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Whenprehistoricmanarrivedinnewpartsoftheworld,somethingstr angehappenedtothelargeanimals.Theysuddenlybecameextinct.Smallerspeciessurvived.Thelarge,slow-growinganimalswereeasygame,andwerequicklyhuntedtoextinction.Nowsomethingsimilarcouldbehappeningintheoceans.
Thattheseasarebeingoverfishedhasbeenknownforyears.Whatr esearcherssuchasRansomMyersandBorisWormhaveshownisjusthowfastthingsarechanging.Theyhavelookedathalfacenturyofdatafromfisheriesaroundtheworld.Theirmethodsdonotattempttoestimatetheactualbiomass(theamountoflivingbiologicalmatter)offishspeciesinparticularpartsoftheocean,butr atherchangesinthatbiomassovertime.AccordingtotheirlatestpaperpublishedinNature,thebiomassoflargepredators(animalsthatkillandeatotheranimals)inanewfisheryisreducedonaverageby80%within15yearsofthestartofexploitation.Insomelong-fishedareas,ithashalvedagainsincethen.
Dr.Wormacknowledgesthatthefiguresareconservative.Onereasonforthisisthatfishingtechnologyhasimproved.Today’svesselscanfindtheirpreyusingsatellitesandsonar,whichwerenotavailable50yearsago.Thatmeansahigherproportionofwhatisintheseaisbeingcaught,sotherealdifferencebetweenpresentandpastislikelytobeworsethantheonerecordedbychangesincatchsizes.Intheearlydays,too,longlineswouldhavebeenmoresaturatedwithfish.Someindividualswouldthereforenothavebeencaught,sincenobaitedhookswouldhavebeenavailabletotr apthem,leadingtoanunderestimateoffishstocksinthepast.Furthermore,intheearlydaysoflonglinefishing,alotoffishwerelosttosharksaftertheyhadbeenhooked.Thatisnolongeraproblem,becausetherearefewersharksaroundnow.
Dr.MyersandDr.Wormarguethattheirworkgivesacorrectbaseline,whichfuturemanagementeffortsmusttakeintoaccount.Theybelievethedatasupportanideacurrentamongmarinebiologists,thatofthe“shiftingbaseline.”Thenotionisthatpeoplehavefailedtodetectthemassivechangeswhichhavehappenedintheoceanbecausetheyhavebeenlookingbackonlyarelativelyshorttimeintothepast.Thatmattersbecausetheorysuggeststhatthemaximumsustainableyieldthatcanbecroppedfromafisherycomeswhenthebiomassofatargetspeciesisabout50%ofitsoriginallevels.Mostfisheriesarewellbelowthat,whichisabadwaytodobusiness.
31.Theextinctionoflargeprehistoricanimalsisnotedtosuggestthat________.
[A]largeanimalwerevulnerabletothechangingenvironment
[B]smallspeciessurvivedaslargeanimalsdisappeared
[C]largeseaanimalsmayfacethesamethreattoday
[D]slow-growingfishoutlivefast-growingones
正确答案：C
