Whenprehistoricmanarrivedinnewpartsoftheworldsomethingstronghappenedtothelargeanimals;theysuddenlybecameextinct.Smallerspeciessurvived;thelargeslow-growinganimalswereeasygameandwerequicklyhuntedtoextinction.

Nowsomethingsimilarcouldbehappeningintheoceans.Thattheseasarebeingover-fishedhasbeenknownforyearsandresearcherssuchasRansomMyersandBorisWormhaveshownhowfastthingsarechanging.Theyhavelookedathalfacenturyofdatafromfisheriesaroundtheworld.Theirmethodsdonotattempttoestimatetheactualbiomass(theamountoflivingbiologicalmatter)offishspeciesinparticularpartsoftheoceanbutratherchangesinthatbiomassovertime.AccordingtotheirlatestpaperpublishedinNaturethebiomassoflargepredators(animalsthatkillandeatotheranimals)inanesfisheryisreducedonaverageby80%within15yearsofthestartofexploitation.Insomelong-fishedareasithashalvedagainsincethen.

Dr.Wormacknowledgesthatthesefiguresareconservative(保守的).Onereasonforthisisthatfishingtechnologyhasimproved.Today’svesselscanfindtheirpreyusingsatellitesandsonarwhichwerenotavailable50yearsago.Thatmeansahigherproportionofwhatisintheseaisbeingcaughtsotherealdifferencebetweenpresentandpastislikelytobeworsethantheonerecordedbychangesincatchsizes.Intheearlydaystoolonglineswouldhavebeenmoresaturatedwithfish.Someindividualswouldthereforenothavebeencaughtsincebaitedhookswouldhavebeenavailabletotrapthemleadingtoanunderestimateoffishstocksinthepast.Furthermoreintheearlydaysoflonglinefishingalotoffishwerelosttosharksaftertheyhadbeenhooked.Thatisnolongeraproblembecausetherearefewersharksaroundnoise.

Dr.MyersandDr.Wormarguethattheirworkgivesacorrectbaselinewhichfuturemanagementeffortsmusttakeintoaccount.Theybelievethedatesupportanideacurrentamongmarinebiologiststhatofthe“shiftingbaseline”.Thenotionisthatpeoplehavefailedtodetectthemassivechangeswhichhavehappenedintheoceanbecausetheyhavebeenlookingbackonlyarelativelyshorttimeintothepast.Thatmattersbecausetheorysuggeststhatthemaximumsustainableyieldthatcanbecroppedfromafisherycomes