Submerging Atlantic salmon cages can do more harm than good
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Atlantic salmon producers may want to rethink their marine cage immersion plans as a lice prevention strategy, new study finds published in the journal Aquaculture. In this trial, the researchers fitted three 1729 m³ mesh pens of air domes and seeded them with approximately 6000 smolts. The researchers submerged the cages 15m below the surface and followed the salmon for a year, measuring their growth rates, swimming behavior, welfare parameters and sea lice levels.
The results of the test showed that the salmon in the submerged cages had significantly lower sea lice levels (93 percent lower than the three control cages kept at the water surface). However, the mean weight harvested for submerged fish was almost half of the control group (2.8 kg vs. 5 kg), and the overall mortality rate was 2.5 times that of the control group. The researchers also found that salmon reared in submerged cages had more eye and mouth injuries than fish reared in conventional cages.
They hypothesized that submerging fish throughout the production cycle exposed them to cooler waters with less dissolved oxygen. This suboptimal environment had significant impacts during the production cycle, resulting in lower fish weights, higher mortalities and lower welfare scores.
Why immerse sea cages?
The salmon farming industry is turning to preventative strategies to control sea lice instead of chemical treatments. Since sea lice tend to live in surface waters, Norwegian fish farmers build barriers such as skirts and lice snorkels to prevent juvenile lice from infecting fish in net pens. A new approach is to use depth as a spatial barrier and to submerge the cages under surface water. This would create a natural barrier between the salmon and the lice living on the surface.
Submerging sea cages could also provide other benefits – keeping fish lower in the water column would make them less susceptible to storms, wave damage, and net deformation from ocean currents. It could also keep fish away from surface algae and jellyfish blooms.
But before the salmon farmers sink the cages, they need to create stable air domes at the top of the structure. Since Atlantic salmon have an open swim bladder, they need access to air to maintain their buoyancy and normal swimming behavior. If producers are successful in incorporating air domes into submerged cages, the salmon industry may have a new way to keep sea lice levels low without using unsustainable delousing methods.
Key results
The researchers note that submerged salmon had significantly lower sea lice numbers than fish reared in conventional cages, but submerged fish faced significant production problems. This resulted in lower growth, higher welfare scores, and higher death rates. The researchers said that “the consequences of keeping the fish submerged throughout the production cycle were more detrimental to overall production,” outweighing the reduced benefits from sea lice.
The study authors highlighted the poor growing conditions in the submerged cages, but felt that the low temperature and dissolved oxygen levels at 15m might not be universal. For submersion to be an effective sea lice prevention strategy, it must be used on rearing sites and at times when deep conditions are best for production. The article‘s conclusion notes that other offshore fjords or ocean environments may have better conditions. Conducting a similar experiment in a more suitable submerged environment may show that the approach is feasible.
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