishers
　　 B．the rough few decades that sharks have suffered
　　 C．the ever-increasing demand for their fins and meat
　　 D．the millions of sharks killed deliberately each year
　　 3．Julia Baum studied marine surveys and fisheries data in order to______
　　 A．prove the fact that shark populations have fallen sharply
　　 B．cast light on the effect of shark decline on ocean ecosystrems
　　 C．document the shark species which have declined most rapidly
　　 D．understand the subtle relationship between predators and prey
　　 4．The rapid increase of the cownose ray results from______
　　 A．the ten-fold increase of its numbers in a short time
　　 B．the abundance of the 12 species that sharks prey on
　　 C．the tremendous decline of the number of its predators
　　 D．the decline of sharks off the eastern seaboard of America
　　 5．The populations of bay scallops, oysters, and clams are threatened directly by______.examda.
　　 A．the high diversity of fishes in the northwest Atlantic
　　 B．the catastrophic changes in the food web in the Atlantic
　　 C．the lethal reaction triggered by the overfishing of sharks
　　 D．the strong desire of many people for them as their delicious food

　难句透析
　　①Overfishing of one of the ocean’S top predators has triggered a lethal chain reaction Ethat threatens to decimate populations of bay scallops，oysters，and clams]，warns a new study．
　　【结构】“warns a new study”是本句的主句。“Oveffishing of…clams”是“warns”的宾语从句；其中的主语是“0ver fishin9”，谓语动词是“has triggered”。方括号所标示的“that threatens…clams”是“reaction”的后置定语从句。
　　【释义】一项新的研究成果警告说，对于一种海洋顶级食肉动物的过量捕捞已经引起了一串致命的连锁反应，可能将把绝大部分海湾扇贝、牡蛎、蛤蜊灭掉。
　　 ②”Demand for their fins and meat has resulted in increasing exploitation，and”the creatures are often accidentally captured by swordfish and tuna fishers．