## 1 Introduction  
Bioactive peptides generally range in molecular size from 2 to 20 amino acid residues and show a large number of potential physiologic functions within biological bodies, such as antioxidant, opioid, antihypertensive, immunomodulatory, antithrombotic, and antibacterial activities [1; 2; 3; 4]. The identification, quantitation and activity evaluation of food-derived bioactive peptides are very important for the characterization of its production, biological value and benefits for consumer health [5; 6]. In general, bioactive peptides can be produced during food manufacturing either by the spontaneous hydrolysis of food proteins with endogenous digestive enzymes, or by designing hydrolysis conditions with commercial proteases [7; 8; 9]. In addition, bioactive peptides can also be released during the gastrointestinal digestion of the ingested foods [10]. Presently, a large number of bioactive peptides has been produced from food proteins of different origins like milk, eggs, soya, fish, mollusk and meat [7; 11; 12; 13]. Among them, antioxidant peptides (APs) derived from aquatics and their byproducts, such as the heads and viscera of sardinella [14], skin and viscera of cuttlefish [15], carcass and viscera of Nile tilapia [16], heads and skins of bluefin leatherjacket (_Navodon septentrionals_) [6; 12], and swim bladders and muscle of miuuy croaker (_Michlthys miuuy_) [8; 17], have attracted much investigative and public interests because they can positively influence health in the body and have huge potential applied in functional and health products [5; 18]. Two hexepeptides of WAFAPA and MYPGLA from the protein hydrolysate of blue-spotted stingray can protect DNA and proteins from oxidation and show stronger capability to restrain H\({}_{2}\)O\({}_{2}\)-induced lipid oxidation than carnosine [19]. The gelatin hydrolysate fraction of Nile tilapia skin (FSGHFs) (molecular weight (MW) \(<\) 1000 Da) can increase cell viability and membrane integrity, suppress reactive oxide species (ROS) production and epithelial barrier damage in H\({}_{2}\)O\({}_{2}\)-induced intestinal porcine epithelial cells (IPEC-J2) by stimulating the nuclear factor erythroid 2-related factor 2 (Nrf2) to increase the level of glutathione and the mRNA and protein expression of catalytic and modulatory subunits of \(\gamma\)-glutamyl cysteine ligase [20]. In addition, a pentapeptide of YGDEY from tilapia gelatin hydrolysates can inhibit ultraviolet B (UVB)-mediated skin photoaging by decreased MMP-1 (collagenase) and MMP-9 (gelatinase) expression, increase antioxidant factors superoxide dismutase (SOD) and glutathione (GSH) expression, and prevent DNA from oxidative damage [21]. [PERSON] et al. reported that GAERP, GAREANVM, and AEVG can effectively scavenge free radicals and protect human liver cancer cell line (HepG2) from H\({}_{2}\)O\({}_{2}\)-induced oxidative damage through reducing the malondialdehydehyde (MDA) level and promoting the contents of glutathione reductase (GSH-Rx), SOD, glutathione peroxidase (GSH-Px), and catalase (CAT) [22]. The literature proved that seafood-derived APs have high antioxidant capability and can be applied in health care food to protect cells and organisms against oxidative stress. As an epipelagic oceanic species, skipjack tuna (_Katsuwonus pelamis_) mainly live in tropical and subtropical seas with over 2 million metric tons of production every year in the Pacific Ocean [23]. In China, approximately 200,000 tons of tuna are processed every year and its byproducts accounted for approximately 50% of the raw material, including fish bones, head, scales, viscera and skins, are generated during tuna can processing [24]. Traditionally, these tune byproducts are processed into low-value fish flour utilized in animal feeds or dumped as harmful wastes [24]. In recent years, the production of bioactive molecules including collagen, gelatin, and bioactive peptides using tuna byproducts were studied as a promising means to increasing tuna value and lowering environmental pollution. [PERSON] et al.