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Lights in fish farms can keep salmon awake
Lighting in fish farms can stress the fish and prevent them from sleeping or developing properly. New research now makes it possible to measure light from the fish’s point of view.
Until now, it has simply been assumed that salmon receive the correct dose of light.
However, the sensory cells in fish eyes react differently to light than those in humans do.
“We have assumed that the salmon are receiving the correct light dosage based on the lamp’s stated power. We take spot samples with a lux metre, but it’s designed for humans, so we don’t have an exact measurement of the amount of light the fish actually receive,” says Harald Ian Muri.
He is a researcher at SINTEF Ocean.
Closer to measuring light on the fish’s terms
The aquaculture industry uses light as one of its most important biological drivers. It does so without a common, biologically based standard for what the correct light actually is.
Light is largely controlled according to lux values or practical experience. That tells us nothing about what the fish actually detect biologically.
Through a dedicated project, researchers are testing how light affects salmon in hatchery facilities.
So what is it that salmon actually see and need?
Melanopsin is a light-sensitive protein found in special cells in the eye.
The researchers have now achieved a breakthrough.
The results so far already make it possible to use melanopic irradiance as a standard for measuring circadian rhythm. Melanopic irradiance refers to the amount of light that affects the melanopsin system in salmon – particularly blue light.
This information allows the researchers to calculate how much of the light from any given lamp actually affects the fish's circadian rhythm, making it possible to measure.
This means light can no longer be described simply as strong or weak. Instead, researchers can determine whether the biological signal is too high, too low, too long, too short, or whether the day-night contrast matches the fish's natural rhythm.
“This is precisely what makes light a possible control parameter, in the same way that we currently measure oxygen,” says Muri.
Light controls both growth and well-being
The fish farming company SalMar, one of the largest producers of farmed salmon along the coast of Norway, started using continuous lighting when the facility opened in 2017. However, they felt that something was not quite right. They therefore took a step back and started over.
They introduced a schedule of 12 hours of moonlight followed by 12 hours of light.
“We’ve seen a great effect by changing from continuous light to controlled light. It has to do with the natural processes in the fish. It’s beneficial for the fish to be able to follow their own rhythms,” says Stein Roar Ernstsen.
He is the production manager at SalMar Settefisk.
Fish are used to extremely different light environments, both in freshwater and saltwater.
Even though they are good at adapting to different light levels, they need to have a rhythm. Without that rhythm, their sleep cycle is disrupted in a way that can make the fish stressed and sleep deprived.
A fish's biological rhythm is regulated by hormones. If the fish lose their natural rhythm, they can no longer regulate these hormones properly, and their health declines.
More testing and a dedicated handbook
As the project now conducts additional testing at Sunndalsøra, the new knowledge is being applied in practice.
“We are measuring light, converting it to melanopic irradiance and using the material as a biologically relevant control parameter in the experiment,” says Muri.
Nofima is also preparing a dedicated handbook as part of the project. It will explain what light is like in water, how light is measured, and how fish absorb and respond to it.
The handbook will also describe current practices in the industry and point towards best practices.
“The handbook will be a useful tool for the aquaculture industry. It will set some frameworks for how controlled light works. We are happy to be able to contribute to that,” says Ernstsen.
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Read the Norwegian version of this article on forskning.no
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