TECHNICAL ARTICLES - AQUACULTURE
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South African aquaculture production, though limited in its contribution to Africa's and global production, has shown a significant increase over the past few years. Total production and value has increased from 3000 tons (R51 million) in 1997, to 4624 tons (R136 million) in 2003 - a 54 percent increase in weight and 167 percent in value (Hoffman, et.al., 2000; Brink, 2003 personal communication). A potential aquafeed market of 3071 tons can be projected from these values when considering realistic feed conversion ratios for respective aquaculture species (Table 1).
Table 1. South African aquaculture production and potential feed consumption projections figures for 2003. 
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The supply and application of specie-specific aquafeeds may play a main role in the development and diversification of the aquaculture industry since it constitutes the most expensive component of the industry.
Feeds may represent from 30 percent of the operational costs in semi-intensive aquaculture where fish are nourished from natural pond productivity as well as supplemental feeding, to up to 85 percent in intensive aquaculture, which depends solely on the supply of artificial feeds.
Division of Aquaculture of the University of Stellenbosch incepted aquafeed development as priority in 1994 after accessing the poor condition of aquafeeds. Under the address of the Feed Technology Project (FTP), activities were directed towards developing better performing cost-effective feeds with reduced environmental impact, to ensure competitiveness on local and international markets.
Forming a close collaboration with the Catholic University of Leuven in Belgium and with the Malmesbury-based aquafeed manufacturer, AquaNutro (Pty) Ltd, the FTP focussed on dedicated feed development support to the local aquaculture industry, which was furthered into the structuring of an industry-oriented course in nutrition for local and distance education students in aquaculture.
This collaboration directly resulted in the wider supply and improvement of aquafeeds, such as the effective reduction in feed conversion ratios of trout feeds from an initial 2:1 to a level of 0.98:1 for up to 1 kg trout (Figure 1). 
Currently, efforts are strongly focussed on the use of “lower-protein-higher-energy” extruded aquafeeds, mainly due to limitations in the application of conventional compress-pelleted feeds in intensive eco-friendly aquaculture.
Inadequate gelatinisation during compress-pelleting often limits optimum water stability and digestibility of aquafeeds, resulting in excessive pollution of water outflow. This is especially an issue in eco-sensitive farming environments. Ironically, these pollutants firstly contaminate the direct living environment of the fish, which may influence health and production potential.
Also, protein levels and unavailable phosphorous levels in feeds have been reduced in order to reduce the amount of excessive nitrogen and phosphorous released to the environment. No antibiotic growth promotants are currently being registered for use in aquaculture, and its use is not foreseen for the near future due to sensitive niche markets.
South Africa has a well-established agriculture industry and related animal feed sector, which provide a wide range of raw materials and by-products that can be used in the formulation of aquafeeds. Essential information on the nutritional value of these products, as well as the nutritional requirements of the candidate species under local conditions, are however not readily available in a complete form for aquafeed formulation and much of these are being extrapolated from other fish species, and even from commercially-important land animals.
Adequate information and technology expertise on aquafeed production are also still limited in South Africa, with little specialised training being on offer, or research being conducted. Due to the relative size of the aquaculture industry very limited funds are available for fundamental research and industry support is limited to product development within companies and evaluation on a contractual basis.
Currently, sub optimal feeding management practices possibly pose an even bigger threat to the economic and environmental optimisation of aquaculture, often overshadowing feed development efforts. Feed wastage from overfeeding is costly and causes pollution to the environment. Table 2 illustrates the effect of poor feed conversion on the concomitant phosphorous pollution potential and incidence cost of various trout producers in South Africa, all using the same feed type and source.
Table 2. The effect of weakening feed conversion ratio on phosphorous waste and incidence cost from various South African trout farmers in 2002.
From Table 2 the economical implication of weakening feed conversion on incidence cost is quite clear. These differences in feed conversion ratio values may directly be related to differing feed management practices on these farms, mainly due to underfeeding of feed wastage from overfeeding. Underfeeding and overfeeding normally results from misjudging the “appetite” of the fish due to a lack of visual observation experience and often due to a lack of dedication when fish are hand-fed.
In attempt to quantify the efficiency of feeding management of a typical local production system, 10 treatments of rainbow trout weighing 200g were stocked at the Elsenburg experimental cage system and fed increasing levels of feed for a period of 3 months. The resultant relationship between feeding level (expressed as percentage of bodyweight per day), feed conversion ratio (FCR) and specific growth rate (SGR) is considered to be an important tool for any producer to evaluate the adequacy of farm-specific feeding management (Figure 2).
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From Figure 2, FCR typically decreases from infinity in fish fed at the level required to satisfy their maintenance requirement, to a ever-increasing value beyond maximum feeding level. When fish that are fed a feeding level in excess of their true appetite, feed wastage occurs and the FCR increases. From its relationship with feeding level, SGR shows an initial increase with increasing feeding level until a distinct plateau is observed at the maximum feeding level.
Since it is often impractical to measure “true” appetite and hence actual food consumption, producers usually assume that the fish eat all of the feed that is presented, and that the levelling off of the SGR curve is due to decreased growth efficiency. This levelling-off may however simply be due to the fish being fed in excess of their appetite, and hence to feed wastage.
In practice, the lowest incidence cost is generally attained when lowest minimum FCR levels are achieved, often when restricted feeding levels are fed according to feed tables of feeding management software. Compensating for low incidence cost, fish however often need to be fed according to maximum SGR instead of minimum FCR when fish need to be “pushed” through the production system. This may typically be the case:
The further apart the “optimum feeding level” (at minimum FCR) and “maximum feeding level” (at maximum SGR) on the graph however, the poorer feeding management (as is the case in Figure 2). In an ideal management situation though, the optimum feeding level with respect to SGR and FCR, should be at the same feeding level, without any feed restriction.
The basic understanding of appetite and feeding behaviour is hence of prime importance to producers in determining optimal feeding regime, feeding level, feeding frequency and duration of feeding cycles - all that should regularly be adjusted in order to optimise SGR and FCR values. Accurate detection of the appetite of all fish in all areas of the production system may lead to less variation in feed consumption and hence less feed wastage. Currently, farmers and workers lacking experience in this field often fail to do just this.
Food distribution by hand is currently the most common method of feeding fish in South Africa. Fish are fed floating or slow sinking feed pellets because fish producers appreciate the advantage of being able to see how vigorously the fish eat. In such feeding management systems, fish are actively fed during a feeding behaviour of aggressive capture of feed pellets with a vicious breaking and splashing of the water surface, the so-called “feeding frenzy” (se Figure 3). This feeding behaviour deteriorates as the fish are fed to apparent satiation of appetite.
The limitation of this system of feeding fish based on surface activities is that the feeder and not the fish usually determine feed allowance. While the feeding activity of dominant individuals are visual in the top layer of the production system, the remaining fish occupying the rest of the system area would only eat when food is made available by the dominants, and has no inputs in their “signal” for appetite satiation.
Although alternative feeding methods, such as mechanised feeding systems (demand feeders and automatic feeders, hatchery feeders, mobile feeders and subsurface feeders) may complement hand feeding, its use is still limited in South Africa. Besides for its often-high implementation cost, producers claim that these systems do not give the operator the chance to observe fish closely and be sensitive for their needs to accommodate the problems associated with feeding fish especially in cage-systems. Hand feeding is in addition labour intensive and time consuming and is limited in its application on large farms.
Much effort and initiative should still be invested in ways to practically quantify true appetite and to detect and feed wastage towards claiming effective feed management. Feed wastage detector systems such as the hydro-acoustic feed detectors have successfully been implemented in aquaculture systems abroad to monitor food sinking through aquaculture cage-systems. Although effective in minimising feed wastage, these systems do not detect fish feeding behaviour or appetite and are therefore limited in its pro-active ability to predict the appetite of the fish as indicated by the start of the next feeding rhythm.
The South African aquaculture industry requires better feeding strategies than those currently available and feed management is therefore seen as an important challenge facing aquaculture production from a nutrition supply point of view. Additionally, environmental considerations are very much in the interest of producers too, as good water quality is also a major prerequisite to long-term survival of the aquaculture industry. The need for the development of improved on-farm feed and water management strategies therefore motivates applying research and technology transfer from developed countries to minimise the costly feed wastage and its global potential negative effect upon the aquatic environment.
References
HOFFMAN LC, SWART JJ, BRINK D., 2000. The 1998 production and status of aquaculture in South Africa. Water SA 2000; 26(1): 133-135.
BRINK, D., 2003. Personal Communication - Chairman of the Aquaculture Association of Southern Africa
Authors: Lourens de Wet a and Dirk van der Linde b
a Feed Technology Project, Division of Aquaculture, University of Stellenbosch
b AquaNutro (Pty) Ltd., Malmesbury
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The previous article is a special collaboration from AFMA South Africa |
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