## 1 Introduction  
Gelatin is produced by partial hydrolysis of collagen with a molecular weight (MW) ranging between 80 and 250 kDa [1]. Traditionally, gelatin has been isolated from the skin and bone collagens of mammalian species, primarily cows and pigs [2]. However, the application of gelatin from mammalian species may be a concern among consumers because of dietary restrictions and infectious disease [3]. These sociocultural and safety concerns have led to intensive research to identify and develop alternatives to mammal derived gelatin [1]. Gelatin from seafood by-products has been thought as an ideal alternative to mammalian gelatin due to its large quantities and safety [4]. In addition, gelatin from marine resources is acceptable for Islam, and can be used with minimal restrictions in Judaism and Hinduism [5]. Therefore, gelatin has been extensively extracted from different fish by-products, such as golden carp skin [2], lizardfish scale [6], unicorn leatherjacket skin [7], cuttlefish skin [8], cod head [9], shark cartilage [10], and red snapper and grouper bones [11]. Nowadays, fish gelatin is wildly applied in the food, pharmaceuticals, cosmetics, biomedical, and biomaterial-based packaging industries for its high nutrition, good moisture-retention, and biocompatibility [2; 12]. However, its properties, including the lower melting temperature and gel strength, and weaker structural stability compared to mammal gelatin, severely limit its application extensively in tissue engineering materials and capsule wall materials [13; 14]. Nevertheless, gelatin with those undesirable physicochemical properties is more easily hydrolyzed by proteases compared to mammal gelatin and is suitable to be prepared for bioactive peptides. Therefore, preparation of bioactive peptides using gelatins attracts broad attention due to its utilization potentiality in biomedical industries. [PERSON] et al. reported that peptides of GPAGPHGPPKDGR, AGPHGPPGKDGR, and AGPAGPARGAR from Pacific cod skin gelatin exhibited a high affinity to ferrous ions, which has a potential application of gelatin-derived peptides as novel carriers to combat iron deficiency [12]. [PERSON] et al. evaluated the effect of tilapia gelatin peptides (TGP) on UV-induced damages to mice skin, and the results indicated that TGP could significantly prevent a decrease of antioxidase activity in a dose-dependent manner. The protective effects of TGP suggested that TGP could be a novel antiphotoaging agent from natural resources [15]. [PERSON] and [PERSON] reported that oral administration of gelatin hydrolysate from Pacific cod skin could suppress ultraviolet (UV) radiation-induced damage to the skin by inhibiting the depletion of endogenous antioxidant enzyme activity, and suppressing the expression of nuclear factor-kB (NF-kB) and NF-kB-mediated expression of pro-inflammatory cytokines [16]. [PERSON] et al. reported that gelatin hydrolysate (FSGH) of Nile tilapia skin prepared using ginger protease exhibited a higher degree of hydrolysis (DH), radical scavenging activity, and lipid peroxidation inhibition capability than those of gelatin hydrolysate from pig skin, pig bone, and bovine skin. Furthermore, tripeptides of Gly-Pro-Ala from FSGH can activate the expression of antioxidant response element (ARE)-driven antioxidant enzyme genes in a dose dependent manner and suppressed the H\({}_{2}\)O\({}_{2}\)-induced intracellular reactive oxygen species (ROS) production in the porcine small intestinal epithelial cell line (IPEC-J2) [17]. Therefore, gelatin peptides derived from seafood by-products exhibit significant antioxidant activity to protect the skin from photoaging through alleviating UV-induced abnormal changes of antioxidant defense systems, repairing endogenous collagen and elastin protein fibers, and decreasing the loss of moisture and lipids. Skipjack tuna (_Katsuvonus pelamis_) belongs to the family of Scombridae, which is an epipelagic oceanic species widely distributed in subtropical and tropical seas [18].