Organic acids as potential growth promoters in abalone culture

[摘要] ENGLISH ABSTRACT: The first successful captive spawning of the South African abalone Haliotis midae occurred inthe 1980's and subsequently the commercial abalone industry in South Africa hasdeveloped, with an estimated investment of US$ 12 million and annual output of 500 to 800tons by 2001, making South Africa the biggest abalone producer outside of Asia. Naturalkelp is currently the major feed and the development of a suitable substitute, and improveddisease management in abalone culture are seen as the primary factors limiting expansion ofthe industry in South Africa. Further, abalone growth rates are very slow and improvementsin growth rate will lead to shortened production times with benefits to producers. Diseases inaquaculture have traditionally been combated using antibiotics as treatment (therapeuticusage) and preventative measure (prophylactic usage). In terrestrial livestock management,antibiotics are also used as growth promoters. The use of antibiotics in aquaculture hasrecently sparked concerns about the development of antibiotic resistance in pathogens ofhumans and aquaculture organisms, and alternative strategies to using antibiotics mainlyfocus on manipulating the microbial composition in the host organism, in order to establish abeneficial microbial population to prevent disease.The role that organic acids and their salts can play as growth promoters in the South Africanabalone Haliotis midae, and as manipulators of the gut microflora of this species of abalonewas investigated and compared to the effects of antibiotics. Three different treatments weretested against a negative control and a positive control containing 30ppm avilamycin, acommercial antibiotic growth promoter (AGP) used in the pig and poultry industry. The 3treatments consisted of 1% acetic and 1% formic acid (treatment AF), 1% sodium benzoateand 1% potassium sorbate (treatment SBPS), and 1% benzoic and 1% sorbic acid(treatment BS). Three different experiments were conducted to test the effects of the differentacids and salts. The first experiment was under controlled optimum water temperatureconditions (16.5ºC), another at elevated water temperature (20.5ºC) in order to test responseduring temperature stress conditions, and the final trial was conducted under uncontrolledpractical production conditions. In an attempt to establish the mechanism by which thetreatments have their effects (if any), the composition of the gut microflora of the abalonewas monitored.It was found that the organic acids and salts investigated can enhance the growth rate ofHaliotis midae in the size class 23 mm to 33 mm mean length significantly when compared toboth control treatments. It was further found that the tested AGP had no effect on growthrate. None of the treatments had a significant effect on feed conversion ratio (FCR),Incidence cost (IC) or feed intake. It could also not be shown that the treatments affected theintestinal microflora of the abalone, although this might be due to inadequate microbiologicalmethods. The mechanism by which the acids and salts have their effects could not beestablished.It was found that the animals in the controlled system underwent an initial adaptation period,which led to improvement in specific growth rate (SGR), FCR and IC as the experimentprogressed during the controlled optimal conditions experiment. Large differences in FCRand IC was seen for controlled optimal conditions and production conditions which meansthat there is still a large scope for developing methods to improve practical on-farm feedutilisation by abalone.SGR, FCR and IC were negatively influenced by raising water temperature from 16.5ºC to20.5ºC. The composition of the gut microflora of the abalone also changed significantly afterthe water temperature was raised. It appears that animal weight gain and shell growthrespond differently to changing water temperatures, which is reflected in a change in Fultoncondition factor.A relationship between the length and weight of abalone between 15 mm and 47 mm wasestablished and it was found that Haliotis midae does not follow an isometric growthrelationship. This relation can be used as a tool to improve farm management and thereforealso profitability.Various micro-organisms were isolated from Haliotis midae during the trial, but theirrelationship and interaction with abalone is not clear. Clear dominance by specific species ofbacteria was observed during certain periods.The current research has clearly showed the potential of organic acids and their salts to actas growth promoters in the South African abalone Haliotis midae, with application in both thelocal aquaculture and feed manufacturing industries. The possibility further exists that someaspects of the current research can be adapted to be applicable in other abalone speciesand even in other aquaculture species.

[发布机构] Stellenbosch University