by Dr John Grayson, Research Scientist, ADM Animal Nutrition, USA
The global aquafeed industry is currently undergoing a period of transformation. During the past three decades, production volumes have increased from 30 metric tonnes per hectare to 150 metric tonnes per hectare, and overall production totals approximately 5.5 million metric tonnes. This remarkable growth has been driven by improvements in genetics, technologies, farming practices, disease resistance and nutrition strategies.
Concerns over the sustainable harvest of marine ‘forage’ fisheries for fishmeal and fish oil production are increasingly relevant to shrimp aquaculture continuation and expansion. Globally, approximately 90 percent of fishmeal and about 65 percent of fish oil produced annually is used in aquaculture feeds and shrimp feeds are a major contributor to this demand (23% of fishmeal and 15% of fish oil usage).
Feed strategies for a growing market
While the global shrimp market has continued to expand at a rate of four to five percent per year, the annual supply of fishmeal and fish oil has remained relatively stagnant for the last 20 years. Most of the wild forage fisheries that fish meal and fish oil are derived from are harvested at or above their sustainable limit, and increasing harvest amounts could lead to population collapse and serious ecological impacts.
Innovations in shrimp nutritional strategies and production are essential for the shrimp market to continue to grow and meet consumer demands. Feed ingredients such as insect meal, plant and algae-based proteins, modified plant and algae oils and single-cell (yeast-based) proteins are being explored and utilised as cost-effective alternatives to traditional marine animal-derived feed ingredients.
Nature of cholesterol as a nutrient
Cholesterol plays key roles in cellular structure, lipid transportation and the regulation of lipid metabolism in shrimp. It is also the essential precursor molecule to an assortment of steroid hormones that regulate moulting, energy use and reproduction. Shrimp cannot synthesise cholesterol from smaller biomolecules, and they need sources of it in their feed to support growth and healthy development (1-2.5g/kg diet for optimal growth, depending on species and life stage).
Feed formulators have typically met the cholesterol requirements of shrimp by including a variety of cholesterol-rich ingredients in diets, such as fish and squid oils, and fishmeal. Formulators also have the option of directly incorporating cholesterol concentrates into feeds (>90% purity, from sheep’s wool grease), but the high cost of these concentrates generally limits their use to specialty shrimp feeds.
The incorporation of lipid emulsifiers in feeds (e.g., lecithins, lysophospholipids, bile acids) has been demonstrated to improve the bioavailability of cholesterol in the diet, contributing to their widespread adoption in feed formulations over the last 30 years. The current options for meeting the cholesterol requirements of shrimp all have their limitations, but there is an emerging solution that has not yet been widely explored—the use of plant sterols in shrimp feed.
An alternative to cholesterol
The incorporation of plant sterols into feed formulations is a promising solution for meeting the cholesterol requirements of shrimp. Plant sterols are similar to cholesterol in molecular structure and perform comparable functions in plant tissues. Several research studies to date have suggested plant sterols have similar growth and health-supporting effects on shrimp as animal cholesterol, but the body of research on this topic is still limited.
A recent study by ADM evaluated the supplementation of practically formulated, extruded test diets with no additional sterols (basal diet), 1.0g/kg of cholesterol concentrate (>92%), or 1.0g/kg of plant sterols concentrate (>95%) and their effects on Pacific Whiteleg Shrimp (Litopenaeus vannamei) growth performance and body composition.
Experimental diets were formulated to reasonably emulate commercial shrimp grow-out diets and were produced as two millimetre sinking pellets using a twin-screw extruder with pre-conditioner. The trial took place in a clear-water RAS setup with 30 120-L rectangular tanks maintained at 30 ppt salinity and 28°C. Whiteleg Shrimp seed stock (PL10) was acquired from a regional supplier and grown out on a nursery diet for three weeks before the start of the trial.
After reaching approximately 0.5g/shrimp in size, shrimp were distributed to the experimental tanks (25 shrimp/tank) and randomly assigned to one of the three experimental diet groups. Shrimp were fed the experimental diets on a semi-restricted feeding rate throughout the trial using automated feeders. All shrimp were harvested and weighed after 62 days of feeding on experimental diets and samples were taken from each tank for body composition analysis.
Comparable growth & feed efficiency
The results of the trial showed that supplementation of the basal diet with either sterols source led to significant improvements in weight gain and feed conversion ratio (FCR) during the feeding period (Figure 1). There were no statistically significant differences in growth performance between shrimp fed cholesterol-supplemented and plant sterols-supplemented diets.
Shrimp fed with the plant sterols-supplemented diet had a similar concentration of cholesterol in hepatopancreas tissues as those fed with the cholesterol-supplemented diet (Figure 1), which suggests that shrimp can convert some fraction of the plant sterols into cholesterol to meet their needs. The results of this study suggest that plant sterols can support the same growth benefits as cholesterol in practical shrimp feeds.
The plant sterols opportunity
The study aligns with other scientific literature in suggesting that plant sterols can be used as an economical solution for meeting the dietary cholesterol requirements of shrimp. Plant sterols concentrates are significantly lower in cost than cholesterol concentrates, and the addition of plant sterols to cholesterol-challenged feed formulations can reduce the feed cost per weight of shrimp produced (through FCR improvements). As cost and sustainable fisheries harvest concerns continue to push the replacement of marine animal-derived feed ingredients in shrimp diets with plant-based ingredients, plant sterols can greatly mitigate the risks of cholesterol deficiency.
To promote farmer efficiencies and environmental stewardship, ADM developed a specialised feed solution, Plant Sterols AN, a highly concentrated plant sterols product (>95% total sterols) that provides a cost-effective way to meet the cholesterol requirements of shrimp. It is a dry powder with high processing stability, a highly stable formulation and a long shelf life. When incorporated in shrimp feeds, the product can support rapid growth and efficient feed utilisation while allowing formulators the flexibility to transition away from cholesterol-heavy ingredients.
