Influence of light and salinity on behaviour of larval Japanese flounder (Paralichthys olivaceus) and implications for inshore migration

Vertical movements are a fundamental part of tidal transport, a migration strategy found in many coastal fishes. Because light is considered important to vertical migration, the ontogeny of eye development, phototaxis and vertically oriented behaviour were examined for different metamorphic stages of Japanese flounder larvae (Paralichthys olivaceus). The timing of rod recruitment to the retina and the development of the ability to dark adapt were investigated histologically. Phototaxis experiments were conducted to determine when during development phototactic behaviour changed. Observations of vertical movements were made using infrared sensitive video equipment. Larvae raised at S=32 were observed at S=30, 32 and 34 under changing light conditions. Light was increased from nocturnal to daytime levels in log steps while swimming and settling behaviour was observed. Settling behaviour was apparent during the early stages of metamorphosis and increased with development. Larvae in the early stages of metamorphosis responded to night time light levels by swimming up. Larvae were more active in water of higher salinity relative to their acclimation salinity and were more likely to settle in water of lower salinity. During the late stages of metamorphosis, the importance of light to activity decreased, increasing the relative importance of salinity. Results are consistent with the hypothesis that tidal migration develops as diurnal vertical migration controlled by light is replaced by tidal movement cued by changes in salinity. Our findings suggest that efficient transport of larvae inshore depends on a clear salinity signal and that variability in such a signal may influence migration success.