BIOLOGICAL AND ECONOMICAL FEASIBILITY STUDIES OF USING SEAWEEDS ULVA LACTUCA (CHLOROPHYTA) IN RECIRCULATION SYSTEMS IN ABALONE FARMING

ABSTRACT Significant effort has been put into the development of cost-effective abalone (Haliotis midae; Gastropoda) cultivation systems in South Africa, but the limited availability of suitable seaweed for abalone food is an obstacle to future development. The aim of this study was to investigate whether a land-based recirculating seaweed-abalone integrated aquaculture system using Ulva lactuca was feasible as well as to test the differences between a commercial gravel bed recirculation system to an existing flow through system. These studies were carried out at two abalone farms: Danger Point (I & J) (140 km east of Cape Town) and at Jacobs Bay (JSP) (120 km north of Cape Town. South Africa). In both studies no significant difference in terms of water quality, abalone growth rates and abalone health were found. It was found that a seaweed labalone recirculating system at the designed water exchange rates (25 %) was nitrogen limited and that the system as designed could be run at 75 % recirculation rate and remove a significant proportion of the dissolved nutrients (ammonium, phosphorus, nitrate and nitrite). It was concluded that seaweed functions well, as a biofilter. The system, both at 25 % and at 75% recirculation, was capable of reducing the effluent concentrations and maintain, or sometimes even improve, water quality as compared to control. Total particle loading did not increase with higher re-circulation, nor does the load of fractions smaller than 35 IJm or the carbon content of the particle load. Since sabellids prefer organic particles smaller than 35 IJm as feed, these results indicate that, at least from a feed perspective, kelp fed re-circulating systems did not favor sabellids or other mobile macro fauna, this was echoed in the health examinations of the animals. In addition, mobile macrofauna diversity and density were similar in a recirculating system compared to a flow through system. Although dissolved oxygen production in the seaweed part of the system was 33 % higher than the flow through system, the oxygen was not being transferred to the abalone tanks. This meant that over the experimental period dissolved oxygen in the integrated system was 5 % lower than the flow through system. Temperature in the flow through system was 1 % lower than the recirculation system. Seaweed production was positively or negatively affected by external environmental effects {e.g. warm water intrusions over the IX western Agulhas bank result in a 7 kg per tank decrease in seaweed production). The risk of spreading disease or rising bacterial levels in the growout through integrated seaweed labalone aquaculture, was considered low. Abalone farms want to supplement the abalone feed with cultivated Ulva and investigate the potential of integrated abalone seaweed systems. The Ulva used in this study was simple to cultivate as it grows vegetatively and was collected from free floating populations in Simons Town Harbor. It has a further benefit in its capacity to absorb nutrients and improve water quality of the aquaculture effluent. However, macroalgae, as feeds for aquacultured abalone produce dimethylsulfoniopropionate (DMSP). DMSP levels are high in Ulva lactuca up to 6977 ± 1161 ~g.g-1 W.wt. DMSP, while Graci/aria gracilis and the kelp Ecklonia maxima contain between 0.8 ± 0.3 and 26.8 ± 20.6 ~g.g-1 w.wt. DMSP. DMSP levels increase in abalone tissue after they feed on these algae. A volatile breakdown product of DMSP, dimethylsulfide (DMS), is formed in abalone during canning, causing repellent tastes and odours in some batches of canned meat. The abalone adductor muscle (the part which is more commonly eaten) displays high DMSP concentrations compared to other tissues. Further, the feeding regime determines to what extent DMSP accumulates. When U. lactuca is fed to cultivated abalone in isolation, DMSP accumUlates in the abalone to a concentration of up to 23 x 103 ~g.g-1W.wt. DMSP, a value of about 1.4 % of the fresh mass of the animal. A depuration phase of 3 - 6 months (depending on water temperature) prior to processing allows for the reduction of tissue DMSP levels to those seen in wild abalone, thereby ameliorating the negative effect on taste and odor. Taste tests showed that Asian people preferred abalone in its raw state with high DMSP contents while this preference changed when the abalone was cooked. Knowing the DMSP levels in feeds and its behavior in abalone tissue will lead to the development of new strategies for controlling abalone taste characteristics. Feed not only affects taste but also affects the cultivation environment. Abalone are wasteful feeders with more than 60 % of their feed intake being converted to waste. This waste is in the form of particulates and dissolved organic nutrients. The sediments have different particle sizes and nutrient values depending on the primary feed source. The higher the nutritional value of the sediments (e.g. a compound pellet feed), the greater mobile fauna, sabellids, vivo bacteria, nitrogen and carbon content the sediments will have. A mixed seaweed diet produced significantly higher phosphate concentrations of the three diets tested, while and Abfeed® diet produced significantly higher total ammonium nitrogen. The abalone farms have had a significant socio-economic impact in the coastal communities in which they are situated. They employ 840 people countrywide but if the feed, canning and seaweed industry figures are included this increases to 1200. Of this 61 % are unskilled workers, which is the highest percentage of people in these communities. The industry has expanded by 500 % in the last 10 years and is predicted to expand by another 100 % in the next 5 years. This should see South Africa retain its position as the leading aquaculture abalone country outside of Asia at least until 2010.