Resource use of two damselfishes, Stegastes planifrons and Stegastes adustus, on Guadeloupean reefs (Lesser Antilles): Inference from stomach content and stable isotope analysis

Stegastes planifrons and Stegastes adustus are two abundant territorial damselfishes of Caribbean coral reefs. Their trophic niches and their food utilisation on Guadeloupean reefs at two contrasting sites were compared on the basis of stomach content and stable isotope analyses (15N/14N and 13C/12C ratios). While ingested food items slightly differed between species, the assimilation of each food source (macroalgae, turf, benthic invertebrates and detritus) calculated with a concentration-dependent mixing model was similar. Macroalgae, that presented the highest nutritional values, contributed to 33% in biomass of their diet. Invertebrates, algal turf and detritus showed similar nutritional values, while they presented different contributions to the diet of Stegastes. Stable isotope ratios indicated similar fish trophic level and similar isotopic structure on a territory scale, showing a similar trophic niche of the two damselfish at the two sites. However, co-occurrence in the same shallow habitat was associated with a reduction in the size of the territory of S. adustus and an increase in the number of species of macroalgae inside their territory, suggesting spatial competitive interactions.

► We determined accurately the trophic niche of two damselfishes with the use of stable isotope and stomach content analyses.
► We compared the trophic structures between the two fish species and between two studied sites.
► The choice of food items by fish was partially explained by their nutritional values, measured by biochemical analyses.