Associations between body condition and seasonal freshwater outflow in a coastal marine fish (Protonibea diacanthus) in the wet-dry tropics of northern Australia

Coastal tropical fisheries can be profoundly affected by freshwater flows, with hydrological variability influencing the quality and quantity of nutrition available to marine consumers, and shaping key biological processes such as recruitment, growth and population productivity. In this study, we examined a range of condition metrics (Fulton’s Condition Factor, RNA:DNA, lipid content, gonado-somatic index (GSI), hepato-somatic index (HSI)) from sampled fish to examine the drivers of population productivity and individual health of the commercially important Sciaenid fish, the black-spotted croaker, Protonibea diacanthus, in the wet-dry tropics of northern Australia. The results showed strong seasonal variation in body condition metrics, with a broad trend for lower body condition, HSI and RNA:DNA, and higher tissue water content in the wet season (Dec-Mar) than in the dry (May-Jul) and ‘build-up’ (Sep-Nov) seasons. These results contrast with findings for other key fishery species in the region whose health and condition respond positively to high rainfall and river discharge during strong monsoonal wet seasons. This study cautions the use of broad conclusions about the importance of linkages between freshwater outflows and coastal fishery productivity, without considering spatial and species-specific responses to seasonal variability.