Stomach content and stable isotope analysis of sailfish (Istiophorus platypterus) diet in eastern Taiwan waters

• Stomach content analysis was determined to examine diet composition of sailfish in Taiwan.
• Carbon and nitrogen isotopes were used to depict the trophic structure of sailfish.
• Stable isotope analysis was used to quantify relationships for sailfish and their prey.
• We estimated trophic level for sailfish and their prey inferred from nitrogen isotopes.
• Stable isotope mixing model was used to estimate importance of diet sources of sailfish.

Stomach content analysis (SCA) and stable isotope analysis (SIA), coupled with isotopic-mixing model analysis, were used to estimate diet composition of sailfish Istiophorus platypterus in eastern Taiwan waters. SCA provided information on diet, but the high occurrence of empty stomachs (48.5%) limited this analysis. According to the index of relative importance (%IRI), the most important prey items were Priacanthus macracanthus (38.7%), followed by Auxis spp. (35.9%), and Trichiurus lepturus (8.5%). However, the most important prey groups for adult sailfish (>181 cm, LJFL) as estimated by the stable isotope-mixing model were T. lepturus (32.6%), Katsuwonus pelamis (15.8%), and P. macracanthus (11.3%), and for maturing sailfish were K. pelamis (12.9%), P. macracanthus (10.4%), and T. lepturus (32.6%), respectively. Juvenile sailfish feed primarily on smaller prey items with lower δ15N values, while adult sailfish preferred larger prey items with higher δ15N values. Our findings suggested that an integrated SCA and SIA is considerably more powerful than using SCA alone in determining diet composition of sailfish over long time scales. In summary, a high diversity in the diet composition of sailfish was found and included an array of coastal, benthic, pelagic, and mesopelagic species. Sailfish are most likely opportunistic feeders consuming the most abundant prey items in eastern Taiwan waters.