Circular bio-economy in aquaculture

The way we produce our food is increasingly scrutinized and evaluated on its possible contribution to the global environmental changes the world is currently witnessing. Scientific evidence about the negative effects of agriculture (including aquaculture) on the world's natural resources^1-5 is mounting. Food production is considered as one of the most significant causes of environmental change.^{6, 7} The planetary boundaries framework^{8, 9} offers a tool to explore to which extent we are still operating within the carrying capacity of our planet. In its most recent update,^{10, 11} six out of the nine boundaries were transgressed already. Even novel planetary health threats such as microplastics are emerging.^{12, 13} Obviously, the current bio-economy needs to be revisited to remain within ‘safe operating space for the future of mankind’. A change to a circular bio-based economy focusing on resource use efficiency and minimizing environmental impacts might be one of the most prospective strategies to achieve that goal.

Muscat et al.¹⁴ presented five principles to transform agro-food systems into a circular bio-economy, for example, safeguarding the health of agro-ecosystems, avoiding waste, prioritizing the use of biomass to avoid a food–feed–fuel competition, recycling by-products and using renewable energy. Products and/or production systems are designed in such a way that they minimize their impact on the environment during their whole lifetime. European Commission, Directorate-General for Maritime Affairs and Fisheries, et al.¹⁵ and Colombo and Turchini¹⁶ transferred this idea to the area of fisheries and aquaculture and presented a framework for a circular economy, focusing on the conceptualization and design of new appropriate production systems, evaluating the possible reuse of wastes and by-products from fisheries/aquaculture, reducing the impact of equipment and products from the sector and exploring possibilities to recycle. It all relates to the so-called three Rs, for example, reduce, reuse, recycle.

Reviews in Aquaculture contributed already to the discourse on circularity in aquaculture. Roy et al.¹⁷ provided a virtual topical collection of 13 review articles published by Reviews in Aquaculture between 2020 and 2021, which together reflected some of the contemporary puzzles of circularity in aquaculture. Following a virtual special issue (here), the journal decided to initiate a physical special issue entitled ‘Circular Bio-economy Framework in Aquaculture’, which we proudly present today.

The special issue was organized around the four key processes in circularity, for example, designing new (circular) aquacultural production systems, reducing the impact of aquaculture, re-using wastes and by-products from aquaculture and recycling materials and biomasses. In our call for papers, we invited state-of-the-art reviews on themes, fitting in each of the above processes.

With regard to Conceptualizing and/or designing circular production systems, we called for ‘Circular production systems, such as aquaponics, integrated multi-trophic aquaculture systems, etc.’. For this topic, four interesting reviews emerged, one relevant for marine aquaculture,¹⁸ two relevant for inland aquaculture¹⁹ and one for both.^{20, 21} Some lesser known approaches were taken to the forefront by Garibay-Valtez et al.,²⁰ Lothmann and Sewilam,¹⁸ and Zhao et al.,²¹ while promising solutions were revisited from a wider industry perspective by Marousek et al.¹⁹

With regard to Reduce, the call stipulated ‘Reducing emissions and footprints of aquaculture’ which was complemented by Arantzamendia et al.²² exploring the feasibility of bio-based solutions for innovations in materials used in aquaculture as a possible emerging new science field. Within this process, the call further stipulated ‘Connecting aquaculture research to side streams (pharmaceutical, biofuel, etc.)’, as a possible theme. The review by Yun et al.²³ presented how seaweed-farming waste streams could be industrially lucrative. An interesting review²⁴ contributed to the process of ‘reduce’ by reviewing marine resources that would reduce the land-use dependency of aquaculture.

With regard to Reuse, themes mentioned were ‘valorization of aquaculture and non-aquaculture waste streams for aquaculture’ and ‘Improving food safety issues of products emanating from circular aquaculture’. Two reviews, one focusing on nutrient reuse at farm level²⁵ and another one focusing on re-using nutrients locked in biomasses at the farm-to-fork level,²⁶ showed several possible routes of upcycling ecosystem nutrients into the human food chain. The latter paper reviewed also intricacies of consumer acceptance, product quality, shelf life and re-manufacturing of certain pond and RAS farmed fish. A third review²⁷ explored the possibilities of re-using the wastewater in fisheries and aquaculture.

The process of Recycling received most submissions. The call themes were ‘Plant and microbial-based solutions’ and ‘Food-feed conflicts in aquaculture and circular feeds’. Altogether five reviews were published under this block,^{11, 28-31} nicely summarizing how aquaculture relied on recycling biomass, from decades ago till now. Colombo et al.²⁸ provided an overview on how farmed blue foods have been fed and how the circularity concepts were gradually integrated in aquafeed, leading to potential candidates for circular feeds. Wang et al.³² showed the potential of transforming industrial gas emissions into edible biomass, through aquaculture. van Riel et al.²⁹ tackled the issue of ‘food-feed competition’ against the background of the expected aquaculture expansion. Ogburn et al.³⁰ showed that substrate-based production systems could be another winner to recycle the bulk of semi-refined agri-food by-products into edible biomass. Sunish and co-authors³¹ gave a perspective of naturally occurring, in situ saprophytic microbes strengthening farmed aquatic animal health.

There were also topics where the editors could not receive enough submissions for. The topics were related to ‘Systems-based thinking and modeling’ and ‘Energy and emergy analysis on potential circular aquaculture models’.

The special issue presents 15 review papers that together prove that circular approaches and bio-based solutions are well adopted in the aquaculture research community. It is now waiting for a widespread application of these innovations in the industry.

The editors cordially acknowledge the immense help of anonymous peer reviewers who have helped us to materialize this special issue. The editors hope readers would find the articles thought-provoking.