© Frode Oppedal
In recent years, it has become more common to use aquaculture facilities where the cages are lowered to a depth between 20 and 60 metres.
“The goal is to keep farmed Atlantic salmon and rainbow trout away from sea-lice larvae,” said Frode Oppedal, a scientist with IMR, in a press release.
He explained that older lice remain on the fish, while the mobile, infectious larvae drift with the current before finding a host. Previous studies have indicated that as pressure increases with depth, most lice larvae move up the water column. At the same time, researchers have documented that certain lice families tend to sink further downward. At some facilities, lice have also been observed on farmed salmon in submerged cages.
Why do some larvae stay deep?
This observation led researchers to wonder why some larvae remain in deeper environments than previously known. Now the first part of the research has been completed, and the results are clear.
“The results show that if fish in submerged cages get lice, and those lice have offspring, then a larger share of the offspring will remain at depth compared with larvae that hatch on fish in surface cages,” said Oppedal.
In addition to some lice sinking deeper, local conditions at the site can also play a role.
“In some areas we see that downward currents or inversion of water masses can occur. Infectious sea-lice larvae can therefore be transported down to fish that are swimming deeper than the lice normally prefer,” he added. “For example, a storm can create turbulent conditions at the surface, and periods of bad weather and upheaval in the water masses can push the lice larvae deeper than where they usually are.”
When they encounter farmed fish in submerged cages at depth, they also find hosts on which they can live on for the rest of their lives.
Tested with commercial farm lice
The project was carried out at the Matre research station as part of a PhD project of Lowri O’Neill from Deakin University in Melbourne. Oppedal was one of the supervisors for the work.
“We collected adult female lice from two different submerged commercial farms and compared them with lice from regular surface farms,” said O’Neill.
Lice are infectious to salmon at a specific stage: when they are copepodite larvae. At that stage, they are about 0.4 millimetres long and drift with the water current, but they can actively swim upward or passively sink. The researchers reared the lice to this stage and then tested where in the water column they chose to position themselves.
“We used transparent, tall vertical pipes, where the larvae were exposed to the same pressure as at a depth of 10 metres,” added O’Neill.
The lice could choose
The goal of this study was to see how the lice distributed themselves in the pipes. Some pipes had lice larvae that were offspring of “surface lice,” while others of “deep lice.” This allowed the researchers to compare the behaviour of the two groups.
After the larvae were placed in the pipies, they were kept in darkness without increased pressure for 15 minutes. The pressure was then increased for five minutes, before which the researchers immediately observed where in the water column the larvae were.
“Five minutes is long enough for the larvae to find their preferred position in the water column. From earlier studies, before deep-drifting cages came along, we know that most will respond to increased pressure by swimming upward and staying there,” said O’Neill.
This time, the results were a little different.
“About 35 percent of copepodid larvae normally prefer to swim upward, but when we looked at larvae from deep-water parents, only 23 percent swam upward,” she added.
Correspondingly, only 19 percent of normal lice sank, while 27 percent of the offspring from deep-cage lice sank.
Further research needed
It's too early to say for sure what these results will mean for deep farming. But this is the first time researchers have seen that the behaviour of louse larvae is influenced by their parents’ growing environment.
“The results give us an indication that the adaptation may be hereditary. We will investigate this further in the next study,” said Oppedal.
If this turns out to be the case, it will be important to take it into account, for example when combining different technologies at aquaculture sites in the same region.
“That way, the long-term negative consequences overall can be kept as small as possible,” concluded Oppedal.
