The Norwegian Institute of Water Research (NIVA) has been monitoring the mercury levels in the Oslofjord cod since 1984. Their latest research article says that there is an increase in mercury over the last thirty years. The average length of the sampled Oslofjord cod has increased over the last decades, and this might be the main explanation of the pronounced increase in mercury levels.
Pure mercury is not readily taken up in organisms; but in aquatic environments where oxygen is not present, small microbes can transform mercury to methyl mercury. This substance easily binds to proteins. By this mechanism mercury enters the food chain, and the concentration builds up for each level in the chain. The compound is stored in protein-rich tissue like muscles.
— source niva.no 2017-10-14