Enzymes for sustainable aquaculture
Posted on 4/11/2004
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Story by: N. Felix and S. Selvaraj, Department of Aquaculture, Fisheries College and Research Institute, Thoothukkudi-628008, Tamilnadu, India
The expansion of global aquaculture production is increasing the demand for aquaculture feeds. Fishmeal is the main and most critical ingredient in aquafeed production. The increasing cost of fishmeal has encouraged feed manufacturers search for cheaper alternative protein sources such as plant proteins. 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. Three that are associated with the most widely used plant materials are 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.
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. Endogenous enzymes found in the fishes digestive system help to break down large organic molecules like starch, cellulose and protein into simpler substances. The addition of enzymes in feed can improve nutrient utilization reducing feed cost and the excretion of nutrients into the environment.
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.
Advantages of phytase
1. Since 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 environmental performance of aquaculture operations is under scrutiny due to the discharge of nutrients into the surrounding ecosystems. Excessive phosphorus in particular is an important factor in the eutrophication of waterways. Phosporus bound in phytate may be unavailable to the fish but it will still ultimately be released into the environment as microbial action breaks down the fishes waste. The addition of 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.
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. Such an approach has been successfully used in poultry diets.
Experimental results using feed enzymes
A number of studies have reported successful use of enzymes to combat anti-nutritional factors in plant proteins for fish feeds. Phytase added diets have been shown to have a higher feed intake, growth and better food conversion efficiency than control diets in Channel catfish, as well as reduced phosphorus load in their faecal matter1. Trout fed with phytase-incorporated soybean based diets have been reported to show a 22% improvement over control fish as phosphorus availability increased from 46% to over 70%2. Microbial phytase added diets containing a higher proportion of plant protein have been shown to improve phosphorus and protein digestibility in Atlantic salmon3.
A feeding trial conducted with tilapia Oreochromis niloticus fingerlings in Brazil showed the significance of phytase in plant protein based diets. The feed was supplemented with commercial phytase enzyme “Natuphas??? at 0, 500, 1500 and 3000 units per kilogram of feed. Fishes fed with 500 units of Natuphas showed higher weight gain and a better food conversion ratio of 1.80. Supplementation of protease-based additive equaled the performance of low protein milk fish diet (24% protein) up to the level of higher protein diets (28% protein)5.
The addition of commercial enzyme Pescazyme TM 5602 in soybean based diets free of fish meal showed equal performance of diets containing 10 or 12% fish meal in carp and tilapia4,5,6.
Aquaculture is fast growing Industry. 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 focussed 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 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.
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.