Opportunistic predation by small fishes on epibiota of jetty pilings in urban waterways

Epibiota were sampled on nine small jetties in the tidal, urban canals of south-east Queensland, Australia, to determine if the small fishes that are associated with these jetties prey on the epibiota on the pilings of the jetties and whether these fishes depend on the epibiota as a source of food. Epibiota was dominated by barnacles, filamentous and foliose algae and ranged in thickness from 4 to 11 mm. The two species of fishes that associated most closely with jetty pilings, Pandaka lidwilli (Gobiidae) and Monodactylus argenteus (Monodactylidae), were sampled twice during the day and twice during the night for analysis of stomach contents. During the day, the diet of P. lidwilli was dominated by amphipods (c. 70%, by mass of organic content), with copepods, bivalves and bryozoans each contributing <10%. At night, amphipods contributed less (c. 45%) and copepods more (c. 35%). The diet of M. argenteus was dominated by filamentous algae (55%) and amphipods (20%) during the day and filamentous algae (70%) and barnacle cirri (23%) at night. Epibiota, therefore, made a substantial contribution to the diet of the fishes but were not the sole source of food for either species. As jetties were the only structures that supported epibiota in the area, fishes probably sourced their epibiota from the pilings of the jetties. Whether fishes depended on the epibiota was, therefore, tested using a manipulative before-after-control-impact (BACI) study. Three jetties were assigned randomly to each of three treatments: (1) epibiota removed from pilings, (2) epibiota cut and damaged (a procedural control) and (3) epibiota left undisturbed. Abundances of P. lidwilli and M. argenteus around jetty pilings remained similar across all treatments from before to after the removal of epibiota. These results indicate that although fishes consumed epibiota on the jetties, they did not depend on the epibiota of the jetties for food.