Age Determination and Growth of Frigate Tuna (Auxis thazard, Lacepede, 1800) in the Celebes Sea

Frigate tuna (Auxis thazard, Lacepede, 1800) is a commercially valuable food fish thriving in tropical marine waters. In the Philippines, despite its abundance, information on its estimated age and growth patterns is lacking. This study reported the relationship between the size (fork length, FL) and age (otolith growth increment) of frigate tuna using the sagittal otoliths. Sampling was conducted in the Celebes Sea, including the inner bay and gulfs within it (e.g., Moro Gulf, Sarangani Bay, and Davao Gulf) from November 2020 to September 2022. A total of 1,046 frigate tuna samples with sizes ranging from 14.5 cmFL to 44.7 cmFL, caught using ring nets, hand lines, and purse seine nets, were examined. Results of the aggregated fish lengthweight relationship (R2 = 0.9819; a = 0.0068; b = 3.2632) indicated a strong positive allometric growth pattern. Similarly, a strong positive correlation (R2 = 0.7130) between the otolith length (n = 344) and fork length relationship indicates that the otolith increases in length as the fish grows. However, for the successfully aged otolith sections, only individuals collected in the western Celebes Sea (n = 113) demonstrated a good result covering ages ranging from one to four years old. From this information on the age-growth relationship, the von Bertalanffy growth function model generated the following values for L∞ at 62.1 cmFL and K at 1.0016/yr. The latter growth parameter estimates from otolith microstructure is the first record for frigate tuna derived from the age-length data.