Predictable hydrology, habitat and food resources determine fish recruitment dynamics in an incised tropical Australian river

The critical role of hydrology in tropical floodplain river systems is well established, but there is limited information on the drivers of larval and juvenile recruitment of freshwater fishes in small tropical rivers. Herein, we describe the patterns of occurrence and abundance of fish larvae, juveniles and adults in the lower reaches of a short, deeply incised coastal river of the Australian Wet Tropics. We expected that several environmental factors, including site proximity to the coast and flow and salinity regimes, would influence the life-history strategies, distribution and abundance of larval and juvenile fishes. Study sites contained the larval stages of 23 of the 37 native species present, including nine species for which there are no previous larval records from Wet Tropics rivers or elsewhere. Larvae and juvenile fishes of most species exhibited maximum abundance in the benign but productive dry season, but some diadromous species showed peak abundance in tidal riverine/estuarine waters during the wet season. Most species exhibited peak larval abundance during the same months of Years 1 and 2 under similar hydrological, physicochemical water quality and habitat conditions in both years. These results are summarised in a conceptual model depicting how seasonal hydrology interacts with channel and habitat characteristics to influence life-history strategies and hence spatial and temporal variations in larval fish abundance in Wet Tropics rivers. Our results highlight the importance of seasonal contrasts in flow, habitat structure, water quality and food resources in supporting the variety of life-history strategies in the fish assemblages.

Automated summary

This study investigates the occurrence and abundance patterns of fish larvae, juveniles, and adults in the lower reaches of a short, deeply incised coastal river in the Australian Wet Tropics. The research finds that most species exhibit peak larval and juvenile abundance during the benign dry season, but some diadromous species show peak abundance in tidal riverine/estuarine waters during the wet season. The results highlight the importance of seasonal hydrology and habitat characteristics in influencing the life-history strategies and abundance variations of larval fish in tropical rivers.