24-hour light in aquaculture - is it 'active euthanasia'?
In this article, biologist Ole Gabriel Kverneland addresses the challenges that can accompany the use of light in land-based farming.
Aquaculture biologist and chief executive at Laksesystemer (Salmon Systems) AS
“The practice of 24-hour light in aquaculture kills millions of farmed fish every year.” This is how I began my talk about the use of light in fish farming at the Fishfarmer conference in Stavanger last year.
Many people probably thought the claim was harsh, but it is based on knowledge of basic biological mechanisms. The point was to raise a necessary debate about a very important topic that can have a great impact on the welfare of animals. I called 24 hours of light a "slow form of active euthanasia". I will return to why we at Salmon Systems believe this - and how we have ended up in a situation where farmed fish are apparently the only organism we know of that does not need darkness.
Why is darkness a necessity of life?
When an organism experiences darkness, or more precisely the absence of melatonin-inhibiting stimuli, a cascade of vital processes is initiated that repair, heal and strengthen. Melatonin is a powerful antioxidant, and there is an overwhelming body of studies that, across species, show that melatonin protects against inflammation, cardiovascular disease, neurodegenerative disease, depression, blood pressure, DNA damage and cancer. Where light causes oxidative stress and forms free radicals, this is counteracted by the antioxidant melatonin to prevent damage. Studies also show that melatonin simply slows down aging and age-related diseases.
Many people associate melatonin with sleep and it is often called the “sleep hormone”. However, in recent times, we have acquired new knowledge that allows us to distinguish between melatonin that is produced in cells throughout the body (extrapineal melatonin) and the melatonin that circulates in the blood at night (endocrine melatonin). The endocrine melatonin circulates in the blood at night and replenishes the stores in cells that have had high consumption and therefore need replenishment.
Melatonin is fat-soluble and diffuses through the cell membrane to the cells that need it most. This is probably what makes it extra difficult to quantify the need for melatonin circulating at night – the need varies in line with the organism's physiological stress and the consequences of a lack of nightly replenishment only become apparent when illness and stress occur.
Recent studies also show that melatonin initiates processes with so-called nanotubes that enable mitochondrial transfer between cells. Mitochondria are known to be energy factories inside each cell that store energy as easily available ATP (Adenosine triphosphate) = the cell's battery. Through these nanotubes, melatonin ensures that damaged cells can borrow energy from neighbouring cells in order to repair themselves – not unlike a car with a flat battery that gets energy through jumper cables to start the engine. Nature is amazingly smart.
Against this backdrop, it seems obvious that all life forms need periodic absence of light.
Active euthanasia?
Of course, humans in all forms of food production have tried 24 hours of light (24L) to increase productivity by being able to feed animals around the clock. The common denominator, however, is that one must slow down and go back to something that for most species is a maximum of 18 hours of light and six hours of darkness (18L:6D) because the animals simply cannot tolerate more light than this for a long time. In cattle, it has been found that more than 18 hours of light weakens the levels of Insulin-like Growth Factor-1 (IGF-1). IGF-1 is known as a hormone that is important in cell growth, differentiation, proliferation, and survival.
IGF-1 is therefore a hormone that is central to an animal being equipped to face challenges and changes. Studies with so-called IGF-1 knockout animals show delayed organ development, reduced body size, and shorter lifespan. Experiments on chickens show that 18:6 increases performance, improves welfare, and reduces levels of stress hormones.
Periodic darkness is so important that in agriculture, it is legally regulated [in Norway] that “Animals shall not be exposed to permanent artificial light”, “The light shall be adapted to the animals’ natural circadian rhythm … and include an uninterrupted period of darkness of at least 8 hours”. In comparison, regulations on the operation of aquaculture facilities state that “the light source shall not be visible from surrounding homes, hospitals, nursing homes and holiday homes”.
I have visited some of the country's leading greenhouses and was met with questioning looks when I told them that salmon often receive 24 hours of light throughout their lives in land-based facilities. "How is that possible? It's when it's dark that tomatoes grow and get stronger?" When we provide the right temperature, optimal lighting conditions and good nutrition, we see that the plants need even more rest, just like a top athlete. This is not just the opinion of the greenhouse industry, it is well documented with scientific studies. Even in tomatoes, it has been proven that phyto-melatonin is an effective protection against stress, promotes growth, and stimulates the immune system.
It is therefore largely the same basic mechanisms with the stress-reducing and repairing effects of melatonin that drive both agriculture and the greenhouse industry's active use of darkness to strengthen organisms.
As a biologist, I find it very difficult to understand that our farmed fish should be the only organism we know of that does not need any form of absence of light and associated rest. Could this be part of the explanation for the industry's challenge with occasionally high mortality?
How did we end up here?
A traditional light-controlled smolt is given a winter signal where it most often receives 12 hours of light and 12 hours of darkness for a period of six weeks. When the fish is then switched to continuous light, after a certain number of degrees Celsius, it will undergo a series of physiological and morphological preadaptations to life in the sea through the process we collectively call smoltification. It is not unnatural that a fish that is fed 12 hours per day during this period grows more slowly than a fish that is fed 24 hours per day. To maximise growth during the fry phase, many have chosen to skip traditional light control and smoltification, which usually gives the fish a six-week period of light/darkness, and instead accustom the fish to life in the sea using various forms of salt or smolt feed and with 24 hours of light also throughout the traditional light control period.
A large part of the reason why we have ended up with these production methods is probably due to a mismatch between the biology of the fish and the technology used. Most of the artificial lighting we surround ourselves with is designed for humans and not for fish. The lights use simpler and cheaper solutions that cause a flicker that we are unable to see with our eyes. Biologically, there is a significant difference between fish and humans in that the fish, in addition to eyes, has a light-sensitive pineal gland exposed to the environment – it simply has an “eye in the neck”. If we set our mobile phones to record in slow motion, we will be able to see how much of the light around us has this type of flickering lighting technology, including in the form of a dimming technology called PWM (Pulse Width Modulation).
If the fish are exposed to a dimming of light that causes stress twice a day, it is not surprising that people have chosen to use 24 hours of light instead. Although 24 hours of light comes with a biological cost, it is probably calculated to be less than the cost of stressing the fish twice a day. However, the best solution would be to listen to biology and adapt our technology to meet the needs of the fish, rather than trying to force the fish's biology to learn to live with our technology - at the expense of fish welfare.
How do we move forward?
Six years ago, Laksesystemer began working systematically and thoroughly on the development of lighting for aquaculture. Through our parent company Fjøssystemer and our supplier network, we have access to the world's leading professional communities when it comes to lighting for other forms of food production. We have spent a lot of time and resources investigating what we in the fish farming industry can learn from others who produce food.
As an aquaculture biologist, the transition from aquaculture to agriculture and the greenhouse industry was a complete shock when it comes to a professional approach to lighting. In agriculture, there are different dimming technologies for different animals based on whether they only see with their eyes or whether they also have exposed epiphysis. Different animals have different needs for intensity and colour, and different life stages are optimised with combinations of lighting regime, colour composition and feeding profile. Specially developed reflector technology is used to create as lighting that is as uniform as possible, and the colour of the light is used to control activity levels. In the chicken industry, this is summarised in a 33-page European industry standard for choosing lighting for chickens. I encourage everyone who works with lighting in aquaculture to read this standard as inspiration for a knowledge-based approach to the field.
Knowledge is available – if we take the time to search, listen and learn
During Tekset 2025, NMBU's Alexander Figenschou clearly showed where the shoe is pressing. Fish that have received 24 hours of light throughout the entire hatchery phase grow better in the smolt producer, but have higher mortality in the sea. The fish survive as long as they are in a closed and protected environment, but they are not prepared for the harsher realities of a more natural environment in the sea. This is easy to recognise; we humans also get sick more easily when we have had underlying stress over a long period of time. The question may be who should foot the bill; is it the hatchery farmer who has to accept lower growth in his part of the production or is it the food fish farmer who has to foot the bill when the fish are less resistant to stress in the sea?
If the industry itself cannot agree on this, perhaps it is time for fish – like all other livestock in Norwegian agriculture – to receive a unified knowledge base for biological needs and a statutory requirement for a natural circadian rhythm?
Recommended further reading
The European Chicken Commitment (ECC) is a series of welfare criteria that more than 400 food companies in Europe have signed up to, and the ECC Natural Lighting Guide for Broiler Chickens is a thorough, knowledge-based approach that takes the biology of the animal and the physics of light as its starting point and explains how we must adapt technology to biology. We need a similar "Norwegian Salmon Commitment: Lighting Guide for Salmon Smolt Production".
