δ15N and δ13C elucidation of size-structured food webs in a Western Arabian Sea demersal trawl assemblage

In aquatic systems, predators are usually larger in body size than their prey, and within a species larger individuals tend to occupy higher trophic levels. This has led to the expectation that communities are strongly size-structured. In the present study, we tested whether size structuring was present in a demersal trawl assemblage from the Western Arabian Sea. Nitrogen and carbon stable isotopes were used to describe trophic level and basal resource utilisation and were assessed against body size (log 2 size class) at both the community (species independent) and species levels. Nitrogen and bulk carbon isotopic values were significantly and positively related to body mass at the community level. Despite sampling over 15 log 2 mass classes (0.001 to 32.8 kg), the trawl assemblage spanned approximately 1 trophic level. The community relationship between 13 C and log 2 body mass was associated with higher lipid content in smaller size classes. In contrast, comparisons of the 15 N and 13 C values of species at a similar stage of their life history (55 to 80% of maximum length) were independent of body size, suggesting reduced niche overlap at different sizes. While some small species feed at high trophic levels and vice versa, they contributed comparatively little to the sampled community biomass, and overall, the community was size-structured in terms of trophic level.