Sonic tagging reveals age and size-specific spatial variation during Pacific herring spawning migrations in northern Japan

Abstract Timing and duration of spawning migration of Pacific herring is known to vary by fish age and length. Older fish mature earlier, but there are few verified examples of differences in the pre-spawning migration of individuals varying in age or size. Here we use sonic tagging to examine the fluctuations in age, fish length and maturity throughout the migration of herring in Akkeshi, Japan. We tracked the horizontal and vertical movement of individual fish using acoustic telemetry and biologging methodology. In Akkeshi Lake, older, larger individuals migrated early, reached peak maturity and remained in the lake until early April, after spawning. The timing and duration of the stay in Akkeshi Lake coincided with the freezing and melting of the surface layer of the lake. Fish tracking indicated no differences in herring in different length classes migrated to and from Akkeshi Lake and offshore and stayed in Akkeshi Bay. For the vertical movement, larger, older herring occupied a wide depth range during diurnal vertical movements as they moved to Akkeshi Lake and to offshore. In contrast, the smaller group occurred frequently in depths shallower than 5 m throughout the day and night. Greater depth variation could result in beneficial effects in predator avoidance and increased prey encounter opportunities though vertical migration. In contrast, smaller herring that were more confined to surface waters could experience more frequent sensing of the environmental cues of spawning grounds such as low-salinity water and olfactory stimulation near the surface. Therefore, there are differences in the benefits and risks during migration for each fish length, which may result in superior spawning grounds and individual mortality in this area.