Size-Related Quality Characteristics of Cage-Reared Butter Catfish (Ompok bimaculatus) in a River Basin

This study evaluated whether body size could serve as a quality and traceability marker for cage-reared butter catfish (Ompok bimaculatus) from the Pak Phanang Basin, Nakhon Si Thammarat, Thailand, in support of Geographical Indication (GI) certification. Fish were classified into three commercial size grades-small (12-15 fish/kg), medium (6-10 fish/kg), and large (3-5 fish/kg)-corresponding to fish harvested after 6, 8, and 12 months of rearing, respectively, with mean body weight and total length of 75 ± 7 g and 19.8 ± 1.1 cm (small), 120 ± 9 g and 25.8 ± 2.1 cm (medium), and 260 ± 10 g and 32.2 ± 2.8 cm (large). Dorsal muscle samples were comparatively analyzed to assess size-related differences in physicochemical properties, nutritional composition, and microbiological quality. Proximate analysis showed that moisture, lipid, ash, and total energy contents increased significantly with fish size (p < 0.05), whereas protein and carbohydrate contents did not differ significantly among size groups (p > 0.05). Small fish exhibited slightly higher muscle pH (7.02 ± 0.18) than medium and large fish (6.65 ± 0.11 to 6.66 ± 0.25) (p < 0.05). Flesh color was characterized by a pale whitish to slightly yellow appearance with high lightness (L*; p < 0.05), while redness (a*) and yellowness (b*) did not differ significantly among size groups (p > 0.05). Mineral composition varied with size, with large fish containing significantly higher levels of calcium and magnesium (p < 0.05). Amino acid profiling identified 17 amino acids, including eight essential amino acids (EAA) and nine non-essential amino acids (NEAA), with EAA being more abundant in large fish-particularly leucine and lysine-indicating improved protein quality, while NEAA were dominated by glutamic and aspartic acids across all size groups. Fatty acid analysis revealed higher proportions of polyunsaturated fatty acids (PUFA) and greater unsaturated fatty acid/saturated fatty acid (UFA/SFA) ratios (1.7-1.8) in medium and large fish. Microbiological assessment showed a decrease in total viable counts with increasing fish size, while Escherichia coli, Staphylococcus aureus, and Salmonella spp. were not detected in any size group, as these microorganisms are commonly used as key hygiene and food-safety indicators in fish products (E. coli for fecal/handling hygiene, S. aureus for human-handling contamination, and Salmonella spp. as a major foodborne pathogen). Overall, body size was associated with consistent variations in physicochemical characteristics, nutritional composition, and microbial quality of Pak Phanang Basin butter catfish. These findings provide baseline compositional and safety markers that can support product specification development and GI documentation.