Population dynamics of Ruditapes philippinarum (A. Adams & Reeve, 1850) after the major ecological crisis of 2018 of the Berre lagoon, southern France

Abstract

ABSTRACT Coastal lagoons are environments with an important ecological richness, which makes them socio-economically highly important environments. However, they are often subject to anthropogenic pressures that may seriously threaten them. The phenomenon of anoxia is regularly the cause of massive mortality and loss of biodiversity. For example, in the Berre lagoon (France), in 2018, an anoxic crisis decimated the Zostera meadow and the stock of Manila clams, Ruditapes philippinarum (A. Adams & Reeve, 1850). As clams are a fished species, this massive mortality has resulted in a fishing cessation and thus is having a significant economic impact. This study is a contribution to our understanding of the resilience of species after a strong perturbation. In our case, we aim to understand the time needed for the stock of R. philippinarum to recover after the ecological crisis of 2018 by providing an analysis of the population dynamics between October 2018 and October 2020. The gonad development stage and condition index highlighted that clam size at sexual maturity was between 20 and 23 mm. The clam's reproduction season is continuous between March and October, with differences observed between sites, and between years at the same site in the Berre lagoon. Long-term field monitoring (in situ cages coupled with sclerochronology) was performed to estimate the growth rate and the mortality. von Bertalanffy growth function parameters show a high growth rate (K = 1.42 yr−1) with a small ${{L}_\infty }$ (35.70 mm). This means that clams need 1.5 yr to reach 30 mm in length: the minimum catch size for the fishery in the Berre lagoon. The mortality rate was 2.52 yr−1, which is possibly due to significant variations in water temperature and episodes of hypoxia and anoxia. These data on the population dynamics of R. philippinarum in the Berre lagoon show that the clam's recovery time can be relatively fast (some years) only if the environment becomes favourable and the anoxic conditions are absent.