UnitI (Stock Concept)POPULATION DYNAMICS

8/7/2019 UnitI (Stock Concept)POPULATION DYNAMICS

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FFBFFB--625625

Fish StockFish Stock

AssessmentAssessment


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Unit 1:

Stock Concept


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PRIMARY OBJECTIVE OFPRIMARY OBJECTIVE OF

FISH STOCK ASSESSMENT:FISH STOCK ASSESSMENT:

TheThe basicbasic purposepurpose ofof fishfish stockstock assessmentassessment

isis toto provideprovide adviceadvice onon thethe optimumoptimum

exploitationexploitation ofof aquaticaquatic livingliving resourcesresources suchsuch asasfishfish && shrimpshrimp.. LivingLiving resourcesresources areare limitedlimited butbut

renewablerenewable && fishfish stockstock assessmentassessment maymay bebe

describeddescribed asas thethe searchsearch forfor exploitationexploitation levellevelwhichwhich inin thethe longlong runrun givesgives thethe maximummaximum yieldyield

inin weightweight fromfrom thethe fisheryfishery..


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ItIt showsshows thatthat upup toto aa certaincertain level level wewe gaingain bybyincreasingincreasing thethe fishingfishing effort,effort, butbut afterafter thatthat level level thethe

renewalrenewal ofof thethe resourcesresources (the(the reproductionreproduction && thethe bodybody

growth)growth) cancan notnot keepkeep pacepace withwith thethe removalremoval causedcaused bybyfishingfishing && aa furtherfurther increaseincrease inin exploitationexploitation levellevel leadsleads

toto aa reductionreduction inin yieldyield.. TheThe fishingfishing efforteffort levellevel whichwhich inin

thethe longlong termterm givesgives thethe highesthighest yieldyield isis indicatedindicated byby

FmsyFmsy && thethe correspondingcorresponding yield yield isis indicated indicated bybyMSYMSY..


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Contii----

Cushing (1968) definesCushing (1968) definesa fish stock as one that has a singlea fish stock as one that has a single

spawning ground to which the adults return year spawning ground tospawning ground to which the adults return year spawning ground to

which the adults return year after year.which the adults return year after year.

Ricker( 1975) definesRicker( 1975) definesa fish stock as the part of a fish populationa fish stock as the part of a fish population

which is under consideration from the point of view of actual orwhich is under consideration from the point of view of actual orpotential utilization.potential utilization.

GullandGulland (1983) defined (1983) defined a subgroup of a species can be treated as aa subgroup of a species can be treated as a

stock if possible differences within the group & interchanges withstock if possible differences within the group & interchanges with

other groups can be ignored without making the conclusions reachedother groups can be ignored without making the conclusions reached

invalid.invalid.This means that it is preferable to start by making stock assessmentThis means that it is preferable to start by making stock assessment

over the entire area of distribution of a species .As long as there areover the entire area of distribution of a species .As long as there are

no indications that separate unit stocks exist in that area.no indications that separate unit stocks exist in that area.


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Contii--

An essential characteristic of a stock is that its growth and mortalityAn essential characteristic of a stock is that its growth and mortalityparameters remain constant throughout its area of distribution.Anparameters remain constant throughout its area of distribution.An

example, partition that area into two parts, sub areas A and B:example, partition that area into two parts, sub areas A and B:

The growth and mortality parameters must be the same in sub areasA andThe growth and mortality parameters must be the same in sub areasA andB or in other words:B or in other words:

The animals in sub areaAmust have the same body growth as theThe animals in sub areaAmust have the same body growth as the

animals in sub area B.animals in sub area B.

The animals in the sub area Amust have the same probability of deathThe animals in the sub area Amust have the same probability of death

as the animals in sub area B.as the animals in sub area B.

If fishing takes place only in sub areaA, it is assumed that eachIf fishing takes place only in sub areaA, it is assumed that each

individual fish in the stock has the same probability of being encounteredindividual fish in the stock has the same probability of being encountered

in sub area A and thereby also that it has the same probability of beingin sub area A and thereby also that it has the same probability of being

caught. The individuals are supposed to move freely between the two subcaught. The individuals are supposed to move freely between the two sub

areas.areas.


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InIn partpart I,I, considerconsider aa fishfish stock,stock, ofof whichwhich thethe distributiondistribution areaarea isis indicatedindicatedbyby aa fullfull lineline.. ItIt isis exploitedexploited byby threethree countries,countries, A,BA,B andand CC.. WeWe looklook atat thethe

stockstock definitiondefinition fromfrom thethe pointpoint ofof viewview ofof thethe islandisland countrycountry CC.. TheThe

brokenbroken lineslines showshow thethe EEZEEZ ofof eacheach countrycountry.. TheThe dotteddotted areaarea indicatesindicates

thethe fishingfishing areaarea ofof countrycountry CC andand thethe hatchedhatched areasareas thosethose ofof countriescountries AA

andand BB.. FromFrom thisthis itit cancan bebe seenseen thatthat ifif countrycountry CC wouldwould basebase itsits

assessmentassessment onon thethe assumptionassumption thatthat thethe unitunit stockstock isis limitedlimited toto itsits ownownfishingfishing area,area, thusthus ignoringignoring thethe fisheriesfisheries ofof countriescountries AA andand b,b, itit isis likelylikely

toto drawdraw wrongwrong conclusionsconclusions.. IfIf forfor ee..gg.. CountriesCountries AA andand BB havehave intensiveintensive

fisheriesfisheries onon thethe stockstock inin questionquestion soso thatthat itit isis overexploitedoverexploited.. ThereThere isis littlelittle

countrycountry CC cancan dodo onon itsits ownown toto improveimprove thethe situationsituation.. FromFrom thethe

assessmentassessment basedbased onon thethe assumptionassumption ofof aa stockstock limitedlimited toto countriescountries CsCswater,water, countrycountry CC maymay concludeconclude thatthat thethe stockstock isis overover exploitedexploited andand itit maymay

introduceintroduce managementmanagement measuresmeasures toto reducereduce fishingfishing.. However,However, thethe

expectedexpected effecteffect ofof thethe managementmanagement measuresmeasures willwill notnot materialize,materialize, ifif

countriescountriesAA andand BB dodo notnot followfollow countrycountry CC..


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InIn partpart II,II, ofof figfig.. 11..22..11 illustratesillustrates thethe casecase wherewhere oneone fisheryfishery

exploitsexploits severalseveral stocksstocks.. InIn thisthis casecase thethe assessmentassessment becomesbecomes

thatthat ofof thethe averageaverage stock,stock, sincesince itit willwill bebe impossibleimpossible toto

separateseparate thethe catchescatches byby stocksstocks.. IfIf thethe fishingfishing efforteffort expendedexpended

isis similarsimilar forfor eacheach stock,stock, thethe resultresult ofof thethe assessmentassessment shouldshould

comecome outout currentlycurrently.. However,However, therethere maymay alsoalso bebe difficultiesdifficulties inin

thisthis casecase.. SupposeSuppose thatthat thethe threethree stocksstocks currentlycurrently fished(fished(11,,22

andand 33)) areare heavilyheavily overfishedoverfished andand thatthat thethe fisheryfishery isis

expandedexpanded toto includeinclude thethe unexploitedunexploited stockstock 44.. inin thatthat casecase thetheaverageaverage catchcatch raterate willwill increaseincrease andand thisthis mightmight leadlead toto wrongwrong

conclusionconclusion regardingregarding thethe statusstatus ofof stocksstocks 11,, 22 andand 33..

UnitI (Stock Concept)POPULATION DYNAMICS

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