Black Soldier Fly Larvae Meal as a Fishmeal Substitute in Juvenile Dusky Kob Diets: Effect on Feed Utilization, Growth Performance, and Blood Parameters

Using black soldier fly larvae meal (BSFM) in place of fishmeal is an ingenious strategy for sustainable fish aquaculture. However, BSFM has not been evaluated for dusky kob (Argyrosomus japonicus), an economically important fish in South Africa. Therefore, this five-week study investigated the effect of partially replacing fishmeal with BSFM on feed utilization, growth performance, and blood parameters of juvenile dusky kob in a recirculating aquaculture system. Four diets were formulated by replacing fishmeal in a commercial dusky kob diet with BSFM at the rate of 0 (BSFM0), 50 (BSFM50), 100 (BSFM100), and 200 g/kg (BSFM200). Fish length and weights were measured weekly, while blood analyses were performed at the end of Week 5. For fish length and weight gain, there were neither linear nor quadratic responses in Weeks 1-4, while quadratic trends (y = 14.77 (+/- 0.19)-0.11 (+/- 0.05)x + 0.01 (+/- 0.00) x(2); R-2 = 0.35 and y = 49.85 (+/- 1.53)-1.22 (+/- 0.39)x + 0.06 (+/- 0.02) x(2); R-2 = 0.47, respectively) were observed in Week 5 in response to BSFM levels. Quadratic effects (y = 1.75 (+/- 0.15) + 0.10 (+/- 0.04)x - 0.01 (+/- 0.00) x(2); R-2 = 0.39) were also observed for feed conversion ratio (FCR) in response to BSFM inclusion levels. Fish fed BSFM200 had a similar overall FCR and specific growth rate as those reared on BSFM0. All blood parameters fell within the normal range for the dusky kob. We concluded that 20% dietary replacement of fishmeal with BSFM does not compromise feed utilization and growth performance of juvenile dusky kob.