Oral Presentation Australian Society for Fish Biology Conference 2025

Negligible influence of terrestrial and freshwater sources on the diet of a coastal marine fish (Protonibea diacanthus) in the wet-dry tropics of northern Australia (125142)

Jo Randall 1 2 3 , David Crook 4 , Brien Roberts 1 , Alison King 5 , R Keller Kopf 2 , Karen Gibb 2 , Mirjam Kaestli 2 , Elizabeth Brewer 6 , Thor Saunders 4 , Brendan Adair 2 , Dion Wedd 4
  1. Fisheries Division, Northern Territory Department of Agriculture and Fisheries, Berrimah, NT, Australia
  2. Research Institute for the Environment and Livelihoods, Charles Darwin University, Casuarina, NT, Australia
  3. Australian Institute of Marine Science, Casuarina, NT, Australia
  4. Narrandera Fisheries Centre, Department of Primary Industries and Regional Development, Narrandera, NSW, Australia
  5. Environment, CSIRO, Albury, NSW, Australia
  6. Environment, CSIRO, Hobart, TAS, Australia

Energy and nutrients from rivers and coastal floodplains can influence the productivity and structure of coastal marine food webs. Hydrological variability affects the quality and quantity of freshwater/ terrestrial derived resources for marine consumers, hence shaping key biological processes. Understanding nutrient sources supporting key fishery species is integral to both fishery and water resource management. Here, we investigate the relative contribution of terrestrial, freshwater, and marine resources to diet in a commercially important Sciaenid, Protonibea diacanthus, in coastal regions of the wet-dry tropics of northern Australia. Bulk (δ13 C, δ34S, and δ15 N) and compound-specific essential amino acid stable isotopes (δ13 C) were analysed to examine food web structure at key P. diacanthus aggregation sites across seasons. The results indicate neither freshwater nor terrestrial nutritional sources contributed to diet, regardless of season or proximity to outflows during wet season. P. diacanthus appear to have strong reliance on marine-derived resources, mostly fishes, with the carbon source predominantly marine phytoplankton. There was also little evidence of freshwater influence in microbial assemblages at P. diacanthus aggregation sites, suggesting lower trophic levels were predominantly marine-derived. The lack of strong linkages between freshwater outflows and productivity in P. diacanthus populations contrasts studies of prawn and coastal finfish fisheries in the region. Our results emphasise the need for caution when drawing broad conclusions about influence of freshwater outflows on coastal fishery productivity and demonstrate the importance of spatial and species-specific factors on trophic patterns in coastal ecosystems.