Ozone is a strong oxidant commonly used for improving water quality and disinfecting pathogens in freshwater fish farms, according to Nofima, an applied research institute in Norway. However, when ozone reacts with certain constituents of seawater, toxic byproducts can severely affect the health of fish populations.

As brackish water is increasingly introduced in farming post-smolt salmon, thresholds for the safe use of ozone need to be established. Suppliers of recirculating aquaculture systems (RAS) are looking for safe, cost-efficient and reliable ways to maintain optimal water quality in brackish water RAS, and with this research, they are now a step closer, Nofima said.

Researchers with Nofima and The Conservation Fund Freshwater Institute (TCFFI) in the U.S. — both central in the CtrlAQUA center for research-based innovation — wanted to determine if ozone is safe for salmon in brackish water and what the safe limits are for post-smolts.

The scientists carried out a trial determining the ozone limit in a flow-through system, with 100 g Atlantic salmon reared for 12 days in brackish water with salinity of 12 parts per thousand. The fish were exposed to ozone levels of: 250 millivolts (mV) -- the control, 280 mV (low), 350 mV (medium), 425 mV (high) and 500 mV (very high).

The researchers identified ozone levels up to 350 mV as potentially safe and 300 mV as safe for the health of post-smolts in flow-through brackish water, Nofima reported.

In a follow-up study by Nofima that has not yet been published, the identified threshold was confirmed for RAS.

Nofima fish health researcher Carlo Lazado will present the results Oct. 21 at the digital conference “Smolt Production in the Future.”

TCFFI scientist Chris Good reported that even lower ozone dosages are sufficient to improve water quality.

“This would depend on the quality of water being treated, but in our experience in replicated freshwater RAS, a lower ozone level of 290 mV still resulted in significant improvements to water quality in general, including reduced biochemical oxygen demand and increased [ultraviolet] transmittance,” Good said. “This new publication from CtrlAQUA highlights the effects of different ozone doses on the fish, therefore complementing our work here in the U.S. on the water quality.”

Too much ozone

There was high fish mortality after a few days in the two highest-ozone groups, and the highest group was terminated for this reason, the researchers said. For the group exposed to 350 mV, the scientists observed changes in the gills, but these were within the acceptable response range. The mortality for this group was 1%, and there was no mortality for the two lowest groups.

The scientists also tracked indicators of fish welfare, such as genes important for antioxidant defense and quality of skin and gills.

“What surprised us was that ozone is much more aggressive on the gills compared to the skin. It means that gills are a good indicator tissue to check out the sensitivity to ozone,” Lazado said.

The recommendation to maintain ozone below 350 mV for 100 g post-smolt salmon in brackish water is consistent with ozone thresholds for other farmed fish species and for salmon post-smolt in freshwater, Nofima said.

“When relying on this limit, it requires that measurements of ozone in the production facility are accurate and reliable,” added Kevin Stiller, a RAS researcher at Nofima.

In the research trial, the scientists needed to take the average from two probes to accurately measure the ozone level. That might also be a tip for the industry when measuring ozone level using today’s technology.

Industrial use

Stiller and Good said their experience is that both in Norway and North America, typical ozone applications are less than 350 mV in commercial RAS facilities.

Should ozone levels become toxic to fish, the morbidity and mortality rates are primarily the result of gill malfunction. The effects are observable to the naked eye, with increased respiratory rate and fish crowding near oxygen-rich tank inlet water, the researchers said.

View the scientific article “The Effects of Ozone on Atlantic Salmon Post-Smolt in Brackish Water — Establishing Welfare Indicators and Thresholds.”

CtrlAQUA SFI has 21 partners from industry and academia. Nofima is the host institution. The center works on building the foundation for the development of closed aquaculture concepts in the future. Given greater control over the production process, problems related to mortality and sea lice will be reduced, as well as reducing the production time for farmed salmon.