## 1 Introduction  
Fish discarding is the practice of returning unwanted catches, either dead or alive, back to the sea. This could occur due to economic reasons such as market demands or fishermen quota levels, or because the sizes of the species do not meet minimum requirements. This is a serious problem according to data provided by the Food and Agricultural Organisation (FAO), which estimates that nearly 50% of the more than 90 million tons of fish and shellfish caught globally each year are discarded [1]. This is not only harmful for fish species themselves, but can also substantially alter the marine environment and even favour the development of parasites such as anisakis [2]. In order to reduce this practice, the European Union put several landing regulations into action that came into force in 2019. Since then, fish discards of the main species have been completely banned [3]. However, this measure left some issues unclear--mainly the use of fish after landing if it cannot be marketed. Additionally, the degradation of the large amount of fish waste generated by fish processing units in coastal cities can have a very negative effect on the environment [4]. Hence, finding an application for fish waste and discards is becoming a very important issue. In recent years, a variety of applications for fish waste have been discovered, such as, obtaining hyaluronic acid from fish eyeballs [5], extracting collagen from fish skin [6; 7], from swim bladders [8] or from cartilaginous edoskeleton [9], or isolating gelatine from scales [10; 11; 12]. However, there are not many utilities for fish bones yet. The most widespread application of fish bones today is the production of animal feed, providing only a limited benefit. In recent years, the possibility of obtaining calcium phosphate from fish bones has been explored [13]. Different species have been used as source of calcium phosphates, including Bigeye tuna (_Thumus obesus_) [14], swordfish (_Xiphias gladius_) [15], Atlantic bluefin tuna (_Thumus thymus_) [15], Atlantic cod (_Gadus morhua_) [16], greater amberjack (_Seriola dimeril_) [17], European sardines (_Sardina pilchardus_) [18], sheelavati river fish (_Rolo labio_) [19], and barramundi (_Lates calcarifer_) [20]. This work tackles the transformation of fish discards, not just fish bones, into calcium phosphates. All these works reported a careful preparation of the fish bones prior to the treatment used to obtain the calcium phosphates. As an example, [PERSON] et al. [14] described that tuna bones were washed with hot water over two days in order to take away the traces of skin and meat. [PERSON] et al. [16; 18] also explained that prior to the thermal treatment, bones from cod fish or sardines were manually cleaned with a brush in hot water to remove all fragments of skin, meat, etc. A similar procedure was reported by [PERSON] and [PERSON] [17]. They boiled and washed the fish bones to remove the soft tissues, such as cartilage, and dried them at room temperature for 1 week prior to the calcination treatment that leads to calcium phosphates isolation. A similar procedure to assure that all remains from the fish (flesh, skin, etc. ), except the bones, were eliminated in order to assure that the precursor material was as pure as possible, was also followed by [PERSON] and [PERSON] [19] and by [PERSON] et al. [20]. In our prior work [15], we also took extra care to guarantee that bones were extracted directly at the factory ships, frozen immediately in each ship, and that the cold chain was not broken. At the lab, we boiled and cleaned the bones and removed all organic matter prior to the thermal treatment. These procedures are excellent from a laboratory point of view, but they are far from the capabilities of the factory ships processing fish or the aquaculture facilities for human consumption. Therefore, the objective of this work is to study the transformation of fish discards (not only bones) into calcium phosphates.