Evaluation of Predictive Equations for Total Height Determination of the Critically Endangered Noble Pen Shell Pinna nobilis (Linnaeus, 1758) in French Coastal Lagoons

Marine species inhabiting lagoons may exhibit different growth dynamics and life expectancies compared with their relatives living in open seas, hence possibly influencing overall population dynamics. The remaining populations of the noble pen shell or fan mussel Pinna nobilis are almost all found in lagoons, of which the Thau lagoon harbors the last dense population of P. nobilis. Population dynamics and growth patterns have been assessed for decades based on the determination of the total height (Ht) of living fan mussels. As these animals are partially buried in the sediment, the total height must be estimated from other length measures. Several equations have been developed to evaluate Ht of individual P. nobilis, yet their applicability to residual lagoon populations remains unassessed. The goal of this study was to evaluate the suitability of five historically established equations for the estimation of the total height of fan mussels in the last sanctuaries in France, the Thau, and Diana Lagoons. A total of 68 still planted, recently dead fan mussel shells from both lagoons were examined. The Garcia-March and Ferrer equation provided the most accurate estimates in both Lagoons, with the highest correlation coefficients (0.97 for Thau and 0.93 for Diana) and lowest error metrics (mean absolute error: 1.86 and 1.87; root mean square error: 2.22 and 2.13, respectively). This equation is thus recommended as the most reliable proxy for estimating the total height of living P. nobilis.