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THE USE OF FEED IN RECIRCULATING AQUACULTURE SYSTEMS (RAS) One of the greatest operating costs in aquaculture is the use of commercial feed pellets, which can comprise of up to 50-60 percent of total expense in some farms.

by Rob J Davies – Aquabiotech Group

n traditional forms of net-pen culture, the composition and wastage of these pellets is very relevant to maintaining good growth of the species being grown, managing expenditure and minimising environmental impacts on the surrounding water bodies. Ultimately, they do not have a large effect overall on their operation as a system. In RAS however, these frequently overlooked considerations, especially by new operators, are extremely important and their lack of understanding can easily lead to the failure and loss of the facility’s economic viability. As a commercial scale farm manager for several RAS farms over the years, I have seen and overcome many of the problems associated with using commercial pellets formulated for netpen culture and the implications of overfeeding of these pellets on a filtration system. Unlike flow- through or net-pen systems, any pellets that are not eaten by the cultured species are retained in the system and must be processed by the RAS. This is potentially a major problem as the filtration systems are designed to process faeces and not pellets, which can be four times as dense. The leeching of oils from the pellets interferes with the foam fractionators or protein skimmers in the systems that are responsible for removing micro-particulate organic material from the water body, reducing the optimal water quality and clarity that must be maintained in order to achieve the fast growth rates required to make a RAS profitable. Another effect of uneaten pellets is the overloading on the mechanical filtration process responsible for removing the macro-particulate organic material from the system, this can cause the overflowing of these filters, which introduce these particles into the bio-filter (often the next filtration process in the sequence in RAS), diminishing its capacity to process the toxic inorganic compounds, such as ammonia, released by the organisms being grown. If the degree of overfeeding is frequent or severe, this bio-filtration process provided by nitrifying bacteria (that take up to eight weeks to establish), can be overcome by heterotrophic bacteria feeding on the supply of organic material and cause a total failure of the system. The outcome being that the toxic inorganic compounds, that are no longer

50 | May | June 2016 - International Aquafeed

May | June 2016 - International Aquafeed