Black Mulberry fruit as a nutraceutical source for juvenile Dusky Kob: Physiological and tissue nutrient responses.

Abstract

There is a need for the aquaculture industry to engineer socially, economically, and environmentally sustainable aquafeeds. This includes minimizing the use of chemotherapeutics and other synthetic feed additives to improve the economic viability of aquaculture, mitigate microbial resistance induced by antibiotic growth promoters, reduce the accumulation of antibiotic residues in fish products, and protect the environment. Accordingly, this dissertation evaluated the potential utility of black mulberry fruit powder (BMFP) as a sustainable nutraceutical source in juvenile Dusky kob fish (Argyrosomus japonicus). Black mulberry fruit (Morus nigra) powder has unique beneficial bioactive compounds with putative antioxidant, immunostimulant, antimicrobial, anti-inflammatory, appetite stimulation, and growth-promoting properties. These bioactive compounds in the powder may also enhance fish health and the shelf life of fish fillets and the nutritional value of fish products for human consumption. However, despite the potential utility of this natural nutraceutical source, it is yet to be evaluated in the Dusky kob. The study is divided into two experimental chapters (3 and 4). Chapter 3 evaluates the utility of BMFP-containing diets and the effect of BMFP-containing diets on feed utilization, growth performance, and haemo-biochemical parameters in juvenile Dusky kob. Chapter 4 gives a narrative of the evaluation of the dietary effects of BMFP-containing diets on the proximate composition, amino acids, minerals, and fatty acids profiles of Dusky kob fish fillets. The fish feeding trial was carried out in a recirculating aquaculture system (RAS) using four isonitrogenous and isoenergetic experimental diets formulated by including BMFP in a Dusky kob commercial diet at 0 (BMFP0), 5 (BMFP5), 10 (BMFP10), and 12% (BMFP12) w/w. In chapter 3, dietary treatment groups were chemically characterized for proximate, amino acid, phytochemical, mineral, and fatty acid content. At the end of the feeding trial, blood samples were collected from five randomly selected kob fish under anaesthesia sampled from each experimental tank iii for haemo-biochemical analysis. In chapter 4, twenty fish were randomly selected from each tank to characterize the proximate composition, amino acid profile, mineral composition, and fatty acid of kob fillets. Diet × week interaction did not show any (p > 0.05) influence on feed intake, feed conversion ratio (FCR) from weeks 1 – 3 and specific growth rate (SGR), but an increase (p < 0.05) of weight gain in weeks 2 and 4, and FCR in week 4 was observed. Weight gain in week 2 showed a positive linear trend in response to incremental levels of BMFP (Figure 3.1). Experimental diets influenced (p < 0.05) lymphocytes, monocytes, and eosinophils counts but not (p > 0.05) haematocrit, thrombocytes, neutrophils, and basophils (Figure 3.2). A positive quadratic trend for lymphocytes and a negative quadratic trend for monocytes was observed, whereas a positive linear trend for eosinophil counts was observed in response to incremental levels of BMFP. A negative quadratic response was observed for blood urea in kob fish reared on BMFP-containing diets. The results suggest that while dietary inclusion of BMFP did not enhance feed utilization and growth performance as expected, it had beneficial effects on some haemo-biochemical parameters (lymphocytes, monocytes, and blood urea) in juvenile Dusky kob. At the end of the four-week feeding trial, quadratic trends were observed for lysine and phenylalanine content. Lower dietary levels of BMFP significantly enhanced lysine (Figure 4.1) and phenylalanine (Figure 4.2) deposition in kob fish fillets. The results also revealed a significant response in the deposition of Mg, Mn, and Zn in fish filets, in which BMFP10 promoted the highest concentration of these minerals. The experimental diets did not affect the fatty acid content of the fish fillets except for stearic acid. It was calculated that the levels of BMFP DM inclusion that optimize lymphocytes, monocytes, and blood urea in juvenile Dusky kob were 6.1, 8.4, and 7.5%, respectively. From the quadratic equation, it was also calculated that 5.6 and 3.9% were the optimal inclusion levels of BMFP that maximized lysine and phenylalanine content in Dusky kob fish fillets, respectively.