The Spainsh mackerel ( Scomberomores commerson ) is one of the major commercial fish species in waters of Taiwan. The three different gear types caught of the Spainsh mackerel, and peak months of catch per unit effort (CPUE) of 16.06 kg/100 km 2for drift in January, 9.54 kg/100hooks for longline in May, and 39.0kg/per-day for trolling line in April, respectively. The total catch annual of Spainsh mackerel in Taiwan has been about 800 metric tones in 2018. The relationships between body weight and fork length forScomberomores commersonBW = 0.00001FL 2.8678(r² = 0.9744, n=896). Stomach contents analysis indicated thatScomberomores commersonmainly feed on larvalDecapterus maruadsi ,Mene maculata ,Scomber australasicusandTrichiurus japonicus . Stable isotope analysis indicated that the δ 15 N values ranged between 7.2and 13.5‰, and the estimated trophic position varied from 2.54 for Spainsh mackerel with FL <40 cm, while it reached 3.9for those with FL58 cm. Based on the distinct diet shift of theScomberomores commerson , demonstrated by stable isotope analysis, this study concluded thatScomberomores commersonshift their diet at approximately 50 cm FL.