Reproductive status of Atlantic bluefin tuna, Thunnus thynnus, during migration off the coast of Sardinia (western Mediterranean)

Fisheries Research 181 (2016) 137–147

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Reproductive status of Atlantic bluefin tuna, Thunnus thynnus, during migration off the coast of Sardinia (western Mediterranean) Piero Addis ∗ , Marco Secci, Cecilia Biancacci, Daniela Loddo, Danila Cuccu, Francesco Palmas, Andrea Sabatini University of Cagliari, Department of Environmental and Life Science, Via Fiorelli 1, 09126 Cagliari, Italy

a r t i c l e

i n f o

Article history: Received 2 February 2016 Received in revised form 31 March 2016 Accepted 12 April 2016 Handled by Prof. George A. Rose Available online 29 April 2016 Keywords: Thunnus thynnus Reproductive biology Trap fishery Western mediterranean

a b s t r a c t From late April until mid-June Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758), migrates along the western coast of Sardinia to reach spawning grounds of the Mediterranean. Despite the substantial information on the reproductive biology of the species, the tracking of its reproductive parameters along the migratory pathways is far to be completed. This study provides some reproductive parameters for adult bluefin tuna collected from the last trap fishery Tonnara of the Mediterranean. The following points summarize our results: (i) gonadosomatic index (GI) was positively correlated with fork length (FL) and varied significantly between sexes, having higher values in males; (ii) ovaries contained a mixture of oocyte stages with primary growth oocytes representing 72%, cortical alveoli 16%, primary and secondary vitellogenic stages 4.6% and 6% respectively, only a small proportion of late vitellogenic oocytes and atretic follicles; (iii) batch fecundity was 95–125 oocytes per gram of body mass and varied from 1.6 million for a female of 105 cm FL, to 29.5 million eggs for a 233 cm FL specimen; (iv) sex ratio differed significantly from 1:1, with males being predominant both for the size classes and years analyzed. These findings suggest that bluefin tuna from western Sardinia exhibits intermediate reproductive characteristics between specimens migrating through the Strait of Gibraltar and those from the spawning grounds of the Balearic Islands. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Atlantic bluefin tuna (Thunnus thynnus Linnaeus, 1758) – ABFT – is a large top-predator fish which inhabits the pelagic ecosystems of the North Atlantic Ocean and the Mediterranean Sea. Much like other large tunas, bluefin tuna is a migratory species, with documented transoceanic and large-scale movements for feeding and reproduction (Mather et al., 1995; Rooker et al., 2008). In particular, the reproductive migration of bluefin tuna is prompted by internal information related to gonadal maturation and is regulated by abiotic factors that control time of departure and grouping of fish in order to reach suitable grounds for spawning (Schaefer 2001). The reproductive biology of bluefin tuna has been described for both the western (Baglin 1976; Baglin and Rivas 1977) and eastern Atlantic Ocean, including the Mediterranean Sea (Rodríguez-Roda 1964). In recent years, significant advancements have been made in understanding the reproductive physiology of this species (Bridges et al., 2000; Susca et al., 2001; Abascal et al., 2004; Corriero et al.,

∗ Corresponding author. E-mail address: [email protected] (P. Addis). http://dx.doi.org/10.1016/j.fishres.2016.04.009 0165-7836/© 2016 Elsevier B.V. All rights reserved.

2003, 2005; Heinisch et al., 2014), which is becoming essential for the purpose of developing captive broodstock (Corriero et al., 2007). Moreover the availability of archival and Pop-Up satellite tag techniques and satellite-derived mapping of surface temperature and Chlorophyll concentration has enhanced our understanding of the geoposition of the species, which in turn has allowed evaluation of biological data in the context of species distribution and environmental preferences for feeding and spawning grounds (Block et al., 2005; Sibert et al., 2006; Boustany et al., 2008; Alemany et al., 2010; ˜ et al., 2015; Abascal et al., 2016). Druon et al., 2011, 2016; Cermeno A number of researchers have tracked the reproductive status and gonad development of bluefin tuna along their migratory pathways, from the eastern Atlantic (off the Strait of Gibraltar) to eastern Mediterranean (Medina et al., 2002; Corriero et al., 2003; Heinisch et al., 2008; Aranda et al., 2013b). For example, Medina et al. (2002) found significant differences in the ovarian mass and gonadosomatic index between the Strait of Gibraltar and the Balearic islands, with incipient ovary development when entering in the Strait of Gibraltar and late ovarian development in the Balearic area. Karakulak et al. (2004) integrated ovarian histology, gonadosomatic index and larvae sampling data to identify a new spawning area for bluefin tuna in the northern Levantine basin

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(eastern Mediterranean). Heinisch et al. (2008) enlarged the spatial mapping of ovarian maturation from the western to the eastern Mediterranean, revealing that gonadosomatic index values were higher in fish caught in the eastern rather than the western locations. Nevertheless there is little information available regarding the reproductive behavior for bluefin tuna in intermediate pathways of migration where the fishery typically harvests mature adults. In the coastal areas of the eastern Atlantic Ocean and the Mediterranean Sea, adult bluefin tuna are still captured by traditional trap fisheries (Tonnara in Italian) during both the pre- and post-spawning migration (Medina et al., 2002). Traps currently are used in Morocco, Spain and Portugal, off the Strait of Gibraltar, and only in three sites in the Mediterranean Sea, specifically in Sardinia (south Italy). In Sardinia, from late April until mid-June, reproductive bluefin tuna migrate along the western coast swimming closer to the bathymetric contour of 40 m, where they are entrapped in the trap fishery of Isola Piana, Portoscuso and Porto Paglia, all deployed in the southwestern area (Fig. 1). These are the last remaining active traps in the Mediterranean, and they provide valuable information about the status of the bluefin tuna stock, and the ecology of the species (Addis et al., 2012, 2013, 2014). The localization of these fishing systems is historically documented by ancient maps and catch data (Cetti 1777; Angotzi 1901). These authors provided evidence that the reproductive pathways of migrating bluefin tuna have been present along the western coast of Sardinia since the sixteenth century, thereby corroborating the existence of a steady “reproductive homing behaviour”, i.e. the behaviour in site and orientation ability of bluefin tuna (Rooker et al., 2014). In these traps, small-medium adult specimens are entrapped in a system of nets and then confined to the small space of the “death chamber” to be pulled up during the final phase of fishing by the “mattanza” (Addis et al., 2013). Peak catches occur between the end of May and early June when large adults have well-developed gonads, which are collected and processed by fishermen for commercial purposes (Addis et al., 2014). This time of the year becomes an expedience for the systematic sampling of tissues, useful to improve understanding of key biological and ecological processes of bluefin tuna (ICCAT, 2013). Despite the uniqueness of this migration pathway and the long history of the trap fishery a comprehensive investigation on the reproductive biology of Atlantic bluefin tuna has never been carried out in this locate. In this paper we provide a wide range of reproduction related biometrics of bluefin tuna collected in the tonnara of Isola Piana during their migration to spawning grounds of the western Mediterranean.

Samples for gonadosomatic index (GI) analysis and histological examination of ovaries were collected from mid-May to mid-June 2000. A total of 311 adult bluefin tuna were eviscerated immediately after landing. Ovaries (n = 238) and testes (n = 73) were weighed to the nearest gram for GI determination. GI was calculated as GI = 100 GW W−1 , where GW is the gonad weight in grams and W is the round weight in kilograms, for the whole sample and by sex, and nine classes of FL were considered (Table 1). For the histological study we used 112 ovaries collected from specimens ranging from 113 to 215 cm FL. We grouped 14 ovaries by class of FL for a total of eight classes (Table 2). Fragments of ovary were collected from the central portion of the right lobule and immediately fixed in Carnoy’s solution for 72 h. After dehydration in ascending concentrations of ethanol and clearing in xylene, the fragments were embedded in synthetic resin (GMA, Technovit 7100, Bio-Optica) in a vacuum for 24 h and sectioned at 5 ␮m using a rotary microtome (LKB, Historange). Sections of gonads were stained with Harris haematoxylin and eosin. The histological staining protocol proposed by Cerri and Sasso-Cerri (2003) was used for glycol methacrylate-embedded tissue sections. Histological sections were viewed with an optical microscope at 40 × magnification (Zeiss Axioskop) for oocyte classification. Classification was based on the developmental sequence described by Brown-Peterson et al. (2011). We classified the samples into six stages based on the dominance of diverse gametogenic cell types and diameters (␮m). For each developmental stage, only oocytes that were sectioned through the nucleus were counted, and their diameters were measured using an ocular micrometer. The monthly development of oocytes was also examined in a subset of 20 ovaries from adult specimens collected on May 20 (n = 10) and June 10 (n = 10). The correlation between FL and oocyte developmental stage was tested using the Spearman rank correlation at 95% C.L. Monthly differences in the number of oocytes in the different developmental stages were analyzed by one-way ANOVA (␣ = 0.05).

2. Materials and methods

2.3. Batch fecundity

2.1. Study area and sampling

Batch fecundity (F) was estimated by the gravimetric method (Hunter et al., 1985) as the product of gonad weight and oocyte density. Ovaries of 14 adult bluefin tuna collected from a single sampling date (May 28, 2000) were used for this analysis. Small slices (∼50 g) of the anterior part of left and right ovaries were removed. Fragments (1 g sample) were subsampled from the outer and central portion of each slice and preserved in Carnoy’s solution (Marte and Lacanilao 1986). Outer fragments were used to evaluate the membrane weight of the full ovary. Fragments from the central portion were used for oocyte counting (large yolked oocyte ≥ 280 ␮m in diameter) which was carried out in a gridded Petri dish under a Leica Wild M10 microscope. The relationship between number of oocytes and FL was expressed by the equation F = ␣ × FL␤, where F is the number of oocytes, FL is the fork length in centimeters, and ␣, and ␤ are the regression parameters.

Specimens of bluefin tuna T. thynnus were caught in the trap fishery of Isola Piana. This trap captures tuna during the prespawning migration in the time range between late April to mid June. The trap fishery consists of a gear placed at coordinates 39.191772◦ N, 8.292655◦ E and a factory building located 2 Nm from the gear location where tuna are processed for fresh and cannery markets (Fig. 1). The gear consists in a system of nylon nets arranged in five chambers, known as the grande, the bordonaro, the bastardo, the camera di ponente and the camera della morte (the “death chamber”) (Fig. 2). Tuna enter the trap swimming naturally. Once inside they are initially enclosed in the largest chamber, the grande, and then cross from east to west into the other four chambers through the narrow passages, which form a system of man-operated mov-

ing nets. Only the death chamber has a net mesh “floor” that is used to draw up tuna during the mattanza. Sampling campaigns were conducted from mid-May to mid June in the period 1993–2000. A total of 8762 bluefin tuna commercially harvested by the trap have been measured for straight fork length (FL, in cm). Estimated age-based distributions were obtained from ordinary length frequency distributions using the estimated growth parameters found in Cort et al. (2013).

2.2. Gonad index and histology

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Fig. 1. Map indicating sampling locations where gonad maturation of Atlantic bluefin tuna across the Mediterranean Sea has been studied by different authors: (1) Barbate, off Strait of Gibraltar (Heinisch et al. 2008; Medina et al. 2002); (2) Balearic Islands (Aranda et al., 2013a,b; Alemany et al., 2010; Heinisch et al., 2008); (3) Southwestern Sardinia (present study); (4) Malta (Heinisch et al., 2008); (5) Northern Levantine Sea between Cyprus and Turkey (Heinisch et al., 2008). In detail (top) pathways where Atlantic bluefin tuna are intercepted by the trap fishery of Isola Piana* and Portoscuso (*trap location where the present study was carried out).

Fig. 2. Scheme of the traditional trap of Isola Piana (Sardinia—W Mediterranean) composed by 5 chambers (modified with permission by J.L. Cort).

2.4. Sex ratio and length-weight relationship Data used to determine the sex ratio were from samples collected in the period 1993–2000. In total, 2619 bluefin tunas (1217 females and 1402 males) were sexed by examining gonads when fishermen gilled and gutted the fish at the trap factory building. The sex ratio describes the proportion of males and females in a population and indicates the dominance of a gender within a given population or school of fish. Assuming that vulnerability (probabil-

ity of capture) to the trap fishery is equal between sexes (expected ratio = 1:1), the Chi-square (␹2 ) test was used to test significant differences between sexes in the sampled specimens (␣ = 0.05). The annual sex ratio and the ratio by FL class (classes of 5 cm) were calculated as total number of males/total number of females. The FL/W relationship was determined for 2158 unsexed fish, 736 females ranging from 87 to 295 cm in FL and 606 males ranging from 89 to 300 cm FL using the exponential model W = a × FL b . Fork length and weight data were log10 transformed, and pairs of

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Table 1 Gonadosomatic index (GI ± S.D.) by size (FL) and round weight (W) for migrant T. thynnus collected in southwestern Sardinia from early May to the end of June 2000. FL (cm)

W (kg)

Female (n = 238)

Min - Max

Male (n = 73)

Min - Max

104–112 113–121 122–130 131–139 140–148 149–157 158–166 167–175 > 175

20–25 26–34 35–40 41–49 50–58 59–68 69–80 81–90 > 90

1.27 ± 0.7 (n = 10) 1.43 ± 0.58 (n = 22) 1.65 ± 0.39 (n = 17) 2.44 ± 0.97 (n = 37) 2.48 ± 0.82 (n = 26) 2.7 ± 0.64 (n = 27) 2.83 ± 0.58 (n = 25) 2.73 ± 0.85 (n = 15) 2.53 ± 1.02 (n = 59)

0.65–1.96 0.8–2.7 0.9–2.65 0.8–5.57 1.2–4.5 1.2–4.5 1.4–3.8 1.7–3.9 0.33–5.3

1.56 (n = 1) 3.38 ± 1.76 (n = 10) 4.05 ± 1.49 (n = 16) 4.2 ± 1.31 (n = 20) 4.5 ± 1.01 (n = 10) 5.11 ± 1.62 (n = 7) 3.88 ± 1.19 (n = 5) 3.85 ± 0.80 (n = 2) 3.78 (n = 2)

– 0.8–6.6 1.8–6.8 1.7–6.0 1.2–4.7 3.0–7.8 3.2–6.0 3.3 –

Table 2 Percentage distribution of mean number of oocytes by maturative stage and size classes. PG (primary growth oocyte); CA1 (cortical alveoli oocyte I); CA2 (cortical alveoli oocyte II); Vtg1 (primary vitellogenic oocyte); Vtg2 (secondary vitellogenic oocyte); Vtg3 (tertiary vitellogenic oocyte). FL (cm)

PG

CA1

CA2

Vtg1

Vtg2

Vtg3

113–121 122–130 131–139 140–148 149–157 158–166 167–175 >175

77.6 67.5 69.0 72.3 76.4 74.1 72.5 63.2

9.3 15.5 9.7 9.1 7.0 6.4 4.9 7.9

5.2 4.5 9.2 5.0 6.1 5.7 10.9 9.3

2.4 4.3 4.1 5.3 3.5 5.6 3.8 7.9

3.9 6.2 6.4 6.3 5.5 6.3 4.6 8.6

1.3 1.7 1.3 1.6 1.2 1.5 3.1 2.9

observations by sex and month were subjected to linear regression. Condition factor K (Ricker, 1975) was estimated for different size ranges. Finally, ANOVA was used to test differences in the regression coefficient and intercept considering a 95% C.L. Data were processed using the Brodgar v. 2.6.6 software package (http://www. brodgar.com). 3. Results 3.1. Length and age frequency distributions The exploration of total catch-at-length highlights that most fish from the studied fishing area were larger than 110 cm FL, indicating that the trap fishery harvests almost mature bluefin tuna (Fig. 3). Age distribution by year (Fig. 4) showed that age classes ≥ 7 years were dominant in the trap catches from 1993 to 1995. From 1996 to 2000, younger individuals of age classes 4, 5, and 6 began to be captured more consistently. These three classes represented 60%, 68%, 58%, 65% and 62% of captures respectively in the period 1996–2000. The proportion of immature fish (<3 years) was very low (<5%), and only a few giant individuals were recorded. For example, one female 295 cm FL weighing 474 kg, and two males 300 cm FL weighing 410 and 412 kg, respectively, were caught. 3.2. Gonad index Bluefin tuna ovaries weighed from 0.144 kg (105 cm FL, 22 kg W) to 11.9 kg (279 cm FL, 340 kg W). Testis weight ranged from 0.266 kg (117 cm FL, 30 kg W) to 7.83 kg (155 cm FL, 55 kg W). In the samples analyzed, ovary growth was isometric (Fig. 5). The regression analysis revealed a strong relationship between ovary weight and body weight (GW/W, R2 = 0.84). Data from males were more scattered (GW/W, R2 = 0.60) (Fig. 6). The estimated GI values for females varied from 0.33 to 5.57, and the highest value was for a 46.6 kg specimen (Table 1). In general, the GI for females ranging from 104 to 130 cm FL was consistent (mean GI = 1.45 ± 0.19 S.D.). Over this size range, i.e. in the size range from 131 to 175 cm FL, the GI increased and became consistent with a mean value of 2.63 (± 0.16 S.D.). GI for adult females larger than 175 cm FL exhibited greater variability (2.53 ± 1.02 S.D.).

Males showed a higher range of GI from 0.8 to 7.8, with the highest value in a specimen that weighed 65 kg (154.7 FL). 3.3. Ovary histology The histological analysis of the ovaries highlighted the presence of a heterogeneous range of maturing oocytes (Fig. 7, Table 2). The Spearman correlation coefficient revealed no correlation (p > 0.05) between the number of oocytes by stage and FL. Based on histological examination and diameter measurements, oocytes were grouped into the following six stages: PG = primary growth oocyte, oocytes that do not contain yolk and have several small nucleoli in the periphery of the nucleus (Ø = 89.27 ± 20.05 ␮m); CA1 = cortical alveoli oocyte I, oocytes with granular structures in the ooplasm increasing (Ø = 144.46 ± 18.40 ␮m); CA2 = cortical alveoli oocyte II, oocytes with alveoli vesicles increasing in size and number to form several peripheral rows, the oocyte became opaque in the area that surrounded the nucleus (Ø = 207·06 ± 31.09 ␮m); Vtg1 = primary vitellogenic oocyte, oocytes with small yolk granules in the periphery (Ø = 286.25 ± 38.03 ␮m); Vtg2 = secondary vitellogenic oocyte, oocytes with large yolk granules distributed in the cytoplasm (Ø = 513.78 ± 68.25 ␮m); and Vtg3 = tertiary vitellogenic oocyte, oocytes with small lipid droplets that have fused to each other to form larger lipid droplets and with a nucleus that has migrated to the periphery (Ø = 685 ± 120.42 ␮m). Sporadic germinal vesicle breakdown (GVBD) and atretic oocytes were identified during the histological examination. The analysis of monthly distribution of percentage of oocytes by stage showed a progressive decrease of primary growth oocytes, the advancement of vitellogenic stages (Vtg1 , Vtg2 and Vtg3 ) and atretic oocytes over time (Fig. 8). One-way ANOVA revealed significant differences between months for all oocyte stages except for CA (CA1 and CA2 data cumulated) and Vtg3 (Table 3). 3.4. Batch fecundity Table 4 shows the biometric features of 14 bluefin tuna collected for batch fecundity analysis and the estimated number of large yolked oocytes (≥ 280 ␮m Ø) used to calculate regression parameters. Estimated number of oocytes varied from 1.6 million for a female of 105 cm FL, to 29.5 million for a 233 cm FL specimen. The

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Fig. 3. Fork length (FL) frequency distributions by year of sampled Atlantic bluefin tuna caught in the trap fishery of southwestern Sardinia between 1993 and 2000.

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Fig. 4. Estimated age-based distributions obtained from ordinary length frequency distributions by year.

14000

y = 32,97x - 485,82 R² = 0,8433

12000

WG (g)

10000 8000 6000 4000 2000 0 0

50

100

150

200

250

300

350

400

W (kg) Fig. 5. Relationship between ovary weight (WG ) and body weight (W) of Atlantic bluefin tuna during reproductive migration in southwestern Sardinia.

relationship between number of oocytes and FL was expressed by the equation:

F = 0.1371418 × FL 3.52 (R 2 = 99.97%)

3.5. Sex ratio and length-weight relationship The size-combined sex ratio in the period 1993–2000 was 1.15, with males being more abundant. The Chi-square test revealed significant differences from the expected 1:1 probability of capture between sexes (␹2 = 13.21, df = 1, p = 2.7 × 10−3 ). There were significant differences in all size classes (␹2 = 82.04, df = 45, p = 6.2 × 10−3 ) and among years analyzed (␹2 = 17.0, df = 6, p = 0.017).

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Fig. 6. Testes weight (WG ) vs. body weight (W) relationship. Table 3 Results of one-way ANOVA testing monthly differences in the number of oocytes in the different developmental stages (Cortical Alveoli stage I and stage II were cumulated in CA). Stage

May

June

F

P-value

PG CA Vtg1 Vtg2 Vtg3 A

76.6 (12.6) 13.8 (6.1) 3.1 (2.6) 4.9 (3.9) 1 (1.4) 0.9 (1.2)

53 (12.2) 16.6 (4.7) 8.0 (3.6) 18 (8.2) 2 (1.0) 2.9 (1.5)

12.73 0.86 8.34 14.6 2.33 7.74

0.004** 0.371 0.013* 0.002** 0.152 0.016*

*: P < 0.05. **: P < 0.005. Table 4 Number of oocytes ≥ 280 ␮m in diameter by fork length (FL) and round weight (W) counted in the ovaries of Atlantic bluefin tuna collected during migration in southwestern Sardinia. FL(cm)

W(kg)

105 113 123 130 162 188 189 200 205 215 218 226 232 233

20 26 32 38 74 145 124 140 150 154 172 210 194 236

Ovary (g) Right

Left

155 140 210 215 850 2500 2800 2700 2200 2700 2100 4150 3250 4700

173 150 160 290 1150 1800 3300 2000 2400 2800 1800 2700 3650 5000

GI

Oocytes× 106

1.642 1.115 1.156 1.320 2.703 2.966 4.919 3.357 3.067 3.571 2.267 3.262 3.557 4.110

1.61 2.31 3.12 3.79 8.22 1.39 14.1 17.3 18.8 22.3 23.4 26.5 29.1 29.5

Table 5 Parameters of the fork length-weight (FL/W) relationship.

Unsexed Male Male - May Male - June Female Female - May Female - June

n

FLmin (cm)

FLmax (cm)

a

b

R2

2158 606 521 85 736 625 111

87 89 97 89 87 87 100

300 300 300 290 295 295 277

5.04 × 10−5 5.84 × 10−5 5.20 × 10−5 3.34 × 10−5 4.90 × 10−5 5.30 × 10−5 3.74 × 10−5

2.79 2.76 2.78 2.872 2.796 2.782 2.849

0.98 0.97 0.98 0.99 0.98 0.98 0.99

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Fig. 7. Photomicrograph of oocyte developmental stages of Atlantic bluefin tuna captured in the trap fishery of southwestern Sardinia: primary growth oocyte (PG); cortical alveoli oocyte (CA); primary vitellogenic oocyte (Vtg1); secondary vitellogenic oocyte (Vtg2); tertiary vitellogenic oocyte (Vtg3); germinal vesicle breakdown (GVBD).

The results of the FL/W relationship are summarized in Table 5. All regressions were highly significant, with the coefficient of determination (R2 ) ranging from 0.97 to 0.99 (p < 0.01). The b values for males and females ranged from 2.76 to 2.87 and 2.78 to 2.84, respectively. The ANOVA used to compare the regression lines between males and females revealed no significant differences for the whole sample (May and June grouped) and the sample separated into months (p > 0.39). However, significant differences were identified for the May and June data when males (F-test = 2.78; p < 0.005) and females (F-test = 2.045; p < 0.05) were considered separately. K = 1.8 ± 0.01 for small adults (113–175 cm), and K = 1.7 ± 0.02 for medium adults (>175 cm), reveal differences between these two groups, the younger fish being in a slightly more advanced stage of fattening. 4. Discussion The Mediterranean Sea is a semi-enclosed system which provides a full suite of seasonal options for the reproductive processes of T. thynnus. In this basin spawning site fidelity and reproductive homing have been widely documented by electronic tagging (Block et al., 2005; De Metrio et al., 2005; Medina et al., 2011; Tudela et al., 2011; Fromentin and Lopuszanski, 2013; Aranda et al., ˜ et al., 2015; Abascal et al., 2016), the micro2013a; Cermeno

chemistry of otoliths (Rooker et al., 2014), studies in population genetics (Carlsson et al., 2004; Riccioni et al., 2010) and historical records from traditional traps (Doumenge 1998; Addis et al., 2008). The utilization of traditional traps have been useful for designing quantitative experimental studies to test specific hypotheses, such as when information is sought regarding migration patterns related to surface circulation during the approach of the species to coastal waters for reproduction (Lemos and Gomes, 2004; Ravier and Fromentin, 2004; Addis et al., 2008). This study provides some comprehensive information of the reproductive-related biological parameters of T. thynnus during migration in southwestern Sardinia where the species is historically harvested by traditional traps since the 16th century. In this area oceanographic studies and surface circulation modeling revealed a stable southward-moving surface current which originated a coastal upwelling (Ribotti et al., 2004; Olita et al., 2015). This upwelling, especially evident in the southern part of Sardinia where the trap fishery is deployed, constitutes the main surface temperature signal of the SST anomalies (Olita et al., 2014). Moreover the surface circulation is strongly affected by the wind-driven circulation from westerly wind (Mistral), producing favorable abiotic conditions for the pathways of reproductive tuna and therefore for the suitability of the trap fishery (Addis et al., 2013). These circumstances justify the persistence of the last trap fishery of the Mediterranean which still provides reasonable yields for the tuna market. In the Mediterranean Sea, considerable progress has now been made in understanding the reproductive status of bluefin tuna from fish sampled along their migration pathways in the central and western basin. For instance, Medina et al. (2002) measured the GI of adult females collected from late April to the end of June in the trap of Barbate de Franco (Cádiz, southern Spain) to the Balearic Islands. In the trap of Barbate (off the Strait of Gibraltar), the GI value was 1.23 ± 0.56 S.D., which was much lower than the value determined in the present study (2.30 ± 0.93 S.D.). Based on histological examination, Medina et al. (2002) classified these specimens as “non-spawning mature” with ovaries containing late vitellogenic oocytes but without spawning activities. From May to early July, ABFT are present in the spawning ground around the Balearic Islands (De la Serna et al., 1996; Aranda et al., 2013b; Gordoa 2015). In this site, the GI value was 4.19 ± 1.65 S.D., which was higher than the value determined in southwestern Sardinia, and Balearic specimens had fully developed gonads and were ready to spawn. Aranda et al. (2013b) evaluated the reproductive status of bluefin tuna in early July relative to the oceanographic conditions

Fig. 8. Percentage distribution of the number of oocytes in ovaries of adult Atlantic bluefin tuna sampled in May (n = 10) and June (n = 10).

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of Balearic waters and found abiotic factors extremely suitable for spawning (i.e., surface waters at a temperature of 24 ◦ C). In a previous study Alemany et al. (2010) pointed out that the spawning of bluefin tuna in the Balearic Islands seems to take place mainly in offshore mixed waters, with salinities between 36.9 and 37.7 and located near frontal areas in the confluence of AW (Atlantic Waters masses) and MW (Mediterranean surface Waters) where surface temperatures ranged between 21.5 and 26.5 ◦ C. Heinisch et al. (2008) studied the reproductive status of bluefin tuna in Barbate, the Balearic Islands, Malta, and the Northern Levantine Sea between Cyprus and Turkey. Their findings confirmed the results presented by Medina et al. (2002) for the western basin. Furthermore, bluefin tuna collected from the Northern Levantine Sea had the highest value of GI for the whole Mediterranean basin (GI varied from a minimum of 4.19 to a max of 5.93), and specimens from Malta exhibited a mixed reproductive pattern. These results corroborated the presence of distinguished reproductive populations of bluefin tuna from the western and eastern Mediterranean basin. Although we did not use a stereological method for mapping the histological features of bluefin tuna ovaries, we were able to identify non-spawning mature specimens and well-developed ovaries based on GI values, thereby corroborating the findings of Medina et al. (2002) and Heinisch et al. (2008). The role of Sardinia as pre-spawning path for migrating bluefin tuna was also corroborated by daily underwater counts of tuna carried out in the chambers of the trap of Isola Piana (Addis et al., 2013). The earliest arrivals of bluefin tuna was in late April, whereas the migration ends in mid-June. No fish is observed in the trap fishery after this date. Once passed through the Sardinian path, the options for bluefin tuna migration are (i) to continue toward the spawning ground of the Balearic area or (ii) toward the southern Tyrrhenian Sea, where spawning occurred in late June and early July. These twofold options are also corroborated by recent data from the recapture of three bluefin tuna tagged (by spaghetti tag) in the trap fishery of Sardinia during the fishing season 2013 and 2014. Indeed two specimens were recaptured on May 26, 2014 in south France (43.18◦ N, 5.51◦ E), and the second one on May 31, 2014 in south Tyrrhenian Sea (39.16◦ N, 15.06◦ E) (Di Natale pers. com.). More recently (Addis et al., 2015), the electronic tagging (miniPATs) of 28 bluefin tuna (average FL 132 ± 24 cm) conducted in the same trap, revealed a wide dispersal behavior of tagged specimens with a prevalence of occurrence in the area between Sardinia and Balearic. Other recoveries occurred in the Strait of Bonifacio, in the north of Tyrrhenian Sea and finally, in the central Mediterranean near the area of Malta. In the past, plankton sampling campaigns conducted in the western Mediterranean, revealed no evidence for spawning off western Sardinia given that no tuna larvae was sampled (Nishida et al., 1997; Piccinetti and Piccinetti-Manfrin, 1994; Tsuji et al., 1997; Ueyanagi et al., 1997). However, spawning of the bluefin tuna should not be excluded in the area as result of the stochastic variation in the environment. Indeed, on June 25, 2014 fishermen from the Sardinian traps observed the occurrence of a spawning event in caged tuna, just before their transfer to the farming site in Malta. Moreover, the occurrence of spawning in caged tunas has been reported recently in the western Mediterranean (Medina et al., 2016). These observations raise the need to control the behavior of bluefin tuna once entrapped into the cages in order to verify if spawning occurs accidentally or regularly and if the variability of abiotic factors may affect the occurrence of spawning. Additional information about the reproductive status of bluefin tuna from North African countries is scarce and fragmentary mainly because the trap fisheries in Tunisia and Libya have been abandoned several years ago and none scientific survey has been carried out in recent times. Hattour and Macias (2002) published the most recent

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data for Tunisian waters. Female bluefin tuna captured in Tunisian waters had mature ovaries with fully yolked oocytes and only incidental atretic stage oocytes. For the period from early May to the end of June, the authors identified these females as “pre-spawning stage individuals.” In the present study we estimated a batch fecundity value of 95–125 oocytes g−1 body weight, which are not far from values reported by Rodriguez-Roda (1967) with a value ranging from 97 to 137 oocytes g−1 body weight for 10 bluefin tuna caught in the eastern Atlantic along the coast of Spain. Medina et al. (2002) examined 24 spawning T. thynnus from the Balearic Islands and reported a value of 93 oocytes g−1 body weight. Similar values (91–135 oocytes g−1 body weight) also were reported for the western Mediterranean (De Metrio et al., 1995). Reported estimates of mean fecundity of large bluefin tuna (>205 cm FL) from the western Atlantic ranged from 30 to 60 million eggs (Baglin 1982). Such values are considerably greater than the estimated fecundity values of spawning individuals from the western Mediterranean and the Strait of Gibraltar (13–15 million eggs) reported by Medina et al. (2002). Maximum mean fecundity reported by Baglin and Rivas (1977) was approximately 45 million eggs, although these authors predicted a fecundity of 75 million eggs for a 25-year-old female. In the present study, counting of large yolked oocytes for a 233 cm FL specimen resulted in an estimated fecundity of about 29 million eggs. Further information on the variation in batch fecundity based on age, season, and the individual spawning period will enable precise estimations of the annual fecundity of stock. The sex ratio of catches from Sardinian traps showed a displacement toward males in all years and size classes analyzed. In contrast, bluefin tuna caught in Spanish and Libyan traps (De La Serna et al., 2003b; El Tawil et al., 2004) and by Maltese long-liners (Fenech et al., 2003) showed a prevalence of females. Baglin (1982), who studied bluefin tuna in the Atlantic Ocean, reported that females were more prevalent than males during spawning time (April–May), whereas males were more frequent in feeding schools. Recently, Aranda et al. (2013b) identified a fractioned sex ratio, with females predominant in the middle size classes (180–210 cm FL) and males predominant at sizes larger than 220–230 cm FL. They speculated that this pattern may be due to different agespecific natural mortality rates associated with reproductive stress mechanisms in females. Kume and Joseph (1966) suggested that differences in sex-ratio may be caused by different catchability by gender. We propose that the observed sex ratio of the Sardinian bluefin tuna may be due to an ecological barrier by gender in the area or to segregation of tuna schools by sex during the reproductive migration. For example, males may prefer a migration pathway closer to the coast due to certain abiotic conditions, or there may be a higher proportion of males within schools in which reproductively active females are present. However this hypothesis should be corroborated by further sampling over time in order to state a final description of the phenomenon. No significant differences in the length-weight relationship were detected by month or gender when considering the data as a whole. Significant differences appeared within each gender when the data from May and June were analyzed. This result is indicative of time-related changes in body mass and corroborates results of a previous study of temporal changes in body shape using geometric morphometrics (Addis et al., 2014). Length distributions obtained in Sardinian traps differ significantly from those obtained in the traps off the Strait of Gibraltar. ABFT caught in the Atlantic traps are far larger than those captured in the traps of Sardinia, according to Rodríguez-Roda (1969), De la Serna et al. (2003a), Santos et al. (2004); and Abid et al. (2012). In the spawning ground of Balearic Islands length frequencies of ABFT caught by purse seiners are also larger (De la Serna et al., 1996; Aranda et al., 2013b; Gordoa, 2015). These facts may

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indicate the ABFT captured by the traps off the Strait of Gibraltar and Sardinia belong to different sub-populations, also known as metapopulations by Fromentin and Powers (2005); the latter would be from local Mediterranean populations. Other results that confirm these facts refer to the values of the condition factor (K) found in both places. Rodríguez-Roda (1967) and Cort et al. (2015) show that the ABFT crossing the Strait of Gibraltar in May have a K = 2.02 ± 0.02. In the present study K was estimated in May for the whole sample (from Table 5) and the values obtained differed markedly from those found in the Strait of Gibraltar in the same month: K = 1.75 ± 0.1, which is consistent with the fact that ABFT from western Sardinia exhibits intermediate reproductive characteristics between ABFT migrating through the Strait of Gibraltar and those from the spawning grounds of the Balearic Islands. 5. Conclusion This investigation describes some reproductive parameters for Atlantic bluefin tuna, especially from migrating schools in the western coast of Sardinia. Results indicate that sampled individuals exhibit intermediate reproductive characteristics between specimens entering in the Mediterranean, off the Strait of Gibraltar, and those present on the reproductive grounds of the Balearic Islands. There are many differences between the three areas in environmental factors such as the oceanographic features, food availability, the size distributions of the fish, and fishing systems. Currently, data on the reproductive biology of T. thynnus are fragmentary, dated, and restricted to the same areas. Spawning areas, maturity and fecundity, and size-specific sex ratios have not been sufficiently studied and should be investigated taking into account the spatial variability of abiotic factors and human-related impacts. The employment of the Mediterranean traps to achieve such research objectives, emphasize its significant contribution for the scientific monitoring of this economically and ecologically important species. Acknowledgments This study was supported by academic funds of the University of Cagliari, Department of Environmental and Life Science. Conventional and Electronic tagging data were provided under the research tasks of the International Commission for the Conservation of Atlantic Tunas (Madrid, Spain) - ICCAT-GBYP tagging programme 2013–2015. We would like to thank the Ligure Sarda Company (Isola Piana trap, Carloforte, Italy), all tonnarotti fisherman, which provided logistical support in the period 1993–2000. We wish to express our gratitude to the diver team of the “Carloforte Tonnare Diving Centre” (Carloforte, Italy) for their helpful support during biological sampling and tagging in the trap. We thank the ICCAT secretariat (Madrid, Spain) and the ICCAT-GBYP coordinator, Dr Antonio Di Natale, for drawing attention to the issue of the trap fishery. References Abascal, F.J., Megina, C., Medina, A., 2004. Testicular development in migrant and spawning bluefin tuna (Thunnus thynnus L.) from the eastern Atlantic and Mediterranean. Fish. Bull. 102, 407–417. Abascal, F.J., Medina, A., De la Serna, J.M., Godoy, D., Aranda, G., 2016. Tracking bluefin tuna reproductive migration into the Mediterranean Sea with electronic pop-up satellite archival tags using two tagging procedures. Fish. Oceanogr. 25 (1), 54–66. Abid, N., Benchoucha, S., Belcaid, S., Lamtai, A., El Fanichi, C., 2012. Moroccan tuna traps: history and current situation. Collective Vol. Sci. Pap. ICCAT 67, 124–138. Addis, P., Dean, J.M., Pesci, P., Locci, I., Cannas, R., Corrias, S., Cau, A., 2008. Effects of local scale perturbations in the Atlantic bluefin tuna (Thunnus thynnus L.) trap fishery of Sardinia (W. Mediterranean). Fish. Res. 92, 242–254. Addis, P., Secci, M., Locci, I., Cau, A., Sabatini, A., 2012. Analysis of Atlantic bluefin tuna catches from the last Tonnara in the Mediterranean Sea: 1993–2010. Fish. Res. 127–128, 133–141.

Addis, P., Secci, M., Cau, A., 2013. The effect of Mistral (a strong NW wind) episodes on the occurrence and abundance of Atlantic bluefin tuna (Thunnus thynnus) in the trap fishery of Sardinia (W Mediterranean). Sci. Mar. 77, 419–427. Addis, P., Secci, M., Pischedda, M., Laconcha, U., Arrizabalaga, H., 2014. Geographic variation of body morphology of the Atlantic bluefin tuna (Thunnus thynnus). J. Appl. Ichthyol. 30, 930–936. Addis, P., Secci, M., Sabatini, A., Palmas, F., Cau, A., Mariani, A., Dell’Aquila, M., Valastro, M., 2015. Electronic tagging of bluefin tuna in the trap fishery of Sardinia (W-Mediterranean). ICCAT-SCRS/2015/193. Alemany, F., Quintanilla, L., Velez-Belchí, P., García, A., Cortés, D., Rodríguez, J.M., Fernández de Puelles, M.L., González-Pola, C., López-Jurado, J.L., 2010. Characterization of the spawning habitat of Atlantic bluefin tuna and related species in the Balearic Sea (western Mediterranean). Prog. Oceanogr. 86, 21–38. Angotzi, F., 1901. L’industria delle tonnare in Sardegna. Pongetti Editore, Bologna. Aranda, G., Abascal, F.J., Varela, J.L., Medina, A., 2013a. Spawning behaviour and post-spawning migration patterns of Atlantic bluefin tuna (Thunnus thynnus) ascertained from satellite archival tags. PLoS One 8 (10), e76445. Aranda, G., Medina, A., Santos, A., Abascal, F.J., Galaz, T., 2013b. Evaluation of Atlantic bluefin tuna reproductive potential in the western Mediterranean Sea. J. Sea Res. 76, 154–160. Baglin, R.E., Rivas, L.R., 1977. Population fecundity of western and eastern North Atlantic bluefin tuna (Thunnus thynnus). ICCAT Coll. Vol. Sci. Pap. 6, 361–365. Baglin, R.E., 1976. A preliminary study of the gonadal development and fecundity of the western Atlantic bluefin tuna. ICCAT Coll. Vol. Sci. Pap. 5, 279–289. Baglin, R.E., 1982. Reproductive biology of western Atlantic bluefin tuna. Fish. Bull. 80, 121–134. Block, B.A., Teo, S.L.H., Walli, A., Boustany, A., Stokesbury, M.J.W., Farwell, C.J., Weng, K.C., Dewar, H., Williams, T.D., 2005. Electronic tagging and population structure of Atlantic bluefin tuna. Nature 434, 1121–1127. Boustany, A.M., Reeb, C.A., Block, B.A., 2008. Mitochondrial DNA and electronic tracking reveal population structure of Atlantic bluefin tuna (Thunnus thynnus). Mar. Biol. 156, 13–24. Bridges, C.R., Schröder, P., Susca, V., Corriero, A., De Florio, M., De Metrio, G., 2000. A new muscle biopsy technique for sex and sexual maturity determination in large pelagic fishes. ICCAT Collective Vol. Sci. Pap. 52, 752–758. Brown-Peterson, N.J., Wyanski, D.M., Saborido-Rey, F., Macewicz, B.J., Lowerre-Barbieri, S.K., 2011. A standardized terminology for describing reproductive development in fishes. Mar. Coast. Fish. 3, 52–70, http://dx.doi. org/10.1080/19425120.2011.555724. Carlsson, J., McDowell, J.R., Díaz-Jaimes, P., Carlsson, J.E.L., Boles, S.B., Gold, J.R., Graves, J.E., 2004. Microsatellite and mitochondrial DNA analyses of Atlantic bluefin tuna (Thunnus thynnus thynnus) population structure in the Mediterranean Sea. Mol. Ecol. 13, 3345–3356, http://dx.doi.org/10.1111/j. 1365-294X.2004.02336.x. ˜ P., Quílez-Badia, G., Ospina-Alvarez, A., Sainz-Trápaga, S., Bustany, A.M., Cermeno, Seitz, A.C., Tudela, S., Block, B.A., 2015. Electronic tagging of Atlantic bluefin tuna (Thunnus thynnus, L.) reveals habitat use and behaviors in the Mediterranean Sea. PLoS One 10 (2), e0116638, http://dx.doi.org/10.1371/ journal.pone.0116638. Cerri, P.S., Sasso-Cerri, E., 2003. Staining methods applied to glycol methacrylate embedded tissue sections. Micron 34, 365–372, http://dx.doi.org/10.1016/ S0968-4328(03)00098-2. Cetti, F., 1777. Anfibi e pesci di Sardegna, vol. 3. Storia naturale di Sardegna, Sassari. Corriero, A., Desantis, S., De Florio, M., Acone, F., Bridges, C.R., De la Serna, J.M., Megalofonou, P., De Metrio, G., 2003. 2Histological investigation on the ovarian cycle of the eastern Atlantic bluefin tuna (Thunnus thynnus L.). J Fish. Biol. 63, 108–119, http://dx.doi.org/10.1046/j.1095-8649.2003.00132.x. Corriero, A., Karakulak, S., Santamaria, N., De Florio, M., Spedicato, D., Addis, P., Desantis, S., Cirillo, F., Fenech-Farrugia, A., Vassallo-Agius, R., De la Serna, J.M., Oray, I., Cau, A., De Metrio, G., 2005. Size and age at sexual maturity of female bluefin tuna (Thunnus thynnus L. 1758) from the Mediterranean Sea. J. Appl. Ichthyol. 21, 483–486, http://dx.doi.org/10.1111/j.1439-0426.2005.00700.x. Corriero, A., Medina, A., Mylonas, C.C., Abascal, F.J., De Florio, M., Aragón, L., Bridges, C.R., Santamaria, N., Heinisch, G., Vassallo-Agius, R., Belmonte, A., Fauvel, C., Garcia, A., Gordin, H., De Metrio, G., 2007. Histological study of the effects of treatment with gonadotropin-releasing hormone agonist (GnRHa) on the reproductive maturation of captive-reared Atlantic bluefin tuna (Thunnus thynnus L.). Aquaculture 272, 675–686, http://dx.doi.org/10.1016/j. aquaculture.2007.08.030. Cort, J.L., Deguara, S., Galaz, T., Mélich, B., Artetxe, I., Arregi, I., Neilson, J., Andrushchenko, I., Hanke, A., Neves Dos Santos, M., Estruch, V., Lutcavage, M., Knapp, J., Compéan-Jiménez, G., Solana-Sansores, R., Belmonte, A., Martínez, D., Piccinetti, C., Kimoto, A., Addis, P., Velasco, M., De La Serna, J.M., Godoy, D., Ceyhan, T., Oray, I., Karakulak, S., Nottestad, L., López, A., Ribalta, O., Abid, N., Idrissi, M., 2013. Determination of Lmax for Atlantic bluefin tuna Thunnus thynnus (L.), from meta-analysis of published and available biometric data. Rev. Fish. Sci. 21, 81–212, http://dx.doi.org/10.1080/10641262.2013.793284. Cort, J.L., Estruch, V.D., Santos, M.N., Di Natale, A., Abid, N., De la Serna, J.M., 2015. On the variability of the length-weight relationship for Atlantic bluefin tuna, Thunnus thynnus (L.). Rev. Fish. Sci. Aquacult. 23 (1), 23–38. De Metrio, G., Cacucci, M., Sion, L., 1995. Characterization of large pelagic stocks (Thunnus thynnus L., Thunnus alalunga Bonn., Sarda sarda Bloch, Xiphias gladius L.) in the Mediterranean. Commission of the European communities, contract n. Xiv med./91/012. De Metrio, G., Arnold, G.P., De la Serna, J.M., Block, B.A., Megalofonou, P., Lutcavage, M., Oray, I., Deflorio, M., 2005. Movements of bluefin tuna (Thunnus thynnus L.)

P. Addis et al. / Fisheries Research 181 (2016) 137–147 tagged in the Mediterranean Sea with pop-up satellite tags. ICCAT Collective Vol. Sci. Pap. 58 (4), 1337–1340. De la Serna, J.M., Platonenko, S., Alot, E., 1996. Observaciones preliminares sobre las capturas de atún rojo (Thunnus thynnus) con artes de cerco en el Mediterráneo occidental. Collective Vol. Sci. Pap ICCAT 45 (2), 117–122. De la Serna, J.M., Farrugia, A., Hattour, A., Tawil, M., Srour, M., 2003a. Results from Project COPEMED Large Pelagic applicable to bluefin tuna aquaculture. In: Workshop on Farming, Management and Conservation of Bluefin Tuna, 5–7 April, Istanbul, Turkey, Available from: Turkish Marine Research Foundation, Istanbul, Turkey, 2003. Publication number: 13, 21–31. De La Serna, J.M., Ortiz De Urbina, J.M., Alot, E., 2003b. Analysis of sex ratio by length-class for bluefin tuna (Thunnus thynnus L.) in the western Mediterranean and eastern Atlantic. ICCAT Collective Vol. Sci. Pap. 55, 166–170. Doumenge, F., 1998. L’histoire des pêches thoniêres. ICCAT Coll. Vol. Sci. Pap 50, 753–802. Druon, J.N., Fromentin, J.M., Aulanier, F., Heikkonen, J., 2011. Potential feeding and spawning habitats of Atlantic bluefin tuna in the Mediterranean Sea. Mar. Ecol. Prog. Ser. 439, 223–240, http://dx.doi.org/10.3354/meps09321. Druon, J.N., Fromentin, J.M., Hanke, A.R., Arrizabalaga, H., Damalasa, D., Tiˇcina, V., Quílez-Badia, G., Ramirez, K., Arregui, I., Tserpes, G., Reglero, P., Deflorio, M., ´ L., MacKenzie, Oray, I., Karakulak, F.S., Megalofonou, P., Ceyhan, T., Grubiˇsic, B.R., Lamkin, J., Afonso, P., Addis, P., 2016. Habitat suitability of the Atlantic bluefin tuna by size class: an ecological niche approach. Prog. Oceanogr. 142, 30–46. El Tawil, M., El Kabir, N., Ortiz de Urbina, J.M., Valeiras, J., Abad, E., 2004. Analysis of sex ratio by length-class for bluefin tuna (Thunnus thynnus L.) caught from the Libyan trap fishery. ICCAT Collective Vol. Sci. Pap. 56, 1189–1191. Fenech, A., De la Serna, J.M., Ortiz de Urbina, J.M., 2003. Sex-ratio by length-class of bluefin tuna (Thunnus thynnus L.) caught by Maltese longliners. ICCAT Collective Vol. Sci. Pap. 55, 1145–1147. Fromentin, J.M., Lopuszanski, D., 2013. Migration, residency, and homing of bluefin tuna in the western Mediterranean Sea. ICES J. Mar. Sci., http://dx.doi.org/10. 1093/icesjms/fst157. Fromentin, J.M., Powers, J., 2005. Atlantic bluefin tuna: population dynamics ecology, fisheries and management. Fish Fish. 6, 281–306. Gordoa, A., 2015. Catch rates and catch structure of the Balfegó purse seine fleet in Balearic waters from 2000 to 2014. Two years of size frequency distribution based on video techniques. ICCAT Collective Vol. Sci. Pap. 71, 1803–1812. Hattour, A., Macias, D., 2002. Bluefin tuna maturity in Tunisian waters: a preliminary approach. ICCAT Collective Vol. Sci. Pap. 54, 545–553. Heinisch, G., Corriero, A., Medina, A., Abascal, F.J., de la Serna, J.M., Vassallo-Agius, R., Belmonte, A., García, A., de la Gándara, F., Fauvel, C., 2008. Spatial-temporal pattern of bluefin tuna (Thunnus thynnus L. 1758) gonad maturation across the Mediterranean Sea. Mar. Biol. 154, 623–630, http://dx.doi.org/10.1007/ s00227-008-0955-6. Heinisch, G., Rosenfeld, H., Knapp, J.M., Gordin, H., Lutcavage, M.E., 2014. Sexual maturity in western Atlantic bluefin tuna. Sci. Rep. 4, 7205, http://dx.doi.org/ 10.1038/srep07205. Hunter, J.R., Lo, N.C.H., Leong, R.J.H., 1985. Batch fecundity in multiple spawning fishes. In: Lasker, R. (Ed.), An Egg Production Method for Estimating Spawning Biomass of Pelagic Fish: Application to the Northern Anchovy, Engruulis mordax, vol. 36. NOAA Tech. Rep. NMFS, pp. 67–77. ICCAT, 2013. Atlantic-Wide Bluefin Tuna Research Programme, Biological and Genetic Sampling and Analyses. ICCAT/GBYP 02/2013. Karakulak, S., Oray, I., Corriero, A., Deflorio, M., Santamaria, N., Desantis, S., De Metrio, G., 2004. Evidence of a spawning area for the bluefin tuna (Thunnus thynnus L.) in the eastern Mediterranean. J. Appl. Ichthyol. 20, 318–320, http:// dx.doi.org/10.1111/j.1439-0426.2004.00561.x. Kume, S., Joseph, J., 1966. Size composition, growth and sexual maturity of bigeye tuna Thunnus obesus (Lowe), from the Japanese long-line fishery in the eastern Pacific Ocean. IATTC Bull. 11, 47–99. Lemos, R.T., Gomes, J.F., 2004. Do local environmental factors induce daily and yearly variability in bluefin tuna (Thunnus thynnus) trap catches? Ecol. Model. 177, 143–156. Marte, C.L., Lacanilao, F., 1986. Spontaneus maturation and spawning of milkfish in floating net cages. Aquaculture 53, 115–132. Mather, F.J., Mason, J.M., Jones, A.C., 1995. Historical Document: Life History and Fisheries of Atlantic Bluefin Tuna. U.S. Dep. Comm., NOAA Tec. Mem., NMFS-SEFSC, pp. 370. Medina, A., Abascal, F.J., Megina, C., Garcia, A., 2002. Stereological assessment of the reproductive status of female Atlantic northern bluefin tuna during migration to Mediterranean spawning grounds through the Strait of Gibraltar. J. Fish. Biol. 60, 203–217, http://dx.doi.org/10.1006/jfbi.2001.1835.

147

Medina, A., Cort, J.L., Aranda, G., Varela, J.L., Aragón, L., Abascal, F.J., 2011. Summary of bluefin tuna tagging activities carried out between 2009 and 2010 in the east Atlantic and Mediterranean. ICCAT Collective Vol. Sci. Pap. 66 (2), 874–882. Medina, A., Aranda, G., Gherardi, S., Santos, A., Mèlich, B., Lara, M., 2016. Assessment of spawning of Atlantic bluefin tuna farmed in the western Mediterranean Sea. Aquacult. Environ. Interact. 8, 89–98. Nishida, T., Tsuji, S., Segawa, K., 1997. Spatial data analyses of Atlantic bluefin tuna larval surveys in the 1994. ICCAT Collective Vol. Sci. Pap. 48, 107–110. Olita, A., Sparnocchia, S., Cusí, S., Fazioli, L., Sorgente, R., Tintoré, J., Ribotti, A., 2014. Observations of a phytoplankton spring bloom onset triggered by a density front in NW Mediterranean. Ocean Sci. 10, 657–666, http://dx.doi.org/10.5194/ os-10-657-2014. Olita, A., Iermano, I., Fazioli, L., Ribotti, A., Tedesco, C., Pessini, F., Sorgente, R., 2015. Impact of currents on surface flux computations and their feedback on dynamics at regional scales. Ocean Sci. 11, 657–666. Piccinetti, C., Piccinetti-Manfrin, G., 1994. Distribution des larves de Thonidés en Mediterranée. FAO Fish. Rep. 494, 186–206. Ravier, C., Fromentin, J.-M., 2004. Are the long-term fluctuations in Atlantic bluefin tuna (Thunnus thynnus) population related to environmental changes? Fish. Oceanogr. 13 (3), 145–160. Ribotti, A., Puillat, I., Sorgente, R., Natale, S., 2004. Mesoscale circulation in the surface layer off the southern and western Sardinia island in 2000. Chem. Ecol. 20, 345–363, http://dx.doi.org/10.1080/02757540410001727963. Riccioni, G., Landi, M., Ferrara, G., Milano, I., Cariani, A., Zane, L., Sella, M., Barbujani, G., Tinti, F., 2010. Spatio-temporal population structuring and genetic diversity retention in depleted Atlantic bluefin tuna of the Mediterranean Sea. Proc. Natl. Acad. Sci. USA 107, 2102–2107, http://dx.doi.org/10.1073/pnas.0908281107. Ricker, W.E., 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Board Can. 191, 1–382. Rodríguez-Roda, J., 1964. Biología del atún Thunnus thynnus (L.), de la costa sud atlantica de Espana. Inv. Pesq. 25, 33–164. Rodríguez-Roda, J., 1967. Fecundidad de atún, Thunnus thynnus (L.), de la costa ˜ Inv. Pesq. 31 (1), 33–52. sudatlántica espanola. ˜ en la Rodríguez-Roda, J., 1969. El atún, Thunnus thynnus (L.), del sur de Espana ˜ 1967, y estudio de la evolución de la pesquería ˜ almadrabera del ano campana de Barbate. Inv. Pesq. 33 (1), 87–96. Rooker, J.R., Secor, D.H., De Metrio, G., Schloesser, R., Block, B.A., Neilson, J.D., 2008. Natal homing and connectivity in Atlantic bluefin tuna populations. Science 322, 742–744, http://dx.doi.org/10.3354/meps07602. Rooker, J.R., Arrizabalaga, H., Fraile, I., Secor, D.H., Dettman, D.L., Abid, N., Addis, P., Deguara, S., Karakulak, F.S., Kimoto, A., Sakai, O., Macías, D., Neves Santos, M., 2014. Crossing the line: migratory and homing behaviors of bluefin tuna in the atlantic ocean and Mediterranean Sea. Mar. Ecol. Prog. Ser. 504, 265–276, http://dx.doi.org/10.3354/meps10781. Santos, M.N., García, A., Gil Lino, P., Hirofumi, M., 2004. Length-weight relationships and weight conversion factors for bluefin tuna (Thunnus thynnus) from Algarve: prior to and after fattening. ICCAT Collective Vol. Sci. Pap. 56 (3), 1089–1095. Schaefer, K.M., 2001. Reproductive biology of tunas. In: Block, B.A., Stevens, E.D. (Eds.), Tuna Physiology, Ecology and Evolution. Academic Press, San Diego, pp. 225–270. Sibert, J.R., Lutcavage, M.E., Nielsen, A., Brill, R.W., Wilson, S.G., 2006. Interannual variation in large-scale movement of Atlantic bluefin tuna (Thunnus thynnus) determined from pop-up satellite archival tags. Can. J. Fish. Aquat. Sci. 63, 2154–2166, http://dx.doi.org/10.3354/meps10781. Susca, V., Corriero, A., Bridges, C.R., De Metrio, G., 2001. Study of the sexual maturity of female bluefin tuna: purification and partial characterization of vitellogenin and its use in an enzyme-linked immunosorbent assay. J. Fish. Biol. 58, 815–831, http://dx.doi.org/10.1006/jfbi.2000.1490. Tsuji, S., Nishikawa, Y., Segawa, K., Hiroe, Y., 1997. Distribution and abundance of Thunnus thynnus larvae and their relation to the oceanographic condition in the Gulf of Mexico and the Mediterranean Sea during May through August of 1994. ICCAT Collective Vol. Sci. Pap. 51, 161–176. ˜ P., Hidas, E., Graupera, E., Quílez-Badia, G., Tudela, S., Sainz-Trápaga, S., Cermeno, 2011. Bluefin tuna migratory behavior in the western and central Mediterranean Sea revealed by electronic tags. ICCAT Collective Vol. Sci. Pap. 66, 1157–1169. Ueyanagi, S.L., Nishikawa, Y., Tsuji, S., 1997. Identification and occurrence of Thunnus larvae collected from the Gulf of Mexico and the Mediterranean Sea by the Shoyo-Maru cruise in 1994 with the review of recent identification study of larval Thunnus. ICCAT Collective Vol. Sci. Pap. 51, 181–185.