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.

Understanding growth rates and other basic life-history information of imperilled species is essential to assessing the extent of threats to a population, but often difficult due to limited access to study subjects. Here we used mark-recapture data to estimate growth rates of juvenile Critically Endangered green sawfish (Pristis zijsron) in a globally important nursery in the eastern Indian Ocean (Western Australia). Our results suggest that growth of juvenile sawfish in this part of the central Western Australian coast is significantly slower compared to populations on the north-eastern coast of Australia. Additionally, growth rates differed between nearby areas within the nursery region, potentially due to differential productivity or anthropogenic effects. Morphological relationships between total length, rostral length, mouth gape, and clasper length are presented, which will allow for greater accuracy in estimating biological parameters in this species, while updated information on size at maturity (> 3200 mm) and size at birth (approximately 750–900 mm) will help to clarify life-history parameters for this data-poor species. Furthermore, there were distinct differences in the number of rostral teeth of green sawfish between this eastern Indian Ocean population and other populations throughout their current distribution, indicating substantial genetic differentiation in this species globally. These results will help to accurately assess growth trajectories and potential impacts of fisheries and other threats to green sawfish. Additionally, results highlight the importance of assessing population-specific growth rates in threatened species and of considering potential long-term life-history impacts of anthropogenic developments and activities.

1. Sawfishes (Family: Pristidae) are one of the most imperilled fish families worldwide. There is an increasingly urgent need to better understand the biology, ecology, and population status of the five sawfish species to develop more effective conservation measures. The dwarf sawfish, Pristis clavata, is one of the least researched members of the pristids, with literature limited to analysing disparate datasets or collations of rare encounters in northern Australia.2. This study examined the spatial ecology of dwarf sawfish using targeted surveys and acoustic telemetry to determine its habitat use in a macrotidal estuary in northern Australia. Seventeen dwarf sawfish were tagged with acoustic transmitters and monitored in the Fitzroy River estuary and adjacent King Sound (Kimberley, Western Australia) between August 2015 and November 2017.3. Dwarf sawfish observed within the Fitzroy River estuary and King Sound were juveniles, ranging between 740 and 2,540 mm in total length. Catch per unit effort of dwarf sawfish in the late dry season was relatively high in the estuary, with the catch rate in 2015 being one of the highest reported for any sawfish species.4. Acoustic detections revealed a distinct seasonal pattern in the use of different parts of the estuary and King Sound, which was found to be driven by salinity.Dwarf sawfish predominately occupied a single large pool near the terminus of the tidal limit in the late dry season (August-November), before transitioning to regions in closer proximity to the river mouth or in King Sound in the wet and early dry seasons (December-July).5. Given the high abundance and residency of dwarf sawfish in the Fitzroy River estuary, this area is an important nursery for the species during the late dry season and should be formally recognized as a habitat protection area for the species.

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.

A new microbothriid monogenean Dermopristis pterophilus n. sp. is described from the skin of the Critically Endangered green sawfish Pristis zijsron Bleeker, 1851 in the Ashburton River delta, northern Western Australia. Analyses of the 28S ribosomal DNA marker and the molecular barcoding markers Histone 3 and Elongation Factor 1 α confirmed position among the Microbothriidae, with close affinity to the only other sequenced representative of Dermopristis Kearn, Whittington and Evans-Groing, 2010. The new species is morphologically consistent with the concept of Dermopristis ; it has two testes, lacks a male copulatory organ and has a simple haptor. It is smaller than its two congeners D . paradoxus Kearn, Whittington and Evans-Gowing, 2010 and D . cairae Whittington and Kearn, 2011 and is most similar to the former, distinguished only in that it lacks the strong, transverse, parallel ridges on the ventral body surface that characterise that species. It is more easily distinguished from D . cairae , differing in body shape, possession of a seminal receptacle, and relative position and size of the haptor. It may further differ from both species by fine details of the gut diverticula, although these details are difficult to ascertain. Spermatophores were observed in the new species, similar to those previously reported for D . cairae . The new species exhibits site attachment preference: infections were greatest on and immediately adjacent to the host pelvic fins (including male reproductive organs, i.e. claspers), moderate in proximity to the dorsal and pectoral fins, few on the caudal fin and peduncle, and infrequently, isolated worms occurred elsewhere on the dorsal and ventral surfaces of the body. There was no incidence of infection on the head (including rostrum). We presume D . pterophilus is restricted to P . zijsron and thus likely faces the same threat of extinction.