Dulse sets the pulse racing for RAS researchers

Growing seaweed in discharge water from recirculating aquaculture systems (RAS) used to raise salmon could be a win-win for fish farmers, new businesses, and the environment, says a scientist who investigated the idea.

Philip James and colleagues at Norwegian research institute Nofima looked at the necessary conditions to successfully produce dulse, a red seaweed with a relatively high value, in RAS discharge water.

In the trial, discharge water from a SalMar land-based facility was used to grow dulse (Palmaria palmata). The research results show that:

• Dulse grew best on 50% discharge water from the facility, compared to seawater, 25% and 100% discharge water from RAS.
• A dilution rate of 50% gave the highest removal efficiency of ammonia and nitrate for the facility. However, for phosphate removal somewhere between 25% and 50% RAS water was most efficient.
• Dulse grew equally well in salinity ranging from brackish water to full seawater (equivalent to 10-32 grams of salt per kg of seawater).

Everything an industry needs

The results show that for every kilo of fish feed used in RAS salmon farming, 257 grams of dulse could be produced. With Norway’s land-based salmon industry consuming around 100,000 tonnes of feed annually, the nutrients in discharge water could provide everything needed for a substantial seaweed industry.

“This underlines the potential for both macroalgae cultivation and utilising RAS nutrients in discharge water as a valuable resource,” said James, who underlines that challenges remain.

In follow-up projects, the scientists will also be testing dulse grown in discharge water from RAS water for nutritional benefits as well as any potential harmful compounds.

The results are from the ValueSøl research project, supported by the Regional Research Fund ARCTIC (RFF). Nofima is working together with the Norwegian University of Life Sciences (NMBU) and SalMar, the world’s second largest Atlantic salmon farmer, as project partners.