There's no doubt that sea lice will adapt to the deep

In November, it was reported that findings from Norway's Institute of Marine Research (IMR) indicate that offspring from lice that have lived in submerged cages do better in deep water.

In addition to some lice sinking to the depths, local conditions where the farm is located can also play a role.

“In some areas we see that downward currents or inversion of water masses can occur. This can transport the infectious salmon lice larvae down to the fish, which basically swim deeper than the lice normally prefer," said IMR researcher Frode Oppedal in the article.

Lice will adapt

There is therefore no reason to believe that the challenges with sea lice will stop even if the industry moves the fish deeper into the cages. That was also the message from Frode Fossøy, senior researcher the Norwegian Institute for Nature Research (NINA), when he gave a presentation at the Tekmar conference in Trondheim, reports Fish Farming Expert's Norwegian sister site, Kyst.no.

Fossøy pointed out that the biology and rapid generation time of lice mean that deeper operations do not necessarily provide a lasting answer.

He referred to new data from the field and DNA-based measurements that document that lice also occur at greater depths than previously thought, including discoveries of high levels of 8 metres and registrations down to 30 metres in a study from Canada.

According to Fossøy, the deepest they have measured in Norway is 18 metres. 

"In Storfjorden we find more lice at 12 and 18 metres compared to 2 and 6 metres deep."

He points out that the louse is incredibly good at adapting, and emphasises that it has a short reproductive cycle. 

"For me as an evolutionary biologist, it is clear that the louse will adapt to most things."

Can sniff out salmon

Fossøy emphasised that this also applies when the industry changes its operating method.

"If we switch to deep farming, which several have chosen to do, there is no doubt that the louse will adapt to that depth."

He also said that the lice not only follow light and salinity, but also odorants from the salmon.

"When salmon are down at the bottom and 'smell good', it is clear that the lice will come down there," he said.

Finally, he stated that adaptability is a fundamental premise that the industry must embrace.

"It is a natural behaviour and reaction for the louse to choose to go deeper, and there is no doubt that it will go down if the fish stays there."

According to Fossøy, this means that deep farming cannot be expected to be a stable solution over time to keep the lice away.