THE EFFECT OF CARBOHYDRATE ON DUSKY KOB Argyrosomus japonicus FED PELLETED DIETS  

The dusky kob, Argyrosomus japonicus, is an emerging commercial marine aquaculture species in South Africa. Feed is a limiting factor in dusky kob production as its nutritional requirements are not well understood. The approximate protein and energy requirements for kob have been established in earlier research, but the ability of kob to utilize carbohydrates as an energy source has not previously been investigated. The aim of this study was to determine the efficiency of dusky kob to utilize dietary carbohydrate in pelleted feeds. The objectives were to determine the effect on i) growth performance and feed utilization, ii) post prandial blood glucose and iii) the health of dusky kob fed diets containing different levels of pregelatinized maize starch (PGMS).

Juvenile dusky kob (5 g) were acclimatized in a recirculating aquaculture system. Isoenergetic, isonitrogenous diets containing 4.1, 8.2, 16.4 and 24.6% carbohydrate, were formulated with pregelatinized maize starch as a carbohydrate source and fishmeal as the main protein source. Fish were fed twice daily at 3.85% body weigh/day for three months.

Specific growth rates and feed conversion rations differed significantly between the four dietary treatments. Growth rate increased with increasing carbohydrate up to 16.4%, after which a decline was observed. The diet with 16.4% carbohydrate produced the highest specific growth rate (SGR) of 1.84±0.05% body weight/day, and the lowest feed conversion ratio (FCR) of 1.28±0.08. While the lowest SGR and FCR were found in the diet with 24.6% carbohydrate. A third order polynomial regression, using SGR, determined the optimum carbohydrate inclusion for dusky kob to be 16.72%.

The general structure of the liver was similar between all fish fed the dietary treatments and all fish liver samples displayed a certain degree of lipid vacuolization of the hepatocytes. Evidence of starvation was observed in the livers of fish fed diet with 24.6% carbohydrate. No differences in the amount of liver glycogen were observed.

Gut bacterial composition did not differ among the different diets or between the different sections of the gut. Differences were however observed in the diversity of the bacterial community structure at the start (when they were fed commercial trout feed) and end of the experimental period

The results demonstrated dusky kob effectively utilize carbohydrate up approximately 20% inclusion level, above which growth performance, feed conversion efficiency and health decline.