Dietary inclusion of partially defatted black soldier fly meal in rainbow trout (Oncorhynchus mykiss) diet and optimization of the flesh fatty acid profile

With the increasing awareness of the importance of omega-3 fatty acids in the human diet and the increase of the world population, fish consumption has more than doubled worldwide since 1973 [40]. This led to the huge expansion of the aquaculture sector, and consequently to the explosion of the demand for aquaculture feeds. Feeding is an essential aspect of aquaculture from economic, environmental and nutritional points of view. Since feed is considered as the biggest issue for the future growth of the aquaculture sector and its sustainability, many studies are currently conducted to find new raw materials for aquaculture feed. Among the possibilities, insects, and more specifically the black soldier fly, appear to be very interesting due to their nutritional characteristics and their great ability to grow on poorly valorized organic waste. This work aimed at evaluating the efficiency of the inclusion of a partially defatted black soldier fly larvae (BSFL) meal in low-fish meal based experimental diets for rainbow trout. The BSFL meal proteins were combined with oils rich in omega-3 long-chain polyunsaturated fatty acids, or in their precursors, to enhance the content of these fatty acids in the fish flesh. Firstly, the BSFL were mechanically pressed to produce a partially defatted BSFL meal. This mechanical defatting increased the crude protein content from 38% in the BSFL to almost 60% in the BSFL meal (on a dry matter basis). Then, four isolipidic, isoproteic and isoenergetic diets were formulated with increasing levels of BSFL meal inclusion and were then manufactured. The chemical compositions and fatty acid profiles of the BSFL, the BSFL meal and the 4 experimental diets were then analyzed. Not surprisingly, the amounts of the most represented fatty acids in the BSFL meal increased in the diets that included it. The « BSF 50 + DHA » diet, which included an oil rich in omega-3 long-chain polyunsaturated fatty acids, increased its eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) contents drastically. The results of the 39-day feeding trial were very promising and demonstrated the interest of insects as recyclers of organic matter in a sustainable production of farmed fish. Indeed, the inclusion of BSFL meal as a protein source in the experimental diets did not have a negative impact on trout growth and even caused a slight increase in the relative weight gain of fish fed with the diet containing the highest percentage of BSFL meal inclusion. The other parameters/indices calculated to assess growth performance and feed utilization did not show significant differences between diets although results were generally better for the « BSF 50 » diet. The fish fed with the experimental diets contained more omega-3 long-chain polyunsaturated fatty acids in their flesh and obtained a better omega-3/omega-6 ratio than the initial fish thanks to the supply of the dietary precursor or by the direct supply of these fatty acids in the diets. The fish fed with the diet including an EPA and DHA rich oil, had the greatest increase in EPA and DHA content in their flesh. The results obtained in this experiment showed that the utilization of locally produced BSFL proteins as a replacement of vegetable proteins, in association with vegetable oils, may be a viable solution for the transition to a sustainable aquaculture system able to provide nutritionally valuable fish.