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1. Introduction

Global use of natural resources has been increasing substantially in recent years, reaching 92.1 billion tons in 2017 and resulting in an increase of 254% from 27 billion in 1970, with the rapid build-up in the annual extraction since 2000 [1]. Because of this, the research is improving its efforts in building a circular bioeconomy, which aims to enhance the value of material flows and to achieve sustainable consumption and production. It represents an emerging concept that elicits great attention for the purpose of efficient and sustainable use of resources, energy and infrastructure to ensure the quality of life of humans. In this framework, biomass wastes play a major role in the implementation of circular bioeconomy, based on the reuse and recycling of materials to reduce waste production.

In the last years, fish products consumption has seen a huge increase following its recognition as a key component of a balanced diet and healthy lifestyle. Based on the Food and Agricultural Organization (FAO), United States, the total fisheries and aquaculture production has shown a considerable increase of more than eightfold between 1954 and 2014, driven by advances in fishing technologies and rapid developments in aquaculture; in 2014, the global fishery production was 93.4 million tons [2]. In the period 1961–2016, the growth in the global fish supply had a notable increase, with an average annual growth rate of 3.2%, higher than the growth rate of world's population (1.6%), even surpassing that of meat from land animals (2.8%). World fish consumption per capita increased from 9.0 kg in 1961 to 20.2 kg in 2015, and preliminary estimates indicate further growth for 2016 and 2017 to approximately 20.3 kg and 20.5 kg, respectively [3]. Consequently, also the amount of fish waste has undergone a dramatic increase across the world. Currently, the post-catch fish losses represent a huge economic and environmental concern occurring in most fish distribution chains, with large amounts of landed fish lost or discarded between landing and consumption [2]. It is important to underline that today the expansion of consumption is driven not only by the increase in production, but also by different factors, including the reduction of wastage. In fact, although the continuous increase in fish consumption in 2016 a small decrease in global capture fisheries production (90.9 million tons) was observed, compared to the previous two years [3]. In order to promote environmentally, economically, and socially sustainable EU fishing practices, the main recent objectives pursued by the EU Common Fisheries Policy (CFP) are the drastic reduction/prohibition of discards and the use of the captured biomass as best as possible [4].

To date, fish waste is partly destined for the production of fishmeal, fertilizers, and fish oil with low profitability or utilized as raw material for direct feeding in aquaculture ^[5][6][7][8][5–8], and partly thrown away [9].

For this reason, a better fish-waste management is needed to overcome environmental issues and for the fully use of biomass for purposes of high-commercial value, at the same time. In this context, the growing recent attention to alternative uses of fish byproducts plays an important role in the economic growth and sustainable development. Several studies have been reported to analyze their possible uses, as they represent a rich source of value-added compounds, including enzymes, bioactive peptides, and bio-polymers, with many possible uses in several fields ^[10][11][10,11].

2. Fish Waste in the Circular Bioeconomy Era

The enormous population growth that has taken place in the last two decades, and the consequent extensive use of nonrenewable resources, has negatively affected the quality of the environment and pushed towards sustainable strategies. In this context, the uses of alternative resources that can replace fossil ones and the development of renewable processes based on sustainability are essential for future generations. The transition from a linear to a circular economy is currently an indispensable aspect for managing resources in an eco-efficient way, since the concept of sustainability is totally based on the circularity of all the necessary materials.

The circular bioeconomy is an integral part of the circular economy and it is fundamental in achieving both sustainability in terms of resources and environmental sustainability. The bioeconomy uses materials of biological origin and imitates or uses processes developed by nature to achieve an efficient use in terms of resources [12]. According to the European Commission, bioeconomy is defined as “the production of renewable biological resources and the conversion of these resources and waste streams into value-added products, including food, feed, bio-based products and bioenergy” [13]. In this way, the anthropogenic consumption of raw materials of fossil origin is reduced, the influx of renewable resources is supported, and the environmental impact is reduced to a minimum.

The greatest strengths of the circular bioeconomy are: awareness by people and industry, involvement of stakeholders and policy makers, support of politics, sustainable production and consumption, resources valorization, and zero waste. In this sense, the bio-waste valorization approach plays a fundamental role in bringing circularity to the bioeconomy. In this area huge efforts are underway by the scientific community, together with the support of the government, and are largely directed at the recovery of resources from biological waste.

In this context, a growing awareness and special attention to the development of greener and more sustainable processes have led to a greater interest in the use of unwanted marine resources, such as the huge amount of waste obtained by fishing and aquaculture, which is a very promising source of products with high market value ^[10][11][14][10,11,14]. Each year, a huge amount of biomass is (i) discarded, generally incinerated, increasing the energy consumption, financial cost, and environmental impact of their management process [15], or (ii) utilized for low-value products; to date, fish waste is used mainly in the fish meal industry, since it contains almost the same amount of proteins as fish meat ^[5][6][7][5–7]. Moreover, the nutritional composition of fish waste allows to supply plant nutrients or to enrich a compost. In fact, fish waste is/can be processed to produce several fertilizers [8], and currently commercial fish-based fertilizers are used for agricultural and horticultural crops [16]. In addition, fish waste has high concentration of biodegradable organics which could be recycled as attractive co-substrate for waste activated sludge, to improve the methane production during anaerobic co-digestion ^[17][18][17,18].

More than 70% of the total fish caught is subjected to further processing before being placed on the market [19], resulting in the production of large amounts (approximately 20–80%) of fish waste, depending on the level of processing (e.g. gutting, scaling, filleting) and species, because each species has a specific composition, size, shape and intrinsic chemistry ^[20][21][20,21]. These operations generate discards which mainly include muscle-trimmings (15–20%), skin and fins (1–3%), bones (9–15%), heads (9–12%), viscera (12–18%), and scales (5%) [22]. Fish processing is an important need for large fish companies both to reduce the costs related to transport of inedible parts of the fish and to increase stability and quality of products, removing parts, such as the viscera, that might contain bacteria and enzymes, which represent a risk for processing and storage of the fish [23]. Preserving the nutritional quality of products represents one of the main challenges for the industry. The degradation of proteins by enzymes is a key aspect that should be minimized, as a high degree of hydrolysis could produce bitter-tasting peptides ^[23][24][23,24], and together with the lipid peroxidation, lead variability in raw materials ^[24][25][24,25]. Acceptable levels of lipid and protein hydrolysis are enormously dependent on the product, based on its end use ^[26][27][26,27]. Hydrolysis determines structural and conformational changes, with possible negative effects on the physicochemical and functional properties of proteins [23]. For this reason, the control of the autolysis and auto-oxidation of these products is fundamental for their use ^[20][28][20,28]. This represents a real challenge for fishing vessels, which require more advanced equipment and technologies for capture and better handling, necessary to maintain the quality of the byproducts ^[29][30][29,30]. Moreover, to ensure food safety and consumer protection, increasingly stringent hygiene measures have been implemented at the National and International level.

In addition, a substantial amount of by-catch is rejected each year, including both low-value species and tons of commercially valuable but undersized fish. However, the amount of fish byproducts is expected to increase in the next years, due to the implementation of the landing obligation, as part of the new reform of the EU-CFP. It requires the obligation of landing all commercially exploited species under the total allowable catch regulations, including undersized fish which cannot be used for direct human consumption, and endangered species, avoiding the great waste of precious fish biomass through the practice of fish discards ^[9][31][9,31]. In fact, it has been estimated that nearly 25% of all the caught fish never reaches the market; every year, approximately 27 million tons of unwanted fish are discarded into the sea, and much of it does not survive.

The final goals of the new landing obligation are also closely related to two other EU strategies, Blue Growth and the 2020 EU Strategy, involved in developing sustainable socio-economic and environmental growth in the marine and maritime EU region [32]. In particular, Blue Growth is a long-term strategy based on the exploitation of seas and oceans, which have great potential for innovation and growth, offering new ways to help the EU emerge from its current crisis and drive the economy. Considering all the activities that depend on the sea, the EU blue economy implies 5.4 million jobs and a gross added value of almost €500 billion per year. The Organization for Economic Co-operation and Development (OECD) expects that many ocean-based industries have the potential to outperform the global economy by 2030, in terms of value added and jobs, and the output of the global ocean economy could be more than double [33].

Part of the Blue Growth strategy is focused on the blue biotechnology that involves the transformation of raw marine materials in order to obtain products with high economic value useful for different biotechnological applications, which could be utilized for the development of innovative markets, contributing to the goals of the EU strategies.

In this context, the valorization strategies for fish discards and fish byproducts could contribute to the economic growth. Likewise, new uses for fish waste could alleviate the costs associated to the landing obligation, and reduce the enormous environmental problems associated to the large amount of waste.

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