Atlantic salmon that are raised in recirculating aquaculture systems (RAS) still face the issue of "off-flavor," according to leading scientists, a factor that could undercut the overall potential of the burgeoning sector.

These earthy and musty flavors in RAS-grown salmon are typically caused by geosmin and 2-methylisoborneol—metabolites released by microbes that grow within the land-based systems.

Removing the off-flavors, however, isn't a straightforward process, and isn't without its costs.

Currently, the most reliable method to mitigate the off-flavor is to purge the fish in a "clean, disinfected system that is well flushed with make-up water," according to The Conservation Fund Freshwater Institute in West Virginia, a non-profit group that researched recirculating aquaculture systems (RAS) and land-based salmon farming in particular.

This "depuration purge system," as its called, includes keeping fish off feed while the flavors cycle through their system. But starving the fish, of course, takes a toll.

"There is the potential for weight loss to occur because the fish are still swimming, active, and using energy," John Davidson, a leading research scientist at the Freshwater Institute, told IntraFish of the process. And that weight loss equates to revenue loss.

What are the impacts?

Davidson pointed to research from a study published in 2012 in the journal Aquacultural Engineering that showed a 4-5 percent weight loss during a 5-10 day off-feed depuration period for Atlantic salmon.

That purging time is still standard for the RAS industry, he said.

For the institute's own RAS testing facility in West Virginia, it currently takes about six days of purging of the fish to remove or reduce off-flavors.

"We use six days here on site and never once had a report back about off-flavor of our fish with that protocol," he said.

But while salmon can lose some weight during the depuration process, it doesn't necessarily harm the fish.

"One thing we want to show through our research is fish welfare is not impacted by putting the fish in depuration," he said.

"It's a typical response for fish in nature to not feed for periods of time. During the winter, for example, fish go off feed or feed very little because they're cold-blooded and their physiology is affected by water temperature."

The 2012 study used fish that were 2.5 kilograms. Davidson said the institute is currently researching how purging affects Atlantic salmon's fin and skin health, scale loss, and other welfare indicators to further prove depuration does not impact the fish.

"A couple other things we're also in the midst of researching is whether we could shorten the depuration time frame, and whether we can rapidly remediate off flavor from Atlantic salmon if we exercise them and/or maintain certain oxygen levels," he said.

"We're also looking at the effect of maintaining higher dissolved oxygen levels, which supports increased metabolism."

The institute is currently researching whether companies can even use a scaled-down RAS that could flush water through systems faster, and thus move bacteria out faster as well.

Off-flavor is site-specific

One thing Davidson makes clear to companies who are interested in how off-flavor could impact their current or future RAS projects is that off-flavor is site-specific.

Water sources drawn from a river or lake have much more potential for imparting off-flavor, he said. These are more nutrient-rich and less biosecure environments compared to underground water sources or open-water farming systems, such as those used for netpen operations.

"Our source comes from an underground spring, it's a clean, pure water source," he said of the the institute's own facility in West Virginia.

In 2014, Davidson helped publish an article in the publication Aquacultural Engineering that established standard procedures for depuration of off flavor that are still used today.

"When we carried out our study published in 2014, we found that it's best to disinfect depuration systems prior to putting fish in," he said.

Freshwater Institute's John Davidson holding a large Atlantic salmon harvested from purge system at FI. Photo: John Davidson

That's because certain bacterial species can be present in RAS biosolids and microbial biofilms inside of pipes and water treatment components, particularly in recirculating depuration systems that use a large amount of surface area for "dispersal of water flows and subsequent gas exchange," according to the 2014 study.

"Salmon cultured to food-size in RAS will likely require transfer to separate, odor-free depuration systems that are flushed with water in a single pass or are operated with limited water recirculation (with no biofilter), in order to purge off-flavors," the study concluded.

A cause for concern

The explosion of proposed land-based salmon farming projects across the globe has generated huge enthusiasm from investors, NGOs and entrepreneurs.

One recent estimate from market analyst Kontali put the potential land-based salmon production in the US alone -- if projections are met -- at 300,000 metric tons by 2025.

Companies such as Atlantic Sapphire, Pure Salmon, Whole Oceans and a range of others have made bullish projections for their production, but lost among the potential for success are seemingly small pitfalls, one of them being off flavor.

Atlantic Sapphire, the world's most valuable land-based salmon farmer by far with a market cap of $1 billion (€900 million), listed off flavors as one of the major risk factors in its IPO and mid-2019 private placement offering documentation.

"If the group is not successfully managing the air and water quality parameters at its production facilities and removal of off-flavour compounds from the fish in its 'finishing' system," the company wrote, "accumulated off-flavours and odours in the fish flesh from the circulating water may decrease the meat quality of the group's products, adversely [impacting] the marketability of the products and the group's business, future profitability and cash flows."

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