Assessment of Dietary Sources in a Tropical Coastal Fish (Blackspotted Croaker Protonibea diacanthus)

Abstract Energy and nutrients from rivers and coastal floodplains are widely recognised as key drivers of productivity in coastal marine food webs, but these inputs may increase productivity for some species while having negligible or negative effects on others. Understanding the sources of nutrition supporting commercially important fishes is therefore critical for effective management. Here, we quantify the relative contribution of terrestrial, freshwater, and marine resources to the diet of the marine sciaenid Protonibea diacanthus in the wet–dry tropics of northern Australia. Bulk stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S) and compound-specific stable isotope analysis of essential amino acids (δ¹³C) were used to resolve trophic pathways, alongside microbial source tracking to assess the influence of freshwater and marine water sources at aggregation sites. Across all approaches, results consistently indicated a dominant reliance on marine-derived resources. Mixing models identified marine sources, particularly marine fish, as the primary contributors to diet, while CSIA-AA independently demonstrated that carbon supporting P. diacanthus was overwhelmingly marine in origin. There was little evidence of freshwater or terrestrial influence in either trophic pathways or microbial assemblages, regardless of season or proximity to river outflows. These findings contrast with studies of other tropical fisheries species that show strong positive relationships with freshwater flow, and suggest that the nutrition of P. diacanthus is supported primarily by marine trophic pathways rather than riverine subsidies. This study highlights the need for caution when generalising the role of freshwater inputs across species and emphasises the importance of species-specific trophic dynamics in coastal ecosystem management.