Japanese sardine and Japanese anchovy are two of the most commercially important small pelagic species in the western North Pacific, which exhibit reciprocal counter–cyclical population oscillations: when one becomes abundant, the other becomes scarce and vice versa. For the first time, the histological condition of the gonads of two species collected in the same area at the same time of year during different periods of their stock status has been compared. During the period of increasing anchovy stocks and decreasing sardine stocks in the late 1980s and early 1990s, a high degree of atresia of primary growth oocytes in sardine ovaries and a normal state of anchovy gonads were observed. A histological analysis of gonads revealed the opposite situation during the period of rising sardine stocks and declining anchovy abundance in 2019–2020. The study found that 91% of Japanese sardine ovaries exhibited no signs of atresia, while the remaining 9% contained 1–2% of oocytes undergoing atresia. In contrast, all anchovy gonads showed atresia of 13–25% of primary growth oocytes. The onset of replacement in Japanese sardine and anchovy stocks occurs in individuals with gonads at the immature and regenerating phases of gonad development prior to spawning and can be identified using a histological method, which can be used in monitoring to predict the status of sardine and anchovy stocks earlier than egg and larval surveys.
Zhukova et al. (Sun,) studied this question.