Primer set for V3–V4 region of the 16S rRNA gene.

This study investigates microbial growth, community composition, and histamine production in freshly caught mackerel stored at 4°C and 10°C. Fish were obtained directly from fishermen and immediately placed in low-temperature storage to minimize external contamination. Microbial activity was assessed using total viable count, amplicon sequencing, and HPLC analysis. The total bacterial count exceeded 5 log CFU/mL after 5 days at 4°C and 2 days at 10°C, indicating the presence of histamine-producing bacteria (HPB), particularly under psychrophilic conditions. Photobacterium was identified as the dominant bacterial genus at both temperatures, suggesting its role as a key psychrophilic HPB. Histamine levels increased progressively, reaching 5.11 ± 2.43 ppm at 4°C and 11.18 ± 4.67 ppm at 10°C, while histidine content declined. Despite cold storage and strict contamination control, naturally occurring bacteria in fish continued to grow and produce histamine. These findings highlight the importance of temperature control in preventing histamine accumulation and provide new insights into microbial dynamics and food safety risks in fresh mackerel.