Fish and shrimp are highly exposed to exchanges of microflora between the environment and the digestive system. This increases the risk for the proliferation of an unfavorable gut microflora or frequent destabilization of the microflora, which can affect the optimal functioning of the digestive system.
Furthermore, the digestive system of fish and shrimp is the main entry port for bacterial, viral and parasitic infections, which remain a major risk for the profitability of aquaculture production. Therefore it is not surprising that the establishment of a stable, well-balanced microflora in the digestive system is a key factor for maximizing production yield and profitability in aquaculture production, writes Peter Coutteau, PhD, Nutriad, Dendermonde, Belgium. 
Sustainable approaches to modulate the gut microflora in aquaculture include the use of a wide variety of natural compounds capable of modulating the microflora towards a favorable composition such as probiotics, prebiotics, organic acids, yeast extracts and phytobiotics.
These strategies may have synergistic effects, for example phytobiotics can enhance the establishment of probiotic bacteria and therefore enhance the efficacy of probiotic inoculations. In this article, we illustrate the beneficial effects from growth promotors with gut modulating properties for aquaculture species.
Natural growth promotion in the absence of disease
A synergistic blend of phytobiotics was selected for their bacteriostatic and bactericidal properties against pathogenic and potentially pathogenic bacteria in vitro. This blend was capable of promoting growth significantly in feeding trials with healthy specimens of different species of fish and shrimp growing under controlled lab conditions (Fig. 1).
These lab results have been confirmed in a number of studies under field conditions with different species of fish and shrimp. Researchers from the Instituto de Pesca, São José do Rio Preto, Brazil evaluated the effect of the natural growth promotor on production parameters and economics of tilapia production in cages (Sampaio Gonçalves et al., 2016; Table 1).
The trial was executed in experimental cages (7 m3 each) during 111 days starting from fish of an average size of 170 g, stocked with 840 fish per cage. Fish fed a commercial diet with or without the addition of the feed additive were evaluated in 5 replicate cages. The control treatment showed excellent production results, confirming the absence of any significant disease challenges during the trial.
At harvest, the treatment receiving the feed additive showed significant improvements compared to the control group regarding survival, feed conversion, and growth, resulting overall in 7.7% increased biomass yield and 3.5% reduced feed costs per kg of fish produced. The application of the feed additive overall increased the revenues for the farmer with 10%.
Fig. 1: Percentage improvement of growth and feed conversion ratio (FCR) due to supplementing a phytobiotic growth promoter (SANACORE® GM) to a practical feed of different aquaculture species. Data show the effect on growth (for fish : SGR, %/day; for shrimp g/week) and feed conversion ratio (FCR) relative to the performance of the non-supplemented control group in a feeding trial with healthy animals (Ceulemans et al., 2010).
Table 1: Effect of a phytobiotic growth promoter (SANACORE® GM) on the production parameters in Tilapia, ongrown in cages from 170 to 700 g during 111 days (Sampaio Gonçalves et al., 2016).
Reduced impact from diseases and parasites on productivity
Functional feeds containing gut health promotors deliver with every meal an adequate concentration of natural antimicrobial activities into the digestive system. These feeds are a key component of any strategy to prevent diseases in aquaculture, particularly when opportunistic bacteria are a major cause of mortality.
However, the success of this approach will depend on the efficacy of the gut health promotor. The gut modulating feed additive ideally is heat stable and can therefore be easily incorporated into the feed at the feedmill and be present in every meal from the starter feed onwards, without requiring major adaptations of the production protocols at the farm.
Natural feed additives combining different action mechanisms such as direct bactericide/bacteriostatic properties as well as Quorum Sensing inhibition properties at concentrations below MIC, are most promising to reduce the impact from bacterial diseases such as Vibriosis and Early Mortality Syndrome (EMS/AHPND) on survival of farmed shrimp (Fig. 2).
The inclusion of such botanical feed additive in a pelletized feed under standard industrial conditions at the feed mill improved survival under production conditions in a semi-intensive shrimp farm in Panama with 24% and 18% compared to the control group during two independent production cycles (Cuellar-Anjel et al., 2011).
In these production trials, the main disease challenge at the farm consisted of White Spot virus and Vibriosis.
A recent pilot production trial executed in small ponds at a shrimp farm located in the Guayas Province, Ecuador showed that the supplementation of a gut health modulator resulted in improved values for survival (with 20.5% compared to control), crop yield (with 14.1%) and food conversion (with 14.9%) (Valle and Coutteau, 2015).
Loc et al. (2015) was able to confirm the effect of the same synergistic phytobiotic product in a controlled challenge trial with Vibrio parahaemolyticus (EMS/AHPND strain) under laboratory conditions; showing 62-107% increased survival in shrimp that had received the additive during 3 weeks prior to the experimental infection, compared to unsupplemented control groups.
In the latter study by Loc and collaborators, the addition of the phytobiotic product in the diet resulted in consistently lower Vibrio counts in the shrimp’s digestive system compared to the control, illustrating the capability of gut modulating additives to protect the shrimp’s gut flora throughout a Vibrio challenge.
Fig. 2: Effect of functional feed additives with combined anti-bacterial/QS inhibition action on survival of Penaeus vannamei in various production trials in the field (in the absence or presence of EMS; left) and a controlled infection trial in the lab (right).
Conclusion
Current aquafeed formulations are mainly focused on nutritional specifications and ingredient choice, whereas the optimal utilization of the nutrients by the fish and the health status of the digestive tract are two areas which are rarely taken into account.
This is in strong contrast with the vast progress made in the agrifeed sector, where nutrient digestibility and gut health are regarded as two focus areas for technological feed development. Lab and field studies demonstrated the potential benefits for aquaculture in terms of productivity and economics of specific feed additives developed to stimulate the establishment of a healthy gut microflora.
References available from the author (p.coutteau@nutriad.com).
