Fish meal has been traditionally the first choice of protein source in aquafeeds for many reasons, including its high protein content, excellent amino acid profile, high nutrient digestibility, general lack of antinutrients and wide availability. However, the production of fish meal seems to have reached its maximum and has shown a stable level over the last decade and prices are increasing due to the increased demand. The unbalance between the fish meal supply and demand has stimulated exploration of low fish meal diet for the aquaculture industry.
Research on low fish meal diet has been an international priority for more than two decades. Among the ingredients investigated as alternatives to fish meal in the diets of farmed fish, soy products are some of the most promising because of the security of supply, price and good reasonable amino acid profile. However, soybean meal of standard quality is used in carnivorous fish diets only at relatively low levels due to its negative effects on feed intake, gut health and immunity in several fish species. A large number of studies have found that: GroPro can alleviate the negative effects of feed intake, gut health and immunity caused by the increase of plant protein level in low fish meal diet.
GroPro is a functional nucleic acid and protein source from saccharomyces cerevisiae. It had been widely used to improve palatability, immunity, intestine health and high quality protein alternative in aquaculture. It is often combined with plant protein raw materials to reduce fish meal in some aquatic feed companies.
1. Improve feed palatability in low fish meal diet
GroPro is rich in flavor compounds that aquatic animals like, such as flavor nucleotides (IMP+GMP) and peptides (Gly-Gly, Gly-His-Gly, Gly-Gly-His etc.). GroPro is also rich in AMP, which is the first compound found to inhibit bitterness. It can inhibit many kinds of peculiar smell in feed raw materials. It is found that AMP can attach to bitter receptor cells and reduce bitter perception by reducing the level of taste nerve transduction. Therefore, when fish meal is reduced and plant materials are increased, GroPro can improve the feed intake and stabilize the production performance of animals.
The two-ball method trial is one of research methods on observing feeding behavior of fishes, which is an important method determining palatability performance of feed additives. However, the touch of fish's body to the ball would affect test results during the two-ball method trial. Therefore, video monitoring equipment was used in the trial in order to improve data precision. Through fine experimental study, we found that GroPro significantly increased biting frequency and relative biting rate in Carassius Auratus Gibeli (Fig.1).
Fig.1. Effect of GroPro on relative biting rate and average biting frequency in Carassius Auratus Gibelio
The result above indicates that GroPro improves feeding times and frequency in Crucian carp. GroPro can be used to improve feed palatability after reducing fish meal in diet.
2. Repair intestinal damage in low fish meal diet
GroPro contains a lot of low molecular weight intracellular compounds, nucleotides, nucleosides, peptides, and ß-glucan. Nucleotides and nucleosides have not been considered as essential nutrients due to endogenous supply through de novo metabolic synthesis and the salvage pathways. Although, during early life stages, stressful conditions, injuries, and nutrient deficiencies, exogenous supply of nucleotides seems necessary. Intestinal morphology has great influence on metabolism and nutrient absorption, as higher intestinal villi length can increase the absorption surface area. Absorbed nucleotides and bases could have degraded within the enterocytes and incorporated into intestinal tissue. Dietary nucleotides could benefit cellular growth andregeneration and reduce energy expenditure for nucleotide synthesis by fish.
In the low fish meal diet, supplementation of soybean meal to the diet of carnivorous fish caused inflammatory conditions in the distal intestine, which was referred as soybean meal induced enteritis(SBMIE). Poor growth performance and feed utilization often accompany SBMIE. Various nutritional strategies have been applied to manage gut health issues. Research from Dr. Ali of Pukyong National University showed that GroPro has the best beneficial effects regarding gut integrity and health among various fish meal alternatives(Fig.2).
Fig.2. Intestinal histology of juvenile olive flounder fed eight experimental diets for eight weeks: CON control,
ESB encapsulated sodium butyrate, GABA, SeY selenium yeast, GRP GroPro, YUM Fyucca meal, SGS song-gang stone, PRT protease.
Intestinal problems often occur due to the decrease of fish meal in shrimp farm. Wei(2017) found that supplementation 1.5% GroPro could improve the intestinal structure of shrimp in low fish meal diet(Tab.1), which was more conducive to stable production.
Tab.1. Effect of GroPro supplementation on intestinal morphology of Litopenaeus vannamei
The above information shows that GroPro contains a lot of beneficial ingredient, which can improve fish gut health in low fish meal diet.
3. Improve disease resistance and reduce mortality
GroPro is a rich source of nucleotides, beta-glucans and mannan oligosaccharides. All of these compounds have shown to improve immune response and growth performance in aquatic species. There already have huge research showed that beta-glucan and MOS can increase immune system in various fish.
Evidence from animal and human studies supports the theory that nucleotides are important for optimal functioning of several components of the immune system. Research on fish also revealed that exogenous nucleotides can influence both humoral and cellular components of the innate immune system. The exact mechanism of humoral and cellular immune stimulation by dietary nucleotides supplementation has not been elucidated. Dietary nucleotides contribute to the circulating pool of nucleotides available to stimulate leukocytes production which are the important cells of the immune system that protect the body against infectious diseases and foreign invaders (Fig. 3).
Fig.3. Hypothesied mechanism of immunomodulation by nucleotides(Hossain,2019)
Survival after challenge with certain pathogens is usually assessed as a measure of disease resistance. Many research found that GroPro can enhance resistance of fish against various pathogens(Fig.4).
Fig.4. Mortality of L. vannamei after challenge
GroPro is a good choice to improve feed palatability, gut health and immunity in low fish meal diet. It can used to reduce fish meal combine with plant protein source and save feed cost.