Cages with deep lice skirts can achieve as good a growth environment as in open cages. Recent full-scale experiments with lifting deep water based on compressed air show that a good cage environment can be maintained, even when using lice skirts.
Lice skirts reduce the onset of lice and parasites, but have also been shown to constrain water exchange in sea cages. According to the company Oxyvision, it has now been proven that Turbolift ensures a more comprehensive and efficient replacement of the water masses behind the lice skirt.
To document the effect of lifting large quantities of deep water with the Turbolift, a full operation test was carried out from October 2019 to January this year at a fish farm in West Finnmark.
More stable oxygen levels
The plant initially used deep lice skirts to ten meters deep, but as a result of reduced oxygen content in the cages the skirts were removed. In one of the cages, however the skirt was kept on and using TurboLift the water lifted from 15 meters deep into the cage. All the cages kept large salmon at normal fish density, and the period was the final part of the growth before harvesting.
The oxygen level at five and ten meters deep was measured several times a day in all cages throughout the three-month period. The measurements showed that the cage with skirt equipped with TurboLift maintained a stable high oxygen level throughout the period. Only exceptionally, levels below 7 mg / l at a few meters depth (see figure at the bottom of the case) were measured. Furthermore, the oxygen levels were at the same levels as in the other cages - without skirts and without TurboLift.
- In general, the oxygen concentration was more stable in the TurboLift cage than in several of the reference cages, where a reduced level was occasionally measured at 4-6 mg / l. This is partly due to the fact that the natural flow conditions in the sea vary, says research leader Asbjørn Bergheim at OxyVision.
- The trial is promising
In the skirt and Turbolift cage mortality rates were very low (<0.8 per cent) throughout the period and actually below the levels at the other cages. At the same time, the feed intake of fish in the test cage(with skirt) remained steadily high and stable throughout the period, gradually decreasing from 0.6 percent / day at eight degrees Celsius in October to 0.4 percent / day at four degrees Celsius in early January.
- This indicates generally good environmental conditions, which did not limit the fish's appetite. Feed intake was also consistently high in the other cages, but varied more from week to week in several cages compared to the cage with lice skirt and TurboLift, says Bergheim. It was also observed that the fish in the skirt cage had high activity and good appetite throughout the period. - The experiment is promising in several ways. It shows that it is possible to maintain a high quality cage environment even with deep lice skirts. At the same time, it shows that a TurboLift in each cage, which is supplied with about 500 liters of air per minute, is sufficient to lift enough deep water to achieve this, concludes Bergheim.
Figure: Distribution of oxygen concentration in Cage 3 at depths of 5 and 10 meters, October - January 2019 -20. Cage with ten meter lace skirt and TurboLift. Temperature: 4-8 degrees Celsius.