Closing the gap – producing black soldier fly larvae on aquaculture side streams

Abstract

The aim of the study was to investigate if black soldier fly (Hermetia illucens) larvae can be grown on sludge from a salmonid fresh-water recirculating aquaculture system facility in a small pilot-scale trial. The growth and transfer of both nutrients and unwanted components from the sludge to the insect larvae was analysed. A commercial chicken feed was used as a control growth substrate and up to 100% of the chicken feed was replaced with aquaculture sludge. Replacing 40% of the chicken feed with aquaculture sludge did not influence the growth of the larvae. A gradual decrease in growth was, however seen at a sludge inclusion of higher than 40%. Proximate (crude protein and fat, dry matter) and amino acid composition of the larvae stayed stable across treatments, while ash content increased with higher sludge inclusion. An accumulation of elements such as Zn, Fe and Se was observed in the larvae with increasing amount of sludge in the growth substrate. The larvae also accumulated unwanted elements regulated by EU feed legislation such as As, Hg and Cd. Both As and Hg in the larvae exceeded the legal EU limits for feed ingredients, but only in the two highest levels of sludge inclusion (80 and 100% sludge). Fatty acid composition reflected the substrate, with the sludge-fed larvae being enriched in marine n-3 poly unsaturated fatty acids. No salmon pathogens were detected in the larvae. The trial showed that it is possible to grow black soldier fly larvae on aquaculture sludge but points at a few feed safety risk factors which need to be further elucidated before a use of sludge as growth material for insects can be considered.