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.

In Chita Bay, western Mikawa Bay, large outbreaks of Fulvia mutica occurred every 5 to 7 years until the 1990s. However, since the 2000s, outbreaks only occurred in 2007 and 2018, and the catch has been relatively low between these outbreaks. Although this has become a major problem in the fishing industry, the mechanism of stock fluctuations and outbreaks has not been clarified. Therefore, we conducted a survey in Chita Bay from 2015 to 2018 to clarify the mechanisms of outbreaks and population dynamics. In 2018, the target year of the survey, a large outbreak of F. mutica occurred (35,000–90,000 inds∙1,000 m⁻², 700–2,800 kg∙1,000 m⁻²). Furthermore, it was determined that F. mutica juveniles that were spawned in Chita Bay in the spring and settled in the shallow area around Chita Bay grew to a shell length of approximately 30 mm. Furthermore, it was determined that F. mutica juveniles moved to fishing grounds at a depth of 10 m or deeper in October, where they grew rapidly and reached 70 mm in March. The outbreak was possibly caused by a decrease in predator starfish due to the development of a strong oxygen-deficient water mass in the summer and decrease in predatory pressure.

We investigated the age and growth of jack mackerel Trachurus japonicus collected in five areas in the sea along the Pacific coast of Japan: Uwa Sea, Kii Channel, Kumano-nada Sea, Sagami Bay, and Joban–Boso area, between 2011 and 2014. Fish were age-determined using transverse sections of sagittal otoliths. Fork length-at-ages were estimated by von Bertalanffy growth function and were compared among the areas. Fish growth in Uwa Sea was fastest among the five areas, while that in the Joban–Boso area was slowest, and those in Kii Channel, Kumano-nada Sea, and Sagami Bay were approximately intermediate between the southwestern area of Uwa Sea and the northeastern area of the Joban–Boso area. In the five areas, most fish were between 1 and 3 years old. No fish older than 9 years occurred in the Uwa Sea, while fish older than 15 years occurred in Sagami Bay and the Joban–Boso area. Fish growth was slower in areas with a longer cool season with sea temperatures lower than 18°C.