Introduction
As aquaculture intensifies, there is increasing pressure to find more sustainable practices that save resources and reduce waste. Combined with increases in resource use efficiency across the aquaculture sector, from feeding methodologies to product storage, nutrient recycling can enable aquaculture to contribute sustainably towards the nutritional requirements of billions of people over the next century. The marine aquaculture sector is expanding towards lower trophic level cultures such as macroalgae and bivalve molluscs, which offer numerous sustainability and human health benefits, with a low environmental footprint and a rich source of essential fatty acids and micronutrient. Despite these clear benefits, the growth of bivalve industry has failed to keep pace with fish aquaculture due to a number of challenges including suboptimal and unreliable diets, yield loss due to disease, and unpredictable quality of product. Indeed, conventional hatchery systems rely on the cultivation of lipid-rich algal diets , which are highly vulnerable to contamination and can occupy half the hatchery footprint. In this project we selected and encapsulated algae as alternative more cost-effective substitutes to conventional microalgal diet. N utritional requirements of mussel Mytilus galloprovincialis spat and conditioning broodstock were researched and growth, gametes development and tissue fatty acid contents were assessed. The aim of this study was to determine the substitution level of microalgae by microencapsulated feeds (BioBullets) in hatchery feeds which could uplift mussel production through a reduction of costs.
Material and method
Optimal nutrients for encapsulation were identified from a wide range of aquaculture side streams, with the marine algae Undaria pinnatifida and Schizochytrium offering complementary nutritional profiles with collectively high EPA and DHA levels. Diets were produced commercially and characterised in the laboratory, showing a spherical shape, a 40-50% loading of nutrients by mass, near-neutral buoyancy and a size of 20-140 µm. Various nutritional feeding studies were designed to identify the optimum level of commercial microalgal substitution (partial or complete) with microencapsulated feeds to support high growth and fast gamete development of M. galloprovincialis at commercial scale. In particular, we firstly conducted feeding trials using blended BioBullets (BB), containing U. pinnatifida and Schizochytrium sp., for mussel spats and compared hatchery performances with spat fed commercial algal diets (A) , the combination of conventional diets and BioBullets (ABB) and unsupplemented spat in the laboratory (NC) and in natural conditions (Mutriku). We further conducted experiments on M. galloprovincialis spat and broodstock using BioBullets containing Schizochytrium sp. at different level of commercial microalgal diet substitution (0 % (A); 60 % (ABL); 80 % (ABM); 100 % (B)).
Results
A circular economy, which allows side streams and by-products to be recycled within the aquaculture sector, will certainly play a key role in increasing production output (Fig. 1A). BioBullets containing Schizochytrium sp. and Undaria pinnatifida supplied to mussel spat at 100 % substitution of commercial microalgae, provided a balanced nutritional diet which supported survival and growth rates similar to mussels fed conventional diets (Fig. 1B). BioBullets containing only Schizochytrium sp. were able to sustain mussel spat growth at 60 % inclusion in the diet (Fig. 1C). At the end of the 6 weeks conditioning , adult mussels fed algae + BioBullets at low and medium inclusion (ABL and ABM) presented 40-60 % of individuals already in resting stages, similarly to A-fed mussels. (Fig.1 C).
Fig. 1. A) Supplies and demands of omega-3 fatty acids (EPA and DHA). Major demand for omega-3 is from aquaculture, which can then substantially contribute to the market supply, through recycling of nutrients from farm wastewaters. Nutrients can be upcycled into biomass through the food web and/or to fertilize omega-3 producing crops. The use of biofilters in the recycling of nutrients from farm effluents not only provides potential for increased biomass yield that can be used as aquafeeds, but also clears water that can then be reused for farmed species, promoting a circular economy within aquaculture. The vertex of each triangle represents the direction of the arrows and the size is proportional to the volume (in MT). B ) Spat t issue growth in the first feeding trial being fed for 6 weeks with different diets ; C) Spat increase in live weight in the second feeding trial being fed for 8 weeks with different diets; D) Percentages of gamete developmental stages (I-VI) in adult mussels before (t0) and after 6 weeks of experiment being fed with different diets. NC=No food; A=conventional microalgae; BB=blended Biobullets containing U. pinnatifida and Schizochytrium sp. ; ABB= conventional microalgae + blended BioBullets ; Mutriku =natural field condition; ABL= 40 % microalgae + 60 % BioBullets containing Schizochytrium sp.; ABM= 20 % microalgae + 80 % BioBullets containing Schizochytrium sp.; B=100 % BioBullets containing Schizochytrium sp.; Different letters represent significant differences among treatment groups.
Discussion and conclusion
Hatchery diets with microencapsulated feeds can provide dependable, contaminant-free diets that reduce costs of seed production and, thus, can sustainably support mussel farming. Additionally, by sourcing the encapsulated algae from side streams of the aquaculture industry, microencapsulated feeds can promote more sustainable practices and circular economies within the sector. Further tailoring of the nutritional composition of microcapsules to specific bivalve species or growth stages could allow microcapsules to replace a greater proportion of or even completely replace algal diets. Inclusion into microcapsules of therapeutics or even flavourings open up additional potential benefits to this important and growing industry.