CARNIVOROUS FISH CAN USE PLANT-BASED DIETS EFFICIENTLY IF SUPPLEMENTED WITH CERTAIN BACTERIA

Successful and profitable aquaculture of a carnivorous fish requires ecologically and economically feasible supply of feeds containing high quality proteins. This requirement has conventionally been met by fish-meals from wild-caught fisheries, containing such high quality proteins. However, considering the problem of overfishing, this is not a sustainable source of high-quality proteins. For many ecological and economic reasons, there is a trend of replacing fish-meals with plant-derived proteins. However, plant-based diets are problematic to carnivorous fish as they contain large amounts of cellulose and starch, which carnivorous fish finds difficult to digest and use efficiently.

Cellulolytic microbes have the ability to break complex lingo-cellolosic bond in plant-based diets. Similarly, amylolytic microbes can convert starch to simple sugar by secreting digestive enzymes in the gut. Such cellololytic and amylolytic activities of gut microbes have been shown from the gastrointestinal tract of freshwater and brackish water fish in many studies. In vitro investigation on bacterial strains secreting cellulase, amylase and protease have shown reduction in the levels of fibre and carbohydrate and, increase in the protein content of the diets. Hence the idea is to use such types of microbes as supplements in the plant-based diets of aquaculture fish to improve the nutrient utilization and growth of fish.

This study examined the effectiveness of the selected cellulolytic and amylolytic bacterial strains as feed supplements for juveniles of Asian seabass (Lates calcarifer). Both of the bacterial strains used, belonged to the genus Bacillus, which were isolated from the intestines of brackish water fish. The juveniles of Asian seabass were fed plant-based diet, supplemented with the bacteria of those selected strains of Bacillus and then compared with the fish at control, that were also fed the same diet, but without the supplementation of bacteria. Apart from control, there was one treatment group supplemented with only cellulolytic bacteria (Bacillus sp.), another group supplemented with only amylolytic bacteria (Bacillus subtilis) and the last group supplemented with a mixture of both strains. 

Comparison with the control revealed that all of the bacteria supplemented groups had higher weight gain, improved nutrient digestibility and survival. Similarly, they showed higher protein efficiency ratio, specific growth rate and lower feed conversion rate. Thus, as we have learned in Daniel’s lectures, the findings of this study clearly showed the importance of gut microbiota of fish as well as the benefits of using probiotic bacteria in fish farming, which has been discussed in details by the authors. Colonisation of such beneficial microbes in the gut of fish, causes increased activity of digestive enzymes, better nutrient digestibility and increased nutrient absorption. Gut microbes play key roles in the digestion process by inducing the endogenous enzyme secretion and also by increasing the total enzyme activity, as this study reported increased levels of amylase and cellulase activity in treatment groups supplemented with bacteria, compared to controls. Besides that, gut microbes also help their host by producing other nutrients such as vitamins, essential amino acids and fatty acids. Vibrio population was lower in the water of bacteria supplemented treatment groups compared to controls, suggesting that the beneficial gut microbes also help their host against pathogens like Vibrio spp.

The benefits of increased activity of digestive enzymes were more pronounced in the fish which were supplemented with the mixture of amylolytic and cellulolytic bacteria, rather than those which were supplemented with either type of bacteria. Compared to controls, protein content of the supplemented groups was also higher, which was again highest in the fish supplemented with the mixture of both strains of bacteria. This highlights the importance of supplementing a cocktail of bacterial strains, rather than providing single strain/type of bacteria to the fish.

De, D., Ghoshal, T. K., Ananda, Raja, R., & Kumar, S. (2013). Growth performance, nutrient digestibility and digestive enzyme activity in Asian seabass, Lates calcarifer juveniles fed diets supplemented with cellulolytic and amylolytic gut bacteria isolated from brackishwater fish. Aquaculture Research 1-11; doi: 10.1111/are.12325.