Upgrading low-quality feeds for tilapia by enzyme and probiotic supplementation

With the expected further growth of aquaculture and the increasing demand for aquafeeds, the trend for reducing levels of fishmeal and fish-oil in these feeds will continue. The use of lower quality ingredients for many fish species, including tilapia, will increase in the future, leading to increased dietary levels of anti-nutritional factors, including non-starch polysaccharides (NSP) and phytate. Both NSP and phytate are considered indigestible, as the enzymes needed to breakdown their bonds are scarce or non-existent in fish. Supplementing feeds with dietary enzymes and probiotics can reduce the antinutritional effects of phytate and NSP. However, compared to other monogastric animals, studies on fish using phytase and NSP-degrading enzymes are scarce. Likewise, multiple studies have been performed, both on broilers and pigs on the improvement of nutrient bioavailability with the use of probiotics. However, for fish similar studies are few in number; little is currently known as to how probiotics can improve nutrient bioavailability of plant-based aquafeeds. In addition, it is expected that in fish there may be synergy between different enzymes, and between enzymes and probiotics, but this has not yet been assessed. Therefore the aim of this thesis is to assess the potential of dietary enzyme and probiotic supplementation in improving the nutritional value of low quality feed ingredients, using Nile tilapia as a model species.

The main conclusions of the thesis are:

• NSP is not inert to digestion.
• Phytase improved the availability of phosphorus, calcium and crude ash fractions in the diet.
• Xylanase has the potential to improve nutrient digestibility.
• β-glucanase did not improve nutrient digestibility.
• Probiotic supplementation improved fat digestibility.
• Probiotics improved growth and reduced the energy requirements for maintenance.
• In Nile tilapia, the available phosphorus requirement is greater than 7 g/kg feed (dry matter).
• The combination of enzymes and probiotics enhanced microbial interactions in the distal gut.
• Enzymes, when supplemented to the diet, are mainly active in the stomach.
• The effect of enzyme supplementation is dependent on diet quality.
• Synergy between phytase and xylanase can occur.

Overall it can be concluded that, in Nile tilapia, the nutritional value of low quality diets was improved with exogenous enzyme and probiotic supplementation, resulting in enhanced growth performance. This implies that an increasing proportion of low quality plant-based co- and by-products could be incorporated into aquafeeds, thus contributing to the development of circular food systems.