Effects of freshwater flow on the year-class strength of a non-diadromous estuarine finfish, king threadfin (Polydactylus macrochir), in a dry-tropical estuary

Abstract

The year-class strength of the commercial catch of king threadfin (Polydactylus macrochir (Gunther, 1876)) was correlated with freshwater flows into a dry-tropical estuary over five consecutive years. The year-class strength of king threadfin, a non-diadromous estuarine species, fluctuated and correlated significantly with freshwater flow and coastal rainfall in spring and summer; a result similar to that found for the catadromous barramundi (Lates calcarifer) within the same estuarine system. All sub-sets general linear models were used to screen relationships between year-class strength and freshwater variables. King threadfin spawn from spring to summer in north-eastern Australia, when hydrological conditions adjacent to estuaries have high salinities and are optimal for egg and post-larval survival. Young-of-the-year enter estuaries during the wet season, enabling them to take advantage of salinity gradients and the seasonal blooms in prey species such as Acetes spp. and juvenile penaeids that are accentuated in wet years. Freshwater flows in spring and summer are important drivers of the year-class strength of estuarine finfish, and reduction in these flows, through the development of water infrastructure and abstraction or long-term climate change, will potentially reduce the size of the population of estuarine fish available for human harvest.