IMAGE: Fieldwork on the Adelaide River. Catches of Speartooth Shark were highest in the Adelaide River, with the South Alligator River and Queensland’s Wenlock River also supporting reasonable catches. Image: Mat Gillfedder.
Populations of sawfishes and river sharks in the Northern Territory are thought to have declined dramatically in recent decades, raising concerns about their viability.
The Largetooth Sawfish, Pristis pristis, and the Dwarf Sawfish, P. clavata, are listed as Vulnerable under the Environmental Protection and Biodiversity Conservation Act 1999; the Speartooth Shark, Glyphis glyphis, is Critically Endangered, and the Northern River Shark, G. garricki, is Endangered. More information is needed about the distribution, ecology and population dynamics of these species to assist in their conservation, management and recovery. This project generated a better ecological understanding of the habitat use and habitat requirements, short and long-term movements, connectivity and spatial dynamics of these priority euryhaline elasmobranch species and collected tissue samples to enable abundance estimation with close-kin mark-recapture. The improved understanding has contributed to the multi-species Recovery Plan for Sawfish and River Sharks being developed by the Department of the Environment.
Video: River Shark Surveys of Kakadu’s West Alligator River
Fisheries-independent surveys in selected river systems were conducted using appropriate sampling gear (primarily gillnet and rod and line). Captured sharks were tagged and monitored with active and passive acoustic telemetry. Active tracking involves mounting a hydrophone to a tracking vessel to follow fish movement patterns and habitat use in real time (short-term telemetry). Passive tracking uses networks of moored acoustic receivers to detect tagged fish when they pass within range of a receiver (long-term telemetry).
Extensive arrays of acoustic receivers (130 receivers) were deployed in seven Northern Territory and Queensland river systems to provide long-term monitoring of tagged animals (some 400 individuals from 10 species tagged).
Mitochondrial genome sequencing of Speartooth Sharks and Largetooth Sawfish was used to help profile their population structure. The mitogenome, which is inherited through the mother, offers clues to how widely the females are dispersing to breed (for example, between river systems). Complete mitochondrial genome sequences have been published for the four sawfish and river shark priority species (as well as for a suite of other coastal and estuarine sharks and rays of northern Australia).
Largetooth Sawfish, P. pristis
Catch per unit effort (CPUE) was used to compare catches of fish species (such as the number of sawfish caught per 100 m of gillnet per day) between surveyed rivers. Low catch rates were recorded throughout the NT, although the CPUE of neonates (newborns) in the Adelaide River was comparable to the Fitzroy River (WA) which is a key nursery area for P. pristis. (Fitzroy River data published by Murdoch University.) Catches were dominated by sawfish less than one-year-old (the 0+ age-class). The very low CPUE of animals aged one year and above is of concern, suggesting that few juveniles survive more than a year. Floodplain billabongs were found to be nursery areas for juvenile P. pristis on the Daly River.
Analysis of mitochondrial DNA revealed strong population structuring of P. pristis across northern Australia: except for rivers flowing into the Gulf of Carpentaria, all river drainages appeared to host a genetically distinct population. This means there is limited capacity for the re-colonisation of localised populations that are depleted due to juvenile and adult mortality (at least for females at this stage).
Dwarf Sawfish, P. clavata
Limited numbers of P. clavata were recorded during the project, mainly due to a lack of fishing effort in its core habitat of coastal and estuarine areas (most sampling for sawfish was undertaken in mid-upstream reaches of rivers to target P. pristis). All known records of P. clavata in the NT were reviewed and mapped, but the species remains poorly known in the NT. Records from mid-reaches of rivers demonstrate the use of this habitat as nursery areas for juvenile fish.
Northern River Shark, G. garricki
Populations of G. garricki were found in the Van Diemen Gulf area (in particular Kakadu National Park’s Alligator Rivers region), and in more rivers than had been documented previously. Catch rates were high in all rivers, with more than 350 individuals recorded (before this project only eight records existed from the NT). This species’ distribution is still limited to less than 10 rivers and estuaries, however, and uncertainty remains over its abundance elsewhere in its range (such as the Kimberley region of Western Australia). The degree of population structuring between rivers and regions across its distribution is also unknown.
Movement data show highly seasonal distribution patterns within the South Alligator River (where 50 sharks were acoustically tagged for long-term telemetry studies). In the dry season, animals were located mainly 40–80 km upstream; in the wet they remained around the river mouth, with limited movement to adjacent rivers.
Speartooth Shark, G. glyphis
G. glyphis catches were highest in the Adelaide River, with the South Alligator River and Queensland’s Wenlock River also supporting reasonable catches. Movement patterns were similar to G. garricki with a downstream migration in the wet season and animals confined to narrow (20–40 km) stretches of river during the dry season.
Analysis of mitochondrial DNA revealed a strong catchment-level separation of all known Australian G. glyphis populations (Adelaide River, Alligator Rivers, Wenlock River) which has important management implications. Each population can be considered a separate management unit, with limited capacity of adjacent populations to repopulate another population were significant declines to occur.
New knowledge and opportunities
Research infrastructure and capabilities developed during this project will support ongoing monitoring and assessment, with acoustic receiver arrays established in the Adelaide and South Alligator Rivers providing an opportunity for long-term monitoring of sawfishes and river sharks. The existing G. garricki tissue sample collection provides the opportunity to assess population structuring across the range of this species (when combined with required surveys of further key rivers).
The occurrence and habitat of adult G. glyphis remains unknown. There is a need to explore how advancing tagging technologies could be applied to find adult populations. Large sub-adults (locatable in key rivers at certain times of year) could be fitted with satellite tags, and larger fishing gear such as set longlines could be trialled to catch (and then tag) adult females at pupping time. (Pupping period has been determined based on the presence of pups in rivers.)
The project demonstrated the benefits of combining the disciplines of field ecology and quantitative genetics. Recent developments in technologies for both disciplines have given rise to the quantitative information on population dynamics and habitat use that will guide management decisions affecting these rare species.
Number of samples collected for close-kin mark-recapture for NERP Marine Biodiversity Hub
Speartooth Shark (NT rivers) 233
Speartooth Shark (Wenlock River, Qld) 102
Largetooth Sawfish 65
White Shark 314
Northern River Shark (for future application) 365
Outputs and outcomes
This project generated improved understanding and knowledge to support the management and recovery of threatened sawfishes and river sharks. Expert advice was regularly provided to the Department. This included contributing to assessments of proposed developments for their potential impact on threatened species (referrals), and to developing a Recovery Plan for Sawfish and River Sharks.
Species information sheets were developed and published for the four species studied. A guide to the sharks of Kakadu rivers, and protocols for surveying and tagging sawfishes and river sharks were also published. More than 10 scientific manuscripts have been produced, and a greater number are being prepared or planned.