2.2. Fish Sampling

Specimens of the golden grey mullet (Chelon auratus) were captured in summer (June 2013) at two sites of the Aveiro lagoon (Figure 1), namely SJ and LAR. A total of eight wild fish were collected at each sampling site. Sampling was carried out during low-tide using a traditional beach-seine net named “chincha”. Juvenile specimens, being not sexually mature, were selected to minimize the interference of gender on metabolome data, and thus in results interpretation. Fish total length at LAR and SJ was 13.6 ± 2.1 and 16.5 ± 2.1 cm, respectively. Immediately after catching, the fish were anesthetized, sacrificed, and properly bled in compliance with the ethical guidelines of the European Union Council (Directive 2010/63/EU), and then the gills and liver were removed. The tissue/organ samples were immediately flash-frozen in liquid nitrogen in the field, while at the lab the samples were stored at −80 °C until further processing for metabolite determinations. As previously reported, the levels of tHg that were accumulated in the gills of mullets from SJ and LAR were 0.08 and 0.22 µg/g dry weight (d.w.), respectively, whereas the concentrations of tHg in the fish liver from SJ and LAR were 0.9 and 2.3 µg/g d.w., respectively [2,3]. Moreover, the level of bioaccumulation of MeHg in the fish gills from SJ and LAR was 0.02 and 0.1 µg/g d.w., respectively, whereas in liver from SJ and LAR it was 0.6 and 1.7 µg/g d.w., respectively [2,3].