Light pollution accelerates development of nuptial colouration in three-spined stickleback (Gasterosteus aculeatus) males

Artificial light at night (ALAN) is a rapidly expanding environmental threat. The ongoing shift from warm yellow light to cool white LEDs is expected to increase its detrimental effects on wildlife. Currently, light pollution is rapidly increasing in densely populated coastal areas, which are important breeding habitats for many animals, especially fishes. Yet, the effects of ALAN on fish reproduction remain poorly known. We investigated if ALAN influences the development of nuptial colouration in three-spined stickleback (Gasterosteus aculeatus Linnaeus, 1758) males at the start of the breeding season, and if any such effect depends on the light spectrum. We found that males developed nuptial colouration faster in the presence of ALAN. This effect was especially pronounced under longer peak wavelength, yellow light, whereas it was weaker under shorter peak wavelength, cool white light. Our results demonstrate that ALAN has the potential to alter the reproductive physiology of stickleback and influence the timing of spawning, and that the strength of this effect depends on the light spectrum. Earlier spawning readiness under ALAN could desynchronise spawning from optimal ecological conditions, potentially affecting individual reproductive success and population dynamics.