Analysis and Evaluation of the Muscle Nutrition of Different Sizes of Wild Yellowfin Tuna (Thunnus albacares)

Yellowfin tuna (Thunnus albacares) is popular with consumers due to its delicious meat and high nutritional value. It is a globally recognized high-end marine economic fish. With the advancement of fishing equipment and the increase in fishing efforts, the amount of tuna resources has decreased significantly. To make up for the insufficient supply of tuna in the market and protect wild populations, it is essential to carry out research on artificial aquaculture technology for tuna and to gradually establish full-cycle cultures of tuna. In order to study the differences in the nutritional components and quality of yellowfin tuna muscles of different sizes, three different-sized yellowfin tuna, J1 (4.2±1.2) kg, J2 (22.5± 2.5) kg, and J3 (50.8±3.9) kg, were used as the research subjects, and conventional biochemical analysis methods were used to compare and analyze the proximate compositions, amino acids, fatty acids, and mineral elements of tuna muscle. The results showed that (1) the moisture level of the J1 group was significantly higher than that of the J2 and J3 groups (P<0.05); the crude protein levels of the J2 and J3 groups were significantly higher than that of the J1 group (P<0.05); and the crude fat level in the J3 group was significantly higher than that in the J1 and J2 groups (P<0.05). (2) A total of 19 common amino acids were detected in yellowfin tuna muscle, including nine essential amino acids (EAA), two semi-essential amino acids, and eight nonessential amino acids. The amino acid with the highest content was glutamic acid (3.04~3.25 g/100 g) of the three tuna sizes; the essential amino acid with the highest content was lysine (2.02~2.15 g/100 g), and the essential amino acid with the lowest content was tryptophan (0.31~0.45 g/100 g). The amino acid content of different-sized yellowfin tuna varied greatly, except for threonine, valine, methionine, isoleucine, tyrosine, and phenylalanine, which were not significantly different among the groups, and the other amino acid contents were mainly J3>J2>J1. The content of nonessential amino acids was J3>J2>J1 (P<0.05). The content of essential amino acids and delicious amino acids in the J1 group was significantly lower than that in the J3 group (P<0.05). The content of semi-essential amino acids in the J1 group was significantly lower than that in the J2 and J3 groups (P<0.05). The ratio of EAA/TAA (total amino acids) in each group was above 40%, and there were no significant differences. According to the Amino acid score (AAS) score, the valine score in each group was the lowest and<1, which was the first limiting amino acid. According to the chemical score (CS) score, except for lysine and tryptophan of the J3 group, the scores of other amino acids were all less than 1. The first limiting amino acid of groups J1 and J2 was tryptophan, while the first limiting amino acids of the J3 group were phenylalanine + tyrosine. (3) A total of 25 fatty acids were detected in the muscles of yellowfin tuna of three sizes, including 10 saturated fatty acids (SFAs), 5 monounsaturated fatty acids (MUFAs), and 10 polyunsaturated fatty acids (PUFAs). There were nine fatty acids whose content was greater than 1%, and their average content ranged from high to low: C22:6n3 docosahexaenoic acid (DHA), C16:0, C18:0, C18:1n9c, C20:5n3 eicosapentaenoic acid (EPA), C20:4n6 arachidonic acid (ARA), C18:2n6c, C24:1n9, and C16:1n7. Among these nine fatty acids, only C16:0, C24:1n9, and C16:1n7 showed no significant differences (P>0.05). The fatty acids in yellowfin tuna muscle were mainly PUFAs, and the content of DHA accounted for 37.46%~39.18% of the total fatty acid content. The DHA content of the J3 group was significantly higher than that of the J1 and J2 groups (P<0.05). The EPA content of the J2 and J3 groups was significantly higher than that of the J1 group (P<0.05). The DHA: EPA ratio of the J1 group was significantly higher than that of the J2 and J3 groups (P<0.05), and the MUFA content was J3>J2>J1 (P<0.05). The PUFA content of the J3 group was significantly higher than that of the J1 and J2 groups (P<0.05). PUFA/SFA and n-3/n-6 ratios of the J2 and J3 groups were significantly higher than those of the J1 group (P<0.05). The h/H ratio of the J3 group was significantly higher than that of the J1 and J2 groups (P<0.05). (4) Among the four macro elements, the contents of sodium and calcium of the J2 and J3 groups were significantly higher than those of the J1 group (P<0.05), and the potassium content of the J1 group was the highest and was significantly higher than that of the J2 and J3 groups (P<0.05). The contents of four heavy metal elements were all far lower than the maximum allowable limit (FAO and WHO) suggested in food, among which the iron content was the highest in the J3 group, and J3>J2>J1 (P<0.05). The copper content was the highest in the J3 group, and was significantly higher than that in the J1 group (P<0.05). Comprehensive analysis showed that large size yellowfin tuna had better nutritional quality, and the results of this study also provide a scientific basis for the selection of residents' diets and the formulation of artificial nutrition feed for yellowfin tuna.