Biophysical drivers of spawning dynamics in estuarine fish

Determining links among estuarine variability, habitat and spawning of estuary-dependent fish is essential for understanding population processes and directing conservation efforts. I used acoustic telemetry, habitat mapping and egg sampling, in the Shoalhaven and Clyde rivers (New South Wales, Australia), to examine biophysical drivers of habitat use and spawning dynamics in estuarine-dependent fish:, Estuary Perch, Percalates colonorum, Sand Whiting, Sillago ciliata , Yellowfin Bream, Acanthopagrus australis, Black Bream, Acanthopagrus butcheri and the Yellowfin Bream and Black Bream hybrid complex. Spawning activity of Estuary Perch was restricted to areas of structurally complex large wooden debris and a concrete ferry landing, adjacent to deep water. Eggs were released at night during the first 2 h of the run-out tide.Spawning movements of Sand Whiting were typified by regular, swift migrations from upstream resident sites to specific areas adjacent to river entrances and deep water. Spawning movements coincided with high water temperatures linked to coastal winds and down-welling oceanographic conditions. Similarly to Estuary Perch, spawning occurred after nocturnal high tides. This strategy increases offshore dispersal and exports eggs and larvae onto coastal waters. At upstream residences, Sand Whiting displayed small core home ranges and high site fidelity to habitat characterised by sediment containing benthic invertebrates. These locations are likely used to maintain reproductive growth between spawning movements.Large-scale tracking of Yellowfin Bream, Black Bream and their hybrids showed high levels of residency and site-fidelity, with peak distributions occurring in the lower and upper-middle estuary. Estuarine movements were correlated with freshwater flow, temperature, and genetic classification. Distinct repetitive spawning migrations were not observed. The data suggests that spawning may occur within estuaries, with inter-estuarine migrations playing a significant role in genetic dispersal and mixing.This study highlights the complex interrelationships between movement, spawning, physicochemical variation and habitat availability. Spawning and recruitment success may be dependent on the selection of spawning locations and the timing of spawning events, which influences dispersal of eggs and larvae within and between estuaries, and can act to isolate populations or facilitate inter-estuarine connectivity and genetic relatedness