VKM indicates that there is a possibility that the water quality in RAS may deteriorate and cause compromised welfare for the fish.

On the other hand, a well-managed RAS can stabilize, or even improve water quality, resulting in better fish welfare compared to some traditional flow-through systems. Based on a semi-quantitative risk assessment, the highest risk factors are considered to be high levels of nitrite and total gas saturation, excessive feeding, and insufficient removal of particles.

VKM emphasize that proper experience and management is essential for safeguarding the welfare of the fish in RAS. Monitoring of key water quality parameters such as dissolved oxygen, pH/CO2, Total Ammonia Nitrogen, nitrite, total gas pressure and temperature is important, as well as routine monitoring of fish behaviour, morphology (e.g. fins, gills and skin), production data and mortality rate.

Proper operational routines for the biofilter are essential in order to provide a healthy and stable aqueous environment. In particular, RAS can be vulnerable in the initial phase after start-up of the biofilter. A quick removal of particles is important. If microorganisms (pathogens) enter RAS, it may be difficult to implement adequate disinfection procedures without affecting the stability of the biofilter.

Adequate dimensioning of the RAS is a prerequisite. The biomass kept in the system shall not exceed the maximum intended level at any time.

It is VKMs opinion that the existing limits for relevant water quality parameters should be considered as guidelines only, since the water quality criteria are not based on results from commercial RAS conditions.

The risk assessment is conducted on request from The Norwegian Food Safety Authority to judge if existing legislation is adequate in safeguarding fish welfare in salmonid hatcheries.