Suitability of blood by-products for use as a sustainable protein source in diets for rainbow trout depends on processing method

Introduction. Fish meal is considered an essential ingredient in feed for farmed carnivorous fish. As demands outgrew availability of fish meal produced from bycatch and trimmings, forage fish is purpose-caught to fill the gap. This led to rising feed costs and growing concerns on environmental sustainability of cultured fish, driving the industry to search for sustainable, cost-effective alternative protein sources [1]. For this purpose, low-ash animal by-products may be suitable ingredients for partial replacement of fish meal in aquafeed. We here evaluate the effect of processing method of porcine blood by-products on protein digestibility in trout: spray drying versus contact drying. Animals, materials and methods. The pepsin-HCl method [2] was used to determine in vitro protein digestibility of spray-dried hemoglobin (Actipro® 95PHS) and contact-dried blood meal (BM). Next, a digestibility trial was performed using 180 rainbow trout (148 8 g) in twelve 250 L-tanks with 15 fish each. Three pelleted diets were prepared: reference diet (RD) with 40% fish meal, 95PHS-diet (70% RD and 30% 95PHS) and BM-diet (70% RD and 30% BM). Fish were fed one of three test diets with 4 replicate tanks per diet. A 2-wk adaptation period and 2-wk feaces collection period were applied using a continuous automatic device and apparent protein digestibility coefficients (aD_CP) of test diets and test compounds calculated [3]. Statistics were done using one-way ANOVA and Tukey post hoc test with significance set at p<0.05. Results and discussion. The in vivo trial revealed an almost double digestible protein content in spray dried hemoglobin compared to contact dried blood meal (p < 0.001), whilst crude protein content and in vitro digestibility coefficients were similar (Table 1). Moreover, protein digestibility was similar between the RD (93 0.3%) and 95PHS-diet (94 0.8%), but significantly lower in the BM-diet (74 1.5%, p<0.001). Seen the large discrepancy observed between in vitro and in vivo protein digestibility, the pepsin-HL method proved to be a non-reliable method to asses digestible protein content of ingredients for aquafeed. Table 1. In vitro (iv) and in vivo (rt, rainbow trout) protein digestibility (mean ± sem) of blood by-products. Conclusion. Spray dried hemoglobin showed to be a suitable ingredient to partially replace fish meal in feed for rainbow trout. High amounts of indigestible protein in contact-dried blood meal, in contrast, would deteriorate feed conversion and increase the amount of nitrogen in culture water as well as the systems’ effluent. References: [1] Cashion et al., 2017. Fish and Fisheries 18:837-844; [2] 72/199/EEC. Official Journal L 123, 29.5.1972, p 6-34; [3] Lemos et al., 2009. Aquaculture 295:89-98