Processed animal by-products as sustainable ingredients in diets for European seabass (Dicentrarchus labrax)

The current low availability and high price of fishmeal (FM) and fish oil (FO) has led to a need for the reduction of the use of these ingredients in fish feed as protein and lipid sources, respectively. This has led to a search for alternative ingredients that must be economically and environmentally more sustainable, but still assure fish nutrient requirements. The agro-food industry generates large amounts of by-products that, after proper processing, could become valid fishmeal and fish oil substitutes in European seabass (Dicentrarchus labrax) diets. Using these locally generated by-products as aquafeed feedstuffs would not only increase the sustainability of this sector by reducing the use of fishmeal and fish oil from fisheries, but also contribute to a functional circular economy by reintroducing such by-products into valued production chains. In Chapter II, several processed agro-food by-products (wheat germ, okara meal, poultry by-product meal, steam and enzymatically hydrolyzed feather meal, β-lactoglobulin and large peptides (>3kDa) obtained from brewer’s yeast and from processing fish by-products) were evaluated as potential alternative protein sources in seabass juveniles. This was done by assessing their chemical composition and their in vivo apparent digestibility coefficients (ADCs) in juveniles. Protein ADC’s were above 84% for steam hydrolyzed feather meal, enzymatically hydrolyzed feather meal, poultry by-product meal, wheat germ, β-lactoglobulin and the peptides from fish by-products. These ingredients also had high essential amino acids’ ADC values (84-98%), while okara and the yeast peptides were poorly digested (73-76%). Considering the digestibility results, market availability and price of the analyzed feedstuffs, the steam hydrolyzed feather meal (HF) was selected as the best candidate to be used as fishmeal replacement for juvenile seabass. The effects of replacing 28, 55 and 76% fishmeal with HF were evaluated in an 18 weeks growth trial (Chapter III). The replacement of 76% fishmeal by HF did not impair growth performance, nutrient gain, muscle fatty acid composition and humoral non-specific immune parameters of seabass juveniles. Furthermore, the dietary inclusion of HF improved phosphorus ADC, significantly decreasing P emissions into the environment. The second stage of this thesis evaluated rendered animal fats as potential candidates to replace fish oil in seabass diets. To do that, three rendered animal fats were selected to assess the in vivo apparent digestibility coefficients of macronutrients and fatty acids in seabass juveniles in Chapter IV. FO from rendering fish by-products (skin, meat and viscera), poultry fat (PF) and mammal fat (MF, a mixture of about 70% lard and 30% beef tallow) from rendering either poultry or mammal by-products (viscera, skin, bruised meat and bones) were added at 14%, as supplemental fats, to a practical feed formulation blend commonly used by the feed industry. Both PF and MF had good overall digestibility results, and therefore they were both tested as fish oil replacements in two separate growth trials: one using PF as single fish oil replacement at 25, 50, 75 and 100% (Chapter V), and the other one using a PF and MF mixture (50:50, PFMF) as 50, 75 and 100% fish oil replacement (Chapter VI). Chapter V and Chapter VI evaluated the effects of replacing fish oil by either PF or PFMF on the nutrient utilization, growth performance, tissue composition and lipid metabolism in seabass juveniles. The use of soy lecithin as an emulsifier agent was also evaluated when PF totally replaced fish oil (Chapter V). Fish oil was completely replaced by PF without impairing feed intake, growth performance and nutrient utilization, but this was only possible up to 75% when the mixture of PF and MF was used. The fatty acid profiles of the tissues analyzed generally reflected those of the diets. Moreover, despite the alterations in the muscle fatty acid profiles, seabass fed diets with up to 75% fish oil replacement (by either PF or PFMF) still provided the recommended EPA and DHA levels for human consumption. Finally, on Chapter VII, a Life Cycle Assessment (LCA) study was implemented to evaluate the environmental impacts of rendering poultry by-products into HF, PF and poultry by-product meal (PBM, a co-product of PF that can also be used as a FM replacement). The results obtained in this chapter show that the life cycles of HF and PBM generate less impacts than the production of fishmeal in all the categories analyzed. Moreover, the life cycle of PF generates less impacts than the life cycle of fish oil. These results indicate that using such land animal sources as FM or FO replacements would substantially decrease the environmental impacts associated with aquafeeds. Furthermore, the LCA study allowed the analysis of some scenarios regarding the use of different fuels in the rendering plants, which enabled the suggestion of some alternatives that could help decrease their environmental impacts. This thesis was performed in an industrial setting, under the partnership of the company Soja de Portugal, and focused on helping increase the sustainability of the aquafeed sector. The combination of the results obtained in the different chapters provides valuable information that enables the feed industry to make more conscientious decisions regarding feed formulations and assure a more sustainable development of the sector.