The Duro river empties into the sea near Porto -- the Duro estuary is peculiar in that the bottom sediment has less mercury than than the top.. like 7ug -1 vs < 1ug-1  usually settles to the bottom being a heavier metal. The transport in the bottom water is more dissolved that's the upper water which is attached to sediments and salt. Still gorgeous and lots of seafood. In the USA have a look at Fish and Shellfish Advisories and safe eating guidelines on EPA.gov -- if they haven't been removed by the current administration in Washington DC.