INFLUENCE OF LIGHT AND SALINITY ON BEHAVIOR OF LARVAL JAPANESE FLOUNDER (PARALICHTHYS-OLIVACEUS) AND IMPLICATIONS FOR INSHORE MIGRATION

Vertical movements are a fundamental part of tidal transport, a migrat ion strategy found in many coastal fishes. Because light is considered important to vertical migration, the ontogeny of eye development, pho totaxis 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 photo tactic 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 swimmin g and settling behaviour was observed. Settling behaviour was apparent during the early stages of metamorphosis and increased with developme nt. Larvae in the early stages of metamorphosis responded to night tim e light levels by swimming up. Larvae were more active in water of hig her salinity relative to their acclimation salinity and were more like ly to settle in water of lower salinity. During the late stages of met amorphosis, the importance of light to activity decreased, increasing the relative importance of salinity. Results are consistent with the h ypothesis that tidal migration develops as diurnal vertical migration controlled by light is replaced by tidal movement cued by changes in s alinity. Our findings suggest that efficient transport of larvae insho re depends on a clear salinity signal and that variability in such a s ignal may influence migration success.