RASTECH Magazine

News Management post-smolt
RASTECH24: Why the smolt stage is critical to RAS business

June 11, 2024  By  Nestor Arellano

Reaping large rewards in recirculating aquaculture system (RAS) depends heavily in getting your act together during the smoltification stage.

This was the gist of the presentations during the Operations room of the RASTECH 2024 Conference and Trade Fair on the first day of the event on June 5, in Charlotte, N.C.

The session titled, “Smolt: Drivers for RAS,” was chaired by Jacob Bregnballe, sales director for AKVA Group – Land Based. Speakers at the session were, Dean Guest, head of RAS technology at Great Northern Salmon (formerly known as Katahdin Salmon), and Greg Lambert, freshwater production manager at Cooke Aquaculture USA.


As salmon mature, they enter an intermediary stage known as smolt. This is when a young fry transforms to prepare to transition from a life in fresh water to a life at sea. This process is marked by rapid physiological changes.

“If you want to produce big fish, do it when they are small,” Bregnballe told session participants.

Bregnballe shared the following pointers:

  • Grow smolt at lower water temperature (12C to 13C maximum) to avoid early maturation.
  • Use light regimes is the most common and more reliable method of promoting smoltification. Adding seawater to tanks can also be combined with light regimes.
  • Normal C02 level for smolt RAS is 12-15 mg/L
  • Grow your smolt larger in RAS before transporting to net pens. This shortens their time spent at sea and reduces chance of exposure to se lice.

“Why would you put a small 100-gram smolt in sea water at nine degrees, when you can have it in a RAS at 12 degrees where they can grow and perform better,” said Bregnballe.

“Don’t overlook early life stage. It’s a good place to spend your money. It has small systems, low biomass, low cost, but big benefits,” according to Guest. “Land based operations have big tanks and research, but it’s in the early life stage that sets the whole thing in motion.”

Key factors for success that he mentioned include:

  • Good fish genetics
  • High water quality
  • Uniform fish sizing
  • Achieving the right number of fish at the right size in a cohort.
  • Develop and adhere to appropriate standard operating procedures.
  • Train staff properly and empower them to make decisions.

If smolt are introduced to saltwater too early, they fail to acclimatize and die. Some studies show that as much as 15% of farmed salmon die each year after being transferred from land-based freshwater tanks to net pens.

Among the tools used by Cooke Aquaculture to determine smoltification are:

  • Calendar and water temperature
  • Saltwater challenge
  • Salt chloride testing
  • ATPase gill testing
  • Salinity adjustment
  • Visual cues

“Fish also notice the change to more sunlight and increase in water temperature around mid-April. If you’re anywhere in North America, when you turn that calendar and you see April, there’s a pretty good chance that your fish are smolting,” said Lambert.

In the saltwater challenge, a sample batch of fry from a tank is placed in another tank with 35 ppt salinity content. “If after 48 hours the fish are alive, it’s a pass. If the fish die, they’re not ready come back next week and try again next week.” This is done several times to identify a trend indicating that the fish are ready.

ATPase gill testing measures the ability of fish to control body fluids for transitions from freshwater to salt water. Lambert said, this third-party test is usually more expensive, and its results does not always align 100% the results of Cooke’s other test.

“But in a lot of places insurers they really like this ATPase test,” he said. “If you can show them that you stocked your fish when they were true smolts according to the test and the fish die 10 days after they were out at sea, your claim is more likely to come through smoothly.”

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