Age determination and growth analyses of freshwater pearl mussel shells from Fallselva, Begna, Numedalslågen, Skauga, Teksdalselva, Borråselva, Mossa and Aursunda, together with published data from Hunnselva, Simoa, Håelva, Ogna and Figga, here enable us to investigate potential effects of water regulation on shell growth.

The shell length of the bivalves in this study was expressed as a function of their age in growth diagrams and then compared with general growth curves for the species. Shell size varied between the mussel localities and even between collection sites within the same water system. As a result, in order to estimate the age of the mussels by their shell length, growth curves need to be constructed for each water system and in some cases even for collection sites in the same river, especially if the populations have different species of host fish.

The compiled results of our measurements showed that annual growth rates of freshwater pearl mussels from regulated river are similar to the ones from non-regulated rivers. Although, when we analysed each river individually and the growth rate through time we did find many examples that indicated that water regulation and construction work related to water regulation, influence negatively the growth rate of freshwater pearl mussel.

Shells from Mossa showed lower growth rate and strongly reduced recruitment of juvenile shells when the construction work at the power plant began in the early 80’s until late 90’s when the water flow was increased. Mossa shells collected in1984, before the construction of the power plant had a higher frequency of growth disturbances in comparison with shells collected in 2010, from the same locality after the power plant started to work. Shells from Borråselva shells had one of the highest frequencies of growth disturbances in comparison to all other shells analysed in the present study. They had also lower growth rate since the reconstruction of the dam began in 2008. In Fallselva the water flow was redirected when the power plant was moved to another location, which caused extreme low water levels at several mussel localities. Shells collected in these localities had low growth rates and high frequencies of growth disturbances. The construction work at the power plant in Begna did not show any indications of negative influence on shell growth and the annual growth had similar variations both upstream and downstream the power plant.

Our results indicate that the regulation of the water flow may affect negatively the growth rate of freshwater pearl mussels when it results in extreme low water levels. There are also indications that the water regulation may indirectly affect the shell growth rate by changing the water temperature, nutrients and the velocity of the water.

Elena Dunca, NRM, Box 50007, 10405 Stocholm, Sverige; elena.dunca@nrm.se;

Bjørn Mejdell Larsen, NINA, Postboks 5685 Sluppen, 7485 Trondheim; bjorn.larsen@nina.no