Unexpected increasing trends in the concentration of contaminants in European perch (Perca fluviatilis) and inactivity of ethoxyresorufin-O-deethylase (EROD) in European perch and eelpout (Zoarces viviparus) have beenobserved at a Swedish coastal reference site. This study uses data from different sources to investigate plausibleexplanations. The results showed that a change in diet and an improved overall condition coincide with anincrease in mercury in European perch. Furthermore, an increase in several organic contaminants in Europeanperch coincided with the introduction of an invasive deep-burrowing polychaete, which likely contributed to therelease of contaminants through bioturbation. The increase in EROD-activity in both species seems to be relatedto contaminants that reach the fish through the water rather than the diet. The results show that for contaminantsthat are taken up via the diet, trends in contamination can be opposite for different species of fish inthe same area.