Preliminary standardization of long-term spawning abundance data for the Pacific stock of Japanese sardine

Monthly egg surveys for Japanese sardine (Sardinops melanostictus) have been conducted since 1978 over a broad area from coastal to offshore waters around Japan. With the initiation of the NPFC small scientific committee on Japanese sardine (SSC JS), we examined the appropriate temporal and spatial coverage of the spawning abundance data for use in the stock assessment of the Pacific stock of Japanese sardine. We also conducted a standardization analysis using spatiotemporal modeling. First, we found that, since the late 1990s, the survey area has remained largely consistent and is considered to have covered the main coastal spawning grounds of Japanese sardine. Next, we examined whether to include spawning abundance data from the waters off southern Kyushu, hereafter referred to as the Satsunan region, where spawning grounds originating from a different stock, the Tsushima Warm Current stock, are thought to form depending on the period. The results suggested that spawning in the Satsunan region has likely been associated with the Tsushima Warm Current stock from 2003 onward. Based on these results, we standardized the spawning abundance data for the period from 2003 to 2025 for the Pacific coastal waters from Tohoku to eastern Kyushu, excluding the Satsunan region off southern Kyushu. The standardized index showed that spawning abundance gradually increased during the 2010s following the low-abundance period in the 2000s, peaked in 2019, and has declined in recent years. This document provides a technical description of the long-term data of spawning abundance and the standardization analysis, as a basis for further discussion on its potential use in stock assessment.