Farmers ‘could slash lice numbers with the correct illumination’

A combination of downward-facing cage lights and underwater feeding could reduce sea lice infestation of farmed salmon by up to 70%, a lighting expert says.

Remco Lansbergen, head of the aquaculture and agriculture department at Dutch company Signify (formerly Philips), told Norsk Fiskeoppdrett (Norwegian Fish Farming) magazine that he gets frustrated when he sees farms “that light up like Christmas trees on the fjord”.

“It seems that many people think that light is light, but they don't think about how the light is used,” said Lansbergen.

There has been a rapid development in the use of LED lights in salmon net pens in the last 10 years, but there is still a lot to be done when it comes to getting the full effect of the lighting, as Lansbergen sees it. This applies in particular to the effect that the correct use of lighting can have to prevent the salmon from being exposed to salmon lice.

Lice layer

Simply explained, it is possible to minimise the time the salmon stay in the lice layer, which is the top few metres of the water column, by lowering LED lights below that layer. The salmon are drawn towards the light, so if there is no light in the lice layer, the salmon will dive down to the light.

For this to happen, however, the light must be directional, i.e., only spread downwards, and not in all directions as is the case with some lamps used in facilities today.

“What must be avoided is spreading light upwards in the cage. If you have lamps that light up in all directions, you also light up the lice layer and then the fish will spend as much time above the light as below,” said Lansbergen.

Although directional lights installed under the lice layer will reduce the time that the fish stay in the lice layer, it does not mean that the fish will not rise to the surface.

Underwater feeding

“The fish will rise to draw air on the surface (for fill its swim bladders), but the right lighting will reduce the time the fish stays in the lice layer,” said Lansbergen. Feeding can also be adapted to keep the fish in deeper water by having the feed discharge under water.

Using the right light and underwater feeding will, according to Lansbergen, lead to a reduction in lice infestation of up to 70%.

The reason why farmers have installed lighting in marine facilities in recent years is not primarily to combat lice, but to increase growth and delay sexual maturation. And for these purposes, too, the light is more effective if it is diffused exclusively downwards in the cage. This is because the salmon not only have receptors for light in the eyes, but mostly in a pineal gland on the top of the head. The light that hits this gland affects hormones in the fish, which in turn affect maturation, and it is therefore important that as much of the light as possible hits the top of the fish.

“If you have light that spreads in all directions, a lot of the fish will swim over the light and the light will then hit the belly of the fish. It produces a poor effect in addition to being not very energy efficient and contributes to light pollution,” said Lansbergen.