Leave The Light On

Whether it be littering, oil spills, chemical pollution from factories, ghost fishing gear or microplastics, the general public are becoming increasingly aware of how we are damaging the natural environment and altering animals' habitats. But what many of us are only just starting to realise is how unnatural light may be a serious form of pollution. Known to alter the movement patterns of some migrating species and confuse hatchlings trying to make their way down the beach, we already know that light pollution can be harmful to marine creatures. So are our lights affecting sharks? Where is this light coming from out in the ocean? And how can we reverse the damage?

Nurse shark in Bimini, The Bahamas (Image property of Sophie Maycock)

Chemical Reaction

In the past we used to imagine pollution as nasty chemicals or toxins leeching into the environment; creeping into water and air from industry waste, oil rig accidents or maybe from unethical dumping (you can learn more about these issues at World Clean-Up Day). Whilst these are undeniably massive problems, in recent years we have become increasingly aware that pollution is not just limited to chemicals. We now know that wild animals and their habitats can be harmed by any stimuli that can affect any of their senses. For example, we now know that noise pollution in the oceans is causing a huge problem (to find out more head over to Keep The Noise Down).

Scientists are starting to learn that sharks are sensitive to underwater noise from boats and other sources (Image Credit: HelloRF Zcool / Shutterstock

Getting Lit

Scientists are now turning their attention to how anthropogenic light pollution may be harmful to the natural environment. In undeveloped areas we can easily see how light conditions would naturally exist; extremely dark nights at new moons, that gradually lift slightly brighter on a monthly cycle thanks to the increasing illumination of the waxing moon (Carroll & Harvey-Carroll, 2023).

Artificial lighting, however, often provides very bright, completely consistent illumination; night and day, regardless of the season. As a result, unnatural light is now a pervasive pollutant to coastal marine environments. Significant coastal urbanisation is the most obvious source; lights in homes, businesses and factories, and around roads, ports and harbours along the shore, providing a continuous glow. Additionally, lighting on water vehicles and shipping vessels can transport light away from more densely populated areas and even out to sea, where offshore platforms and fishing vessels also contribute to the problem (Tinari et al, 2026).

Increasing development is contributing to enormous amounts of light pollution around the globe (Image Credit: Alejandro Sanchez de Miguel, Source: Dark Sky)

Light Me Up

Scientists are becoming increasingly curious about the impact that coastal light pollution may have on sharks... and their findings are certainly very concerning.

There is concern that artificial light pollution could affect the reproduction of already-threatened nurse sharks.

Studies of nurse sharks (Ginglymostoma cirratum) off the coast of Miami, Florida have shown that exposure to artificial light around a coastal city inhibits the normal production of a hormone called melatonin. Levels of this hormone should naturally be at night, as it plays a role in regulating the natural diel pattern of rest and sleep. Over longer time periods, it also contributes to synchronising the body's reaction to seasonal changes (Carroll & Harvey-Carroll, 2023; Tinari et al, 2026).

Therefore, there is concern that continuing coastal urbanisation may confuse these sharks' natural daily and seasonal rhythms and behaviours. This could potentially have an impact on reproduction - a great concern when you consider that this species is already flagged as Vulnerable according to the IUCN Red List of Threatened Species (Tinari et al, 2026).

In The Dark

Interestingly, this study showed us that the responses to light pollution can vary significantly in different species, as the melatonin levels of blacktip sharks (Carcharhinus limbatus) sampled in the same area were not as seriously affected by light pollution. This is likely because they are a more mobile species that do spend all their time around the urbanised coastal areas, where the nurse sharks have a much smaller home range and tend to stick to a smaller area - right under these bright lights (Tinari et al, 2026).

As they are a highly mobile species - moving from place to place - blacktip sharks are not impacted by light pollution in the same way as coastal species with a smaller home range around urbanised areas (Image Credit: Martin Prochazkacz / Shutterstock)

Experts investigating how light affects brain activity in small spotted catsharks (Scyliorhinus canicula) found that even very small amounts of illumination can have an impact - with light as low as a single candle affecting the sharks' brain activity (Hamasaki & Streck, 1971).

Therefore, whilst we have not found any evidence that melatonin was affected in this study, this does not mean that there are not other impacts that we do not understand, and even the smallest amount of artificial light may be having some sort of unforeseen impact on these sharks (Tinari et al, 2026).

Only more research will allow us to understand how serious and widespread the affects may be...

Artificial light as dim as a single candle has been shown to alter the brain activity of small spotted catsharks (Image Credit: Martin Fisch / WikimediaCommons)

References

Carroll D & Harvey-Carroll J (2023). The influence of light on elasmobranch behavior and physiology: a review. Frontiers in Marine Science, 10, 1225067. Access online.

Hamasaki DI & Streck P (1971). Properties of the epiphysis cerebri of the small-spotted dogfish shark, Scyliorhinus caniculus L. Vision Research, 11:3. Access online.

Tinari AM, McDonald MD, Cooke SJ, Gallagher AJ & Hammerschlag N (2026). Sharks at night, exposed to city light: Melatonin concentrations in two shark species differ in response to artificial light at night. Science of The Total Environment, 1015, 181446. Access online.