Spatiotemporal variations in the distribution of round herring eggs in the East China and Japan Seas during 1997–2013

Abstract

Spawning stock biomass (SSB) and environmental factors, such as water temperature, can affect the size and location of spawning grounds of small pelagic fishes. We examined the effects of SSB, water temperature, and day length on the probability of egg presence of the round herring Etrumeus teres to clarify spatiotemporal variation in its spawning grounds in the Tsushima Warm Current region off the west coast of Japan. Egg presence probability (EPP) was estimated from monthly egg surveys and annual stock assessments from 1997 to 2013. Our results indicated that EPP was positively correlated with sea surface temperature (SST), and increased between 12 and approximately 17 °C, where it became constant, and then decreased when SST exceeded 24 °C. From February to June, locations of the main spawning grounds gradually shifted northeast from the waters off western Kyushu to the Japan Sea as the areas with water temperatures suitable for successful egg development (≥17 °C) expanded. However, few eggs were observed during the season when day length shortened daily, even when SST was suitable. The main spawning grounds expanded more towards northern waters in warmer years than in cooler years. In addition, EPP was positively correlated with SSB, which resulted in an expansion of the spawning grounds into coastal regions in years with high SSB. These results indicate that spawning of the round herring responds to seasonal changes in SST and day length, and interannual fluctuations in SST and SSB in the Tsushima Warm Current region.