Distribution of Symbolophorus californiensis (Teleostei: Myctophidae) in the Kuroshio region during late winter: Evidence of a southward spawning migration

Abstract Symbolophorus californiensis is a typical trans-Pacific transitional water myctophid, occurring from off Japan to North America, and is one of the dominant mesopelagic fishes in the transition region of the western North Pacific. This species has been considered to spawn in the warm southern part of the transition region during spring. The present study reports the occurrence of reproductively mature adults of S. californiensis (89.7–118.5 mm standard length) in the Kuroshio region, south of the transition region, during late winter for the first time, based on 111 specimens (1.58:1 F:M) collected in the surface layer at night. More than 30% of females had ovaries containing migratory nucleus oocytes or postovulatory follicles, suggesting that spawning occurs in the Kuroshio region. They were multiple spawners, with a spawning frequency of approximately 3 days and mean relative batch fecundity of 505 eggs g−1. Stomach content analysis revealed that S. californiensis adults feed actively during the spawning season, showing characteristics of multiple spawners. The nighttime ration was estimated to be 2.0–2.4% of the myctophid body weight. They preyed heavily upon copepods (mainly Candacia bipinnata and Calanus sinicus) and euphausiids (mainly Euphausia similis). Our study suggests that at least a part of the S. californiensis adult population undergoes a southward spawning migration from the feeding ground in the northern part of the transition region to the Kuroshio region during winter. Such a migration of myctophids would potentially contribute to the energy flow between these adjacent ecosystems.