Traceability markers in Atlantic bluefin tuna ("Thunnus thynnus")

Zusammenfassung

This Thesis presents the main characteristics of the Atlantic bluefin tuna species (ABFT, Thunnus thynnus), the status of its captivity breeding and explores possible traceability tracers and markers. Our general objective was to discriminate between ABFT batches using non-invasive identification techniques and markings. Some of the studies presented have never been pursued in ABFT. For the develop of this Thesis, first, ABFT weighing less than 1500 g were taken in 2018. The ABFT were of three different batches: i) onshore tanks (called farmed), ii) sea cages and ii) wild, but only i) wild and ii) farmed in some of the studies. From the tunas, several soft tissue samples were taken (liver, kidney, muscle and brain), also hard tissue samples (gill and bone), and otoliths (biletral and symmetrical ‘earstones’). In the First Section of the Thesis, we envisaged the study of the elemental composition of different tissues as non-invasive and natural tags. In Chapter I, copper (Cu) was the only element with statistically significant differences between groups in all the tissues (liver, kidney, muscle and brain). Regarding the literature, food is the major source of the Cu and therefore these differences would be due to the differing diet of the batches. In the discrimination analysis (DCA), batches were differenced with more than 80% of success in some tissues. In resume, the essential elemental composition in soft tissues in ABFT could be used to discriminate different tuna batches, especially because DCA can generate formulas for identifying the possible batch of specimens. In relation to these results, Chapter II envisages the use of accessible and more commercially worthless tissues (gill and bone). Again, copper was the only element with statistically significant differences between groups in both. In this study, bone was the tissue with higher statistically significant differences between groups, but gills the one with higher discrimination success. In the last Chapter of this Section (Chapter III), we found statistically significant differences in the otolith’s Na, Mg, P, Sr which were higher in farmed and Rb, higher in wild tunas. In the DCA, P and Sr by themselves had more than 75% of discrimination success. In resume, Sr and P are the elements that could drive batch discrimination in western Mediterranean waters (specifically from the Mazarrón Bay) using the whole otolith composition profile. In the Second Section of the Thesis, we analysed deeply the morphometry of the otolith and some related parameters like the otolith asymmetry (morphological differences between right and left otoliths of an individual), and the vaterite presence (aragonite polymorph, related to abnormal morphologies and dysfunctionalities). In the first Chapter of this Section, Chapter IV, we found differences between batches in one trait for right and five for left otoliths. The differences by side, in fact, could be explained due to some pathologies within the otoliths. And the differences by batch could be due to the rearing conditions, given that the otolith morphometry depends on fish genotype and environment. In the DCA, the weight and eccentricity were selected as morphological variables to discriminate. In the right otoliths, the 63.4% of tunas were successfully classified, meanwhile 57.4% of tunas in left otoliths. Finally, the obtained formulae in this analysis can be applied to unidentified tunas, given that when introducing the traits’ values in the formula the result can tell us the probable tuna batch. Therefore, the morphometry of the otolith has been proved to be a useful group biomarker, being the otolith weight as trait or the left side otoliths of choice for a group differentiation analysis based on otoliths. In Chapter V, it was the first time that two types of asymmetry were found in ABFT otoliths. In general, there is a described link between high asymmetry values and rearing conditions, however, open waters’ conditions can also cause asymmetry, because the environmental problems causing stress (i.e., pollution, marine traffic…) are the mostly mentioned asymmetry triggers. In our study, farmed specimens showed higher asymmetry, and the most probable factors would be the differing water chemistry, diet and environmental stressors (ie., substances accumulation, water changes, oxygenation, photoperiod or stocking densities). In the Chapter VI, abnormal morphologies and vaterite were found in ABFT juveniles otoliths. The vaterite presence (26.67% in farmed vs. 4.55% in wild) and the vaterite quantity by otolith (88.15% in farmed vs. 12% in wild) were higher in farmed tunas than wild specimens. In the Third Section of the Thesis we developed two different artificial markings. Firstly, in Chapter VII, we injected oxitetracycline (in two concentrations: 100 and 200 ppm), in ABFT juveniles and found 100% of marking success without statistical differences in the mark intensity between concentrations. Finally, in Chapter VIII we tested alizarin red marking in ABFT eggs (50 ppm and 3h of immersion). We also obtained 100% of marking success. Comparing these results with the previous study (Chapter VII), this method had an easier application: no direct handling of the fish was needed and grinding the samples was unnecessary. As far as we know, no other chemical marking experiences have ever been conducted in ABFT.