## 1 Introduction  
Bigeye tuna (_Thunnus obesus_) as an important species dominates the aquatic market due to its high protein and low fat [1]. With the growth of market demand and the achievement of good taste, the quality of bigeye tuna has been paid more attention. Among varieties of preservation techniques, cold storage (\(-1\)-\(8\)\({}^{\circ}\)C) is the most convenient storage method currently used to delay fresh fish spoilage and prolong its shelf life [2]. According to its high nutritional value and delicious meat, tuna has been investigated for quality changes, including chemical composition, microbial growth, enzymatic activities and physicochemical properties, during cold storage [3].  
Water is an important factor for determining the quality of aquatic products. Water accounts for up to 70% of the seafood muscle [4]. Water distribution and mobility in muscle have significant effects on appearance, flavour, ammonia nitrogen content and textural parameters of seafood during storage.  
High water content can affect quality deterioration of seafood, which leads to a limited shelf-life [5]. Based on the characteristics of sensitivity, non-invasiveness and low cost, LF-NMR (low-field nuclear magnetic resonance) has been applied to aquatic food during processing and storage [4]. Magnetic resonance imaging (MRI) is an accurate and non-destructive method for visualizing the internal aquatic product structure [6]. Accordingly, T\({}_{2}\) relaxation time has been proven to quantify the water dynamics combining with chemometric models to predict shelf life, thus, has helped to explain how these changes are correlated with fish quality [7].  
ATP degradation is used to evaluate the taste or freshness of aquatic products [8]. Aquatic products easily spoil as the initial catabolism of ATP normally results in a fast and temporary accumulation of inosine monophosphate (IMP) [8; 9]. IMP and hypoxthine (Hx) accumulation play an essential role in the flavour of aquatic products, especially for umami or sweet flavours [10]. Volatile organic compounds (VOCs) are formed during the spoilage of aquatic products which is originated from physical, chemical and microbial changes. In addition, VOCs of aquatic products, including aldehydes, ketones, alcohols, aromatic compounds, sulphur compounds, amines, esters and acids can be formed during cold storage, which were analysed by solid phase microextraction coupled with gas chromatography/mass spectrometry (SPME-GC/MS) [11; 12; 13]. Many researchers have explored that fish spoilage can produce some VOCs, such as 1-pentan-3-ol, 1-octen-3-ol, 1,5-octadien-3-one, (Z)-4-heptenal and benzaldehyde [8; 14; 15]. Hexanal, 2-nonanone and nonanal in yellowfin tuna (_Thunmus albacares_) could be used to evaluate quality changes at 30 \({}^{\circ}\)C and in ice using SPME-GC/MS [15]. Therefore, VOCs and ATP-related compounds of fish have been considered important in quality. In addition, VOCs and water are both assessed to quality changes of fish. Water loss can affect the flavour of the fish [4]. However, the affection of different status of water on the specific volatile substance remains unknown. No previous study has been focused on the correlation between water dynamics and VOCs of bigeye tuna throughout cold storage. Therefore, this aspect deserves more attention.  
This study aimed to investigate the changes in water distribution and VOCs and their correlation in bigeye tuna to determine the quality during cold storage. Water distribution and mobility were measured by LF-NMR as a rapid and non-destructive method. Changes of physical and chemical property including drip loss, colour, ATP-related compounds and histamine of bigeye tuna during cold storage were also analysed. The study provides novel insights into spoilage mechanisms, that specific VOCs are affected by different status of water.