Aquarium lighting: protecting the biological night

Lighting in a public aquarium serves several legitimate purposes. It supports animal viewing, exhibit interpretation, plant and coral photosynthesis, staff operations and visitor safety. Yet light is also one of the most important biological time signals. Its intensity, spectrum, duration and timing help organise daily and seasonal rhythms across aquatic species.

Managing lighting therefore involves more than switching off the main fixtures. Corridor lights, emergency signage, display screens, decorative projectors, equipment-room lamps and LED status lights may create persistent illumination after closing time. Levels that appear negligible to staff can still be biologically meaningful underwater.

Darkness is part of the habitat

In fish, light-dark cycles contribute to the organisation of activity, rest, feeding and reproduction. Responses are species-specific. A diurnal reef fish, a crepuscular predator and a nocturnal benthic species will not use the same light environment in the same way. Age, season, social conditions and feeding schedules can also modify their rhythms.

A field experiment on European perch (Perca fluviatilis) and roach (Rutilus rutilus) exposed fish to approximately 15 lux at the water surface during the night. The researchers did not detect a clear difference in melatonin under their experimental conditions, but they reported reductions in circulating sex steroids and in pituitary expression of gonadotropins. The study does not provide a universal threshold for aquarium collections. It does, however, demonstrate that relatively dim night lighting may affect reproductive physiology without producing an obvious clinical sign.

This is a useful warning for animal-care teams. Continued swimming or feeding does not necessarily show that a lighting regime is harmless. Chronobiological disruption may remain subtle and emerge through changes in reproductive performance, space use or daily activity patterns.

Corals depend on natural lunar information

Broadcast-spawning corals use several environmental cues to coordinate gamete release, including temperature, seasonal photoperiod and moonlight. A global study published in 2023 analysed 2,135 spawning observations from 156 species. In most of the genera examined, corals exposed to artificial light at night spawned one to three days closer to the full moon than corals on unlit reefs.

Even a modest shift can reduce synchrony between neighbouring colonies and lower the probability that eggs and sperm meet under favourable conditions. This is directly relevant to aquariums involved in ex situ conservation, coral research or captive propagation. Their lighting programmes may need to reproduce not only an adequate dark period, but also credible seasonal and lunar variation.

A permanent “moonlight” channel is not equivalent to the natural moon. Lunar illumination changes across the month, moonrise occurs at different times, and dark intervals remain present. A blue LED channel left on every night for visual effect may mask precisely the temporal information that corals use.

Conducting a practical night-light audit

A meaningful assessment should take place after the venue has closed and the main exhibit lighting has shut down. The enclosure should be viewed from the water surface and, where possible, from the animals’ perspective rather than only from the visitor walkway.

The review should check:

• direct spill from corridors, emergency exits and service areas;
• reflections from glass, ceilings and pale surfaces;
• screens, signs, indicator lights and decorative fixtures;
• the actual duration of dawn and dusk transitions;
• whether software settings match the light reaching the water;
• access to shaded areas, shelters or dark refuges;
• accidental persistence of a blue LED channel after other channels switch off.

A lux meter is useful for documenting change, but lux alone does not define biological exposure. Light transmission varies with depth, turbidity, water colour, enclosure geometry and LED spectrum. Species also differ in spectral sensitivity. Instrument readings should therefore be combined with species knowledge and observations of night-time behaviour.

Corrective action does not require complete redesign

The primary objective is a consistent period of genuine darkness. Night work should be limited to necessary interventions and performed with the lowest practical level of light. Where permanent safety lighting is required, fixtures can be directed towards walkways, shielded from the water and positioned to avoid illuminating the surface.

Programmes should be reviewed enclosure by enclosure. Gradual transitions may reduce startle responses at switching times, but excessively long ramps can unintentionally shorten the dark phase. For corals targeted for reproduction, lunar and seasonal schedules should be documented, tested and aligned with the geographical origin and biology of the colonies.

Infrared video can support behavioural monitoring without adding visible light. It may reveal persistent activity, unusual distribution, repeated surfacing or failure to use normal resting sites. Changes in lighting should be recorded alongside spawning, feeding response, aggression, injuries, mortality and space use, so that delayed effects are not overlooked.

A risk-based lighting procedure

A simple internal procedure can assign each enclosure a biological lighting profile: species present, natural activity period, required day length, minimum dark phase, transition times, need for lunar simulation and unavoidable sources of night light. This turns a general concern into a maintainable operating standard.

Temporary deviations should also be managed. Evening events, maintenance, veterinary procedures, filming and emergency work may expose animals outside the normal schedule. Recording these events helps teams distinguish one-off disturbance from a persistent design problem and supports better interpretation of behavioural or reproductive changes.

Conclusion

In a public aquarium, lighting should be managed as a core environmental parameter. A suitable photoperiod is not defined only by the timetable of the main fixtures. It also requires control of residual light, transitions, spectrum and, where biologically relevant, lunar cues.

Protecting the biological night is usually compatible with visitor experience and operational safety. The essential step is to assess the facility after closing time and from the perspective of the animals.

How Vetofish can support public aquariums

Vetofish can audit lighting schedules and night-time exposure, relate them to behaviour, reproduction and health records, and recommend enclosure-specific adjustments. Support can include operating procedures, staff training and integration of lighting into animal-welfare assessments and preventive veterinary programmes.

References

• Davies T.W., Levy O., Tidau S., Marangoni L.F.B., Wiedenmann J., D’Angelo C., Smyth T. (2023). “Global disruption of coral broadcast spawning associated with artificial light at night.” Nature Communications, 14, 2511. DOI 10.1038/s41467-023-38070-y.
• Brüning A., Kloas W., Preuer T., Hölker F. (2018). “Influence of artificially induced light pollution on the hormone system of two common fish species, perch and roach, in a rural habitat.” Conservation Physiology, 6(1), coy016. DOI 10.1093/conphys/coy016.
• Kaniewska P., Alon S., Karako-Lampert S., Hoegh-Guldberg O., Levy O. (2015). “Signaling cascades and the importance of moonlight in coral broadcast mass spawning.” eLife, 4, e09991. DOI 10.7554/eLife.09991.