01.01.16- Re-examining feeds and water quality

By Thomas R. Zeigler and Chris Stock

Feed efficiency and FCR management, critical for shrimp health should change with current shrimp and challenging farming conditions.

Water quality, along with aqua feed, are the most important factors affecting shrimp performance and health in shrimp aquaculture systems. However, even with readily available technical information on the importance of maintaining optimum water quality in shrimp aquaculture systems, including remedial technologies, we still hear of the many reports of crop failures or low production resulting from undesirable water environments.

Shrimp aquaculture production conditions have changed over the last 4 decades. Advanced genetic breeding programs allow shrimp to grow faster. The types of containments used for production have evolved, as have management methods, and stocking densities have increased significantly. We now have higher biomass harvested per unit of time and space. These developing changes are necessary in order to maintain an economically viable and growing shrimp aquaculture industry.

In order to accommodate many of these changes, the feeding rates have increased resulting in higher oxygen demand, greater CO2 production, higher ammonia and nitrite production. Unless managed, the consequence is increased levels of dissolved, suspended and settled solids. These can rapidly reach concentrations that the animals cannot tolerate.

“FCRs must be evaluated in real time, such as on a daily or weekly basis in order to be relevant.”

A primary cost of production, feed either as uneaten, undigested or as metabolic byproducts, is the major contributor to the rapid decline of water quality. In turn, this can quickly promote disease outbreaks. Accordingly, feed and feeding techniques require continuous review and improvement.

Feed efficiency

A feed with a high feed conversion ratio or FCR results in less of the feed going to shrimp growth and more of the feed going into the water environment. FCRs must be evaluated in real time, such as on a daily or weekly basis in order to be relevant. If one just uses the FCR values at the end of a crop, this number has economic importance, but its value in terms of measuring feed efficiency is compromised because of the impact of mortality, unless, the weight of the mortality is added to the weight of the crop. It is suggested that if real time FCRs approach 1.4 :1 then this is an indicator that there is opportunity to improve.

By applying best feed formulation and manufacturing practices in conjunction with carefully monitored feed management techniques today’s feeds are capable of resulting in a real time FCR of 1.1:1 in most commercially produced shrimp. This should be our goal in managing our shrimp production practices

Waste production

The prediction on waste products generation from aquaculture was studied by Dr George Klontz and associates in 1978. Table 1 clearly illustrates that an increase in FCR by 0.2 units increases the amount of solids by approximately 8.5 kg/100 kg of feed fed or an increase in FCR from 1.2 to 1.6 increases the amount of water solids by 20 kg/100 kg of aqua feed applied to the pond.

However, the quantity of waste predicted is reported as solid waste and does not consider soluble wastes. The extra waste must be dealt with in some way or another, to maintain optimal water quality. Although the researchers conducted their work with trout, it is suggested that as the FCRs increase the relative increase in the amount of solids produced would be similar for most aquaculture species including shrimp.

Managing FCRs

How can farmers improve FCRs without compromising the performance, growth rate and survival of shrimp? This should be a continuing, top-priority objective of management. Several important tools and techniques can be applied to accomplish these objectives. The most important ones are mentioned below.

Eliminate overfeeding

This is the single most important way to improve FCRs in shrimp farming. This involves selecting the appropriate, highly palatable, highly digestible and complete feed as well as selecting the right particle size. For the shrimp, more frequent feeding is better. However, it is important that shrimp can easily and quickly access the feed when delivered to them.

Nutrient dense feeds

Feed a nutrient dense feed formulated to avoid both nutrient excesses and nutrient deficiencies. The feed should be highly digestible and formulated using highly digestible ingredients. The scientific literature reports that the digestibility of dry matter and protein can vary as much as 20% or more for different sources of the same ingredient.

Gut health

Maintain optimum digestive track physiology and health. This is where the nutrients move from the feed into the lymphatic or the circulatory systems through the process of digestion and absorption. If the lining of the digestive track becomes irritated and dysfunctional- because of toxins, bacterial or parasitic infections - the digestive and absorptive processes can be significantly reduced and the digestibility of the feed becomes impaired, resulting in greater faecal waste.

Avoid under feeding

Although it is well known that under-feeding gives a lower FCR value, this practice is definitely not recommended for several reasons. Hungry shrimp will consume the detritus that is on the bottom of the ponds and can be a significant source of disease.

Hungry shrimp typically will aggressively consume small, weak or dead animals that can also be infected with pathogenic organisms. Lastly, if under-feeding is not very carefully controlled it will result in smaller, less uniform animals, greater overall size variability and lower biomass at harvest. The resulting economic loss can easily be greater than the value of the feed saved.

Relevance of water quality to EMS / AHPND management

There is growing and strong anecdotal evidence that organic loads in pond bottoms, from uneaten feed, faeces and other sources, play a significant role in the occurrence of early mortality syndrome/acute hepatopancreatic necrosis disease (EMS/AHPND), as a source of nutrients for pathogenic Vibrioparahaemolyticus. Therefore, farmers should adapt pond management methods and feed management to minimize organic loads in shrimp ponds. This is to keep the density of the EMS pathogen below 104 CFU/mL. No EMS toxicity has been reported to occur below this bacterial density.

To minimize organic loading of ponds, some of the more relevant management recommendations include the prevention of overfeeding, the digestion of excess organic matter using probiotic products, and the frequent/constant removal of uneaten feed and organic sludge from the pond bottoms.

Automated feeders can significantly improve feeding efficiency as well as shrimp growth, survival and FCR, in ponds of all sizes, when compared to traditional feed broadcasting methods. Shrimp learn to approach the feeding station, and only one unit is needed for every 500-750,000 shrimp at the high stocking densities typically used throughout Asia. In other shrimp farming regions like Latin America, with lower stocking densities and biomass, 1-2 units/ha can service the larger ponds used.

In addition, automated feeders can broadcast small pulses of feed pellets at varying time intervals. This means that most if not all pellets will be caught and consumed and very little feed waste accumulating on the pond bottom or dissolving in the water. The result is reduced organic waste and sludge buildup, better pond water and pond bottom quality. This reduces the potential for pathogenic bacteria like EMS-causing Vibrio parahaemolyticus populations to grow and reach threshold densities where toxin production is triggered.

For frequent or constant removal of uneaten feed and organic sludge from pond bottoms, a technique that is gaining popularity is the construction and operation of so-called ‘pond toilets’. These are depressions on the pond bottom, usually at the centre of the pond or where or organic matter and sludge accumulates(which can be promoted and located by proper positioning ofaerators). The sludge can be frequently or continuously pumpedout from these depressions using hoses and PVC pipes and smallpumps operated from pond dykes.

Concluding remarks

A clear understanding of the role feed and feeding have on water quality is important for shrimp farmers. A lower FCR number is better as it normally indicates higher levels of feed utilisation and lower levels of solids going into the water column. It can also indicate less feed wastage.

Preventing water quality problems through proper feeds and feeding should be the first priority as the cost of remediation can be quite significant. In either case, the water quality standards for optimum performance of aquatic animals must be maintained.

AQUA Culture Asia Pacific Magazine - January/February 2016 Voume 12 #1