Gut content- and stable isotope-derived diets of four commercially and recreationally important fish species in two intermittently open estuaries

Despite remaining closed for variable periods, intermittently open estuaries provide habitat for estuarine and marine fish species of commercial and recreational value. To better understand how these systems trophically support their fish assemblages, the diets of four valued fish species, namely Acanthopagrus australis, Platycephalus fuscus, Sillago ciliata and Mugil cephalus, were examined in two intermittently open estuaries in New South Wales, Australia. Fish diets were determined using both gut contents and stable isotope analyses because the different temporal resolutions afforded by these methods can provide insight into the flexibility of fish diets. Stable isotope signatures of prey and fish proved to be particularly useful in analyses of the diets of M. cephalus and P. fuscus, because these species consume large quantities of unidentifiable organic matter and have high incidences of empty guts respectively. Diet reconstructions across methods were generally consistent for A. australis, but differed substantially for S. ciliata, with fewer prey taxa identified in the guts than expected. This result suggests that individual S. ciliata switch between local resources on the basis of their fluctuating temporal availability. Trophic flexibility, coupled with broad physicochemical tolerances, enables these species to flourish in the challenging environment of intermittently open estuaries.