We live in an increasingly busy world. Where once the night sky was lit only by the predictable movements of the moon and stars, it is now scored by the blinking lights of aeroplanes and satellites. The oceans are some of the last true wildernesses left on Earth. Even in its deepest recesses, however, human influence can be found. Our plastic litter has sunk to the bottom of the Mariana Trench and is washed onto the shores of even the most remote uninhabited islands.

The noise of our mining activities echoes around the globe, drowning out whale songs. In coastal waters, where more than a quarter of humanity competes for space with 90% of marine species, the dark of night has long offered respite to marine wildlife from the drone of boats and danger of nets. Increasingly, artificial lights are shining into the ocean, illuminating processes that have evolved to go unseen.

In the warm shallow waters of the Maldives, the world’s largest fish, the whale shark, has had a chequered history of interactions with humans. The slow growing species, which can reach up to 18 metres in length and take between thirty and forty years to mature, was hunted to the brink of extinction for its meat and the oil from its liver, which was used to waterproof boats and cisterns. More recently, a duty of care for the species has developed among Maldivians as the ecotourism industry has made whale shark sightings one of the country’s most valuable natural resources. Tourism is the largest industry in the country, with shark focused ecotourism bringing in a minimum estimate of 145.83 million US dollars per year.

The conservation success of a wholehearted switch from hunting to protecting a threatened species cannot be emphasised enough. However, the current system is not without flaws. Ecotourism based conservation places an economic burden on animals – requiring them to ‘pay their way.’ A decrease in the number of tourists threatens conservation efforts, as occurred in the Maldives when the global lockdown in 2020 led to a sharp increase in shark hunting and finning. Ecotourism can also be a victim of its own success.

Collisions between busy boat traffic and slow-moving whale sharks leads to fatalities and injuries, potentially hampering global conservation efforts. At the same time, a constant stream of snorkellers and divers surrounding sharks causes them to change their behaviour, sacrificing the warmth and abundant food of the surface for the peace of less crowded deeper waters. Wildlife tour operators are under constant pressure to provide clients with Instagram worthy moments.

Recently, a clever method for guaranteeing shark sightings has been developed. Originally employed as a fishing technique, light pooling involves shining bright spotlights into the sea at night. The light attracts plankton which in turn attracts smaller fish, setting up an irresistible buffet for filter feeding animals like whale sharks and manta rays. The technique is so successful that many tour operators are now offering multiple light pooling excursions seven days a week throughout the night. Similar tours are being offered in the Philippians, Hawaii, and Palau.

Animals live their lives according to fixed patterns of behaviour. Over the course of a day, different species coordinate their sleeping and feeding to maximise their chance of survival. The ocean never fully sleeps. While the sun’s light and heat promote ‘diurnal’ animals to wake and gather resources, the darkness is an alarm bell to ‘nocturnal’ creatures. In the vast spaces of the oceans, many species of sharks and rays spend most of their lives alone. Over the course of a year, the gradual shift in day length, as well as the cycling of the moon, tells animals when it is time to abandon this solitary existence and gather in rare but predictable aggregations, such as the annual gathering of more than 1,000 black tip reef sharks in the Gulf of California.

Light has been a lone constant and reliable cue for coordinating behaviour for millions of years. When viewed across a geological time scale, our climate is in a state of flux. Glaciers reach from the poles towards the equator before retreating, and even the shape of our continents is not permanent. The solar and lunar cycles are independent of these changes. Although days have lengthened since the Earth’s formation 4.5 billion years ago, these changes are so gradual that entire dynasties of animal have evolved and gone extinct with little to no change in annual cycles of day length.

Light is such a fundamental cue for organising behaviour that light sensitive, or ‘photosensitive’, cells can be found in even the simplest animals, such as sponges. This indicates that the common ancestors of all animals were capable of sensing and responding to light. Sharks and rays belong to the elasmobranchs. This is the clade of the cartilaginous fish, which are as closely related to humans as they are to the other species of bony fish, with whom they share the seas. As well as eyes, elasmobranch possess photosensitive cells concentrated in the pineal gland – often referred to as a ‘third eye,’ as well as smaller patches throughout their skin. Studies have shown that being exposed to irregular light cycles can disrupt elasmobranch behaviour and physiology.

The problem of artificial light at night – often simply referred to as ‘light pollution’ – has only become apparent this century. On land, we know that light pollution increases cases of mortality and disease among animals, including birds, mammals, and reptiles. In extreme cases, it leads to the collapse of ecosystems as the lack of coordination between species knocks delicate systems out of balance. Light pollution in the sea is less well studied, despite the fact that the area exposed to artificial light is increasing by between two and six percent annually. Isolated studies indicate that this is doing worrying damage to coastal species. Juvenile damselfish are 36% less likely to survive, while orange-fin anemonefish lose 51% of their mass with exposure to artificial light. Unfortunately, the impact of light pollution of most marine species, including whale sharks and manta rays, has not yet been studied.

So, what can be done? Ecotourism is a necessary conservation tool if coastal communities of humans are to live alongside abundant and healthy marine life. But every kind of human activity has the chance of damaging nature if carried out without oversight. Light pooling will harm marine species, including whale sharks and manta rays. We can say this based on the importance of light for cueing elasmobranch behaviour. What these harms will be remains to be seen.

Often, conservation is an arms race. As new harmful practices emerge, researchers and legislators must rush to find proof of the exact mechanisms and severity of that harm before any action can be taken. In the time this takes, irreparable damage is done to individual animals, species, or entire ecosystems. There is another way. The precautionary approach is a system which is increasingly adopted in conservation. Under this system, emerging practices are assumed to be harmful until proof has been gathered that they are not. The precautionary approach essentially ends the arms race by shifting the burden of proof from conservationists to industries.

Adopting a precautionary approach to light pooling wouldn’t mean completely banning it and asking people to sacrifice the extra income it provides. Instead, it would mean accepting that light pooling is probably harmful and limiting how often it is done. Leaving certain days of the week free of light pooling, limiting its occurrence to a few hours after sunset, or limiting the number of operators at any given time are all options. While these compromises will not completely negate harm to sharks and rays, they would certainly prove a safer for marine life than a complete lack of regulation.

Light is unique among the long list of emissions we humans generate in its temporary nature. When we turn a light off, there is no lingering poison in the atmosphere, no legacy of greenhouse effect that future generations must deal with. With sufficient coordination and goodwill, the problem of light pollution could be solved in a single night though the flick of several million switches.

The oceans resources are vast. The success of ecotourism proves that the wonder of seeing wild animals in all their majesty is one of these resources. This resource is finite, and sensible management is needed today so that incredible animals like whale sharks and manta rays share our waters in the future. By regulating light pooling we can give animals a chance to rest and recover from human interactions in the last pieces of temporal wilderness left on Earth. So, for the sake of nature, let us leave at least some of the night in darkness.