© Inter-Research 2015. This study used primary producer abundance surveys, stomach content analysis, stable isotope analysis, and mixing models to investigate the ecological niche of western striped trumpeters Pelates octolineatus in the subtropical seagrass ecosystem of Shark Bay, Australia. The percent cover of seagrass and macroalgae in the study area was 35.9 ± 4.3 and 28.59 ± 3.8% (mean ± SE), respectively. Of the 2 dominant seagrass species, Amphibolis antarctica was present at a higher number of stations than Posidonia australia, and although fast-growing, smaller-bodied seagrass species occurred infrequently, only 3 stations surveyed did not contain seagrass. Algae also occurred at the majority of stations (47 of 49 stations). P. octolineatus stomach contents consisted mainly of primary producers (87.7%). The mean relative amount of seagrass and algae in P. octolineatus stomachs was similar (43.7 ± 44.1 and 44.0 ± 43.8%, respectively), although there was considerable variation among individuals. There was a positive relationship between fish length and percentage of seagrass consumed. Stable isotope values suggested that algae contributed a larger portion of assimilated food rather than seagrass, but predictions from mixing models more closely coincided with stomach content analysis when the standard deviation of potential food sources was increased. P. octolineatus exhibited a similar, but smaller, isotopic niche space to sympatric, largely herbivorous, green turtles Chelonia mydas (27.3 and 62.4 units, respectively). These data suggest that P. octolineatus function as herbivores in this subtropical seagrass system, thereby having the potential to structure primary producer communities and facilitate the transfer of primary production to higher trophic levels.