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

Nowsomethingsimilarcouldbehappeningintheoceans.Thattheseasarebeingover-fishedhasbeenknownforyearsandresearcherssuchasRansomMyersandBorisWormhaveshownhowfastthingsarechanging.Theyhavelookedathalfacenturyofdatafromfisheriesaroundtheworld.Theirmethodsdonotattempttoestimatetheactualbiomass(theamountoflivingbiologicalmatter)offishspeciesinparticularpartsoftheocean,butratherchangesinthatbiomassovertime.AccordingtotheirlatestpaperpublishedinNature,thebiomassoflargepredators(animalsthatkillandeatotheranimals)inanesfisheryisreducedonaverageby80%within15yearsofthestartofexploitation.Insomelong-fishedareas,ithashalvedagainsincethen.

Dr.Wormacknowledgesthatthesefiguresareconservative(保守的).Onereasonforthisisthatfishingtechnologyhasimproved.Today’svesselscanfindtheirpreyusingsatellitesandsonar,whichwerenotavailable50yearsago.Thatmeansahigherproportionofwhatisintheseaisbeingcaught,sotherealdifferencebetweenpresentandpastislikelytobeworsethantheonerecordedbychangesincatchsizes.Intheearlydays,too,longlineswouldhavebeenmoresaturatedwithfish.Someindividualswouldthereforenothavebeencaught,sincebaitedhookswouldhavebeenavailabletotrapthem,leadingtoanunderestimateoffishstocksinthepast.Furthermore,intheearlydaysoflonglinefishing,alotoffishwerelosttosharksaftertheyhadbeenhooked.Thatisnolongeraproblem,becausetherearefewersharksaroundnoise.

Dr.MyersandDr.Wormarguethattheirworkgivesacorrectbaseline,whichfuturemanagementeffortsmusttakeintoaccount.Theybelievethedatesupportanideacurrentamongmarinebiologists,thatofthe“shiftingbaseline”.Thenotionisthatpeoplehavefailedtodetectthemassivechangeswhichhavehappenedintheoceanbecausetheyhavebeenlookingbackonlyarelativelyshorttimeintothepast.Thatmattersbecausetheorysuggeststhatthemaximumsustainableyieldtha