Importance of feed Enzymes for Shrimp and Fish–In Sustainable Aquaculture

The expansion of global aquaculture production is increasing the demand for aquaculture feeds. Fishmeal is the main and most critical ingredient in aqua-feed production. The increasing cost of fishmeal has encouraged feed manufacturers search for cheaper alternative protein sources such as plant proteins. Commercial fish feeds usually contain high fish meal as the major protein source, ranging from 30-50% (Hardy, 1995). But now-a-days, fish meal is generally avoided in the feed due to its scarcity and high cost. Hence, aquaculture nutrition have been trying to improve the nutritional value of fish feed by enzyme supplementation, to find suitable alternatives to fish meal, since last decades.

 Feeding enzymes to shrimps and fishes is one of the major nutritional advances in the aquaculture sector since last few years. Exogenous enzymes are now extensively used throughout the world as additives in animal diets. Also, supplementation with enzymes can help to eliminate the effects of antinutritional factors and improve the utilization of dietary energy and amino acids, resulting in improved performance of fish/shrimps (Farhangi and Carter, 2007; Lin et al., 2007; Soltan, 2009).

The primary purpose of enzyme application in feeds is to improve digestion. The digestive processes will work better and result shown  in improved feed efficiency by providing an extra dose of enzymes. Further, aquatic animals are lack certain digestive enzymes during early development or throughout their life. In the case of fishes / shrimps lacking certain enzymes even in adulthood, application of these enzymes results in better utilization of nutrient fractions that are digested by the enzymes. 

Enzymes are one of the many types of protein in biological systems. Their primary characteristic is to catalyze the rate of a reaction  but is not themselves altered by it. They are involved  in all types of anabolic and catabolic pathways of digestion and metabolism. Enzymes tend to be very specific catalysts  that act on one or ,at most a limited group of compounds known as substrates.  Enzymes provide additional powerful tools that can inactivate anti-nutritional factors and enhance the nutritional value of plant-based protein in feeds. They provide a natural way to transform complex feed components into absorbable nutrients.

The addition of enzymes in feed can improve nutrient utilization , reducing feed cost and the excretion of nutrients into the environment.

Enzymes are  produced  in every living organisms from the higher animals and plants to the simplest unicellular forms of life as they are essential for metabolic process. Microorganisms that generally involved in production of various enzymes are:

Bacteria : Bacillus subtilis, Bacillus lentus, Bacillus amyloliquifaciens and Bacillus stearothermophils.

Fungus : Triochoderma longibrachiatum, Asperigillus oryzae , Asperigillus niger and yeast

In animals, digestion of food is carried out by the animal's digestive system and by microorganisms that inhabit the intestinal tract. The bacteria present in the gastrointestinal tract of fish/shrimp are potent producers of proteolytic enzymes. They may also produce cellulase moderately. The adding of live microorganisms to diets to produce enzymes is possible in specialty feed applications. Large scale commercial enzyme applications are rely on enzymes produced by microbial fermentation technology. 

Feed ingredients from plants sources contain some compounds that either the shrimp/fish cannot digest or which hinder its digestive system because they cannot produce the require enzymes to degrade .Though the palatability of many plant materials has demerits, anti-nutritional factors are the most serious concern in replacing the fishmeal completely in feed formulations. Anti-nutritional factors have an adverse impact on the digestion of feed and its efficiency. There are many kinds of anti-nutritional factors and they are associated with the most widely used plant materials like trypsin inhibitor proteins, glucosinolates and phytate.

Heat inactivation and water soaking are the two common detoxification methods used to overcome most of the anti-nutritional factors. 

Many enzymes  have been used in fish/shrimp  nutrition over the past several years which includes cellulose , (β-glucanases) ,xylanases and associated enzymes like; phytase, proteases, lipases and galactosidases. Enzymes in the feed industry have mostly been used for culture animals to neutralize the effects of the viscous ,nonstrach polysaccharides in cereals and other food grains.

Endogenous enzymes found in the fish/shrimp digestive system which help to break down large organic molecules like starch, cellulose and protein into simpler substances.

The  carbohydrate digestion improves by  using microbial enzymes. Addition of exogenous carbohydrates enzymes to feed increase utilization of unavailable dietary carbohydrates .High levels of non-starch polysaccharides (NSP) such as cellulose, xylans and mannans reduce the nutritive value of many plant ingredients. Intestinal enzymes to digest these carbohydrates are not produced by most animals.

Application of enzymes is probably a solution to high larval mortality in aquatic animals. The intestinal tract of aquatic animal larvae is shorter and relatively undeveloped when compared to that of the adults. The larval feeding would benefit from enzyme application.

Phytic acid is one of the most powerful anti-nutritional factors in plant ingredients. The anti-nutritional activity of phytic acid can be eliminated by the addition of relevant enzymes, for example phytase. The phytic acid or phytate found in cereals, legume grains and oil seeds is bound with phosphorus and also with calcium and magnesium, trace elements like iron and zinc, protein and amino acids. Most fishes do not possess their own enzymes to break down the phytate and release the nutrients so they pass through the fish undigested. This is why higher proportions of valuable nutrients from vegetable sources are not utilized by the animals and are wasted as excreta. The feed enzyme phytase not only releases phosphorus from the phytate but also releases minerals and amino acids that are also bound, paving the way for maximum utilization of nutrients. 

The application and benefits of phytase in animal species is well documented. Up to 80% of phosphorus in plant seeds is in the form of phytate. This form cannot be digested by the intestinal enzymes of  fish/shrimp. Further, phytate forms chelates with a large number of mineral cations (K, Mg, Ca, Zn, Fe and Cu) and complexes with proteins and amino acids, thereby reducing the bioavailability of other minerals and digestibility of proteins. 

Advantages of phytase

1.As the phosphorus bound in phytate becomes available as nutrient due to the addition of phytase, the inclusion of inorganic phosphorus such as fishmeal can be drastically reduced. 
2. The phytase reduces the release of nutrients into the environment by making the bound phosphorus available to the fish for growth – so it is incorporated into the fishes body instead.
3. Phytase added to the diets improves protein and amino acid digestion in fishes.
4. Phytase can improve the metabolisable energy of feeds by breaking down the phytate-lipid complex.
5. Cheaper plant based protein sources can be substituted for fishmeal lowering feed costs.
6. Phytase added diets have been shown to have a higher feed intake, growth and better food conversion efficiency than control diets.
7. Phytase reduces phosphorus load in their faecal matter of fishes and shrimps.
8. Phytase  improves  phosphorus and protein digestibility . 

Non – starch polysaccharides (NSP)
Another important anti-nutritional factor that can be addressed with feed enzymes is non-starch polysaccharides (NSP), present in the plant materials and found to reduce the performance of animals. Their anti-nutritive effects are mainly due to the increased viscosity of the digest in the intestine and the enclosure of nutrients making them unavailable to digestion. Since the animals lack the intestinal enzymes for the degradation of non-starch polysaccharides, the supplementation of degrading enzymes in the diet will break down these anti-nutritive factors and result in better feed utilization.

Successful and sustainable aquaculture depends on economically viable and environmental friendly feeds. Feed is the major operational cost involving 50 to 60% of the total cost in intensive farming. The major feed ingredient, fishmeal, is expensive and there is increasing competition with other livestock industries for the available supply.

Hence, research work has been focused to find alternatives to fishmeal. One alternative is to substitute fishmeal with plant proteins supplemented with feed enzymes. Phytase enzyme is able to release the phosphorus bound in phytate and this permits feed manufacturers to reduce the fishmeal and lower the cost of feed production. Improved phosphorus utilization can also help reduce the discharge of nutrients into the environment.

Enzymes have been used in aqua feed because they are natural products of fermentation and therefore pose no threat to fish / shrimp health and pond environment. Enzymes reduces  high phosphorus output problems in pond environments.  It reduces ammonia levels in pond water. 

Enzymes can therefore play an important role in formulating eco-friendly aquafeeds. Currently, the use of enzymes is able to reduce fishmeal inclusion by around 5% in most aquafeeds with potential for more as techniques are refined. This may help to reduce the demand for fishmeal from the aquaculture sector in coming years.

The use of enzymes as a feed additive has rapidly expanded and significant  instrument for the use in aqua feed. Although  the economic and social benefits of enzymes have been well established, the future of feed enzymes is a bright one in aquaculture industry. 

References

1. Jackson, L.S.; Li, M.H and Robinson, E.H. (1996).Use of microbial phytase in channel catfish Ictalurus punctatus diets to improve utilisation of phytate phosphorus. Journal of the World Aquaculture Society.27:309-313.
2. Forster, I.; Higgs, D.A.; Dosanjh,B.S.; Rowshandeli, M. and Parr, J.(1999). Potential for dietary phytase to improve the nutritive value of canola protein concentrate and decrease phosphorus output in rainbow trout Oncorhynchus mykiss held in 11°C freshwater. Aquaculture 179:109-125.
3. Carter, C.G. and Hauler, R.C. (1998). In: Fish meal replacement in aquaculture feeds for Atlantic Salmon: 23-45
4. Finnfeeds International. (1997 & 1998). Internal reports.
5. Feord, J.C. (1996). Exogenous enzymes improve performance of carp and tilapia when fed diets containing high levels of soyabean meal. VII International Symposium on Nutrition and Feeding of Fish.
6. Viola, S.; Angeoni, H. and Lahav, E. (1994). Present limits of protein sparing by aminoacid supplementation of practical carp and tilapia feeds. Israeli Journal of Aquaculture, 46: 212-222.