Facilitating Smoltification of Atlantic salmon in Freshwater RAS Systems

by John Carberry-  CO-CEO Sustainable Nutrition Inc. 

In June 2020 Sustainable Nutrition Inc.(“SN) together with its sister company, Sustainable Aquatics Inc (“SA”) began culturing its first cohort of Atlantic salmon triploids supplied by Benchmark Genetics, followed by a second cohort that hatched November 2020.  Both cohorts went through smoltification when fish reached 60 grams, completing the transition in 10 days, mimicking the smolt transition observed in wild salmon.1, 2

 Pre Smolt Picture
Post Smolt Picture
In this article, we describe how we use water engineering, husbandry and nutrition protocols that mimic what is observed in nature, which together enable rapid smoltification and maximum growth rate over a fish’s lifetime.  We also hypothesize that existing RAS aquaculture is failing on multiple dimensions to create the ideal conditions for rapid smoltification, delaying the harvest cycle by up to 180 days.

Water Engineering

pH. Most of the freshwater streams in northern regions have a pH of 8.03.   Both of our cohorts were hatched and cultured through smoltification in RAS systems operating at a pH of 8.0.

Alkalinity and pH. 

Alkalinity refers to the capacity of water to buffer or neutralize an acid.  Fresh water streams in Alaska have high alkalinity levels, typically between 340-440 ppm.   High alkalinity comes from carbonate-rich soils (carbonates and bicarbonates) such as limestone.  High alkalinity acts as a buffer to help maintain pH levels, even in the presence of acid rain or pollution. 

Both of our cohorts were raised in freshwater RAS systems with high alkalinity levels exceeding >300 ppm.

Ammonia/Ammonium.  Biofilters in our RAS system encourage the growth of two different aerobic bacteria species, the first converts ammonia to nitrite and the second converts nitrite into nitrate. We maintained ammonia and nitrite levels at less than 2 ppm and 1ppm, respectively, even at biomass loads of >50 kg/m3.  Low nitrite levels allow oxygen to be efficiently transferred through the gills, contributing to the ability of the parrs to put more energy into the smolt transition.

Common industry practice is to operate RAS farms at a pH of 7.2, attempting to drive the chemical equilibrium toward NH4+, which is much less toxic than NH3.  We believe that high acidity levels slow smoltification 4  and is an important factor adding days to the overall hatch-to-harvest cycle.


Allometric Development.  Upon hatching, genetic signals from the DNA of salmon instruct how energy (mitochondria) is allocated to different stem cell lines. Growth during this phase enables some stem cell lines to gain early preference over others. Early beneficiaries are the adaptive immune system, eyes, mouth gape/size, liver, and tail.  If the fish is compromised during this phase of development, it does not recover and is compromised for the remainder of its life. This slows fish growth rate and increases the time required for smoltification.

Temperature. Both cohorts hatched and were raised at 12-14°C. Common industry practice is to hatch and grow at 6°C, which improves yield by slowing down growth rates.

First Exogenous Feeding.  Exogenous feeding was begun for both cohorts at 420 degree-days post-hatch. This contrasts with common industry practice which is to start feeding at 840 degree-days when the yolk is zipped-up and the alevins begin to rise from the redd in search of food.

As the DNA instructions for smoltification are tightly scheduled, reducing temperature to slow growth rate causes distortions in the timing of this process.  Moreover, by not providing the micro- and macro-nutrients that alevins require for proper allometric development, further slows growth, leading to higher yield losses and slower transitions through smoltification.


Micronutrient Supplements.   
Both cohorts were fed a commercial hatchery diet coated with Amplified(tm) Topcoat, a micronutrient supplement added at 1-part Amplifeed Topcoat for every 10 parts commercial feed. The most critical micronutrient in Amplifeed Topcoat is di-esterified 3S, 3’S astaxanthin.  Upon hatching in streams, alevins consume substantial amounts of this compound (hence why the eggs are deep red). Early exogenous feeding also provides high doses of this compound, coming from the decomposition of old adult salmon releasing astaxanthin into the food chain.  Smoltification is a stressful event for fish. The presence of astaxanthin in their diet facilitates the smoltification process by reducing the effects of reactive oxygen species (ROS) .

Traditional RAS systems are using feeds where astaxanthin and other micronutrients are either absent or denatured, so that they are not bioavailable to the salmon.


We have demonstrated that a 10-day smoltification is possible when salmon reach 60 grams in freshwater RAS systems that follow the water engineering, husbandry and nutrition protocols that we have developed at SN/SA.  Smooth smoltification is possible because we changed pH, nitrite levels, start date of first feeding, hatching temperature and nutrition. These changes have allowed us to attain the best survival, highest growth rates, and lowest deformities reported in the industry. 


  • Jorma Piironen et al Comparison of smoltification in Atlantic salmon (Salmo salar) from anadromous and landlocked populations under common garden conditions 2013 Ann. Zool. Fennici 50: 1–15
  • Sigurd O Stefansson, Björn Th Björnsson, Lars OE Ebbesson, Stephen D McCormick Fish Larval Physiology Chapter 20 2008 CRC Press
  • Timothy P. Brabets, Bronwen Wang, and Robert H. Meade Environmental and Hydrologic Overview of the Yukon River Basin, Alaska and Canada 2000 U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 99-4204 page 85
  • Björn Thrandur Björnsson et al Environmental endocrinology of salmon smoltification 2011 General and Comparative Endocrinology 170 (2011) 290–298