Variation and Ecological Drivers of Fish Biomass Estimated Acoustically In Two Inter-Connected Australian Estuaries

Spatial series of total fish biomass (TFB) were obtained in two estuaries of subtropical Australia using a split-beam sonar during four seasons. Logan River (main estuary) and Albert River (tributary) were surveyed upstream of their confluence. On most occasions, TFB was much higher in Albert River due to greater target strengths of larger fish. Discharge, salinity and pH were significantly lower, while phosphorus concentrations higher in Albert River. TFB was strongly negatively correlated to salinity for all data pooled for two rivers. TFB was positively correlated to discharge in the Albert River, but not in the Logan River. Turbidity was also correlated to TFB, but proved to be an insignificant predictor in stepwise multiple regression. We hypothesise that salinity was the major factor of fish distribution. While lower salinity of Albert River generally attracted more large fish, variation of TFB there was governed by discharge. Positive responses of TFB to discharge did not seem to depend on primary production, which seemed to be driven by light penetration rather then nutrients.