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Impacted fish movement into tropical freshwater wetlands discharging water low in dissolved oxygen; the tail of two wetlands

Coastal freshwater wetlands are important ecosystems which support myriad of plants and animals. In north-eastern Australia freshwater wetlands comprise a mixture of natural and modified habitats. Ponded pastures are a common type of modified wetland in central Queensland and were created to provide improved grazing pastures for cattle. The wetlands were formed when earthen bunds were constructed across portions of upper estuaries and lower flood plains. Despite the modified nature, ponded pastures provide the same ecological services as natural freshwater wetlands. The extensive nature of many ponded pastures often make them the largest freshwater wetlands within a catchment. Like natural freshwater wetlands, ponded pastures are commonly impacted by poor water quality, in particular low dissolved oxygen (DO). Low DO levels have been linked to fish kills within wetlands, it has also been suggested that fish are unlikely to move up flow paths that are low in DO. Concurrent monitoring of fishways at two ponded pastures within the same catchment identified a considerable disparity in catch rates and species composition. Fishways were monitored at Tedlands and Boundary wetlands, central Queensland, between 2016 and 2018. Pooled average daily catch rates from Boundary were 770 fish/day, while Tedlands recorded 290 fish/day. A total of 21 species were captured from Boundary, while 12 species in total were recorded in captures from Tedlands. The DO (% saturation) concentrations of water exiting Boundary ranged from 13.0% - 128.2%, while at Tedlands DO levels ranged from 0.6% - 21.6%. Temperature, pH, and conductivity were relatively consistent between the two wetlands. Our observation support suggestions that low DO concentrations in water exiting freshwater wetlands negatively impact fish movement. Given that coastal freshwater wetlands provide critical habitat for many important socio-economic species and provide primary habitat for many others, it is imperative that causes of low DO be addressed.