6 minute read
Improving salmon growth in RAS systems
Terry W. Snell, Matthew Carberry, John Carberry, Tim Wilson, Sustainable Nutrition Inc.
Introduction
Recent trials at the Sustainable Aquatics marine hatchery have demonstrated that it is possible to grow Atlantic salmon to maturity faster in RAS systems with less than 1% deformities and greater than 98% survival rates as compared to the industry standard in ocean net pens. In our system, we observed growth from eggs to 22 g fish in 25 weeks posthatch, suggesting that it is possible to grow alevins to harvestable adults in 12 to 20 months, as compared to an industry average of 36 months.
The keys to our success include fine-tuning allometric growth in alevin stages, providing optimal nutrition in juvenile and adult stages, and maintaining excellent water quality in a RAS system to avoid growth inhibition. Our approach is in contrast to most of the scientific effort to improve salmon yields which has been directed at genetic improvements of stocks
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Day 16 Day 8
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Figure 1. Sequence of salmon larvae development at Sustainable Aquatics hatchery
Figure 2. Graph of average salmon weight vs. weeks post-hatch. Week 25 survival rate = 99%, deformity rate <0.5%. This exponential model fits the data exceptionally well (R2 = 0.991) through 25 weeks of growth. The model allows us to predict that these salmon will reach a size 200 g by week 34. We are continuing grow-out in larger RAS systems where bioload can be kept at <50 kg/cubic meter and predict that salmon will reach a harvestable size of 5 kg by week 48.
(Stokstad, 2020). However, it is becoming increasingly clear that nutritional programming, especially in juvenile alevin on yolk stages, is key for promoting the health, disease resistance and growth rates of salmon in aquaculture (Skjærven et al., 2020).
Fine-tuning allometric growth in alevin stages
Like many fish, the growth of salmon is allometric. Eyes, mouth, gills, liver and tail develop faster than other body parts and long before the yolk is exhausted (Osse & van den Boogaart, 1995). These patterns reflect the developmental priority given to vital functions, like feeding, locomotion and respiration. This allometric growth is driven by the rapidly changing balance between inertial and viscous forces experienced by salmon larvae as they develop in relation to their body length and swimming velocity. Allometric growth is genetically programmed and must occur in the proper sequence in order for fish to thrive throughout their lives. It is becoming clear that larvae that do not conform to this allometric sequence of growth do not recover and are compromised their entire lives.
To better understand allometric growth of salmon larvae, we need to appreciate their early life history. Salmon eggs are spawned and hatch after about 450-degree days (a degree day is a day times temperature in centigrade). Initially, nutrition is completely supplied by yolk. Since there is no exogenous feeding, the larvae hide in the gravel (redd) to avoid predators. After about another 400-degree days, the external yolk is “zipped up” and alevins are given their first exogenous feed, as is typical for salmon industry protocols. However, in nature, alevins begin to find food in the gravel like rotifers, copepods, insect larvae and worms and continue to avoid light while still on the yolk. Recent literature indicates the digestive system is functional and operating well before the yolk is “zipped up” (Sahlmann et al., 2015). As the alevins develop, the yolk is not adequate to support allometric development and the yolk is complemented by exogenous feeding
while in the redd. As the yolk is exhausted, these small prey become inadequate to fully meet their nutritional needs, so they rise to hunt in the water column where they begin to encounter predators. Natural selection has programmed salmon larvae to develop allometrically so that they are better prepared to avoid predators when they are forced to venture into the water column by hunger.
The aquaculture industry raises about 2.6 million tons of salmon a year. For the most part, they feed the newly hatched fish only after the egg yolk has been consumed, and they feed them with industrial feeds lacking some essential nutrients like highly bioavailable astaxanthin. We know from observations of natural salmon alevins that they are photonegative for a long time, avoiding light to avoid predation. Alevins in nature eat exogenously for about threequarters of the time they are on the yolk in the redd (Solberg et al., 2014), and the prey that they eat are rich in astaxanthin. Our innovation has been to begin feeding salmon alevin much earlier (100 degree days post-hatch) than the industry standard and feeding them foods enriched with highly bioavailable astaxanthin. Astaxanthin is a critical, but little appreciated, an ingredient in aquaculture feeds (Snell et al., 2019).
Over the past year, we conducted feeding trials at Sustainable Aquatics hatchery using Atlantic salmon, Salmo salar. Our objective was to grow these fish from egg to adult with high survival, low deformities and fast growth rates using protocols for RAS systems that we have perfected over the past 15 years. The main parameters were: egg source: triploid, all-female Atlantic salmon eggs supplied by BMK Genetics, water temperature: 13°C, water: freshwater RAS system with high alkalinity (350-550) and pH of 8.0 or higher, bioload maintained at <30 kg/cubic meter, feed – commercial hatchery feed starting at 500 microns, coated with Amplifeed Topcoat supplying a lipid-rich nano emulsified astaxanthin and seven essential minerals, metals and vitamins as a supplement. Each week, salmon length and weight were measured, along with survival and deformity rates. From these data, we developed a model to describe growth from alevins through growout (Fig. 1,2).
Land-based, closed, freshwater recirculating aquaculture systems (RAS) have been demonstrated to be a viable production technology to raise Atlantic salmon from egg to harvestable size (5 kg) (Liu et al., 2016, Davidson et al., 2016). RAS systems have many advantages over ocean net pens, including the elimination of sea lice, growth at higher temperatures and less food waste. The challenge of RAS systems is to maintain good water quality at the high biological loads required for profitable aquaculture systems. Sustainable Aquatics hatchery has developed extensive expertise in operating RAS systems over the past 15 years while raising more than 200 species of marine tropical fish for the aquarium market. We have optimized the design and application of protein skimmers, biofilters, and pH and alkalinity control so that high bioloads can be maintained without periodic crashes. In rearing salmon, we estimate that our systems can maintain up to 50 kg biomass/cubic meter without inhibitory effects on growth and maturation.
References available on request
More information: Terry W. Snell
Professor Emeritus Georgia Institute of Technology, USA E: terry.snell@biosci.gatech.edu
John Carberry
Co-CEO Sustainable Nutrition, USA E: johnc@ mosseycreekenterprises.com
