Age and growth of Atlantic bluefin tuna, Thunnus thynnus (Osteichthyes: Thunnidae), in the Mediterranean Sea

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1 J. Appl. Ichthyol. 25 (2009), Ó 2009 The Authors Journal compilation Ó 2009 Blackwell Verlag, Berlin ISSN Received: January 21, 2008 Accepted: June 30, 2008 doi: /j x Age and growth of Atlantic bluefin tuna, Thunnus thynnus (Osteichthyes: Thunnidae), in the Mediterranean Sea By N. Santamaria 1, G. Bello 2, A. Corriero 1, M. Deflorio 1, R. Vassallo-Agius 3,T.Bo k 4 and G. De Metrio 1 1 Department of Animal Health and Well-being, University of Bari, Valenzano (BA), Italy; 2 Arion, Mola di Bari, Italy; 3 Malta Centre for Fisheries Sciences, Fort San Lucjan, Malta; 4 Faculty of Fisheries, Istanbul University, Istanbul, Turkey Summary The objective of the study was to describe the biometry of Mediterranean bluefin tuna, Thunnus thynnus, the biology of which is not yet well understood. A total of 504 specimens was collected from 1998 to 2005 in the central part of the Mediterranean basin. They were sexed and measured; fork lengths (FL) ranged from 51.0 to cm while body weights (W) ranged from 2.6 to kg. The first spiniform ray (spine) of the first dorsal fin was removed and cross-sectioned near the condyle base in order to count annuli for age estimation. The regression coefficient (b) of the female FL W relationship was significantly higher than that of the male, and both sexes displayed a negatively allometric growth (b < 3); male regression equation: ln W = ) ln FL; female regression equation: ln W = ) ln FL. Based on counts of the translucent zones in the sections of the first ray of the first dorsal fin, estimated ages ranged from 1 to 15 years for males and 1 to 14 years for females. The correlation between the spine ray (R) and FL fit the allometric model best; the R FL regression equations of the two sexes did not differ significantly and the overall equation was: ln FL = ln R. Due to the R FL allometric correlation, estimates of fork lengths at previous ages, FL i, were back-calculated with a body proportional hypothesis. Von Bertalanffy growth equations were derived from both observed and back-calculated FLs-at-age, which did not differ significantly. Moreover, no significant difference was found between the growth equations of the two sexes; the overall equation was FL t = [1)e )0.07(t ) ]. Weight-at-age values were derived from the von Bertalanffy predicted FLs-atage by the FL W correlation equations for males and females. The paper represents the first comprehensive study on the biometry, including age and growth, of bluefin tuna captured in the Mediterranean Sea. Introduction The bluefin tuna, Thunnus thynnus Linnaeus, 1758, a highly developed species (Cort and Liorzou, 1991), is one of the fastest, largest and most wide-ranging teleost fish in the oceans. It can grow up to 700 kg, travel on trans-oceanic migrations and swim at 90 km per hour (Safina, 1993). Atlantic bluefin tuna distribution extends over an extraordinarily large area ranging off the Atlantic coasts of Europe and Africa, from the North Cape to the Cape of Good Hope, and off the North American coasts from Newfoundland to a latitude of 40 S, in addition to most of the intervening oceanic areas (Mather et al., 1995). The International Commission for the Conservation of Atlantic Tunas (ICCAT) regulates the bluefin tuna fishery and, primarily on the basis of their spawning sites, currently recognizes two stocks: those of the west and the east Atlantic (the latter including the Mediterranean sea), separated by the 45 W meridian (Nemerson et al., 2000). Recent evidence indicates, however, that the two populations overlap in the North Atlantic foraging grounds (Block et al., 2005). Because of its commercial importance, bluefin tuna is intensely fished and actually overexploited. Since 1970 the biomass of bluefin tuna broodstocks declined by 77% and 14% in the western and eastern populations, respectively (ICCAT, 2005). The main objective of age and growth studies in fish is to estimate their mean size at each age class and determine their growth parameters (Mather et al., 1995). The growth rate of fish is an essential component of models used in stock assessment of fish populations; small variations in growth rates can have a significant impact on modelling outcomes for population analysis (Megalofonou, 2000). This is particularly important in the Atlantic bluefin tuna, as the actual existence of two stocks is a matter of debate. Biometric and age estimation data on the eastern Atlantic bluefin tuna are sparse and fragmentary, most being published in technical reports issued by international institutions such as ICCAT or the FAO (Food and Agriculture Organization of the United Nations) conference proceedings and other grey literature not readily available to the scientific community at large. There are several ways of modelling fish growth (Ricker, 1975). However, the von Bertalanffy growth function is by far the most studied and most widely applied of all length-age models in fish biology (King, 1995). To age bluefin tuna, a variety of methods have been used based either on the size analyses of caught individuals (cohort analysis) or interpretation of the discontinuities of hard structures (or hard parts) of the fish. The reading of hard parts, such as otoliths, scales, spines and vertebrae, is based on the number of marks, usually called annuli, which are interpreted as periodic events (Sella, 1929; Compea n-jimenez and Bard, 1983; Megalofonou and De Metrio, 2000). In tuna species, fin spines have often been chosen for growth studies because they are easy to sample and have obvious and well-defined growth marks (Megalofonou, 2000; Megalofonou and De Metrio, 2000). In this paper updated statistical methodologies have been applied to provide bluefin tuna biometric and growth data useful to improve stock assessment and fishery management of this endangered species. U.S. Copyright Clearance Centre Code Statement: /2009/ $15.00/0

2 Age and growth of bluefin tuna in the Mediterranean 39 Materials and methods Bluefin tuna (N = 504) were sampled over an 8-year period from 1998 to 2005 in several central Mediterranean Sea sites (North Ionian, South Adriatic, South Tyrrhenian seas and Ionian waters around Malta). Thunnus thynnus were caught by commercial long-line and purse seine extending over a number of years in order to collect an adequate size sampling. Fork length (FL) and total body weight (W) were measured in each tuna to the nearest cm and 100 g, respectively, and the date and place of capture were recorded. The first spiniform ray (spine) of the first dorsal fin was removed from each fish. Whenever possible sex was determined by macroscopic observation of the gonads and subsequently confirmed by histological analysis. A low-speed saw with diamond blades was used to obtain three serial cross-sections, each about 0.7 mm thick, from each spine at the point near the condyle base. The sections were mounted with Eukitt Mounting Medium (Electron Microscopy Sciences, Hatfield, PA) on glass slides and observed with a binocular lens microscope Wild M3C (Leitz, Heerbrugg, Switzerland) under transmitted light, connected by digital camera DC 300 (Leica, Wetzlar, Germany) to the image analyser Quantiment 500 W (Leica, Wetzlar, Germany). Growth bands observed under these optical conditions were measured (lm). Interpretation of growth bands was based upon recognition of the narrow translucent and wider opaque zones assumed to represent periods of slow and fast growth, respectively. Occasionally a double translucent zone was observed and interpreted as representing a single periodic event of slow growth. A translucent zone, either single or double, and the associated opaque zone together were assumed to represent an annual growth band (Compea n-jimenez and Bard, 1983; Cort, 1991; Megalofonou and De Metrio, 2000). The number of translucent zones (herein referred to as rings or annuli) was counted in order to assign an estimated age to the fish and build a size-age key. As the nucleus of the spine is reabsorbed and the first rings begin to disappear at age 3, the mean diameters of the first rings of younger specimens were used to infer the age of the first visible ring of older specimens (Compea n-jimenez and Bard, 1983; Cort, 1991; Corriero et al., 2005). Two readings of each spine were independently made by one person; additional readings were carried out when the first two counts did not agree. The radius of the spine (R) was defined as the distance between the estimated centre of the cross-section and the edge of the section (Compea n-jimenez and Bard, 1983). Due to the difficulties in detection of the centre of the section that is located in a vacuolated (i.e. reabsorbed) area of the spine, R was calculated as d 2, where d = diameter of the spine. The radius of each growth ring (R i ) was calculated measuring the distance between the outside edge of each ring and the opposite edge of the cross-section (Cayre and Diouf, 1983). These distances were converted into radii by the formula (Cayre and Diouf, 1981): R i ¼ d i d=2 where R i = radius of the ring i; d i = distance from the outside edge of the translucent zone i to the opposite edge of the crosssection; d = diameter of the spine. equations, i.e. FL = aw b, whose parameters were computed following the natural log-transformation of the FL i and W i data and their fitting to a regression line by the least square method (predictive linear regression or model I of Sokal and Rohlf, 1981). The predictive regression model was preferable to the geometric model (cfr. Ricker, 1973), to make the conversion of the FL-time von Bertalanffy growth equation into the W-time equation feasible. Residuals were analyzed and outliers rejected (Sokal and Rohlf, 1981). The male and female regression equations were compared by the t-test proposed by Mayrat (1959). Proportionality between the first dorsal spine radius (R) and body size during growth was verified by analysis of the FL R correlations for both sexes. Linear and power regression equations (of the types FL = a + br and FL = a R b, respectively) were tested; residuals were analyzed and outliers rejected (Sokal and Rohlf, 1981). Comparison between FL R relationships of the two sexes was carried out by t-tests. The estimate of fork lengths at previous ages, FL i, viz. backcalculation, was performed according to the Whitney and Carlander (1956) models as recommended by Ricc (1990). Due to the non-linear, viz. power, relationship between fork length and spine radius, with a body proportional hypothesis (BPH) (cfr. Ricc, 1990), the following formula was used: ln FL i ¼ b lnðr i =RÞþln FL where, FL i = estimated fork length at age i; FL = fork length at capture; b = bias adjustment parameter (corresponding to the slope of the FL R regression equation following log-transformation of data); R i = radius of the ring i; R = radius of the spine section. Estimates of theoretical growth in length were obtained using the FiSAT II package (Gayanilo et al., 2002) by fitting the von Bertalanffy growth model (Bertalanffy von, 1938) to the mean lengths at estimated age: FL t ¼ FL 1 ½1 e kðt t0þ Š where, FL t = predicted fork length at age t, FL = mean asymptotic fork length, k = growth constant (year )1 ), and t o = theoretical age at which the fish would have been 0 length. An analysis of co-variance (ANCOVA) was performed to test differences in growth performances between males and females. Theoretical growth in weight was obtained by converting fork length to weight by the fork length weight, FL W, predictive regression equation. The growth performance index phi-prime (/ ) was estimated to compare growth parameters obtained in the present work with those reported by other authors. This index was calculated using the formula (Pauly and Munro, 1984): / 0 ¼ log 10 k þ 2 log 10 FL 1 The potential longevity of the species was calculated using Pauly and MunroÕs formula (1984): AGE MAX ¼ 3=k: Statistical treatment of data The FL W correlations for males and females were analyzed. Regressions of FL on W were best described by power Results Fork length (FL) and weight (W) of sampled bluefin tuna ranged from 51.0 to cm and from 2.6 to kg,

3 40 N. Santamaria et al. ln W (g) ln W = ln FL r = n = ln FL (cm) Fig. 1. Fork length weight, FL (cm) W (g), relationship of male bluefin tuna (T. thynnus) sampled in the Mediterranean sea, (a) (b) ln W (g) ln W = ln FL r = n = ln FL (cm) Fig. 2. Fork length weight, FL (cm) W (g), relationship of female bluefin tuna (T. thynnus) sampled in the Mediterranean sea, (c) respectively. Sex was determined in 378 specimens: 201 males and 177 females. The FL W relationships of male and female bluefin tuna captured between 1998 and 2005 are shown in Figs 1 and 2, respectively, and their parameters are reported in Table 1. The slope of the female regression equation is significantly higher than that of the male (Table 1). Moreover, both male and female slopes are significantly different from 3 (males: t = 133.9; d.f. = 199; P < ; females: t = 105.3; d.f. = 175; P < ), which indicate allometric growth in both sexes. Based on the counts of the translucent zones in the first ray sections of the first dorsal fin (Fig. 3), the estimated age of the examined T. thunnus ranged from 1 to 15 years in males and from 1 to 14 years in females. Mean FLs-at-age were computed for males, females and overall specimens (males + females + unsexed) (Table 2). Sizes of the rings corresponding to estimated ages (years 1 15) are given in Table 3. The R FL correlation was found to best fit the allometric model. The equations for males and females did not differ significantly from each other (Table 4); hence, the data of Fig. 3. Images of bluefin tuna (T. thynnus) spine sections. (a) Age 1 specimen, 58 cm FL, captured in North Ionian sea, 7 July One ring visible. (b) Age 5 bluefin tuna, 138 cm FL, captured in North Ionian sea, 19 May Four rings visible, one ring reabsorbed. (c) Age 7 specimen, 165 cm FL, captured in North Ionian sea, 23 October Five rings visible, two rings reabsorbed. Arrows indicate visible rings. Magnification bar = 3 mm Table 1 Regression equations for male and female bluefin tuna (T. thynnus) sampled in the Mediterranean sea Equations N r s b L 1 L 2 Males ln W = ) ln FL Females ln W = ) ln FL t slope = d.f. = 374 P < N, number of specimens; r, correlation coefficient; s b, standard error of slope; L 1 and L 2 = 95% confidence limits of slope; t slope,studentõs t-test for significance of slope difference; d.f., degrees of freedom; P, significance level of StudentÕs t-test.

4 Age and growth of bluefin tuna in the Mediterranean 41 Table 2 Mean observed fork lengths-at-age, FL (cm), and back-calculated mean overall fork lengths-at-age for bluefin tuna (T. thynnus) sampled in the Mediterranean sea ln FL = ln R r = Estimated age group Mean observed FL Males Females Overall (N) Back-calculated mean overall FL ln FL (cm) n = (23) (33) (27) (35) (18) (13) (5) (5) (2) (5) (11) (21) (14) (12) (4) Table 3 Mean R (radius from focus to distal edge of each annulus) of bluefin tuna (T. thynnus) spine sampled in the Mediterranean sea, Annulus Radius, R (mm) Min Max Mean SD I II III IV V VI VII VIII IX X XI XII XIII XIV XV 7.72 Table 4 StudentÕs t-test for differences between male and female bluefin tuna (T. thynnus) R FL correlation equations R FL correlation equations t-test Males ln FL = t slope = d.f. = 53 P = ln R Females ln FL = ln R males, females and unsexed individuals were pooled and the overall equation parameters calculated (Fig. 4). Estimates of fork lengths at previous ages, FL i, viz. backcalculated values, are reported in Table 2. The mean observed fork lengths at estimated ages were fitted to the von Bertalanffy growth model by sex. The ANCOVA test did not show any significant differences in growth patterns between sexes (F = 0.127; P = 0.725, n.s.), thus the overall (males + females + unsexed specimens) growth equation ln R (mm) Fig. 4. Relationship between spine radius (R, mm) of first dorsal fin and fork length (FL, cm) of bluefin tuna (T. thynnus) sampled in Mediterranean sea, parameters were derived (Table 5; Fig. 5). A second set of von Bertalanffy growth model parameters was computed by back-calculated FLs-at-age, (Table 5). No significant difference was found between the growth equation obtained from the observed FLs-at-age data and that derived from the backcalculated FLs-at-age data (ANCOVA test: F = 0.010; P = 0.921). Table 6 reports the fork lengths-at-age predicted by the von Bertalanffy equation for observed FLs-at-age and weightsat-age derived from the predicted fork lengths-at-age by the FL W correlation equations for males and females. Discussion As a comprehensive, scientifically rigorous study on age and growth of bluefin tuna captured in the Mediterranean sea was lacking in the literature, the present study reviews the length weight relationship, spine radius fork length relationship, mean fork length-at-estimated ages, and von Bertalanffy growth equations for bluefin tuna captured in the Mediterranean Sea over an 8-year period. The analysis of the length weight relationships of the two sexes showed that after sexual maturity [100% maturity is reached above 135 cm FL (Corriero et al., 2005)], the female weight-at-length is higher than the maleõs (regression coefficient, b, is higher in females). In both sexes growth is negatively allometric (slope, b < 3); that is, males and females become slightly more slender as they increase in size. Based on an 8-year sampling from the central Mediterranean, the present work shows for the first time the occurrence of sexual dimorphic length weight relationships for bluefin tuna inhabiting the Mediterranean sea. To our knowledge, the only sexspecific length weight relationships for eastern Atlantic bluefin tuna are those provided by Tawil et al. (2004). However, Tawil et al. (2004) did not report any tests to show whether the differences between the sex-specific FL W relationships were statistically significant; moreover, no information was provided on the number of sexed specimens used in their study. Indeed, sex-specific fork length weight (FL W) relationships are not easy to calculate for bluefin tuna because this species does not show any external sexual dimorphism and generally fish are not dressed onboard, thus preventing on-site observers from determining the sex by visual inspection of the gonads. Age estimation through examination of the first dorsal fin spine showed that the tuna were from 1- to 15-year-old males and 1- to 14-year-old females, ages much lower than the maximum reported in the literature (Cort, 1991) or those derived from growth equation models (see further).

5 42 N. Santamaria et al. Table 5 Summary of parameter estimates for von Bertalanffy growth equations derived from observed FLs-at-age data and back-calculated FLs-at-age data of bluefin tuna (T. thynnus) sampled in Mediterranean sea, Derived from observed data Derived from back-calculated data Parameter Estimate L 1 L 2 Parameter Estimate L 1 L 2 FL FL k k t 0 )1.76 )2.16 )1.36 t 0 )1.63 )1.94 )1.32 FL (cm) (23) FL = [1-e 0.07 (t ) ] (33) (27) (18) (13) (5) (35) (5) Age (year) (2) (5) (11) (21) (14) (12) (4) n = 228 Fig. 5. Von Bertalanffy growth curves for pooled Mediterranean Sea male and female bluefin tuna (T. thynnus). Mean (black dots), range (lines) and number (in parentheses) of observed fork lengths, FL (cm) Table 6 Fork-lengths-at-age, FL (cm), predicted by von Bertalanffy equation calculated from observed overall data for bluefin tuna (T. thynnus) sampled in Mediterranean sea, Estimated age group Predicted FL Predicted W Males Females Predicted weights-at-age, W (kg) derived from conversion of predicted fork lengths using the length weight relationship. The fork length radius of the first dorsal fin spine, FL R, relationship is best described by a power or allometric equation, a type of relationship also reported in other large pelagic fish such as the swordfish (DeMartini et al., 2007); no significant difference was found between FL R relationships in female and male bluefin tuna. This fact substantiates the Whitney and Carlander (1956) models rather than LeeÕs (1920) model for back-calculation (Ricc, 1990). Back-calculation of an animalõs growth history is based on the assumption that there is a predictable and unchanging relationship between the size of the hard parts (skeletal pieces, scales, otoliths, fin rays) and the size of the animal (Smith, 1983). In the present case the spine radius size-at-age also proved to be a powerful tool for making back-calculations of previous growth, as in other studies (Gonza lez-garce s and Farin a-perez, 1983). Our data, as well the von Bertalanffy growth equation based on backcalculated fork lengths-at-age, is very similar and does not differ significantly from that based on observed data. The parameters of the von Bertalanffy growth equation, based on observed data of this study, indicate a theoretical maximum length of 382 and 349 cm FL for males and females, respectively, with a theoretical longevity of 50 years for males and 43 years for females. If this estimated longevity seems to be surprising for a teleost fish, it must be considered that the largest fish in the study was 255 cm FL and ca. 247 kg weight with an estimated age of 15 years, but much older specimens have been recorded in the past: Hamre et al. (1971) measured animals as long as 315 cm at the Istanbul market; and 625 and 685 kg specimens have been captured in tuna traps of Sardinia (Sara`, 1969). Another feature of bluefin tuna is very rapid growth during the first year of life, when it reaches about 60 cm FL and 4.0 kg W. The growth parameters of bluefin tuna sampled in the Mediterranean and eastern Atlantic as calculated by various authors using the hard parts, as well as the calculated quantity /, showed slight differences (Table 7). Von Bertalanffy growth curves of both eastern and western Atlantic bluefin tuna stocks, based on different ageing methods, are shown in Fig. 6. Indeed, the curve by Rodríguez-Roda (1971) is not highly reliable because of the very small sampling (only 28

6 Age and growth of bluefin tuna in the Mediterranean 43 Table 7 Parameter estimates of von Bertalanffy growth curve for eastern Atlantic bluefin tuna (T. thynnus) management unit from various sources Growth parameters Sex Ageing methods FL k t 0 Area / Longivity Rodríguez-Roda (1971) Sexes combined Vertebrae )0.89 Northeast Compeán-Jimenez and Bard (1980) Sexes combined Spines )1.58 Northeast Farrugio (1980) Sexes combined Vertebrae )1.09 Mediterranean Compeán-Jimenez and Bard (1980) Sexes combined Spines )1.71 Northeast Cort (1991) Sexes combined Spines )0.97 Northeast Present study Sexes combined Spines )1.76 Mediterranean Males Spines )1.75 Mediterranean Females Spines )1.63 Mediterranean c a e d FL (cm) b (a) Rodriguez-Roda (1971) (b) Farrugio (1980) (c) Compéan-Jimenez and Bard (1983) (d) Cort (1991) (e) Present study Farber and Lee (1981) Hurley and Iles (1983) - males Hurley and Iles (1983) - females Nagai (1985) Age (year) Fig. 6. Von Bertalanffy growth curves for western (- - -) and eastern ( ) Atlantic bluefin tuna (T. thynnus) management units from various sources specimens). Some differences are evident comparing the curves for the western Atlantic (Faber and Lee, 1981; Hurley and Iles, 1983) and those from the eastern Atlantic samples (Farrugio, 1980; Compea n-jimenez and Bard, 1983; Cort, 1991; present results). In particular, the curves derived by Hurley and Iles (1983) had higher values of the parameter K; moreover, they seemed to show a difference between the sexes (such a difference was not observed in the present study). The geographical differences might be due to differences in growth rates of the two populations. However, the age structure of the samples may have a higher determinant on growth modelling, since it is known that fish may change growth rate parameters during their lives (Ricker, 1975). The fact that the eastern Atlantic-Mediterranean and Faber and Lee (1981) samples included fish only up to 15 years of age, whereas the Hurley and Iles (1983) sample comprised mostly older fish up to 32 years of age, might have contributed to the different von Bertalanffy curve parameters of the latter authors. Lastly, the role of the seasonal component in sampling, especially for younger ages, should also be considered when comparing growth curves (Hurley and Iles, 1983). Different ageing procedures are generally applied to fish species (Campana, 2001): size frequency analysis, taggingrecapture data, and reading of periodical marks on hard parts. The latter method is generally deemed a good compromise between good results and economic effort. Reading of the hard parts has been the most extensively used ageing method, despite that it may be affected by various sources of errors caused by the coalescence or disappearance of the first marks or to the presence of multiple marks due to migration patterns (Compea n-jimenez and Bard, 1983; Cort, 1991). The first attempt of ageing eastern Atlantic bluefin tuna by reading the yearly hard part markings was performed by Sella (1929), who counted the annuli on vertebrae of more than 1500 individuals and estimated the mean length of fish at ages 1 14.

7 44 N. Santamaria et al. However, of course no equation parameters were calculated. Subsequently, the hard parts were applied for estimates made by Rodrı guez-roda (1971) and Farrugio (1980). Fin ray sections of bluefin tuna were analyzed to estimate age by the authors Compea n-jimenez and Bard (1983) (Eastern Atlantic Ocean and Mediterranean bluefin tuna), Cort (1991), Farrugia and Rodrı guez-cabello (2001), El-Kebir et al. (2002), and Rodrı guez-marı n et al. (2004). The von Bertalanffy equation parameters provided by Cort (1991) are still considered by ICCAT as reference data for eastern Atlantic bluefin tuna stock. As mentioned, there are some drawbacks that affect the readings of hard parts to estimate fish age. Periodicity of the growth band deposition must be validated by the use of a bone tracer such as oxytetracycline or by a marginal increment analysis (Campana, 2001). With reference to the eastern Atlantic bluefin tuna, no tagged, oxytetracycline-treated, individuals have as yet been recaptured (G. De Metrio, unpubl. data). Due to the high cost of this type of research and the poor results obtained, it is unlikely that further experiments applying this method will be performed in the near future. However, analysis of marginal increments demonstrated that only one translucent band is formed per year in the spines of juvenile bluefin tuna (age 1 3) captured in the Mediterranean Sea (Megalofonou and De Metrio, 2000); strong evidence indicates that the skeleton growth bands are annual events, as reported by Faber and Lee (1981), who found in western Atlantic bluefin tuna that lengthsat-age back-calculated from vertebra and otolith analyses were in close agreement with the theoretical fork lengths for ages 1 10 derived from tag release-recapture data fitted to the von Bertalanffy growth model. Compea n-jimenez and Bard (1983), Cort (1991), and Corriero et al. (2005) reported the disappearance of the first annuli in the first dorsal fin spine from age 3 onward because of the reabsorption of the nucleus. Such a phenomenon was also experienced in the present study, but it was adequately overcome by using the mean radius of the first rings of younger specimens to date the first visible ring of older specimens. Finally, when two translucent bands are formed in just 1 year, due to either Atlantic-Mediterranean migrations (Compea n-jimenez and Bard, 1983) or different diet regimes between autumn and early spring (Cort, 1991), they are separated by a thin opaque band that cannot be mistaken for the larger summer opaque band. In addition to the established effectiveness of the use of spiniform rays for fish age estimation, the removal of fin spines is an easy operation with minimum interference to the fish body, one more reason why this technique is widely used for age estimation of high-value fish such as the bluefin tuna. We believe that the present results will contribute to the proper management of this resource. Nonetheless, further research is needed to obtain finer details of its biological parameters. In particular, a definitive validation of the methodology used in the present study, by means of administration of a bone tracer such as oxytetracycline in individuals raised in captivity, is desirable and will be a challenge for future research. This is a goal for the near future with an Italian research project on bluefin tuna domestication funded by the Government of the Regione Puglia. References Bertalanffy von, L., 1938: A quantitative theory of organic growth. Human. Biol. 10, Block, B. 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