1 II-Main scientific and management results expected from the tagging programme 1) Stock structure and migrations 2) Tuna growth 3) Natural mortality as a function of age and sex 4) Behaviour of tunas as a function of their environment 5) Size of stocks and fishing mortality 6) Impact of FADs on the resource 7) Interactions between fisheries

2 Various types of tags will be used n Large numbers of dart tags (100,000+) will be used to estimate mortality and growth rates, stock structure and interactions between fisheries n Small numbers of archival tags, internal & pop up (a few hundred) will be used to tag large tunas (yellowfin and bigeye) in various areas Dart tag Archival tag Pop up tag In tagging, fish are released alive after a tag has been attached. With dart and internal archival tags, the fish must be recaptured. Pop-up tags release themselves automatically and transmit stored data by satellite.

3 Dart tags for stock structure, movement and growth n Many fish are tagged and the tag information is given to IOTC if a fish is recaptured n The location of tagging and of recapture gives movement and stock structure n The fish are measured when tagged and when recaptured, providing growth n When tagged fish are injected with a chemical marker, the number of growth rings in the otoliths between tagging and recapture permit ageing n Attrition with time gives an estimate of natural mortality

4 Tagging will provide knowledge of stock structure of Indian Ocean tropical tunas and of their movement patterns, as illustrated by dart tag tagging in the Pacific ocean (taken from Hampton and Fournier 2000, SPC, Secretariat of the Pacific Community) Stock structure and migration

5 Growth Tagging is a very efficient and reliable way allowing to estimate the growth of tunas as a function of their sex and of the fishing zone (tropical or equatorial)

6 Example of the two stanza Atlantic yellowfin growth shown by tagging results Example of the Indian Ocean skipjack growth from tagging in Maldives (Adam 2000) Modeling of growth using recovery data

7 The tagging program will deliver estimates of natural mortality as a function of age and sex n Natural mortality (from causes other than fishing) tends to vary as a function of age n Stock assessment are highly dependent on estimates of natural mortality Age M Natural mortality

8 Pop up - Archival tags independent of fisheries Dart tags suggest east to west and west to east transatlantic movements Pop up tags placed on giant bluefin around Cape Cod were popped up in an area where bluefin was never fished before (is this a spawning zone?) Small numbers can provide understanding of movement patterns; movement patterns suggested for the Atlantic bluefin by dart tags and by pop up tags are quite different

9 Archival tags show both the geographical migrations and vertical movements of tunas (taken from Kurt Schaeffer 2000, IATTC, Eastern Pacific Ocean) thermocline night Tuna behaviour

10 Stock size and fishing mortality n Large scale tagging programmes permit estimation of stock size (biomass) and then of fishing mortality (F) n Tagging is the best method available to estimate these parameters in a reliable way for tunas Age F

11 Tagging and FADs There are serious concerns about the long term negative impact of FAD fishing on the resources since: => FADs may modify the behaviour of tunas: “biological trap” hypothesis => they increase fishing mortality of juvenile tunas (bigeye, yellowfin) => they increase the mortality of various by-catch species. => FAD catches are increasing in the Indian Ocean and now produce more than 70% of the total purse seine catch Natural logs Artificial FADs % of FAD associated catches

12 FAD = biological trap ? A triple cascading hypothesis Hypothesis 1 : the aggregation of small size tunas ( cm) under FADs is a rapid, strong and long-lasting process Hypothesis 2 : the large numbers of floating objects placed in the equatorial zone can alter the natural migration of these tunas Hypothesis 3 : FADs negatively affect growth and natural mortality of small tunas associated to FADs as access to food may be compromised and small tunas may be more exposed to predation

13 FADs will be widely used when tagging…. ….this is an easy way to tag tunas! => Dart, acoustic & archival tagging  Comparison of tuna migration of FAD-associated and free school tuna  Comparison of growth rates of FAD and free school tunas  Assessment of the fidelity of tunas to FADs in the short and medium term What tagging should be done on FADs?

14 Tagging results are essential to model the exploitation of migratory stocks. Traditional assessment models: Even simple production models need a good hypothesis concerning stock structure New statistical models used to evaluate stock status (for instance MULTIFAN CL developed by the SPC) are based upon large scale tagging. Tagging results are essential to estimate most of the basic parameters

15 III-What major management questions can be answered by a tagging programme? a) Interaction between fisheries: purse seine, artisanal, longline b) Sustainability of present large scale FAD fisheries c) Realistic modelling of resources tropical tunas which are migratory and are exploited by multi-gear fisheries

16 Interactions between fisheries? This is a key issue for coastal countries, and a key responsibility for industrialized fishing countries # The potential of interaction between large scale industrialized fleets and artisanal fleets remains unknown # The potential interaction between purse seine and longline catches is also unknown

17 # Catches of skipjack are steadily increasing for both the artisanal and purse seine fisheries. # A major pending question: what potential interaction is there between purse seiners and artisanal fisheries? # Interaction depends mainly on stock biomass, natural mortality and movement patterns A potential interaction: Skipjack tuna Movements and catch locations Artisanal Purse seine Yearly catches

18 Bigeye (Thunnus obesus): potential interaction between purse seiners & longliners Purse catches during recent years: small bigeye Longline catches: large bigeye Yearly catches in weight of bigeye by purse seiners and longliners The potential effect of increased catches of small bigeye by purse seiners on the adult stock, exploited by longliners, remains unknown. Longline Purse seine

19 Tagging would permit evaluation of this interaction Age: Since 1993, much larger numbers of small bigeye are taken by purse seiners than the number of large bigeye taken by longliners Bigeye taken by purse seiners are much smaller (average 6kg) than than bigeye taken by longliners (average about 40kg) FL 1 Catches in numbers