Temporal and spatial distribution of diarrheic shellfish toxins in blue mussels, Mytilusedulis, along the Swedish Skagerrak coast

Bivalves are ancient animals that feed by filtering large volumes of water. In this way, phytoplankton in the water column are concentrated in the mussels. The hazards associated with the consumption of mussels are thus dependent on the occurrence and composition of toxic algae in the areas where shellfish are grown. Diarrheic shellfish toxins have occurred regularly in Sweden since 1987. A rapid intoxication versus slow detoxification of mussels is a common phenomenon in Swedish waters. Concentrations of DST in mussels are normally low from March to August (<160 μg/kg mussel meat) and high from October to December (>160 μg/kg mussel meat). Some years minor peaks above the limit can be observed from mid June to mid July. Peaks of toxins in mussels are mostly recorded from October to December, but the pattern can, however, differ significantly due to location and year. In 1997 mussel farmers in the Tångesund/Nösund region experienced very low levels of DST. However, the following year, 1998, high levels were recorded 27 weeks in a row. A particularly interesting area is the Kolje Fjord. This region had low levels of toxins until 1999, despite regular recordings of potentially DST producing algae. Today mussels grown and harvested in this area have similar toxin levels as mussels from other fjords in the Skagerrak region.