Modelling growth in the critically endangered freshwater mussel Margaritifera auricularia (Spengler, 1793) in the Ebro basin

Here we analyse in detail, for the first time, the growth pattern of the endangered freshwater mussel Margaritifera auricularia in the Ebro basin, a life history trait essential for conservation purposes. We combined information on size and age from captive bred juveniles, together with growth annuli from living preadults studied in the field, and empty shells of preadult and adult individuals. We compared the fit of six types of (asymptotic and sigmoid) non-linear growth models and, in terms of residual errors and AIC values, the sigmoid ones were superior, being the Generalized von Bertalanffy and Richards models the best fitted ones. After an initial exponential growth phase in juveniles, growth rate starts to decelerate at an inflection point corresponding to an age of seven years. At an age of about 30 years, the growth rate markedly declines and attains an asymptote at c. 150 mm shell length. Global growth rate is relatively low but comparable to other members of the family, and the maximum age estimated from ring counts was 68 years. We demonstrate that the use of sigmoid models provides more accurate estimation of growth patterns in freshwater mussels, as previously observed for other bivalves.