Nutritional strategies in open water aquaculture

Arun B Patel *

Introduction

There is a huge deficit between demand and supply of fishes in the NE region and the region is importing fish almost on a daily basis not only from different states of India but also from the neighbouring countries including Bangladesh and Myanmar. Furthermore, locally produced fishes are in greater demand and fetch higher prices compared to imported/frozen fishes. Therefore, it is imperative to increase the local production.

Since, the catch from the capture fisheries is stagnating or even declining in some part of the region, the increased production has to come from the aquaculture. The earthen ponds remain the favoured fish production system. However, the region's landscape is dominated by undulating and /or hilly terrain, somewhat limited plain lands available for pond construction.

On the other hand, the region is bestowed with rich and diverse open water aquatic resources including rivers (20,050 km), floodplain wetlands and lakes (143,792 ha), reservoirs (23,972 ha) against about 40,809 its of ponds and tanks. These resources are relatively pristine and fish productivity in these natural water bodies remains rather low.

Productivity from these resources can be enhanced by employing suitable production systems such as pens and cages where extensive to intensive forms of rearing or growout can be taken up through appropriate measures including stocking, fertilization, feeding, protection from predators, etc. Nutritional interventions in pen and cage culture Artificial diets are provided so as to further enhance the fish production per unit area per unit time.

The development of suitable feed and feeding strategy would depend on the species, size, price of the product, local availability of resources, etc. Traditionally protein has attracted the central attention in case of fish diets because it is the most important macro-nutrient as far as fishes are concerned because of multiple reasons including protein being source of indispensable/essential amino acids, being more expensive than particularly the carbohydrate, and can also be utilized by fishes very efficiently for energy.

In addition, fishes as such need much less maintenance energy compared to warm blooded land animals and most of the fishes particularly carnivorous fishes are not great utilizers of carbohydrate. While artificial feeds enhance productivity, usually it also constitutes the single largest operating cost. Therefore, it is important to bear in mind that the utilization efficiency is crucial in determining the economic success of artificial feeding in pen or cage culture.

In this regard, it is important to note that feed conversion ratio (amount of feed to produce one kilogram of wet meat) is substantially lower compared to that for terrestrial animals. For instance, ca. 4-8 kg of diet is required to produce one kg of beef cattle or hog/pig, whereas only 1-2 kg of diet may be required to produce one kg of fish. However, considering the markedly higher cost of protein in comparison to carbohydrate, the cost of a fish diet usually is higher than that of diets for cattle/pig because of higher protein content of the fish diet.

Further, feed cost of production would also depend on the fact that how much of nutritional requirement of the cultured fish is being met by natural food. Greater the contribution from natural flood, lower would be the feed cost of fish production. Pen culture is usually of extensive to semi-intensive types with relatively low stocking density and natural food supplies a substantial part of the nutritional requirement of the cultured animal.

The main interventions are stocking and supplementary feeding with simple mixtures of locally available plant origin materials including rice bran and oil cakes. Depending on the hydrological characteristics of the ecosystem in which pens are established, fertilization can be undertaken if water current/wave is not strong. In contrast, cage culture is usually more of an intensive type where stocking densities are usually high and need more complex feeds containing all the essential nutrients.

As can be easily visualized, fertilization is almost not utilized for productivity enhancement in cage farming. Therefore nutritional strategy in the form of provision of artificial feed is the main intervention apart from the stocking in cage farming. In general, diets for a typical high stocking density cage culture systems which should have a balanced profile of all the nutritional components, more careful formulation using multiple ingredients is a necessity because no single ingredient contains a balanced profile of all nutrients which can be cost effective.

Various levels of formulation, starting from simple Pearson Square Method where only one level of one nutritional component, generally protein, is optimized, to a much more sophisticated Least Cost Method using linear programming and /or special computer software. Fish meal has traditionally been used as the source of protein in diets but considering the high price as well as finite availability of fish meal, alternative protein rich plant materials such as soybean meal has been aggressively evaluated.

Since, plant origin proteins lack some of the essential amino acids in addition to having some anti-nutritional factors which reduces the utilization of ingested feed, the complete replacement of fish meal has not yielded the same production of the fish. Nevertheless, it has been possible to replace substantial parts of fish meal by utilizing soybean or other plant origin protein rich materials. In addition to protein, further attention also is given to pellet size, water stability, nutrient leaching, threat to remove/ deactivate anti-nutritional factors, and etc in association with feeding behaviour of the cultured fish.

For instance, pellet size has to be smaller and more water stable having less leaching of nutrients for prawns which are a slow eater compared to fish. From a cost optimization point of view, maximizing the utilization of protein components of the diet only for growth and minimizing its utilization for meeting the energy requirement is one of the most important strategies to improve the profitability of culture practices.

To achieve this sufficient level of energy should be provided in the diet through incorporation of sufficient carbohydrate for herbivorous and omnivorous fishes or lipid for carnivorous fishes. This particular strategy is based on the concept of protein sparing effect of the carbohydrate and /or lipid. The carbohydrate is the cheapest source, therefore, it has special relevance in the culture of herbivorous or omnivorous species.

The first logical step after having the understanding of the above-mentioned concept is to formulate a suitable diet based on the nutritional requirement of the cultured species. For instance, in a clear water system the optimum levels of macro nutrients for carps may be summarized as; adults need ca/ 30% protein for proper growth whereas fmgerlings and fry require 35% and 40% dietary protein for good growth and survival.

The gross requirement of lipid of juvenile & adult carps is 7-8% with young carps requiring relatively higher lipid content. The juveniles may utilize dietary carbohydrate levels upto 35-40% of the diet. The dietary protein requirements of carnivorous fishes differ from that of carps primarily on two accounts that the diets have to have markedly higher protein content (35-45%) and should not contain high level of carbohydrate on account of the fact that carnivorous fishes are poor utilizers of carbohydrate.

Supplementary feeds of 20-24% protein content are good enough to support optimum fish growth in a productive fish pond. It is important to realize that a diet consisting of 1:1 rice bran and oil cake which is often soaked in water for a couple of hours before it is offered to the fishes, is not a nutritionally balanced diet.

The formulation of a supplementary fish diet on a least cost basis using linear programming/computer software utilizing few more locally available ingredients of plant and animal origins to ensure optimum protein level and a better amino acid profile of the resulted formulation would be a highly desirable supplementary feed for carps. The mixture can be converted into a pellet of desirable shape, size, water stability of sinking or floating using simple compression or a highly sophisticated extrusion process.

For instance, a more appropriate diet can be prepared using locally available resources such as wheat bran (10-20%), dry fish (ca. 10%) and peas (5-10%) in addition to rice bran and mustard oil cake. Formulating and producing a good quality diet is only but part of the issue, and due attention must be given to the proper storage of theproduced diets. Ifnot storedproperly, the quality of diet would deteriorate drastically within a short dine.

The deteriorated diets not only result in poor utilization or wastage ofmaterials but also can cause serious health problems through spread ofpathogens and toxins, and may even cause mass mortality. Therefore, diets and related ingredients must be stored properly to avoid quantitative, quAitative and economic losses. Preventivemeasures during storage can be listed as:

Check the ingredients for contamination/quality deterioration
Store under clean and dry condition
Avoid high temperature (insulation and ventilation)
Packaging (multilayer if possible)
Control stack size
Do not store on wet floor/Use a wooden stage
First in first out
Human error reduction/less handling
Keep bags away from wall
Perform fumigation if apparent signs of insects/rodents

Once a good quality diet/feed ingredient is available, the next important point that needs our attention is feeding strategy. Feeding can be up to satiation (ad libitum) and or a fixed quantity. It generally involves different components including feeding time, feeding amount/ration, feeding method, feeding frequency, method of application, etc. Feeding time would generally depend on cultured organisms.

Fish should be generally fed during the daytime but for nocturnal organisms like prawns/shrimps, a major portion of ration should be provided in the evening/early morning hours. In general, fishes should be fed when dissolved oxygen in the water is high and increasing. In the event of persistent cloudy days and/or rainy days, feeding should be reduced and/or completely stopped. Feed ration as % of bodyweight and feeding frequency generally depend on the age of the animal.

Younger animals need to be fed at a higher% of their body weight and more frequently. For instance, the younger fishes/fry stage needs to be fed ca. 10-20% of their body weight and total ration should be split into 4-6 meals a day with juveniles ca. 2-4% body weight and whole feed ration may be split into 1-2 meals. The feeding method could be manual & simple broadcasting (especially for floating pellets), and or putting the sinking pellets into a feeding tray. Other methods could be simple demand feeders where fish are trained to feed from the feed bags hung or fixed into the culture system.

Conclusion

Natural foods may contribute substantially to the fish growth in pen culture where mostly extensive to semi-intensive form of aquaculture is performed, therefore, whenever possible efforts may be made to stimulate the production of natural foods through application of organic manures and /or inorganic fertilizers.

The presence of natural foods usually reduces the cost of fish production. However, it must be applied keeping the hydrology of the culture system in mind. If wind action, current or waves are strong, this strategy may not be judicious and supplementary feeding should be taken up in priority manner.

On the other hand, in cage culture where stocking density or standing biomass usually is higher, the artificial feeding with quality diet is required for sustaining or further enhancement of the growth of the carps/cultured animals in.

Further, the formulation of appropriate fish diets should be done after keeping in mind different factors including the nutritional requirement of the target species, availability of ingredients along with applicable costs, and the price of the produce so as to get the maximum biological and /or economic efficiency. The development of suitable diets and efficient feeding strategies in pen and cage cultures should consider the relative roles of natural foods.

Further, attention should also be paid towards neutralizing the anti-nutritional factors in the plant origin ingredients by simple methods such as asking, pressure cooking, drying etc. so as to utilize easily available and relatively cheaper plant origin nutrient sources.


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