Fluctuation of the cockle, <i>Fulvia mutica</i>, stock related to the characteristics of hypoxia in Mikawa Bay, Japan

<p>The cockle Fulvia mutica exhibits large fluctuations in catch volume. However, the source of the annual fluctuation in their abundance is unclear. In this study, we analyzed the relationship among spawner abundance (Ct), recruit abundance (Ct₊₁), larval density (Lt) of this species and the maximum area of hypoxia (At) based on the data of 2006–2018, to elucidate the cause of fluctuation in the species’ abundance in Mikawa Bay. Additionally, the relationships between the densities of larvae, juvenile cockles, their competitors and predators, and hypoxia area were investigated during 2016–2018 inclusive of the 2017 dominant year class. Our 2006–2018 data showed that the larval density (Lt) and recruitment (Ct₊₁) decreased during high catch years. In addition, larval densities per catch (Lt/Ct) increased during years with large hypoxic areas, but recruitment per larva (Ct₊₁/Lt) decreased. In the 2017, the hypoxic area (At) was larger than in usual years, but the highest larval density was observed, and the juveniles were predominantly distributed at the areas where hypoxic conditions disappeared early. Overall, these results suggest that there was a relationship which depended on the juvenile population density was assumued to be dependent on larval density and the hypoxia, and the existence of a blank niche after the elimination of hypoxia seemed to be one of the important factors for the increase in abundance.</p>