Dietery Utilization of Black Soldier Fly Larvae (Hermetia illucens) in African Catfish (Clarias gariepinus) Cultured Under Spinach (Spinacia oleracea) Propagated Aquaponic System Using Different Substrates

Abstract

The rising demand of fish due to its nutritive value has led to over exploitation of capture fisheries thus, paving way for aquaculture development. Being the world’s fastest growing agro-food sector, aquaculture faces a number of challenges that hamper the sector from meeting the ever-rising demand of fish and fish products. High cost of fish feeds, use of outdated culture systems that require large tracts of land and plenty of water are some of the factors limiting aquaculture from attaining its full potential. Reduction of fish feeds cost by use of cheaper ingredients and use of modern fish farming technologies such as aquaponics can address the challenges to enhance aquacultural productivity. The study was conducted at a private fish farm in Baringo County, Kenya. The study investigated the effect of partial replacement of fishmeal (FM) with black soldier fly larvae (BSFL) on survival, growth performance and fatty acid composition of Clarias gariepinus. Also, the study assessed the suitability of pumice, a mixture of (pumice and charcoal) mixed homogenously at 50 % ratio and charcoal substrates on nutrients removal from water for C. gariepinus culture and Spinacia oleracea propagation under aquaponic system. In the first experiment; three test diets 35% crude protein content (CP) in which FM was substituted by BSFL at 25%, 50% and 75% were formulated and experimented with commercial diet of 35% CP manufactured by Unga Farm Care Limited. Four weeks old C. gariepinus of average weight 5.6±0.5 g and average length 9.63±0.5cm were stocked in 12 tanks at a density of 50 fish/tank and subjected to the diets. Whole body, liver and muscles tissues were obtained from experimental fish and taken to the laboratory where they were analysed for fatty acids profile using gas chromatography at the end of the four months experimental duration. In the second experiment, C. gariepinus post fingerlings weighing 14.97±0.5g and length 14.05±0.5cm were stocked at 50 fish/tank in 12, 1000L tanks under aquaponic system. Experimental fish were subjected to a commercial diet manufactured by Unga Farm Care Limited for four months duration. The commercial diet was analysed for its proximate composition and found to contain 35% crude proteins content (CP). In both experiments, fish sampling was conducted every three weeks; water quality parameters were analyzed weekly while mortality was recorded on occurrence. Results on growth performance of fish fed on diets containing BSFL showed 50% BSFL diet obtained food conversion ratio (FCR) of 1.2 compared to FCR of 1.3 obtained by diets containing 25% and 75% BSFL. Weight gain obtained for control diet varied significantly (p<0.001) in comparison to weight gain obtained for formulated diets. The results on fatty acids profile showed inclusion of BSFL at different rates influenced fatty acid composition. Saturated fatty acids composition increased while unsaturated fatty acids decreased with increasing BSFL ratio in diets. Total saturated and unsaturated fatty acids varied significantly (p<0.05) among the diets and fish tissues. Palmitic acid was the dominant saturated fatty acid while oleic and linolenic were the dominant monounsaturated and polyunsaturated fatty acids in fish tissues respectively. Docosahexaenoic acid (DHA) was the dominant omega-3 fatty acid in fish tissues with liver having significantly higher (P<0.05) accumulation than whole body and muscle tissues. Under different substrate types, the mean weight gained by fish for pumice (93.81g), pumice charcoal (77.57g), charcoal (69.79g) and control (55.37g) treatments showed significant difference (p<0.03). The final breadth, length and number of leaves in S. oleracea showed significant difference obtaining p values of (0.023), (0.045) and (0.003) respectively between different substrate treatments. Survival rates for C. gariepinus ranged between 92-98%. The least survival rate was observed for control treatment (92%), followed by treatment with charcoal substrate (94%), then treatment with pumice substrate (96%) while the highest survival rate was obtained for the mixture of pumice and charcoal substrate (98%). Growth performance and survival demonstrated that BSFL has potential to replace FM up to 75%. Catfish productivity can be improved and feed cost reduced by incorporating fully defatted BSFL with higher CP content compared to 25.3% CP content used for the diets in the present study. The study on fatty acid profile of diets and fish tissues showed that BSFL can replace fishmeal in diets without affecting the composition fatty acid profile of cultured catfish muscles. Thus, BSFL is a suitable protein sourced ingredient for partial FM replacement in catfish diets. The observed variation in growth performance of; C. gariepinus and S. oleracea and nutrients reduction efficiency suggest pumice is a better substrate for catfish and spinach production under aquaponic system.