2022 Volume 86 Issue 3 Pages 110-122
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.
