###### Abstract  
The by-product of needlefish (_Tylosaurus acus melanotus_) waste possesses important characteristics that could be used in food applications. Fish by-product collagen may be used in place of mammalian collagen due to ethical and religious considerations over environmental degradation. Different forms of acid-soluble collagen (ASC) were successfully extracted from needlefish skin. Based on dry weight, the collagen extracted using acetic acid (AAC), lactic acid (LAC), and citric acid (CAC) treatments was 3.13% with a significantly difference (_p_ < 0.05), followed by 0.56% and 1.03%, respectively. Based on proximate analysis, the needlefish skin composition was found to be significantly different (_p_ < 0.05) between compositions, with the highest moisture content at 61.65%, followed by protein (27.39%), fat (8.59%), and ash (2.16%). According to the SDS-PAGE results, all extracted collagen were identified as a type 1 collagen. Additionally, ATR-FITR revealed that all collagens had amide \(\Lambda\), B, amide I, II, and III peaks. AAC significantly outperforms LAC and CAC in terms of yield following physicochemical characterisation, including pH determination, colour (L* value), and hydroxyproline content. All collagens demonstrated strong heat resistance and structural stability with T\({}_{\text{max}}\) above 38 \({}^{\circ}\)C. Collagen was most soluble at pH 5 for AAC, pH 3 for LAC, and pH 7 for CAC. The effect of collagen solubility on NaCl concentration was discovered to be significantly reduced to 50 g/L for all collagen samples. All collagens can be used as alternatives to terrestrial collagen in a diverse range of applications. by-product; needlefish (_Tylosaurus acus melanotus_); acid-soluble collagen extraction; physicochemical; characterization +
Footnote †: journal: Applied Sciences  
fish by-products such as scales, bones, fins, and skin are the most secure sources for collagen production, and no fish-related diseases have been reported. Furthermore, crude protein levels in fish by-products can reach up to 35%, and depending on the species, age and season, collagen can constitute up to 70% of the fish's dry weight [2]. As a result, these have the potential to serve as a source of collagen, polyunsaturated fatty acids (PUFA), gelatine, enzymes, and essential amino acids [5]. Therefore, there is a need for collagen that can be made from marine sources that has been demonstrated to be comparable to traditional collagen made from mammalian sources in terms of amino acid content and biocompatibility [6]. Wastes often created by fish processing that have been utilised; include the skin, bone and muscle of leather jackets (_Odonus niger_) [7]; skin, scales and fins of _Catla catla_ and _Cirrhinus mrigala_[8]; skin of tilapia (_Oreochromis niloticus_) [9]; and several other parts which have texture and are promising sources of collagen. Due to its unique properties, collagen is important to many sectors, including those that provide healthy meals, cosmetics, tissue engineering, and wound-care products [10]. Collagen makes up nearly 30% of the total protein in mammalian skin and bone connective tissue [6] and governs tissue growth, supports structural integrity, and offers texture, shape, thickening, durability, and gel function [11]. Recently, [PERSON] et al. [2] discovered a range of techniques for extracting collagen from fish, including acid solubilization, enzyme solubilization, ultrasound, and the extrusion-hydro-extraction (EHE) approaches. In order to extract collagen, acids and enzymes are commonly used. Generally, collagen dissolves in acidic liquids; however, when exposed to a very acidic pH, its solubility diminishes, and certain chemicals can be hazardous or toxic [12]. By attaching an amine group (-COOH) on the collagen protein, weak acids or organic acids, such as acetic acid, citric acid, and lactic acid, are utilised to facilitate the collagen extraction process [13].