Observations of the Threatened Australian Snubfin Dolphin ( <scp> <i>Orcaella heinsohni</i> </scp> ) in the Roper River, Northern Territory, Australia

Australian Snubfin Dolphins (Orcaella heinsohni) are known to occur in shallow coastal waters close to river and creek mouths throughout Queensland (QLD), the Northern Territory (NT), and northern Western Australia (WA), as well as southern Papua New Guinea (Parra et al. 2002; Beasley et al. 2002; Beasley, Jedensjö, et al. 2025). The species was previously thought to be synonymous with the Irrawaddy Dolphin (O. brevirostris) distributed throughout Asia, from the Bay of Bengal through to Java in Indonesia, until taxonomic delineation in 2005 (Beasley et al. 2005). The Australian Snubfin Dolphin (hereafter, “snubfin dolphin”) is currently listed as Vulnerable under the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species (Parra et al. 2017; IUCN 2025), and under the Australian Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act; DCCEEW 2025). Conservation concern for O. heinsohni stems from its small, fragmented population of less than 10,000 mature individuals which is distributed in groups of 50–200 individuals, loss and degradation of habitat by humans, habitat pollution, and incidental capture in recreational and commercial fishing gear (e.g., Brown et al. 2016; Parra et al. 2017; Beasley, Amepou, et al. 2025; DCCEEW 2025). Recent research has improved our understanding of abundance and connectivity, with genetic analyses demonstrating limited gene flow across some populations in WA despite relatively small geographic distances (Brown et al. 2017; Chabanne et al. 2021). Local abundances of O. heinsohni are small, for example ranging from 50 to 130 in Cygnet and Roebuck Bays, respectively, WA (Brown et al. 2016), 19–70 in the Darwin region, NT (Brooks et al. 2017), and less than 90 in the Fitzroy River, QLD (Cagnazzi et al. 2013). Gaps remain in our knowledge of the abundance and population structure of snubfin dolphins at local scales across their range (Brown et al. 2017). Snubfin dolphins are long-lived (at least 28–30 years), with late maturity (8–10 years) and low reproductive rates, producing one calf every 2–5 years (Moore 2015). A preference for shallow waters close to river mouths, potentially due to abundance of resources, may increase the risk of encounters with large predators such as sharks, as well as increasing proximity to threatening anthropogenic activities (Parra 2006; Nicholls et al. 2023). Snubfin dolphins also have a complex social structure, driven primarily by strong associations between individuals and preferred companionships (Parra et al. 2011). They demonstrate site fidelity and residency over both long- (years) and short-terms (months; D'Cruz et al. 2022), suggesting that localized threats may have a substantial impact on individuals. Their life history, habitat use, and behaviours may therefore contribute to their Vulnerable IUCN conservation status. The Roper River in the south-west Gulf of Carpentaria, NT (Figure 1), is one of the largest river systems in northern Australia, with a catchment of over 80,000 km2 (Faulks 2001; CSIRO 2023). Unlike most rivers in the NT, the system is perennial (permanently flowing), fed by groundwater discharge in the dry season (roughly, May–October), and by highly variable monsoonal rainfall in the wet season (November–April; Petheram et al. 2008; CSIRO 2023). Tidal influence reaches 150 km upstream, while groundwater baseflows extend 200 km upstream of the tidal limit (Faulks 2001; CSIRO 2023). Flow is currently unregulated; however, there is considerable interest in substantially increasing the use of ground- and surface water from the Roper River catchment for the development of agriculture and aquaculture in the region (CSIRO 2023). During shark and ray surveys (Constance et al. 2024), snubfin dolphins were sighted in the Roper River over two six-day periods in October and November 2024 (8–13 October, 5–10 November; Figure 2). Our research vessel was anchored with the engine off, often for > 9 h per day. This allowed for opportunistic observations of dolphins, their behaviour, and for photographs to be taken to allow individual identification, from which we identified thirteen snubfin dolphins (Figure 2). Dolphins were regularly seen (almost every day) travelling between ~22 and ~38 km upstream from the mouth of the Roper River (Figure 1) and were also frequently observed foraging at the Roper/Phelp confluence, 34 km upstream from the mouth, where photographs were taken and water quality data were collected periodically. Here, snubfin dolphins were observed in brackish (~12–16 psu), warm (30.2°C–31.7°C), often turbid (39.8–90.0 NTU [New Turbidity Units]) water. Individual dolphins were identified from approximately 200 high-quality photographs based on distinguishing features on the leading and trailing edges of the dorsal fin such as nicks, notches, and marks, as well as overall fin shape (see Figure 2; e.g., Hammond and Thompson 1991; Parra and Corkeron 2001). Several videos were also recorded, however, lower footage quality resulted in only one identification. Of thirteen individuals identified, four (30.7%) were observed in both October and November, and an additional one individual was observed on multiple days (within the same month). Sightings and re-sightings may be limited due to only one high quality camera being present, limiting the number of dolphins which could be photographed at a time. Foraging behavior, as defined by Parra (2006), was regularly observed, and prey species such as catfish (Neoarius spp.) were observed being carried and tossed at the water surface. Additionally, dolphins were observed and filmed spitting water towards the riverbank, potentially another foraging tactic used to herd fish, as has also been seen in O. brevirostris (e.g., Stacey and Hvenegaard 2002; Azizul et al. 2022). Dolphins were regularly observed foraging in a relatively deep section (~15 m) at the Roper/Phelp confluence which often held schools of bottom-dwelling fish, as well as moving upstream in the Phelp River. On October 12, 2024, snubfin dolphins were observed possibly foraging on an outgoing tide approximately 64 km upstream from the mouth, and approximately 600 m downstream of the lower end of Green Island (Figure 1). Water at this location was 30.8°C, relatively turbid (39.8 NTU), and fresh (4.7 psu). A group of 4–5 dolphins, including a calf, was also recorded 56 km upstream at Blackfella Creek in the Roper River in October 2017 (M. Lee & P. Sainsbury; ALA 2025). Field observations in the Roper River provide an insight into the occurrence of snubfin dolphins in the river and its tributaries, where the species was previously recorded up to 20 km upstream (Palmer et al. 2014). Snubfin dolphins have previously been observed in the East and South Alligator Rivers in Kakadu National Park in turbid waters up to 50 km upstream (Palmer et al. 2014); however, the tidal reaches of these rivers (103 km in the South Alligator River) are well-mixed due to large tides (Vertessy 1990). It is therefore likely that dolphins extending 50 km upstream in the Alligator Rivers experience higher salinity than those observed near Green Island in the Roper River. The Roper River experiences smaller tidal ranges and has freshwater input from groundwater throughout the dry season, indicating that dolphins in this system may regularly occur in low salinity levels. The health of dolphins in this system should be monitored into the future to determine any potential outbreaks of diseases such as freshwater skin disease, which has been documented in Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) occupying lower salinity environments (Duignan et al. 2020). Sightings of snubfin dolphins upstream in the Roper River pose conservation implications given interest in the development of water resources in the region (CSIRO 2023). Due to limited survey and photographic effort, long-term monitoring and dedicated research is required to confirm dolphin abundance and spatial and temporal extent of use of the river system. Small subpopulations restricted to riverine environments have been found in this species, with decreased genetic diversity indicating poor connectivity between riverine and marine subpopulations (Minton et al. 2017; Dai et al. 2021). Riverine populations are considered to have an increased risk of extinction (Minton et al. 2017), as has been demonstrated in the Mekong River, Cambodia, where the Irrawaddy dolphin population is under serious threat of extirpation due to its small, declining population with reduced genetic diversity (Krützen et al. 2018). While individuals utilizing the Roper River may be connected to a larger population which also uses coastal waters, geneflow throughout the wider region may be limited (Brown et al. 2017; Bouchet et al. 2021; Chabanne et al. 2021). Snubfin dolphins in the Roper River may therefore be sensitive to population decline as a result of anthropogenic impacts such as habitat loss and degradation, and bycatch and entanglement in fishing gear (DCCEEW 2025). The Roper River contains habitat which may be critical to the survival of this population of snubfin dolphins given its shallow waters and high abundance of prey (DCCEEW 2025). The Roper River is facing intense interest in the extraction of groundwater, including the Georgina Wiso and Mataranka Water Allocation Plans. Combined, these plans may allow the extraction of 272,474 megalitres per year from the regional Cambrian Limestone Aquifer (CLA) (NTG 2023; NTG 2024) which supplies baseflows to the Roper River (CSIRO 2023). These allocations may cause a substantial decrease in discharge to the Roper River, driving loss of connectivity, alteration of magnitude, timing, and duration of high and low flow events, and productivity (CSIRO 2023). The availability of prey consumed by snubfin dolphins may also be reduced by water extraction (see O'Mara et al. 2023 for review). Persistence of dolphins in rivers can be significantly impacted by water extraction, causing habitat fragmentation and changes to flow regimes (Smith and Smith 1998). In O. brevirostris, large gaps in the occurrence of individuals within their range have been attributed to a lack of freshwater input, indicating local extirpations due to water extraction and environmental change (Minton et al. 2017). Bycatch and entanglement in fishing gear pose a significant concern for the conservation of O. heinsohni given its small, fragmented populations throughout its range (DCCEEW 2025). For O. brevirostris, entanglement in fishing gear, primarily gillnets, is considered the most severe threat to the species (Minton et al. 2017). Common bottlenose dolphins (Tursiops truncatus) are incidentally captured in gillnets in the inshore NT Barramundi Fishery (DAFF 2023), which regularly operates at the Roper River mouth. There are no bycatch data for O. heinsohni in the NT; however, capture as bycatch is likely given capture of T. truncatus, their preference to occur close to river mouths (Parra et al. 2006), and capture and mortality of O. heinsohni in similar fisheries in Papua New Guinea (Beasley, Amepou, et al. 2025) and QLD (DCCEEW 2025). Due to their slow population recovery rates resulting from conservative life history characteristics (Moore 2015), snubfin dolphins are highly vulnerable to population decline as a result of mortality in fisheries. The loss of a single individual every 2.7 years from anthropogenic threats is considered unsustainable (PIDU Project 2022). Further research in the Roper River is required to understand the potential impacts of anthropogenic threats. Orcaella heinsohni is a threatened species with limited connectivity between populations, and limited data on its use of river systems. Observations in the Roper River and its tributaries, including the Phelp River, extend our knowledge of the occurrence of snubfin dolphins to 64 km upstream, near Green Island. Dolphins were observed foraging in a range of conditions, from brackish to freshwater, and in turbid water. As development in the Roper region progresses (CSIRO 2023), dolphins occupying the river system may be affected by changes to their environment (DCCEEW 2025), as well as being vulnerable to entanglement in fisheries. As a priority, further research and targeted surveys in the Roper River and its tributaries are critical to developing a thorough understanding of the abundance, occupancy (including whether river occurrence is seasonal or year-round), and connectivity with other populations of snubfin dolphins to ensure the survival the population into the future. Julia M. Constance: conceptualization, funding acquisition, writing – original draft, visualization, writing – review and editing, investigation. Riley D. Farris: investigation, writing – review and editing, visualization. We acknowledge the Traditional Custodians of the lands and waters on which this research was undertaken, including the Marra, Yugul, and Mangarrayi peoples, and acknowledge the ongoing connection to culture that the Roper River and its catchment represent. We thank the Yugul Mangi Rangers for their ongoing support of our research. We also thank Erica Garcia for support and advice during the preparation of this manuscript. Open access publishing facilitated by Charles Darwin University, as part of the Wiley - Charles Darwin University agreement via the Council of Australasian University Librarians. Dolphins were sighted opportunistically during shark surveys funded by the Department of Climate Change, Energy, the Environment and Water, Ocean Leadership Package for the Threatened and Migratory Species Fisheries Bycatch Mitigation Program. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.