Description:
<jats:p>This study used three different methods to ascertain dietary composition for 21 Chaetodontidae species co‐occurring on a single fringing reef in Derawan Island, East Kalimantan, Indonesia. The combination of <jats:italic>in situ</jats:italic> feeding observations, examination of gut contents and stable‐isotope analyses was expected to identify previously unresolved prey items that contribute to dietary separation. <jats:italic>In situ</jats:italic> feeding observations identified five species that feed predominantly on anthozoans (<jats:italic>Chaetodon baronessa</jats:italic>, <jats:italic>Chaetodon bennetti</jats:italic>, <jats:italic>Chaetodon lunulatus</jats:italic>, <jats:italic>Chaetodon punctatofasciatus</jats:italic> and <jats:italic>Chaetodon speculum</jats:italic>). Stable‐isotope ratios for these species, as well as for <jats:italic>Chaetodon ornatissimus</jats:italic> (for which no feeding observations were completed), were very similar and consistent with diets comprising mostly anthozoans. Feeding observations, however, showed that they mostly fed on different coral species, while the identifiable portion of their gut contents showed clear separation based on cryptic dietary components. For example, <jats:italic>C. baronessa</jats:italic> and <jats:italic>C. bennetti</jats:italic> appeared to ingest annelid worms during the course of coral feeding, whereas gut contents of <jats:italic>C. punctatofasciatus</jats:italic> and <jats:italic>C. speculum</jats:italic> were dominated by crustaceans. <jats:italic>In situ</jats:italic> feeding observations further identified the following groups: coral–bottom feeders, bottom feeders, sponge feeders and pelagic feeders, feeding on a wide variety of prey items such as Annelida, Crustacea, Cnidaria, Mollusca and macroalgae. Overall, many chaetodontid species had similar reliance on major prey items (<jats:italic>e.g</jats:italic>. anthozoans or polychaetes) but differed greatly in the minor prey items that they utilized. Partitioning of minor prey items may be important in reducing interspecific competition and facilitating coexistence of chaetodontids on coral reefs.</jats:p>