Inter-decadal variation in diadromous and potamodromous fish assemblages in a near pristine tropical dryland river

Freshwater ecosystems are both incredibly biodiverse and highly threatened globally. Variation in environmental parameters including habitat and flow can substantially affect many ecological processes within riverine aquatic communities, but the ties between such parameters and ecology are neither well studied nor understood. In highly variable tropical dryland river systems, assessing such relationships requires data collection over inter-decadal time scales, which is not typically permitted on development schedules driven over short periods (including election and funding cycles). Here, we used seine net sampling data collected over an 18-year period in the tropical dryland Fitzroy River, Western Australia, to assess how environmental and temporal factors including habitat, seasonality, and inter-annual variation in wet season magnitude affect the community assemblage structure, recruitment, and growth of aquatic species in dryland rivers. Results demonstrated that macrohabitat (main channel vs floodplain creek) and the magnitude of wet season rains and resultant flooding both had a substantial influence on biotic communities, alongside seasonal and diel variation. The magnitude of wet season flooding (measured as river discharge volume) had the greatest impact on assemblage composition within floodplain creek habitats and was a significant driver of recruitment rates and growth of recruits and adults of several species examined. This study highlights key considerations for conserving dryland river systems and constituent biota. Specifically, these are maintaining (a) rhythmicity of flow within each year, (b) diversity of flow volume between years, and (c) a variety of habitat types including ephemeral, semi-permanent, and permanent shallow floodplain and deeper main channel pools, in order to support a diverse array of generalist and specialist diadromous and potamodromous fishes

Recruitment of a critically endangered sawfish into a riverine nursery depends on natural flow regimes

The freshwater sawfish (Pristis pristis) was recently listed as the most Evolutionarily Distinct and Globally Endangered (EDGE) animal. The Fitzroy River in the remote Kimberley region of north-western Australia represents a significant stronghold for the species, which uses the freshwater reaches of the river as a nursery. There is also mounting pressure to develop the water resources of the region for agriculture that may substantially affect life history dynamics of sawfish in this system. However, the relationship between hydrology and population dynamics of freshwater sawfish was unknown. We used standardized catch data collected over 17 years to determine how wet season volume influences recruitment of freshwater sawfish into their riverine nursery. Negligible recruitment occurred in years with few days of high flood levels (above 98th percentile of cease-to-flow stage height), and relatively high recruitment occurred in years with 14 or more days of high flood levels. This relationship is indicative of a distinct boom-or-bust cycle, whereby freshwater sawfish rely almost entirely on the few years with large wet season floods, and the brief periods of highest water levels within these years, to replenish juvenile populations in the Fitzroy River nursery. This has direct implications for sustainable water resource management for the Fitzroy River basin in order to preserve one of the last known intact nursery habitats for this globally threatened species