Effect of dietary additives on growth performance, gut histology, blood parameters and tissue nutrient composition of the South African dusky kob, Argyrosomus japonicus
Madibana, Molatelo J.
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The dusky kob, Argyrosomus japonicus, has been identified as a viable marine aquaculture species whose sustainable production can contribute to food and nutrition security in South Africa. For local dusky kob aquaculture to flourish, its sustainable production would require cost-effective feeding strategies and interventions. Therefore, the current study was designed to assess the effect of green macroalgae seaweed Ulva sp., inactivated yeasts (Brewer's and torula yeast) and commercial herbal products on fish growth performance, haemo-biochemical parameters, gut histology and fillet fatty acids concentration. The three experimental chapters (chapters 3, 4 & 5) in this thesis evaluated alternative, unconventional feed additives previously shown to promote growth, health and welfare in other aquaculture species. Experiments were carried out at the Marine Research Aquarium of the department of Agriculture, Forestry and Fisheries (DAFF) in Sea Point (33.9169°S, l 8.3875°E), Cape Town, South Africa. The experimental system was a recirculating aquaculture system (RAS) consisting of 20 black, high-density polyethylene grow-out tanks ( 465 L capacity, 67 cm deep and 94 cm diameter) with flattened conical floors coated with white fibreglass resin to allow for better fish visibility. The seawater temperature was maintained at 25 °C via a heat pump and with dissolved oxygen at 5.5-6.0 mg/1 via air lines. Fish of 6-9 g used for the study either were sourced from dusky kob breeders or were of DAFF Marine Research Aquarium breed. For all the experiments, fish were offered feed at a rate of 2.8 % of their body weight, weighed and measured for length to assess weekly growth performance. All diets across the three experiments were hand prepared. The drawing of blood, distal intestine sampling for gut histology, fillet sampling for fatty acids concentration were performed at the end of each experiment. In chapter 3, five diets were formulated to contain graded levels of Ulva, 0 (Ulva0), 50 (Ulva50), 100 (Ulval00), 150 (Ulva150) and 200 (Ulva200) g seaweed meal/kg commercial kob feed on a dry matter basis. There were four replicate tanks per treatment and each had 77 fish. The experiment was run for nine weeks. There was a decline in protein content as the level of seaweed supplementation increased. Repeated measures analysis revealed that the interaction between fish age (weeks) and diet had a significant effect on fish weight gain (P < 0.05). No significant (P >0.05) dietary effect was detected on blood haematological parameters. Blood alkaline phosphatase declined (P < 0.05) in response to Ulva inclusion in fish diets but all blood parameters were within the normal range for fish. The histological evaluation of the distal intestine revealed normal gut structure for all the dietary treatments. Eleven SF A, seven MUF A and nine PUF A were detected from the fillet samples of all fish groups. It was concluded that 50g Ulva/kg kob commercial was the optimum inclusion in dusky kob diets for unimpaired fish growth. The growth performance results from this study, especially when fish were fed U/va-supplemented diets beyond 50g/kg, showed a need to test other potential feed additives such as brewer's yeast (Saccharomyces cerevisiae), which have shown growth promoting potential in other previous studies. Therefore, in chapter 4, five isonitrogenous and isoenergetic diets, consisting of three brewer's yeast-containing diets at rates of 50, 150, and 300 g/kg DM (BY5P0, BYl 5P0, and BY30P0, respectively), a commercial dusky kob diet containing 10% pro biotic mix but no brewer's yeast (BY0P 1, positive control), and a commercial dusky kob diet with neither the probiotic mix nor the yeast (BY0P0, negative control), were formulated. Each treatment tanks for six weeks. Repeated measures analysis revealed a significant interactive effect between dietary treatments and age of the fish (weeks) (P< 0.05) on fish weight, but not on caudal length. Fish growth decreased with an increase in yeast inclusion. The effect of dietary treatments on FCE was not significantly different. There was a notable decrease in haematocrit levels and alanine transaminase concentration with incremental levels of brewer's yeast. However, all the haemo-biochemical parameters were within normal range for fish. Similar to the conclusion reached in Chapter 3, 50g Brewer's yeast/kg kob commercial was the optimum inclusion level for unimpaired dusky kob growth. It was therefore important to investigate the effect of combining brewer's yeast, torula yeast (Candida utilis) and commercial herbs in the last experimental chapter (5). Torula yeast has been found to be beneficial in terms of fish growth when included at higher levels ( 45% of the diet), and herbal products are accredited with stimulating both the digestive enzymes and the immune systems of fish, thereby promoting fish growth. In chapter 5,a base diet of commercial Skretting Active ZA feed (62.9% of the diet) combined with torula yeast (25.9%) and brewer's yeast (5.2%) was formulated.Five commercial additives which included Aquapro® herbal powder, Liv up® herbal granules, Liv up® herbal liquid, Ultra Natural PlusPB-20® (UNP PB-20) herbal liquid and 2-strain Probitlora®(Lactobacillus acidophilus and Bifidobacterium bifidus) were respectively added to the base diet. The base diet served as a positive control and the Skretting Active ZA feed served as the negative control, bringing the total number of diets to seven. Each treatment, except the negative control (two replicate tanks) was offered to three replicate tanks containing 50 fingerlings each. The highest weight gain was achieved with the base diet (43.38 ± 2.49 g), followed by the Probiflora® group and the Liv up® herbal granules group (40.35 ± 1.42 and 40.35 ± 4.86 g, respectively). The least weight gain was observed for the negative control diet (4.38 ± 0.80 g). In conclusion, 50 g/kg kob commercial diet of both Ulva and brewer's yeast are recommended because they did not impaired fish growth. The dietary inclusion of the two inactivated yeast products is recommended for dusky kob commercial diets since they enhanced growth performance. In conclusion, dilution of the diet protein quality by higher Ulva inclusion did not have an effect on fish fatty acid concentration, haemo-biochemical parameters and gut morphology. The combination of brewer's and torula yeast in future dusky kob is highly recommended and on the other hand, future studies should investigate the optimum inclusion levels of herbal products such as Aquapro® herbal powder in aquaculture feeds in South Africa.