Análisis de la distribución de metales pesados (Hg, Cd, y Pb) en atún rojo del Atlántico(Thunus thynnus L. 1758) e impacto de las condiciones hidrodinámicas locales en su acumulación muscular

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OBJECTIVES The overall objective of this Doctoral Thesis was to advance the understanding of the tissue kinetics of major heavy metals of environmental and health interest in a species of significant economic, social, and consumption interest, the Atlantic bluefin tuna (Thunnus thynnus). This includes potential differences associated with the hydrodynamic characteristics of the areas where aquaculture farms are located. To this end, the following specific objectives were set: • To understand the bioaccumulation and tissue distribution of mercury (Hg), cadmium (Cd), and lead (Pb) in the main organs and tissues of fattened tuna in aquaculture farms (blood, liver, kidney, muscle, brain, gill, and bone). • To identify differences in muscle accumulation of Hg, Cd, and Pb associated with the hydrodynamic characteristics of the areas where the fattening farms are located. The starting hypothesis is that variables such as tissue type and the hydrodynamic characteristics of the area where the farms are located influence the concentration and tissue distribution of the heavy metals of greatest health and environmental interest. MATERIALS AND METHODS The aquaculture facilities from which the tuna samples were taken are located in two distinctly different bodies of water, due to the underwater morphology: the San Pedro del Pinatar Marine Cultivation Polygon and the El Gorguel Marine Cultivation Polygon (Cartagena). For the studies described in Chapters 1 and 2 of this Thesis, 43 specimens of Atlantic bluefin tuna were used, from which samples of blood, brain, muscle, bone, liver, kidney, and gill were obtained. For the study described in Chapter 3, 67 specimens of Atlantic bluefin tuna from the mentioned polygons and 32 wild adult specimens were used; muscle samples were taken from all of them. Age, sex, and biometric data were obtained from all sampled specimens. The Hg content in each tissue was determined using pyrolysis atomic absorption spectrometry with a gold amalgam. For Cd and Pb analysis, samples were analyzed using inductively coupled plasma optical emission spectrometry. This same technique was used to determine selenium (Se) as it is an antagonist of Hg toxicity. To characterize the hydrodynamic conditions of the environment, two oceanographic buoys located in each of the aquaculture polygons were used to collect information on water temperature, waves (height, period, and direction), and marine currents at different depths (speed and direction). FINAL CONCLUSIONS This Doctoral Thesis has provided simultaneous insights into the distribution kinetics of the most environmentally and health-relevant metallic contaminants in the blood and main tissues (liver, kidney, muscle, brain, bone, and gill) of the Atlantic bluefin tuna. FIRST: Despite not identifying a specific bioaccumulation pattern for Hg or Se, findings suggest that muscle is the tissue that accumulates these elements the least. SECOND: Se plays an important role in mitigating the effects of Hg, both in blood and in the studied tissues, highlighting the importance of this trace element in food products and possible by-products that can be used from fish remains (bones, liver, etc.). THIRD: In the case of Cd, the tissue sequence based on metal concentration was well-defined when addressing the data without considering sex. For Pb, the tissues with the concentrations were the same regardless of whether the study was sex-dependent or not. Sex is a variable to be considered in the kinetic evaluation of these metals in the species. FOURTH: In the individualized study, bone was the tissue that accumulated the least Cd and Pb, while for Hg, the brain should also be considered. In all cases, blood was positioned in an intermediate-low place in the sequence from highest to lowest concentration. FIFTH: The differences in hydrodynamic conditions between the San Pedro del Pinatar Marine Cultivation Polygon and the El Gorguel Polygon significantly affect the bioaccumulation of Hg, Cd, and Pb in Atlantic bluefin tuna, with a more pronounced effect for Cd and Pb. These and other variables associated with hydrodynamics should be monitored, given the trends observed regarding the muscle concentration of Cd and Pb. SIXTH: The study of tissue kinetics of heavy metals in Atlantic bluefin tuna is complex for various reasons, including differences associated with sex, weight, diet (wild and farmed), and the hydrodynamic characteristics of fattening areas. All these variables should be carefully considered in future studies on the species.