Diet composition of bluefin tuna (Thunnus thynnus L. 1758) in the Eastern Mediterranean Sea,Turkey

This study gives relevant information on the diet composition of the bluefin tuna (Thunnus thynnus) during the spawning period in the eastern Mediterranean Sea. The stomach contents of 218 bluefin tuna were sampled from 2003 to 2006 during the fishing season (May–June) aboard purse seiners operating in the northern Levantine Sea off the coast of Turkey. Stomachs were removed from the fish soon after landing and kept frozen at ?18°C until analysis. Prey items were classified into large taxonomic categories and preserved in 70% ethanol. A total of 745 different prey specimens belonging to 47 taxa were identified, including 34 species of fish, 11 of squid, and two of crustaceans. The most important fish and cephalopod prey belonged to the families Myctophidae, Carangidae, Chauliodontidae, Paralepididae, and Octopoda. This study marks the observation of myctophid fish in the stomach contents of bluefin tuna from the Mediterranean Sea. The paper offers some new information of regional importance and compares the feeding habits of the species to other regions, bringing confirmation on the opportunistic feeding ecology of the species in the enclosed Mediterranean Sea, where bluefin tuna seasonally occur as a strong cohort. New information on the diet composition of T. thynnus in the eastern Mediterranean Sea is revealed; the findings indicate that, depending on the abundance of the different prey species in the habitat, the dominant prey species can be distinctive.     

Feeding habits of the Atlantic bluefin tuna, Thunnus thynnus (L. 1758), in the central Mediterranean Sea (Strait of Messina)

The study of feeding habits of the Atlantic bluefin tuna was carried out in 123 specimens, ranging from 115 to 222 cm fork length (FL) and collected during spring seasons of 2010 and 2011 in the central Mediterranean Sea (Strait of Messina). The analysis of stomach contents allowed us to identify 91 taxa of prey items, mainly belonging to Teleostea (54), Cephalopoda (20) and Crustacea (13). The percentage of index of relative abundance (IRI) shows the highest values for the myctophid Hygophum benoiti (%IRI = 22.854) and the stomiid Chauliodus sloani (%IRI = 15.124), followed by the oegopsid squid Illex coindetii (%IRI = 14.316). The broad spectrum of prey items could suggest a generalist behavior of this predator, with several species that occasionally occurs in its diet. However, if prey are grouped into food categories, the importance of mesopelagic and benthopelagic fishes can be appreciated (54.41 % of %IRI). The assessment of the hypothetical foraging rhythm of the Atlantic bluefin tuna highlighted that its feeding activity is concentrated on diel migrating fauna during night and on larger preys upon daylight. The predation on the high-energetic food as mesopelagic and bathypelagic fishes during the pre-spawning and the spawning period may bring an energetic advantage in tuna metabolism and gonadal maturation     

First record of female germ cells in the testis of a wild Mediterranean northern bluefin tuna Thunnus thynnus L. 1758

The presence of female germ cells was observed in the central portion of the testis of a Mediterranean northern bluefin tuna Thunnus thynnus , 2250 mm fork length and 217 kg. This is the first report of intersex in this species.     

Diet of young-of-the-year bluefin tuna, Thunnus thynnus (Linnaeus, 1758), in the southern Tyrrhenian (Mediterranean) Sea

The diet of juvenile bluefin tuna (Thunnus thynnus) caught 2 to 8 miles off NE Sicily was investigated in order to improve knowledge of the species’ early life history. From 1998 to 2000, 107 specimens ranging from 63 to 495 mm (total length) were fished between July and November. Fishes were caught by trolling line or purse seine in a Fish Aggregating Devices (FADs) area 2 to 8 miles offshore and as by‐catch of the purse‐seine sardine fishery 2 to 4 miles offshore. Most frequently found items in the T. thynnus stomachs were fishes (84.5%), crustaceans (54.6%) and cephalopods (50.5%). The largest contribution in weight was provided by cephalopods (47.3%) and fishes (46.5%), while the most abundant items were fishes (51%), cephalopods (27.2%) and crustaceans (21.1%). These results suggest that young‐of‐the‐year tuna have an essentially piscivorous diet, although invertebrate prey provide a substantial contribution to the food array. Prey show little relationship with FADs, although one prey species (blue runner, Caranx crysos) is associated with FADs in the Mediterranean.     

Microsatellite and mitochondrial DNA analyses of Atlantic bluefin tuna (Thunnus thynnus thynnus) population structure in the Mediterranean Sea

Genetic variation was surveyed at nine microsatellite loci and the mitochondrial control region (868 bp) to test for the presence of genetic stock structure in young-of-the-year Atlantic bluefin tuna (Thunnus thynnus thynnus) from the Mediterranean Sea. Bluefin tuna were sampled over a period of 5 years from the Balearic and Tyrrhenian seas in the western basin of the Mediterranean Sea, and from the southern Ionian Sea in the eastern basin of the Mediterranean Sea. Analyses of multilocus microsatellite genotypes and mitochondrial control region sequences revealed no significant heterogeneity among collections taken from the same location in different years; however, significant spatial genetic heterogeneity was observed across all samples for both microsatellite markers and mitochondrial control region sequences (FST=0.0023, P=0.038 and PhiST=0.0233, P=0.000, respectively). Significant genetic differentiation between the Tyrrhenian and Ionian collections was found for both microsatellite and mitochondrial markers (FST=0.0087, P=0.015 and PhiST=0.0367, P=0.030, respectively). These results suggest the possibility of a genetically discrete population in the eastern basin of the Mediterranean Sea.     

A histological investigation of the occurrence of non-reproductive female bluefin tuna Thunnus thynnus in the Mediterranean Sea

The presence of non-reproductive Atlantic bluefin tuna Thunnus thynnus females in the Mediterranean Sea was investigated through histological analysis of the gonads. Three hundred and twenty-six ovary samples were collected from adults captured at different locations in the Mediterranean Sea during the reproductive seasons between 1998 and 2008. Only three specimens were considered to be in a non-reproductive state: two of them were in a reabsorbing state showing ovaries with early vitellogenic oocytes and extensive α and β atresia of vitellogenic follicles; the third showed gonads with perinucleolar oocytes and was considered to be in a resting state. The low occurrence of non-reproductive individuals found in this study makes it unlikely that non-reproductive individuals aggregate with reproductive ones during their migration towards spawning grounds. Further research is suggested in order to investigate the potential presence of non-reproductive individuals on non-spawning grounds during the reproductive season.     

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

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 255.0 cm while body weights (W) ranged from 2.6 to 247.0 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 = ?2.942 + 2.730 ln FL; female regression equation: ln W = ?3.660 + 2.878 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 = 3.721 + 0.851 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 = 373.08 [1?e^{?0.07(t + 1.76)}]. Weight‐at‐age values were derived from the von Bertalanffy predicted FLs‐at‐age 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.     

Feeding habits and trophic levels of bluefin tuna Thunnus thynnus of different size classes in the Mediterranean Sea

Possible changes in diet and trophic levels in relation to size of Mediterranean bluefin tuna, Thunnus thynnus, were investigated using labelled carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes. Samples were obtained from two locations in the southern Tyrrhenian Sea (Western Mediterranean Sea) in May and October 2004. The δ¹³C and δ¹⁵N analyses revealed at least three significant isotopic groups [small juveniles (0.7–2.2 kg), sub‐adults (15–50 kg) and adults (70 to 225 kg)]. δ¹³C was negatively dependent on weight, while δ¹⁵N was positively dependent on weight [TW = 8.2 (±0.16) + 0.03 (± 0.0) *δ¹⁵N (n = 49; r = 0.91; P < 0.001)]. Different prey contribution to the diet was highlighted for each class. The diet of juveniles comprised zooplankton, small pelagic fish and some coastal fish; sub‐adults relied on medium pelagic fish, shrimps and cephalopods, and adults relied mainly on cephalopods and larger fish. The trophic level (TL) of tunas belonging to each size class was closely correlated to weight, starting from ca 3.0 TL for Group I and reaching 4.4–4.8 TL for the giants. Bluefin tuna, from small juveniles to giants, showed a shift in feeding preferences due to different use of habitats and food items as a function of the life stage.     

Muscle temperature in free-swimming giant Atlantic bluefin tuna (Thunnus thynnus L.)

Muscle temperature was measured by telemetry in giant Atlantic bluefin tuna whilst the tuna were free-swimming in large pounds. Muscle temperature tended to remain steady at about 24°C; water temperature ranged from 9 to 17°C. Muscle temperature was much less variable than stomach temperature in these fish. Muscle temperature varied less than 3°C whereas stomach temperature varied by as much as 14°C.     

Geographic variation of body morphology of the Atlantic bluefin tuna, (Thunnus thynnus,Linnaeus, 1758)

Geometric morphometric methods were used to explore body shape morphology in 260 Atlantic bluefin tuna, Thunnus thynnus, collected in Sardinia (Western Mediterranean) during the breeding phase and in the Bay of Biscay (North Eastern Atlantic) during the feeding phase. The shape of each specimen was captured by high resolution digital images and recording the 2‐D coordinates of seven morphological landmarks. A general procruste analysis (GPA) was applied in order to eliminate any morphological variations resulting from size, position or orientation of specimens. A thin plate‐spline (TPS) method was then used to provide a graphical representation of the shape conformation between two sets of data. Results of the regression model between the direct and indirect measurements accounted for a R² = 0.98. The Principal Components Analysis shows differences linked to the two sampling areas, accounting for 37% and 19.97% of the body shape variation in the first (PC1) and second (PC2) principal component, respectively. Specifically, the deformation grid projection highlights the major differences regarding the anterior‐ventral part of the body (landmark 5‐6‐7). These differences might not necessarily be linked to an actual population substructure. Instead, it was hypothesized that such body shape differences were due to the diverse life phases during which specimens were collected, since the reproductive specimens show a ‘pot‐bellied’ shape, which was larger than for the feeding specimens that showed a ‘slimmer’ shape. Analyses of likely sexual dimorphism conducted on Sardinian specimens did not reveal any significant differences; whereas body shape differences related to the pre‐ and post‐reproductive sizes were detected.     

Amino acid composition and physico-chemical properties of bluefin tuna (Thunnus thynnus) myoglobin.

1. The heart ventricle myoglobin of Atlantic bluefin tuna has been purified and its amino acid composition has been determined. 2. The perturbing effect of guanidine hydrochloride on the molecular structure of tuna ferrimyoglobin and its corresponding apoprotein has been investigated by Soret absorbance and ultraviolet fluorescence. 3. The conformation-free energy of unfolding delta G0 has been calculated by thermodynamic treatments of the data concerning guanidine unfolding. 4. The results have been compared with other known myoglobins, particularly those of yellowfin tuna.     

Immunocytohistochemical characterization of pituitary cells of the bluefin tuna, Thunnus thynnus L

In this paper we report the first complete mapping of the pituitary in a tuna species. The various different adenohypophysis cell types of the bluefin tuna, Thunnus thynnus L. have been identified and located using different antisera against mammalian and piscine hormones and various histochemical techniques: PAS, Alcian Blue pH 2.5 and lectins -ConA and WGA(Neutral and Acidic Glycoproteins); Bromophenol Blue (Proteins) and Tioglycollate-Ferric-Ferricianide-FeIII (-S-S- groups). Prolactin (PRL) and adrenocorticotrophic (ACTH) cells were located in the rostral pars distalis (RPD) of the pituitary, while the proximal pars distalis (PPD) displayed gonadotrophic (GTH), thyrotrophic (TSH), somatotrophic (GH) and also a few PRL cells. Moreover, somatolactin (SL) and melanotrophic (MSH) cells were identified inside the pars intermedia (PI). Interestingly, some SL-immunoreactive fibers were also detected in the neurohypophysis. Some GTH cells were also located on the exterior surface of the PI. Glycoproteins containing mannose (Man) and/or glucose (Glc); N-acetyl-glucosamine (GlcNAc) and/or sialic acid sugar residues, as well as -S-S- groups, were observed in GTH, TSH and SL cells. The Bromophenol Blue technique stained amphiphilic SL, acidophilic GH cells and weakly ACTH cells. GH and ACTH cells were unreactive to PAS, Alcian Blue, Tioglycollate-Ferric-Ferricianide-FeIII and lectin (Con A and WGA) techniques. Finally, PAS reaction was positive in amphiphilic SL cells, which were PbH unreactive, while MSH and ACTH cells were stained with PbH technique.     

Ultrastructure of oogenesis in the bluefin tuna, Thunnus thynnus

Ovarian ultrastructure of the Atlantic bluefin tuna (Thunnus thynnus) was investigated during the reproductive season with the aim of improving our understanding of the reproductive biology in this species. The bluefin, like the other tunas, has an asynchronous mode of ovarian development; therefore, all developmental stages of the oocyte can be found in mature ovaries. The process of oocyte development can be divided into five distinct stages (formation of oocytes from oogonia, primary growth, lipid stage, vitellogenesis, and maturation). Although histological and ultrastructural features of most these stages are similar among all studied teleosts, the transitional period between primary growth and vitellogenesis exhibits interspecific morphological differences that depend on the egg physiology. Although the most remarkable feature of this stage in many teleosts is the occurrence of cortical alveoli, in the bluefin tuna, as is common in marine fishes, the predominant cytoplasmic inclusions are lipid droplets. Nests of early meiotic oocytes derive from the germinal epithelium that borders the ovarian lumen. Each oocyte in the nest becomes surrounded by extensions of prefollicle cells derived from somatic epithelial cells and these form the follicle that is located in the stromal tissue. The primary growth stage is characterized by intense RNA synthesis and the differentiation of the vitelline envelope. Secondary growth commences with the accumulation of lipid droplets in the oocyte cytoplasm (lipid stage), which is then followed by massive uptake and processing of proteins into yolk platelets (vitellogenic stage). During the maturation stage the lipid inclusions coalesce into a single oil droplet, and hydrolysis of the yolk platelets leads to the formation of a homogeneous mass of fluid yolk in mature eggs.     

Structure of the inner ear of bluefin tuna Thunnus thynnus

The ears of five large bluefin tuna Thunnus thynnus were examined by light and scanning electron microscopy (SEM). The gross structure of the ear is similar to that in other fishes. The ears, however, appear to be held more rigidly in place than in other species through the presence of an extensive connective tissue between the membranous ear and the surrounding bone. Moreover, unlike other fishes, the semicircular canals and otolithic end organs have thick cartilaginous walls and there is a dense matrix surrounding the otoliths rather than a more watery fluid found in other species. SEM revealed that the saccular epithelium has a 'standard' hair cell orientation pattern. The hair cell orientation patterns in the lagena and utricle resemble those found in most other fishes. Ciliary bundle density and length vary in different epithelial regions and each ear had >2 × 10⁶ sensory cells. The morphological results support the hypothesis that bluefin tuna probably do not detect sounds to much over 1000 Hz (if that high) and that only very loud anthropogenic sounds have the potential to affect hearing in this species.     

Oogenesis in the bluefin tuna,Thunnus thynnus L.: a histological and histochemical study

Histology and histochemistry are useful tools to study reproductive mechanisms in fish and they have been applied in this study. In the bluefin tuna, Thunnus thymus L., oocyte development can be divided into 4 principal phases based on the morphological features of developing oocytes and follicles. The primary growth phase includes oogonia and basophilic or previtellogenic oocytes classified as chromatin-nucleolus and perinucleolus stages. The secondary growth phase is represented by vitellogenic oocytes at early (lipid globule and yolk granule 1), mid (yolk granule 2) and late (yolk granule 3) vitellogenesis stages. The maturation phase involves postvitellogenic oocytes undergoing maturation process. During the spawning period, both postovulatory follicles, which indicate spawning, and atretic follicles can be distinguished in the ovary. Carbohydrates, lipids, proteins and specially those rich in tyrosine, tryptophan, cystine, arginine, lysine and cysteine, as well phospholipids and/or glycolipids and neutral glycoproteins were detected in yolk granules. Moreover, affinity for different lectins (ConA, WGA, DBA and UEA) was detected in vitellogenic oocytes (yolk granules, cortical alveoli, follicular layer and zona radiata), indicating the presence of glycoconjugates with different sugar residues (Mannose- Man- and/or Glucose -Glc-; N-acetyl-D-glucosamine- GlcNAc- and/or sialic acid- NANA-; N-acetyl-D-galactosamine- GalNAc-; L-Fucose -Fuc-). Histochemical techniques also demonstrated the presence of neutral lipids in globules (vacuoles in paraffin sections) and neutral and carboxylated mucosubstances in cortical alveoli. By using anti-vitellogenin (VTG) serum, immunohistochemical positive results were demonstrated in yolk granules, granular cytoplasm and follicular cells of vitellogenic oocytes. Calcium was also detected in yolk granules and weakly in follicular envelope. In females, the gonadosomatic index (GSI) increased progressively from May, during early vitellogenesis, until June during mid and late vitellogenesis, where the highest values were reached. Subsequently, throughout the maturation-spawning phases (July), GSI decreased progressively reaching the minimal values during recovering-resting period (October).     

Isolation and characterization of seven tetranucleotide microsatellite loci from Atlantic northern bluefin tuna Thunnus thynnus thynnus

Microsatellite‐enriched genomic libraries were obtained from the Atlantic northern bluefin tuna, Thunnus thynnus thynnus and seven tetranucleotide markers were successfully isolated and characterized from this library. These markers were found to have between 1 and 17 alleles in Atlantic northern bluefin tuna and heterozygosity ranged from 0 to 0.85. No deviations from the expectation of Hardy‐Weinburg equilibrium were found for any marker. Several of these markers amplify reliably in other tuna species.