Sustainable diets for shrimp aquaculture utilizing local by-products

Abstract

The global food system, including the aquaculture industry, is challenging the planetary boundaries. Aquaculture has become the fastest-growing food sector in the world. To meet the demand of aquatic protein for a growing human population by 2050, aquaculture production levels will need to increase by more than 50%. Most farmed species depend on external feed supply, but feed drives the majority of the environmental footprint of this sector. Additionally, many of the aquafeed ingredients directly or indirectly compete with human food. Therefore, utilization of by-products as aquafeed ingredients can help to reduce waste, recycle nutrients, add value, and promote circularity of the aquaculture sector. The objective of this thesis was to identify, characterize, and test by-products as aquafeed ingredients for the high-value Pacific Whiteleg shrimp, Litopenaeus vannamei. Under-utilized by-products originating from the three different sectors seafood processing, insect farming, and the cosmetic industry were identified and tested. These include processing remains of the brown shrimp, Crangon crangon, (BSPR); exuviae (shed exoskeletons) of growing larvae, cocoons, and dead adult flies (imagines) of the black soldier fly, Hermetia illucens; and algae pomace, which remains after aqueous extraction of marine macroalgae, Saccharina latissima used to produce skincare products. Analysis of the chemical composition of these materials revealed high protein contents with favorable amino acid profiles in BSPR and imagines, supporting their potential as a dietary protein source. High levels of chitin and microminerals in cocoons, and potentially bioactive substances in macroalgae suggest their use as feed additives to enhance health and overall performance of L. vannamei. Controlled feeding trials were conducted in clear water recirculating aquaculture systems to examine the bioavailability of key nutrients, as well as their effects on growth parameters and health. Initial experiments with BSPR showed improved growth of shrimp without adverse effects on health when 50% of the conventional fish meal was replaced in the diet. A second, more elaborated controlled feeding experiment revealed that BSPR enhances shrimp growth at a fish meal replacement level of 80%, while concomitantly improving the feed efficiency. Examination of physiological and health parameters (digestive enzymes, phenoloxidase activities, hemolymph parameters) failed to elucidate the underlying mechanism for the enhanced growth performance of shrimp. The inclusion of defatted meal made from the adult black soldier fly resulted in poor shrimp growth at all tested dietary inclusion levels. Furthermore, diets containing imagines were less attractive for shrimp, resulting in a reduced feed intake. Significant changes in the chemical composition of the muscle and midgut gland of shrimp further indicated malnutrition induced by the replacement of fish meal with imagines. The suitability of cocoons and algae pomace as feed additives was tested at supplementation levels ranging from 1 to 5% in low fish meal containing diets. Both ingredients enhanced shrimp survival at the highest inclusion level in a controlled feeding trial conducted in a clear water recirculating aquaculture system. However, supplementation of algae pomace resulted in decreased growth performance. Health indices, including the phenoloxidase system and hemolymph parameters, did not show a significant response to the experimental diet treatment. To further validate the suitability of cocoons and algae pomace as feed additives, controlled feeding experiments were conducted in biofloc systems with specifically formulated diets. Dietary effects on gut bacteria counts, hemolymph parameters, and resilience of shrimp to cold temperatures were examined. In the biofloc system experiment, growth was impaired in both diet treatments containing cocoons and algae pomace and no effect on survival was observed. Furthermore, no differences between treatments were detected in any of the analyzed parameters. The potential of cocoons and algae pomace as feed additives thus appears to be limited and likely depends on the rearing system, and applied feed formulation. To holistically consider the potential of these by-product-based feed ingredients, a variety of factors were evaluated and classified as strengths, weaknesses, opportunities, and threats. This thorough assessment revealed that BSPR originating from a traditional and well-established fishery is characterized by an exceptional nutritional value and a high potential to serve as proteinaceous aquafeed ingredient for shrimp in the current situation. At present, the amount of BSPR that is annually generated from the North Sea regions could cover the entire protein demand required by European shrimp aquaculture. However, limited scalability, as well as potential environmental regulatory constraints restrict future opportunities for this resource. On the contrary, the by-products originating from the novel insect, and cosmetic seaweed industries have a low nutritive value for shrimp, which is the main weaknesses of these resources. Yet, methods exist to improve their suitability as aquafeed ingredients that warrant examination and further development. With the expected growth of these sectors, both of which are capable of addressing many environmental and societal challenges, the role of their by-products as aquafeed ingredient could become more relevant in future.