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　　Whenprehistoricmanarrivedinnewpartsoftheworld，somethingstronghappenedtothelargeanimals；theysuddenlybecameextinct.Smallerspeciessurvived，thelarge，slow-growinganimalswereeasygame，andwerequicklyhuntedtoextinction.

　　Nowsomethingsimilarcouldbehappeningintheoceansthattheseasarebeingover-fishedhasbeenknownforyearswhatresearcherssuchasRansomMyersandBorisWormhaveshownisjusthowfastthingsarechanging.Theyhavelookedathalfacenturyofdatafromfisheriesaroundtheworld.Theirmethodsdenotattempttoestimatetheactualbiomass（theamountoflivingbiologicalmatter）offishspeciesinparticularpartsoftheocean，butratherchangesinthatbiomassovertime.AccordingtotheirlatestpaperpublishedinNature，thebiomassoflargepredators（animalsthatkillandeatotheranimals）inanesfisheryisreducedonaverageby80within15yearsofthestartofexploitation.Insomelong-fishedareas，ithashalvedagainsincethenDr.Wormacknowledgesthatthesefiguresareconservative，onereasonforthisisthatfishingtechnologyhasimprovedToday‘svesselscanfindtheirpreyusingsatellitesandsonar，whichwerenotavailable50yearsagothatmeansahigherproportionofwhatisintheseaisbeingcaught，sotherealdifferencebetweenpresentandpastislikelytobeworsethantheonerecordedbychangesincatchsizes.Intheearlydays，too，longlineswouldhavebeenmoresaturatedwithfish.Someindividualswouldthereforenothavebeencaught，sincetobaitedhookswouldhavebeenavailabletotrapthem，leadingtoanunderestimateoffishstocksinthepast.Furthermore，intheearlydaysoflonglinefishing，alotoffishwerelosttosharksaftertheyhadbeenhooked.Thatisnolongeraproblem，becausetherearefewersharksaroundnoise.

　　Dr.MyersandDr.wormarguethattheirworkgivesacorrectbaseline，whichfuturemanagementeffortsmusttakeintoaccount.Theybelievethedatesupportanideacurrentamongmarinebiologists，thatofthe“shiftingbaseline”。Thenotionisthatpeoplehavefailedtodetectthemassivechangeswhichhavehappenedintheoceanbecausetheyhavebeenlookingbackonlyarelativelyshorttimeintothepast.Thatmattersbecausetheorysuggeststhatthemaximumsustainableyieldthatcanbecroppedformafisherycomeswhenthebiomassofatargetspeciesisabout50ofitsoriginallevels.Mostfisheriesarewellbelowthat，whichisabadwaytodebusiness.

　　31、Theextinctionoflargeprehistoricanimalsisnotedtosuggestthat

　　A、largeanimalwerevulnerabletothechangingenvironment

　　B、smallspeciessurvivedaslargeanimalsdisappeared

　　C、largeseaanimalsmayfacethesamethreattoday.

　　D、Slow-growingfishoutlivefast-growingones

　　32、whocaninferformDrMyersandDr.Worm‘spaperthat

　　A、thestockoflargepredatorsinsomeoldfisherieshasreducedby90

　　B、thereareonlyhalfasmanyfisheriesaretherewere15yearsago

　　C、thecatchsizesinnewfisheriesareonly20oftheoriginalamount

　　D、thenumberoflargerpredatorsdroppedfasterinnewfisherishthanintheold

　　33、Bysayingthesefiguresareconservative（linein，paragragf-3），Drwormmeansthat

　　A、fishingtechnologyhasimprovedrapidly

　　B、thencatch-sizesareactuallysmallerthenrecorded

　　C、themarinebiomasshassufferedagreaterloss

　　D、thedatecollectedsofararepitpfdate.

　　34、DrMyersandotherresearchersholdthat