Abalone (Haliotis midae) aquaculture in South Africa is an example in which IMTA is practiced at a large scale. These systems have evolved from both the 'trial and error' approach and small scale scientific experiments. It is still in its early stage but the potential is high as South Africa is the largest producer of cultured abalone outside Asia. Abalone is cultured in land based flow through tank systems and fed kelps (Ecklonia maxima) or pellet feeds. Over 6000 tonnes of kelps are harvested annually on the South African west coast for abalone feed, and some kelp beds are now reaching sustainable limits of exploitation (Figure 8). A growing body of evidence suggests that a mixed diet of kelps and other seaweeds can induce growth
rates at least as good as with artificial feed, can improve abalone quality, and reduce parasite loads. Ulva and Gracilaria are cultivated in the wastewater from the aba lone and one farm on the southeast coast currently grows most of its feed (4 tonnes per working day of Ulva) in shallow raceways (Figure 9). Seaweeds grown in abalone wastewater have an increased nitrogen content, resulting in value added seaweeds with over 40% protein dry weight content and, hence, of excellent quality to feed abalone.
The general benefit from IMTA, that is, reduction of nutrient release to the environment, is also true for inte grated seaweed-abalone culture. Furthermore, as seaweeds remove ammonium from the seawater and add oxygen, the abalone wastewater passing through seaweed ponds can be partially re circulated back to the abalone tanks, thus potentially reducing pumping costs. The abil ity to operate in re circulation mode is important as red tides occasionally occur along the South African coast. Moreover, some coastal areas experience heavy traffic of tanker boats, which represent potential risks for oil spills. It has been shown that a farm can operate successfully at 50% re circulation, and even higher recirculation (up to
100%) can be sustained for shorter periods. This can, of course, be optimized, depending on what the main objec tive is with re circulation. The re circulation through seaweed tanks/ponds also has the potential to raise water temperature, which can stimulate abalone growth in areas of cold coastal waters. Compared to many other aquaculture operations, there is currently no real envi ronmental pressure from abalone wastewater release in South Africa. Wastes from abalone operations are differ ent from those of fish, with significantly lower concentrations of both nitrogen and phosphorus. This implies that the seawater in the seaweed tanks needs to be fertilized to sustain seaweed growth. This additional input of nutrients would not be needed if seawater from fish tanks were to be used (this has been tested with success). The development of IMTA in South Africa has, in fact, been driven by other incentives, such as future limitation of wild kelp harvesting and the proven eco nomic benefits from improved abalone growth and quality with seaweed diets.
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