Jean-Marc Fromentin. Atlantic bluefin tuna Some issues related to stock delimitation

Transcription

1 Jean-Marc Fromentin Atlantic bluefin tuna Some issues related to stock delimitation

2 Three connected issues Complex population dynamics Resource shared by many countries/fisheries Stock delimitation

3 Complex Population Dynamics Maximum size > 3m and up to 7kg Longevity 2 to 4 years Maturity East ~4 years (~11cm, 35kg) West ~8 years (~19cm, 13kg) Fecundity is related to age and weight (up to 45,, eggs/ female) Thermoregulation (environment: 3 C to 3 C; body T ~25 C) Large spatial distribution: North Atlantic and Mediterranean Pelagic species (mainly in surface but performing deep dives) Highly migratory fish (straight-line distance of 77 km within 5 days, mean speed ~ 6 km/h up to 3 km/h), but migration patterns still unclear

4 Complex Population Dynamics Time (month) 1 12 In comparison to other tunas; BFT Distance (1 n.miles) Large feeding zones, primarily cold to temperate waters Small spawning zones & short spawning season (in warm waters >24 ) spawning occurs in a small spatial and temporal window Spawners of bluefin tunas are at low average density in the North Atlantic: a relatively low biomass (probably < 4 millions fishes) in a wide habitat of about 34 millions nautical squares, about 12 fish per 1 miles² This characteristic probably implies a homing behaviour: adults bluefin probably need to «remember» time and area of birth and be able to migrate in due time towards this strata

5 Complex Population Dynamics Mysterious past events What s happen to the Nordic BFT? For Tiews (1978), Marsac (1999) and Pusineri Yields (tonnes) Sweden Denmark Germany Norway et al. (22) more likely related to changes in migration patterns Large catches ~ 15 years in a an unusual area 2 8 How do we explain the Brazilian longline fishery? Still no satisfying hypothesis Brazil 1-1 Catch (number) years later Large catches ~ 5 years in a an unexpected area Why pop-off of tags East of the Bermuda triangle?

6 Complex Population Dynamics European programme STROMBOLI Collection of 11 series of trap catches Favignana - Formica (Sicilia-Italy) Dominant secular cycles (11-12 yr) South Sardinia (Italy) Synchronous long-term fluctuations all around the western Med. and near North Atlantic Changes in catchability unlikely : variations in recruitment or migrations??? BFT long-term fluctuations are strongly related to those in temperature Number of Tuna caught Changes in migration patterns is the most Medo das Casas (Portugal) likely hypothesis (variations in homing behavior) 4. Sidi - Daoud (Tunisia)

7 Complex Population Dynamics The lack/difficulty of observations make the migration patterns partially (poorly?) known qualitatively and consequently vaguely estimated quantitatively This precise quantification of movement is, however, essential for stock assessment and management Major uncertainties concern: (1) Spatial and temporal variations in migration patterns (2) Site fidelity: Is BFT homing behaviour strict (such as salmon)? (3) Proportion of spawners doing yearly migrations and this being permanent in a given area (e.g., Western vs Eastern Mediterranean, East vs West Atlantic)

8 Resource shared by many countries/fisheries Long fishery history Exploitation in the Mediterranean since the Antiquity (beach seine and hand line) Traps: Important fisheries from the XVII th to mid-xx th century A bait boat fishery since the mid- XIX th century in the Bay of Biscay Purse seiners and long liners about a century later, firstly in the East and West Atlantic, then in the Mediterranean (traps became then secondary)

9 Resource shared by many countries/fisheries Estimation from historical trap catches (STROMBOLI): ~11, BFT/yr (+/- 5,) Catches (Number of BFT) Reference period: ~15, tonnes/yr [7,-25,] Portugal Spain Morocco Tunisia Sicily Sardinia 6 45 In the 199s, catches exceeded the historical level of 5, tons/year

10 Resource shared by many countries/fisheries In a context of high value Medium to low value until the development of deep freezing and transport by planes Since the 198s, Atlantic BFT is imported by Japan (sushi and sashimi market) In 21, a single bluefin tuna has been sold 145,US$ at the Tokyo market Steep increase of «farmed tuna»in recent years This high value is now a major threat for BFT fishery because it allows to fish BFT even at low catch rates $ /k g Mean Price at Tokyo market An Conflicts of interest between various fishing countries

11 Resource shared by many countries/fisheries Large uncertainties in the official (ICCAT) catch database Poor reporting of non-iccat members Illegal fishing (under-sized and flag of convenience) Under-reporting of ICCAT members due to quotas and high value Total East Atlantic BFT yields Before 1998 revision After 1998 revision Difference of ~2% What are the real recent yields? Loss of information due to the caging system (number, size, weight and origin of fish going into cages)

12 Resource shared by many countries/fisheries Difficult to get scientific survey FRANCE Montpellier information (STROMBOLI) Marseille Aerial surveys appear unsuitable to compute fishery-independent indices of abundance of BFT spawners in the Mediterranean Sea. More encouraging on BFT juveniles, but would imply SPAIN Gulf of Lions 2 to: Reduction of observation errors FRANCE Montpellier Description of the BFT habitat Marseille Understanding of BFT spatial Dynamics Good and trustworthy fisheries data are indispensable SPAIN Gulf of Lions 21

13 Stock delimitation Population = For ecologists, it is simply a group of individuals of one species in an area. For geneticists, it is the group of individuals which are close enough to each other for there to be at least occasional mating between them (Begon et al. 1996). Stock = A unit stock is an arbitrary collection of populations of fish that is large enough to be essentially self-producing (abundance changes are not dominated by immigration and emigration), with members of the collection showing similar patterns of growth, migration, and dispersal. The unit should not be so large as to contain many genetically distinct races or sub-populations within it, although it may be smaller than any genetically distinct racial unit of the species (as measured through morphologic or biochemical indicators, Hilborn and Walters 1992)

14 Stock delimitation Methods for stock identification (Jennings et al. 21): Distribution and abundance (provided by commercial data and scientific survey) Natural marks and tags (parasites, chemical contamination) Artificial marks and tags (conventional and electronic) Mersitics and morphometrics differences (e.g., size ratios) Calcified structures and scales (e.g., otholith and scales chemistry) Genetics (whether stocks differ genetically depends on their isolation: a transfer rate of 5% of individuals each year between 2 stocks may lead to genetic homogeneity) Life history traits (e.g., growth rate, maximum size)

15 Stock delimitation A brief history of Atlantic BFT stock delimitation Cetti (1777): Unique stock between Mediterranean and North Atlantic (BFT comes into the Mediterranean from the North Atlantic to spawn around Sicily and then goes back) Pavesi (1889): postulated the native hypothesis, i.e., the occurrence of a separated Mediterranean BFT stock (endorsed by Roule 1917, de Buen 1925, Scordia 1938) Sella (1929) and Heldt (1929) refuted the native hypothesis on the basis of occurrence of hooks used in the Atlantic (endorsed by Sara 1963 and SCRS) SCRS (198): Current two stocks option, separated by the 45 W meridian (distinct spawning sites, different age-at-maturity, traditional fishing grounds along the coasts, low trans-atlantic migration but, obvious and persistent connection between Mediterranean Sea and East Atlantic) Still an opening issue

16 Stock delimitation Scientific arguments for two stocks (East and West) Contrasted age-at-maturity, 4 and 8 years for the East and West, respectively Two main distinct spawning and nurseries areas, the Western Mediterranean and the Gulf of Mexico (recent confirmation by otolith chemistry, Rooker et al. 23) Lithium Highest concentrations of both juveniles and adults on the both sides of the Atlantic: no clear continuum across the ocean Some differences in the growth curves, but

17 Stock delimitation Scientific arguments for one stock Trans-Atlantic migration occurs but remains difficult to quantify because of the scarcity of data and variability among age-classes and years or decades (estimations from ICCAT database 199s = 7.6%; 198s = 1.2%; 197s =.84%; 196s = 2.3%) The present boundary cut several homogenous oceanic provinces, such as the Gulf Stream or North Atlantic sub tropical gyre provinces (longhurst 1998) The actual boundary was based on past fisheries distributions but these are changed and LL fisheries now occur in the middle of the Atlantic prises palangre BFT prises palangre BFT

18 Stock delimitation Conclusion Non unequivocal scientific basis to decide between both options. The Atlantic BFT population may be composed by one, two and even more than two stocks The present boundary makes little sense (does not take into account the current fisheries distribution and homogenous oceanographic units) Any progress on the stock issue would involve: 1. To get accurate and trustworthy fisheries information ( efficient control of the current management measures) to fit the models (with or without mixing between stocks) 2. To instigate an international and coordinated scientific programme (including: large-scale tagging experiments, modelling and time series analyses, studies on biology, genetic, fingerprints (otolith chemistry) and the habitat)

19 Impressive life history traits, but Late age-at-maturity: 4 to 8 years Short spawning season: 1.5 months Relative slow juvenile growth Skipjack, a small tropical tuna Early age-at-maturity: 1.5 years Long spawning season: ~12 months Relative rapid juvenile growth This makes bluefin more fragile to exploitation than tropical tuna. Why is it so?

20 Impressive life history traits, but Simulation modelling (STROMBOLI): SKJ and BFT populations of the same size submitted to the same fishing pressure with (1) Constant moderate fishing on juveniles & adults SKJ stocks = 16*BFT stocks SKJ yields = 9*BFT yields Sapwning Stock (tons) Skipjack x 1 8 x 1 7 Bluefin x 1 8 x Long-term fluctuations in BFT Yields (tons) Time (in year) Time (in year)

21 Impressive life history traits, but SKJ and BFT populations of the same size submitted to the same fishing pressure with (2) increasing fishing on juveniles & adults BFT collapses but SKJ not Long-term fluctuations in BFT make difficult to detect overfishing Spawning Stock (tons) Yields (tons) x Skipjack x Bluefin x 1 8 x Time (in year) Time (in year)

22 Impressive life history traits, but SKJ and BFT populations of the same size submitted to the same fishing pressure with (3) Exponential fishing on adults only BFT does not collapse anymore BFT yields on adults = 2*BFT yields on adults + juveniles SKJ yields = 4*BFT yields Sapwning Stock (tons) Yields (tons) x Skipjack Bluefin x 1 8 x x Time (in year) Time (in year)

23 Impressive life history traits, but Short live species (SKJ) are more resistant to exploitation than long-lived species (BFT) because they are more productive Long-term fluctuations in BFT stock and yields make more difficult to detect overfishing and depletion risks Fishing juveniles of long-lived species, such as BFT, is: risky (reinforces the possibility of overfishing) less productive in a fishery viewpoint Current measures on size limit are particularly relevant for BFT (whatever the stock delimitation)