Specificities of herbivorous species
General considerations
The diets administered in aqua production are especially designed depending on the needs of the target species so that they supply the essential nutrients for fish to grow properly and remain healthy.
These needs vary broadly depending on the target species and productive stage. This is because the nutritional requirements are different for each species and because of the singularities of the digestive tract.
Specificities of the digestive tract in fishes depending on their feeding habits
Generally, species with more specialized feeding habits, the carnivorous and herbivorous, produce a lesser variety of enzymes and these are less active. In these cases, it is necessary to include enzymes in the diets to make sure they are used properly.
On the other hand, fish with varied diets in nature, such as omnivorous fish or other species whose habits change as they develop, can secret a greater diversity of enzymes, so that they can more easily adapt to all types of diets.
The anatomy of the digestive tract is also adapted to the type of diet, so that the species that are mainly herbivorous, such as common carp, have a more developed intestine and a smaller stomach than carnivorous species.
Herbivorous species have a small stomach, which means that they cannot ingest large amounts of feed, since the fullness stomach regulates the appetite; and a very long intestinal tract, which means that it needs more time to digest feed. Therefore, they need to be fed continuously as they are constantly digesting the feed. The ideal frequency to administer feed varies depending on the development phase, too, since the first ages need to be fed continuously, while juveniles and older fish require lower feeding frequencies.
Nutritional composition of the diets for herbivorous fishes
The protein requirements in herbivorous species are between 30 and 40% of the diet in dry matter, far from the 40-55% required in carnivorous species [2]. Besides, the protein ingredients in diets for herbivorous fish are usually of plant origin, since they tend to have fewer problems than carnivores with this type of protein sources. This represents an advantage over carnivorous fish farming because vegetable protein is usually more economical and sustainable [3], [4].
Carbohydrate levels in omnivorous or herbivorous fish diets can be high, as these species are more tolerant to high levels of these compounds. Otherwise, the inclusion of lipids is usually low, in carp it should be below 6%, since these fish do not usually tolerate high levels of triglycerides and they can lead to fatty carcasses.
| Species | Crude protein (%) | Crude energy(kJ/g) | Protein:energy rate |
| Common carp (Cyprinus carpio) adult | >25 | 12.8-22.7 | 15.3-29.6 |
| Tilapia (Oreochromis aureus) fry | 34-36 | 13.4 | 20.5 |
| Tilapia (Oreochromis aureus) juveniles | 56 | 19.3 | 27.5 |
| Hybrid tilapia (O. niloticus x O. aureus) | 30-35 | 17.3 | 30.3 |
| Nile tilapia (O. niloticus) | 25-35 | ||
| Milkfish (Chanos chanos) juveniles | 40 | 15.3 | 26.3 |
| Rainbow trout (Oncorhynchus mykiss) | 40-45 | 19.1-20.8 | 20.5-22.5 |
Regarding the ingredients used, diets usually contain cereals as a source of energy (corn, barley, wheat), sunflower or other vegetable oils as a source of energy and triglycerides, and protein sources of vegetable origin such as soy flour.
Some diets intended for omnivorous but mainly herbivorous fish may also contain fishmeal, MBM, or feather meal to increase the protein content in the feed.
Lastly, enzymes like carbohydrases (such as xylanases and amylases) are highly recommended in herbivorous species because the pH in their digestive tract is more basic and, under these conditions, carbohydrases are more active. Proteases like pepsin are recommended if high protein diets are administered in adult herbivorous fish. Proteases are more active at an acidic pH and in species with larger stomachs, so they are less effective in herbivores.
Enzyme supplementation is a common practice that is more frequent in larval stages and early stages (fry and fingerling) when the endogenous synthesis of these enzymes is not sufficient for proper feed digestion.
Ingredients of vegetal origin: limitations in carnivorous species
One of the priorities of the poultry industry is to improve the sustainability of production, and this more achievable when growing herbivorous species, as they do not require animal protein in their diets. In other words, herbivorous fish are ultimately more efficient converters of plant protein to fish meat.
This way, it is easier to use protein vegetal sources such as soybean, instead of fishmeal or fish by-products that lead to the exploitation of rivers and seas. In contrast, carnivorous species such as salmon or trout require relatively higher amounts of resources.
Moreover, these species tend to require less investment for their cultivation and are more affordable for the general population, compared to salmon, because the latter requires highly concentrated protein feed and very specific rearing conditions.
Despite this, herbivorous fish farming currently represents only 5% of the total fish reared in aquaculture, probably since they are species that are less appreciated by consumers and with less value in the market.
Conclusions
Diets intended for herbivorous fish species require a lower content of protein and have higher carbohydrates inclusions. Besides, these species are more efficient in converting vegetal protein to animal protein, which means that their diets can be exclusively based in ingredients of a vegetal origin.
To optimize feed digestion, especially during the first development stages when the digestive system is not yet completely developed, carbohydrases can be used to help to digest carbohydrates.
Delta Labs offers a feed analysis and formulation service to optimize feed composition while considering the inclusion of the best functional additives. To use the service, please contact us through our webpage www.globalvetslab.com/en or via email at info@globalvetslab.com
References
- P. G. White, T. A. Shipton, P. B. Bueno, and M. R. Hasan, Better management practices for feed production and management of Nile tilapia and milkfish in the Philippines, no. September. 2018.
- M. R. Hasan, “Nutrition and Feeding for Sustainable Aquaculture Development in the Third Millenium,” in Technical Proceedings of the Conference on Aquaculture in the Third Millenium, 2000, pp. 193–219.
- GVL, “Fuentes alternativas de proteína en cría de peces carnívoros. Uso de enzimas.,” Delta Labs, 2021. [Online].
- NOAA, “Feed for Aquaculture,” NOAA Fisheries. [Online]. Available