## 1 Introduction  
Fish consumption has increased tremendously, and its demand by 2050 is estimated to be 9.8 billion tons [1]. During the processing of fish, more than 60% of fish mass is generated as leftovers, including viscera (liver, kidney, and roe), frames, trimmings (containing muscle, bone, and skin), heads (containing the gills), and mince [2]. Salmon (_Salmo salar_) constitutes a large portion of the fish globally served due to its high market demand. It is usually sold as a fillet or as whole, which often leads to the generation of frames attached with remaining meat. Consequently, a large amount of waste is generated [3]. The hydrolysis of salmon frames prepared in the mince and chunk form has been studied [3]. After hydrolysis, a high amount of fish bones remains as residues, particularly when chunk form is used. These residual bones could be used as a starting material for the production of bio-calcium for calcium supplement. Calcium is an essential element that is abundantly found in the structure of human bones and teeth. It is also involved in the numerous physio-logical activities of the human system, including maintaining nerve impulse transfer and heart rate, facilitating blood flow within capillaries, participating in blood coagulation, and modulating muscle function [4]. Deficiency of calcium is a general problem associated with reduced bone mass and osteoporosis [5]. This is due to inadequate calcium in most regular meals consumed by people. Therefore, an alternative source of calcium in necessitated.  
In general, fish bones are rich in calcium and phosphorus as well as other trace elements such as Na, Mg, Fe, etc. [6]. Calcination at high temperature has been used to produce the hydroxyapatite (HAp: Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)), Ca:P = 1.67) compound from natural bio-mass, such as animal bones and shells [7]. This compound has been widely used in dental and bio-medical applications [7; 8; 9], as well as in catalysis, ion exchange, and heavy-metal removal sorbents [10]. HAp produced from different fish bone species resulted in a variation of chemical composition and a deviation of Ca/P ratios from 1.67. Several studies have reported the formation of other compounds in addition to HAp such as CaO and \(\beta\)-tricalcium phosphate (\(\beta\)-TCP) when Ca/P ratios are higher and lower than 1.67, respectively [10]. The HAp/\(\beta\)-TCP biphasic compound has been extensively studied due to its ability to promote osteoconductive property [11].  
Extraction of HAp from fish and other animal bones usually involves the elimination of organic matters during the pre-treatment process. Alkaline treatment has been documented to be one of the effective means to remove proteinaceous substance from the bone matrix [3; 4]. Nevertheless, no information exists regarding whether there is an influence of alkaline treatment on the Ca/P ratios of treated bones, which could determine the formation of HAp and \(\beta\)-TCP upon calcination. Therefore, this work aims at elucidating the effect of alkaline treatment used in the extraction process of bio-calcium and HAp from salmon bone on their characteristics, including chemical compositions, Ca/P ratios, and phase morphology.