Variability in transport processes of Pacific saury Cololabis saira larvae leading to their broad dispersal: Implications for their ecological role in the western North Pacific

Abstract

Transport of Pacific saury Cololabis saira larvae from spawning to nursery grounds was examined based on datasets from larval distribution surveys during winter seasons 1990–2011. In the surveys, a neuston net was towed for 10min over broad areas off the southern Pacific coast of Japan. Transport trajectories of saury larvae and temperature they experienced were analyzed based on a numerical particle-tracking model from 1993 to 2011 using satellite-based sea surface current and temperature data. Larval growth during the transport was estimated from the relationship of larval growth to environmental factors obtained in a previous study. Results of the particle tracking experiments indicated that a large amount of saury juveniles (knob length, KnL, 40mm) were transported to an area south of the Kuroshio axis. Only a relatively small amount (14.6%) of the 40mm KnL stage juveniles were transported in and around the Kuroshio extension, although it has previously been considered important that the juveniles are transported to the area north of the Kuroshio extension. Abundance of juveniles transported to the area north of the Kuroshio extension indicated a positive relationship to the abundance of medium size class saury in the autumn fishing season. Even though a very small part (3.9%) of hatched larvae were transported to this area, recruitment of the saury population depends on this amount of transported hatched larvae. On the other hand, the high juvenile transport rate to the areas south of the Kuroshio axis may suggest a broad dispersal mechanism to ensure that some larvae find a suitable habitat. Subsequently, the importance of saury in the marine ecosystem as a food item for predatory species in the sub-tropical region is discussed.