Trophic size-structure of sailfish Istiophorus platypterus in eastern Taiwan estimated by stable isotope analysis

To examine trophic dynamics over different size classes, an isotopic study of sailfish Istiophorus platypterus life‐history stages was carried out. Samples were collected from eastern Taiwan and the South China Sea during April 2009 and February 2012. A total of 263 samples (111–245 cm, lower jaw fork length, LLJFL) were examined for changes in trophic structure in relation to LLJFL by using stable isotope analysis of carbon (δ13C) and nitrogen (δ15N). The δ15N values for I. platypterus ranged from 7·51 to 14·19‰ (mean ± s.d. = 12·06 ± 1·16‰) and the δ13C values ranged from −22·04 to −15·48‰ (mean ± s.d. = −17·62 ± 1·10‰). The δ15N values were positively dependent on LLJFL (r2 = 0·377), whereas δ13C were negatively dependent on LLJFL (r2 = 0·063). There were significantly different seasonal changes in nitrogen and carbon isotopic concentration, but no significant differences in concentrations between eastern Taiwan and the South China Sea were reported. The trophic level (TL) of each LLJFL class was correlated, starting from 2·84 TL for size class I (LLJFL 221 cm). The mean ± s.d. TL was 4·43 ± 0·19 for all samples. The results reveal that I. platypterus occupies a wide range of trophic levels and different size classes occupy different trophic positions in the pelagic ecosystem.