The influence of size on the trophic interactions between juveniles of two syntopic Trachinotus species

Syntopic species with similar anatomic configuration may face strong competition for trophic resources, thus relying on developed mechanisms to ensure coexistence. The present study investigated the influence of body size on trophic interactions between juveniles of two closely related fish species at three sandy beaches in south-eastern Brazil. A total of 150 fish were sampled, where 103 were identified asTrachinotus carolinus(mean +/- SE: weight = 9 g +/- 1.13) and 47 asTrachinotus goodei(weight = 46.7 g +/- 3.34). A significant size-difference between juvenileTrachinotuswas detected by a null-model analysis (P= 0.04), withT. carolinus(TL = 79.6 mm +/- 2.4) presenting a smaller body size thanT. goodei(TL = 147.7 mm +/- 4.2). The main prey items consumed byT. carolinuswerePerna perna(IAi = 0.76) andEmerita brasiliensis(IAi = 0.18), whereas the latter was the majorT. goodeidietary prey (IAi = 0.71). Both prey were correlated with larger-sized juveniles of each pompano species, whereas smaller fish shared non-preferred trophic items. Such opportunistic behaviour of smaller juveniles may account for the dietary overlap detected between theTrachinotusspecies (P= 0.09). Size-related dietary partitioning was observed for the largestT. goodeijuveniles, which displayed only a slight overlap withT. carolinus, and between juveniles belonging to the small and medium size groups of each pompano species. Therefore, food partitioning related to pompano body size seems to be especially important for the smallest juveniles, as they present the highest vacuity (particularlyT. carolinus) in the sampled beaches, suggesting that these individuals are under intra- and interspecific competitive pressure, which may affect local coexistence.