Does sympatry affect trophic resource use in congeneric tidepool fishes? A tale of two gobies Favonigobius lentiginosus and Favonigobius exquisitus

The feeding ecology of two sympatric gobies, Favonigobius lentiginosus and Favonigobius exquisitus, which inhabit soft substrate pools was studied in Moreton Bay, Australia. Favonigobius spp. and sediment cores were collected from three locations within the bay and fish gut contents were analysed to explore potential competition and ontogenetic dietary shifts. The most abundant prey at all sites was nematodes at 6·33 ± 0·38 cm⁻³ at Dunwich, 33·58 ± 0·26 cm⁻³ at Manly and 6·36 ± 0·849 cm⁻³ (mean ±s.e.) at Godwin Beach. Nevertheless, they were not a dominant component of the diets. Volumetric percent contribution of prey showed that copepods and decapod shrimps dominated F. lentiginosus diets at Dunwich (7·8 and 6·6%, respectively) and Godwin Beach (6·5 and 14·3%, respectively) and the diets of F. exquisitus at Manly (9·2 and 9·5%, respectively) and Godwin Beach (10·4 and 11·8%, respectively). Schoener's index of dietary overlap between the two species, when sympatric, was 0·85 indicating a high similarity. An ontogenetic shift towards larger prey items occurred as Favonigobius spp. reached larger sizes. Gut fullness indices showed significant differences between time of day (two-way ANOVA, P < 0·01) and species (two-way ANOVA, P < 0·05) but Bonferroni's multiple comparison test showed that the only significant difference was between F. lentiginosus at Dunwich and F. exquisitus at Godwin Beach at 1800 hours. Food resource competition and temporal resource partitioning did not appear to be a limiting factor between F. lentiginosus and F. exquisitus despite cohabitation in such restricted environments.