Salmon parr tolerate more ammonia in the water than expected
More and more farmed salmon will have their early life in recirculating aquaculture systems. The most common production method to date in Norway has been flow-through systems, in which the water only passes the fish tank once. The recirculated water contains some ammonia which is excreted via the gills of the fish. “Today’s recommended limit of how much ammonia the salmon can tolerate is probably not adapted to the smolt production of the future,” the scientist says.
Facts about ammonia
- Ammonia originates primarily from the catabolism of amino acids in the fish
- Ammonia is excreted via the gills and a large proportion is removed by the biofilter in the recirculating aquaculture system.
- Ammonia is toxic as it can easily enter the blood stream of the fish and be converted to ammonium.
- Ammonium disturbs the metabolism, the acid-base balance, and the nerve signals in the brain. In the event of an overdose, the fish loses control of its nervous system, develops cramps and can die. This can occur in both traditional flow-through tanks and in recirculating aquaculture systems if the level of control is inadequate.
- In order to maintain optimal growth, behavior and welfare in production of large and robust smolts, it is of importance that the tolerance level of the fish is not exceeded.
“Trials we have carried out at Nofima give us reason to believe that salmon in recirculating aquaculture systems can handle relatively high concentrations of ammonia,” says Bendik Fyhn Terjesen, Senior Scientist at Nofima and Project Manager of the Research Council of Norway project “Fish welfare and performance in recirculating aquaculture systems”.
He believes that the recommended limit for the concentration of ammonia the fish can tolerate, and the way this is expressed, should be reassessed as we now know that the fish have comprehensive mechanisms to deal with high concentrations of ammonia. If the limit is unnecessarily low, the recirculation systems that are dimensioned have unnecessarily large biofilters or excessive water flow rate through the biofilters, and as a result cost more to build and operate than necessary.
The recommended limit in Norway for the concentration that Atlantic salmon can tolerate is set at 2 milligram total ammonia nitrogen (TAN) per liter. However, such a limit explains little about the most toxic form, namely ambient unionized ammonia nitrogen (NH3-N). This is due to the fact that the proportion of NH3-N in TAN relies strongly on factors such as the pH of the water. It is, therefore, important to also state limits in micrograms of NH3-N per liter.
The fish adapt
Little research exists about the tolerance level of salmon parr in fresh water to long-term ammonia exposure, so the scientists at Nofima wanted to find this limit.
The scientists carried out trials involving five concentrations of ammonia in the water during the salmon’s parr stage (the stage between fry and smolt). When the 15-week trial ended, none of the concentrations had led to a reduced growth rate of the fish.
However, it was observed that the salmon parr exposed to the extreme concentration of 32 micrograms of unionized ammonia nitrogen per liter of water had temporary gill damage and reduced growth after three weeks of the trial. The development after that period reveals that the fish have natural mechanisms to combat the ammonia. For the first time, the scientists have established that also salmon has several mechanisms in the brain and gills that are involved in excreting the ammonia and controlling the level in the nerve tissue.
“The fish adapted to the ammonia level after three weeks of being exposed and even the fish swimming in the highest concentration of ammonia appeared to excrete increasingly more ammonia via their gills without us being able to detect any negative consequences in growth or fish welfare,” says Fyhn Terjesen.
The optimal level appears to be 14 micrograms of unionized ammonia nitrogen per liter of water. The scientists found no effect of previous ammonia exposure when it came to how well the smolts tolerated transfer to seawater.
The Nofima trial was performed in flow-through tanks. The scientists want to use the results of this study as a basis for designing a long-term experiment with ammonia at the Nofima Centre for Recirculation in Aquaculture at Sunndalsøra, with relevant levels of other potential stressors such as nitrite and CO2.
Theme of conference
The optimal water quality in recirculating aquaculture systems and possible closed-containment sea-based systems will be one of the topics at the conference “Smolt production in the future” at Sunndalsøra on October 23-24.
“We believe that the research provides a basis for studying these limits in recirculating aquaculture systems, and that we need to develop more knowledge in order to dimension efficient recirculating aquaculture systems.”
Check out the publication of november 2012 in Aquatic toxicology by following the link to the right.
By; Jelena Kolarevic, Harald Takle, Olga Felip, Elisabeth Ytteborg, Roger Selset, Christopher M. Good, Grete Baeverfjord, Torbjørn Åsgård, Bendik Fyhn Terjesen