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Increasing fish stocking density while maintaining welfare

Increasing fish stocking density and at the same time maintining good welfare will give semi closed facilities a competitive advantage: Results from production in Aquatraz show that the fish stocking density can be increased without registering any negative consequences for water quality or fish welfare.

Aquatraz G3. Photo Steinar Johansen/MNH

Aquatraz is a semi closed escape-proof cage in steel that protects the salmon against salmon lice and improves fish health. Good water exchange is achieved by using flow setters that ensure that oxygen-rich water is pulled into the cage  from water depths nearly free from  salmon lice  which are then circulated in the cage. The protection agains salmon escape and large water exchange, Aquatraz is suitable for increasing stocking density and total biomass compared to conventional cages. The first Aquatraz cages in operation have been granted a dispensation for densities of up to 45 kg/m3and 400,000 individuals, as compared to conventional cages which are allowed a debsity of 25 kg/m3 and 400.000 fish. In 2019-2020, the water quality was documented during experiments with higher density in the first three Aquatraz cages at MNH’s site Kyrøyene. The experiment where the density was increased to 30 kg/m3 was carried out in close collaboration between MNH, professionals from NIVA and Aqua Kompetanse. You can read more about the experiment and the results in the article linked to below. The experiments will continue and the Norwegian Food Safety Authority has extended the exemption for density up to 45 kg/m3 for the Aquatraz cages until July 2024.

 The experiment and results are documented in this comprehensive article in the Norwegian magazine “Norsk Fiskeoppdrett 02.2021

Unfortunately the article is only in Norwegian. Please contact us if you would like more information. 

The experiments show that we can produce salmon with a density of 30 kg/m3 in Aquatraz cages with acceptable results in terms of fish welfare, mortality, water quality and growth. With better protection against lice (i.e. deeper lice skirts), combined with good water exchange, it should be possible to produce salmon with further increased density and with better results than what we achieved in this experiment.

The oxygen concentration was generally high and relatively stable in the Aquatraz cages, which hydrographic profiles taken with CTDO instrument every week showed. In cases where the fish stocking  density was above 25 kg/m3, the oxygen profiles show no systematic decrease or increased variation in any of the Aquatraz cages. Fixed oxygen sensors logging every 10 minutes, show that all Aquatraz cages have managed to maintain a good water exchange and good oxygen values ​​for the fish even at fish densities above 25 kg/m3