Light is a fundamental environmental factor shaping the physiology, behavior, and overall welfare of fish. Beyond its role in vision, light acts as a primary regulator of biological rhythms and endocrine processes, influencing a wide range of functions including growth, reproduction, immune activity, and stress responses. This review synthesizes current knowledge on the relationship between light conditions and fish welfare, with a particular focus on photoperiod, light intensity, and spectral composition.
The photoperiod is one of the most predictable environmental cues in aquatic systems and plays a central role in synchronizing circadian and seasonal rhythms in fish (S��nchez-V��zquez 2019). Light signals are transduced via the retina and pineal organ, leading to rhythmic melatonin secretion, which typically peaks during darkness and conveys temporal information to the organism (Bayarri et al 2004). Melatonin acts as a key endocrine mediator, regulating physiological processes such as reproduction, metabolism, and immune function (Maitra and Hasan 2016). In addition, several other hormones and metabolites (e.g. cortisol, GnRH, LH, lactate, glucose, etc.) are affected by photoperiod (Rodriguez et al 2001). Disruption of natural light–dark cycles, for example through continuous illumination, can therefore alter hormonal balance and impair welfare (Liu et al 2026).
In aquaculture, manipulation of photoperiod is widely used to optimize production. Delay of sexual maturation can enhance of growth , in the other hand lightning regime is used for synchronize sexual maturation (Bayarri et al 2004). However, these production-oriented practices may conflict with welfare considerations, as prolonged or unnatural light conditions can disrupt circadian organization and increase chronic stress.
Light intensity and spectral composition further modulate fish responses. Excessive light intensity can induce avoidance behavior and physiological stress, while insufficient light may impair feeding efficiency, particularly in visually oriented species (Li et al 2025). Emerging research indicates that different wavelengths of light can differentially affect physiological processes; for instance, specific light spectra may influence oxidative stress and immune responses in fish, potentially via melatonin-mediated pathways (Jung et al 2016) . These findings highlight the importance of considering not only the duration but also the quality of light.
Behavioral responses to light provide additional insight into welfare status. Fish often exhibit clear preferences for specific light environments, such as shaded or low-intensity zones, and changes in light conditions can alter activity patterns, spatial distribution, and social behavior. Such behavioral indicators are increasingly recognized as valuable tools for assessing welfare in both experimental and applied settings (Marchesan et al. 2004).
Despite significant advances, important knowledge gaps remain. Optimal lighting conditions are highly species-specific and depend on life stage, ecological background, and interactions with other environmental factors such as temperature and stocking density. Furthermore, long-term welfare consequences of artificial lighting regimes are still poorly understood. While continuous light may enhance short-term production, it may also lead to maladaptive physiological states and reduced resilience under more natural conditions.
In conclusion, light is a key but complex driver of fish welfare, acting through integrated neuroendocrine and behavioral pathways. Future research should aim to develop species-specific, welfare-oriented lighting strategies that balance production goals with the biological needs of fish, incorporating a more holistic understanding of environmental interactions.
References
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