Growth characteristics of freshwater pearl mussels, Margaritifera margaritifera (L.)

1. Shell growth in the freshwater pearl mussel, Margaritifera margaritifera, was investigated. Three non-linear growth models (i.e. power, logistic and von Bertalanffy) were fitted to Scottish length-at-age data sets and compared. 2. Overall, the von Bertalanffy model outperformed the other two approaches, generating the smallest residuals in eight out of 11 samples (the logistic model provided slightly better fits to the other three). It was concluded that individual M. margaritifera appear to grow in an approximately asymptotic fashion and that the von Bertalanffy equation is an appropriate growth model to fit to freshwater pearl mussel length-at-age data. 3. The ranges in von Bertalanffy parameter estimates observed (k = 0.023-0.075 year(-1), L(infinity) = 77-158 mm, t(o) = -3.93-4.33 years) are typical of those reported in northern European populations. 4. Most of the populations investigated had relatively low k-values and high maximum age (A(max)) estimates. This feature, which suggests high long-term productivity and less vulnerability to decline (i.e. larger, longer-living mussels produce more offspring), may be a reason why these populations have survived until now. The population which appears to be the most vulnerable (i.e. which has the highest k and lowest A(max)) is probably not recruiting adequately at present. 5. An index of absolute growth (mean shell length-at-age) was also used for comparing different populations. Observed between- and within-river differences in mussel growth patterns may be associated with a number of environmental factors, particularly water temperature and productivity. 6. A significant positive relationship between river length and mean mussel length-at-age was observed. In general, mussels grow large in large, cold rivers and vice versa, although there are exceptions which suggest that additional factors may be involved.