Age determination and annual growth analyses of two M. margaritifera shells from river Haukåselva, Norway, was carried on thin transversal sections prepared using the method developed at the Swedish Museum of Natural History in Stockholm, Sweden. In one of the shells (the oldest) elemental distribution was analysed using ICP-MS. 

The growth analyses showed that mussel B is younger and had higher growth rate than mussel A indicating that the two locations where the shells were found had different growth conditions. As such, the annual growth pattern of the two shells had a poor match but in both shells the growth was found decreasing after 2008, which indicated that some environmental factors had caused distress in both locations in the last living years.

Variations in annual growth showed lower growth in the periods 1961-1978 (except 1967) and 1996-2013. In the same periods of time an increased amount of Fe, Mn, P, Ba and Sr was measured in the shell, while the levels of Cu, Co and Pb were lower. This together with the absence of heavy metals (such as Al, As, Cd and Cr) in the shell indicated that the freshwater pearl mussels in Haukåselva are not affected by the pH condition of the water, but highly affected by low O levels possibly due to eutrophication.