Effects of light wavelength on feeding and surface death in larval Pacific bluefin tuna (Thunnus orientalis)

We investigated the expression patterns of visual opsin genes in Pacific bluefin tuna ( Thunnus orientalis ; PBT) larvae at the onset of feeding and examined how light wavelength influences their swimming behavior, feeding rate, and surface-death rate. At 2 days post-hatch (dph), all opsin genes except for the green-sensitive RH2A2 were expressed, suggesting that larvae can perceive a broad range of wavelengths. 2 dph larvae were then reared in 2 L beakers under white, yellow, green, blue, or ultraviolet (UV) light-emitting diodes (LEDs). The proportion of midwater-swimming individuals and their swimming speed were significantly higher under blue, green, and white light, indicating enhanced activity, whereas these metrics were significantly lower under yellow and UV light, suggesting avoidance behavior. Feeding experiments were subsequently conducted under the same five wavelengths, as well as under combined white + UV and dark control conditions, both with and without aeration. Regardless of water agitation, feeding rates were consistently higher in the UV, white + UV, and yellow groups than in the others. Finally, the effects of light wavelength on the surface-death rate of 3-dph larvae were examined: rates were high under blue and green light but significantly lower under UV, white + UV, yellow, and dark conditions. Validation trials in 200 L tanks confirmed that the surface-death rate was significantly higher under green than white light, and significantly lower under yellow light. Overall, these findings indicate that irradiation with UV and yellow light promotes feeding in PBT larvae while inducing an avoidance response that may help prevent surface mortality. • Nine opsin genes were expressed at the onset of feeding in PBT larvae. • Larvae showed wavelength-dependent swimming and phototactic responses. • Blue–green light enhanced activity, whereas UV and yellow light induced avoidance. • Surface-death rate was reduced under UV and yellow light illumination. • UV and yellow light significantly increased feeding rates regardless of aeration.