Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic

Methylmercury (MeHg) determinations in hake, its food-chain, and the surrounding waters and sediments allowed us to show that the higher length or age normalized mercury concentrations of Northwestern Mediterranean (Gulf of Lions: GoL) muscle hakes compared to its Northeastern Atlantic (Bay of Biscay: BoB) counterpart are due to both biotic and abiotic differences between their ecosystems. Bioenergetic modeling reveals that the slower growth rate of Mediterranean hake favors the MeHg bioaccumulation in the fish muscle and explains most of the difference between GOL and BoB hake populations. In addition, the waters of the Mediterranean hake habitat favor a higher MeHg exposition, due to the upper position of the thermohalocline, where MeHg is formed. Furthermore, we show that, within the Mediterranean hake population, a major increase in the biomagnification power (the slope of the relationships between logMeHg and δ(15)N), from 0.36 up to 1.12, occurs when individuals enter adulthood, resulting from the combined effects of lowering growth rate and change in feeding habits. Finally, δ(15)N normalized Hg concentrations indicate that the highest Hg concentrations are for hake from the shelf edge and the lowest are for hake from the Rhône prodelta area, suggesting a lower Hg bioavailability in inshore environments, consistent with MeHg distributions in water, sediment, and preys.

Tributyltin along the coasts of Corsica (Western Mediterranean): a persistent problem

Despite optimistic forecasts by various scientists after regulatory measures were taken in the 1980s, coastal tributyltin (TBT) contamination is still a major problem. The present study concerning Corsica (Western Mediterranean) shows that contamination is not limited to harbour areas, but extends along the coast, involving protected nature reserves. The concentrations measured in harbours, which can reach 200 ng TBT l(-1), tend to incriminate both pleasure craft and ferries providing regular service between the island and the continent. Contamination as high as 7 ng TBT l(-1) has been measured in waters of the Scandola nature reserve, which is quite excessive given the no effect concentrations (NOEC) for marine fauna are around 1-2 ng TBT l(-1). The inadequacy of current regulations and their application are a major factor in this situation.