Mercury levels in commercial mid-trophic level fishes along the Portuguese coast – Relationships with trophic niche and oxidative damage

Abstract Sardines (Sardina pilchardus) and mackerels (Scomber spp. and Trachurus spp.) are amongst the most common fish caught along the Iberian coast for human consumption and, as mid-trophic level organisms, they constitute essential prey in the diet of several marine predators. Thus, the assessment of trophic ecology, contamination exposure, and stress responses in such organisms provides valuable information on the environmental health in the mid-trophic chain and the potential risks for consumers. This study aims to contribute for an integrative mercury assessment of the marine ecosystem by measuring stable isotopes, mercury levels, and oxidative stress parameters in five commercial fish species in the northeast Atlantic. Samples of muscle and liver were collected from a total of 116 individuals in two fisheries assessment surveys (Spring and Autumn) along the whole Portuguese coast. Mercury concentrations and stable isotope analysis (δ13C and δ15N) were performed in the muscle, while oxidative stress parameters (lipid peroxidation and DNA damage) were measured in both muscle and liver. Mercury concentrations were found to be considerably lower than values recorded in previous studies and the limits set by the European Union law, which confirms the non-toxic status of these species along the Portuguese coast, concerning this harmful element. Interestingly, no differences were found among the three genera, despite clear differences in trophic niches. Still, mercury concentrations were significantly higher during Autumn than Spring, possibly due to upwelling events during the Summer. Moreover, no relationships between the mercury concentrations and oxidative stress parameters were found. Overall, this work provided an integrative mercury assessment of commercial fishes from the mid-trophic chain, particularly from pelagic (and neritic) regions in the northeast Atlantic, showing that mercury concentration in these species is low. However, it is important to note that this study focused on small- and medium-sized specimens (overall ranging from 13–25 cm in length), and thus higher concentrations of mercury may be present in larger fish due to bioaccumulation.