Partial Substitution of Fishmeal with Mopane Worm Meal in Dusky Kob Fingerling (Argyrosomus japonicus) Diets: Feed Utilization, Digestive Enzyme Activity, Blood Parameters, and Growth Performance

The use of protein-rich mopane worm meal (MPWM) in marine fish diets has the potential to reduce the overall demand for fishmeal (FM) and thus promote economically and ecologically sustainable dusky kob aquaculture. Therefore, this study investigated the effect of graded levels of MPWM on feed and nutrient utilization, digestive enzymes, blood parameters, and growth performance in dusky kob fingerlings (Argyrosomus japonicus, Temminck and Schlegel, 1843) over a 7-week feeding trial. Five isonitrogenous and isoenergetic commercial dusky kob diets were formulated by substituting 0 (MPWM0), 3 (MPWM3), 6 (MPWM6), 9 (MPWM9), and 18% (MPWM18) of FM with MPWM and offered at 2.8% of fish body weight. Two-thousand fingerlings (average weight 15.68 ± 0.25 g) were evenly distributed into 20 experimental tanks of a recirculating aquaculture system. Weight measurements were taken weekly, while blood and intestinal samples were taken at the end of the experiment. The inclusion of MPWM quadratically influenced (y = 39.25 (±1.27) + 0.89 (±0.36)x − 0.04 (0.02) x2; R2 = 0.23; p = 0.03) overall feed intake. Overall weight gain and specific growth rate (SGR) linearly increased while overall feed conversion ratio (FCR) linearly declined with MPWM inclusion levels. Substituting FM with MPWM did not alter (p > 0.05) digestive enzyme activities and haematological parameters. Among the serum biochemical components measured, only urea increased linearly in response to MPWM levels, while no trends were observed for the other components. It was concluded that replacing 11.13% of FM with MPWM in commercial dusky kob diet formulations does not compromise feed intake, feed utilization efficiency, growth rate, and physiological status of juvenile dusky kob.