Nofima Centre manager, Roger Selset, checking the water quality. (Photo: Kjell Merok, Nofima)
Salmon parr naturally excrete high amounts of ammonia
Friday, August 31, 2012, 23:40 (GMT + 9)
Researchers are studying how to adapt today’s recommended limit of how much ammonia farmed salmon can tolerate to the increasing popularity of smolt production. They have found that salmon parr have natural mechanisms that help them excrete ammonia.
The most common recirculating aquaculture production method in Norway has been flow-through systems, in which the water only passes through the fish tank once. The recirculated water contains some ammonia that is excreted by fish via their gills.
“The trials we have carried out at Nofima Mat, the Norwegian Institute of Food, Fisheries and Aquaculture Research, give us reason to believe that salmon in recirculating aquaculture systems can handle relatively high concentrations of ammonia,” said 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.'
Terjesen believes that the recommended limit for the concentration of ammonia the fish can tolerate and how this is expressed should be reassessed, since it has been established that the fish have ways to deal with high concentrations of the compound. The problem with needlessly low limits is that the recirculation systems that are dimensioned have unnecessarily large biofilters or excessive water flow rate through the biofilters, and thus are more expensive to build and operate.
|The biofilter is an important component of the water treatment in a recirculating aquaculture system. It contains bacteria that “eat” and transform the ammonia to a less toxic compound. (Photo: Kjell Merok, Nofima)
The recommended limit in Norway that Atlantic salmon can tolerate is now 2 mg total ammonia nitrogen (TAN) per l. What is needed is the effect of ammonia’s most toxic form, ambient unionized ammonia nitrogen (NH3-N), which relies strongly on factors such as the pH of the water.
Therefore, it is also crucial to set limits in mcg of NH3-N per litre.
Scientists at Nofima set out to find the tolerance level of salmon parr in fresh water to long-term ammonia exposure.
Although when the 15-week trial ended, none of the concentrations had led to a reduced growth rate of the fish, scientists saw that the salmon parr exposed to the extreme concentration of 32 mcg of unionized ammonia nitrogen per l had temporary gill damage and reduced growth after three weeks. The subsequent development shows the fish have natural mechanisms to combat the ammonia.
This means that, for the first time, it has been seen that salmon has several mechanisms in the brain and gills involved in excreting ammonia and controlling its level in the nerve tissue.
“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,” said Terjesen.
The optimal level appears to be 14 mcg of unionized ammonia nitrogen per litre.
The scientists want to use the results of this study as a basis for designing a long-term experiment with ammonia with relevant levels of potential stressors such as nitrite and CO2.