Introduction
Low trophic aquaculture, such as bivalve species, can be candidate species for the minimization of environmental impact of worldwide growing aquaculture, while providing consumers with high quality and nutritive seafood. Nowadays, mussels are cultured suspended mainly in raft and longline systems, using ropes made of non-biodegradable fossil-based plastics , which use could result in augmenting marine litter and microplastics entering the oceans . The EU BIOGEARS project addresses the challenge of minimizing the use of fossil -based plastics by developing alternative biobased and compostable ropes , biogears. The aim of this study is to assess and compare the sustainability of two biogears prototypes vs. commercial fossil-based rope counterparts in mussel offshore longline productions , under technical, economic and environmental perspectives.
Material and methods
Two biogears ropes prototypes (B1 and B2) were developed based on compounds of commercially available biopolymers and manufactured with industrial processes to be fit-for-purpose , with technical and mechanical properties similar to commercial counterparts, for mussel productions . For the technical assessment , the upscaling of their potential implementation by minimizing technical risks at rope production and the aquaculture performance phases were studied. For aquaculture validation, a one-year mussel longline production was performed using biobased B1 and B2 ropes and commercial fossil-based rope counterparts (GROPE). In the economic assessment, the costs and benefits of the use of biogears in mussel aquaculture were analysed along the value chain , comparing them to fossil-based ropes ( market trends of raw materials, rope processing costs, aquaculture production, and End of Life (EoL) options). Additionally, the eco-efficiency indicator of the ropes was calculated (ISO 14045:2012) . In the environmental assessment, rope biodegradation trials and the environmental profile via Life Cycle Analysis (LCA) methodology (ISO 14040/14044 ) were studied.
Technical assessment : Biobased ropes promoted similar mussel growth but higher mussel productions per rope linear meter ( 85% in B2 and 23% in B1) with respect to the fossil-based rope counterpart (4.29 kg/m). Overall, mussel a bundance per rope linear meter decreased over the experimental period in all types of rope , and especially in fossil-based ropes . Mussel quality was not compromised by using biobased ropes , as similar Condition Index, meat yield, proximal composition and fatty acid profiles were observed among experimental groups. Mechanical properties (Load at Break and elongation) of biogears decreased in the first two months of the sea tests, although not compromising rope functionality in one-year mussel production. T he correlation between total mussel weight held per rope and the Load at Break results demonstrated that a 40% higher mussel weight held by B2 ropes had no significant effect on the variation of their mechanical properties.
Economic assessment : Currently, the raw materials used in biogears are economically more costly than the raw materials of fossil-based counterparts. Future trends envisage increasing biopolymer production s to meet market demand, which could in turn reduce their price and make biogears even more competitive than fossil-based ropes. Mussel productivity gains in biobased B2 ropes offset the over-cost of the biogears production, making mussel production more profitable with biogears than with the commercial fossil-based ropes. B2 prototype rope production and use should be encouraged, rather than biobased B1 and fossil-based ropes, due to their best eco-efficiency indicator (environmental impact and economic value of the ropes by kilogram of harvested mussel) (Figure 1).
Environmental assessment : B iodegradability tests in marine conditions indicate that biogears will not biodegrade in sea water at 20-30 °C or below. Compostability tests simulated in laboratory conditions (58±2 ºC) concluded that industrial composting of biogears is technically feasible . However, i t should be validated in industrial composting facilities. Composting, as EoL option for biobased ropes, w ould reduce by 10% the carbon footprint of their life cycle and the impact on the use of fossil resources, due to the benefits of avoided impact s and the use of the produced compost . So, actions for the c ollection, sorting and transport of biogears to industrial composting facilities are encouraged for a correct management at their EoL . Finally, considering rope aquaculture production (impact/kg mussels produced per rope linear meter), B2 rope shows the best environmental performance, reducing by 34% the carbon footprint of mussel productions compared to conventional fossil-based ropes.
It can be concluded that biogears , and specifically biobased B2 ropes , can be technical, economic and environmentally sustainable alternatives to currently used fossil-based ropes in mussel offshore productions promoting aquaculture decarbonization.
BIOGEARS is supported by funding from the EU European Maritime and Fisheries Fund (EMFF).