Abstract (English Version)

This study provides a comprehensive analysis of the visibility of red light (600–940 nm) in fish and its influence on behavioral responses, integrating multi‑year field experiments, aquarium observations, optical measurements, and visual physiology.
Across numerous trials involving seabass, black sea bream, rockfish, flatfish, crustaceans, and amphibians, the results consistently demonstrate that red light is largely invisible to fish under low‑light conditions, where vision is dominated by rod photoreceptors (rhodopsin), which detect brightness but not color.

At night, fish rely almost exclusively on rod‑based scotopic vision, resulting in minimal sensitivity to long‑wavelength light, including wavelengths near the conventional visible boundary (≈600 nm). This explains the frequently observed phenomenon in field reports:
fish show no initial reaction to red illumination but begin responding slowly after prolonged exposure, a behavior attributed to the delayed activation of cone photoreceptors as retinal illumination increases.

Species‑specific differences were also identified.
Rockfish and greenling exhibit complete insensitivity to wavelengths ≥660 nm even during daytime, whereas seabass and black sea bream retain limited daytime sensitivity but show markedly delayed responses at night. Environmental factors further modulate perception: fish inhabiting brackish or freshwater environments may express different opsin sets, altering their effective visible spectrum.

Optical comparisons revealed that pure‑wavelength red LEDs produce narrow‑band, high‑purity red light that maintains intensity and invisibility characteristics, whereas red filters applied to white LEDs drastically reduce brightness and yield low‑purity, mixed‑spectrum light, making them unsuitable substitutes for true red illumination.

Overall, the findings indicate that red light functions as an effectively “invisible” illumination source for many fish species at night, enabling anglers to observe or land fish with minimal disturbance. The study highlights the importance of wavelength selection, species‑specific visual physiology, environmental conditions, and optical purity in understanding and applying red‑light technology in night fishing.