Many marine species undertake extensive oceanic and continental-scale movements, which are influenced by a variety of biological and environmental factors, often related to resource needs (e.g., Block et al., 2011; Espinoza et al., 2016). In species exhibiting large-scale movements, local or regional management measures may not provide adequate protection if the species is exploited in other regions. These species are also more likely to move between different jurisdictions, further complicating management and conservation efforts (Heupel et al., 2015). Defining the extent and occurrence of long-range movements is therefore necessary for a comprehensive understanding of a species’ spatial ecology and, within a resource management context, for determining the extent of connectivity among stocks and ensuring sustainable resource use (Lascelles et al., 2014). Various approaches have been applied to define movements and connectivity between stocks: genetics (e.g., Junge et al., 2019), body morphology (e.g., Turan, 2004), microchemistry (Izzo et al., 2016; McMillan et al., 2018), parasites (e.g., Barton et al., 2018), and tracking (e.g., Block et al., 2011; Hussey et al., 2015). For fisheries management, there is a need to ensure that stock delineation and connectivity are measured at the same temporal scale at which movements occur, since fisheries management operates at the ecological rather than the evolutionary scale. Genetic methods can struggle to distinguish the degree of connectivity below which separate stock management is required if migration rates are high (Waples et al., 2008), because demographically-independent populations might still be connected genetically over many generations (Ovenden, 2013). Results from tracking studies might therefore be more relevant to fisheries management, in determining whether stocks are demographically-independent units that should be managed separately (Lédée et al., 2021).

There is an increasing number of acoustic tracking networks worldwide, including in North America (Pacific Ocean Shelf Tracking – POST, Florida Acoustic Cooperative Telemetry – FACT, Integrated Tracking of Aquatic Animals – iTag), South Africa (Acoustic Tracking Array Platform – ATAP), and Europe (European Tracking Network – ETN), with most of these large-scale arrays being affiliated to the global Ocean Tracking Network (OTN; O’Dor and Stokesbury, 2009). In Australia, the Integrated Marine Observing System (IMOS) Animal Tracking Facility enables large-scale collaborative animal tracking research through the deployment of continental-scale curtains and grids of IMOS-owned acoustic receivers. The IMOS Animal Tracking Facility network is complemented by a large number of independent, project-based, non-IMOS acoustic receivers that are deployed by individual researchers and research teams to address regional research needs. All IMOS data and the voluntarily supplied detections from non-IMOS receivers are hosted by and publicly accessible through the Australian Ocean Data Network (AODN) and the IMOS Animal Tracking database (Hoenner et al., 2018)¹. These data enable large-scale studies of animal movements (Heupel et al., 2015), the ability to reveal intra-specific differences in movement profiles and site residency of a wide range of species (Brodie et al., 2018), and to determine how changes in human activity impact animal populations during global disruptions such as the COVID-19 pandemic (Huveneers et al., 2021). The IMOS Animal Tracking Facility provides an opportunity to quantify the extent of movement and connectivity of marine species and determine the most suitable spatial scale for stock assessments and management purposes (Lédée et al., 2021).

The bronze whaler Carcharhinus brachyurus and the dusky shark Carcharhinus obscurus are globally distributed species which are commercially and recreationally targeted in many parts of their distributions, as well as being taken with other more productive shark species in mixed-species fisheries (see Rogers et al., 2013a; Bradshaw et al., 2018). Bronze whaler and dusky shark have life history traits that make them highly susceptible to overexploitation (e.g., slow growth, late age-at-maturity) (Romine et al., 2009; Rogers et al., 2013a; Drew et al., 2017). At a global level, both species are of conservation concern, with the bronze whaler listed as Vulnerable by the IUCN Red List due to declining population trends over most of its range (Huveneers et al., 2020) and the dusky shark listed as Endangered due to a global population reduction of ∼72% (Rigby et al., 2019; Pacoureau et al., 2021). In Australia, bronze whaler and dusky sharks are commercially targeted in New South Wales (NSW), South Australia (SA), and Western Australia (WA) (Simpfendorfer, 1999; Macbeth et al., 2009; Rogers et al., 2013a). In NSW, mostly adult dusky sharks are caught by longline fisheries (Macbeth et al., 2009; Pleizier et al., 2015; Barnes et al., 2016). Catches in NSW appear to have declined due to management action and reduced fishing effort in recent years. In SA, there are no species-specific regulations managing commercial catches of bronze whalers and dusky sharks. However, these species are managed under input controls, with measures aimed at limiting fishing effort and mortality of large mature individuals. These include limits on the daily number of hooks that can be set (200 hooks), on leader diameter for longlines (2 mm), and mesh size restrictions for demersal gill nets (150 mm). Bronze whalers and dusky sharks (reported together as whaler sharks) are considered secondary species, with sustainability of the fishery and stock status assessed using performance indicators. Demographic models suggest that current catch levels of bronze whaler are likely to be sustainable, but that population decline could occur if total catches increase (Bradshaw et al., 2018). However, the need to differentiate the two species remains a key uncertainty in estimating current levels of fishing mortality. The status of the SA whaler shark fishery is therefore classified as undefined (Steer et al., 2020). In WA, due to the rapid increase in shark catches between the mid-1970s and 1990s and other sustainability concerns, a range of management measures have been introduced since the early 1990s (limited entry, effort limits, a maximum size limit, state-wide commercial protection of sharks in most non-target fisheries, prohibition of metal trace wire and large hooks, mesh-size limits in target gillnet fisheries, spatial closures, and recreational bag limits) (Braccini et al., 2021). Weight of evidence assessments are conducted on four indicator species every 5 years, including dusky sharks but not bronze whalers. Stock status of dusky sharks is sustainable – recovering with current management arrangements considered suitable to allow the gradual recovery of the breeding stock from historic overfishing (Braccini et al., 2021). However, dusky shark stock assessment only includes WA catches (Braccini et al., 2021).

Genetic studies have suggested large-scale stock segregation of bronze whalers between Australia–New Zealand, South Africa–Namibia, and Peru (Benavides et al., 2011b), and some delineation within Australia between WA and the rest of the Australian population (Junge et al., 2019). Dusky sharks are currently considered to have an eastern and western stock in Australia, with conventional and electronic tracking showing that dusky sharks move between SA and WA (Rogers et al., 2013b), and genetic analyses suggesting restricted gene flow between eastern and western Australia (Geraghty et al., 2014). However, previous and recent genetic analyses have also proposed panmixia within Australia (Ovenden et al., 2009; Benavides et al., 2011a; Junge et al., 2019). The appropriate spatial scale to assess and manage bronze whaler and dusky shark stocks, and the extent of their movements across each species distribution are still unknown.

We investigated the broad-scale movements and levels of connectivity among bronze whalers and dusky sharks in Australian waters between WA, SA, Victoria (VIC), Tasmania (TAS), NSW, and Queensland (QLD), a spatial scale of 7,300 km, using the network of acoustic receivers facilitated through the IMOS Animal Tracking Facility. We also assessed the benefits of the IMOS Animal Tracking Facility by comparing our ability to detect cross-jurisdictional movements using receivers from the IMOS Animal Tracking Facility (IMOS receivers), community-owned receivers (non-IMOS receivers), or a combination of both. The results from this study provide movement information over spatial and temporal scales relevant to fishery management decisions (Crossin et al., 2017) and insights into the importance of having a continental-scale network of acoustic receivers to support the assessment of species with broad movement patterns for effective fisheries management.

A total of 210 sharks (117 bronze whalers and 93 dusky sharks) were tagged between 31 January 2010 and 19 October 2016 in NSW, SA, and WA (Table 1) and tracked until 5 November 2020. Based on known size-at-maturity (Last and Stevens, 2009; Drew et al., 2017), most tagged bronze whalers (mean ± standard deviation: 200.9 ± 86.3 cm total length) and dusky sharks (279.9 ± 60.7 cm total length) were immature. Similar numbers of bronze whalers were tagged in WA (51.3%; N = 60) and SA (48.7%; N = 57), whereas most dusky sharks were tagged in WA (75.3%; N = 70) (Table 1). From all sharks tagged, 112 (95.7%) bronze whalers and 92 (98.9%) dusky sharks were detected for at least 1 day during the monitoring period. For these sharks, bronze whalers (4,819 ± 19,750 detections/individual) were detected between 3 and 3,127 days (mean = 577.2 ± 801.6 days), whereas dusky sharks (327 ± 551 detections/individual) were detected between 1 and 2,730 days (mean = 539.1 ± 734.9 days).

Our study shows that 25 out of 117 tagged bronze whalers (21.4%) and eight out of 93 tagged dusky sharks (8.6%) showed inter-state movements, with bronze whaler showing more complex and frequent inter-state movements than dusky shark. For bronze whaler, inter-state movements occurred among all five of the southern states, whereas SA- and WA-tagged dusky sharks only moved between WA and SA, and NSW-tagged dusky sharks remained on the east and southeast coasts. While the number of tagged individuals moving among jurisdictions is relatively low, our findings highlight the wide-ranging nature of these two shark species, which needs to be considered in future stock assessments.

For dusky shark, although 8.6% of all tagged individuals undertook inter-state movements, ∼50% of dusky sharks tagged in SA went to WA. Similarly, a previous study found that all satellite-tracked dusky sharks tagged in SA went to WA (Rogers et al., 2013b). While the number of dusky sharks tagged in SA is small (seven dusky sharks tagged with acoustic tags – this study; three dusky sharks tagged with pop-up satellite tags – Rogers et al., 2013b), the high percentage of dusky sharks tagged in SA going to WA [50% of the ten tagged dusky sharks across Rogers et al. (2013b) and our study] provides evidence of connectivity between the two states. Previous studies reporting movements from conventional tags also show that 3% of recaptured dusky sharks tagged in WA (2,470 dusky sharks tagged since 1994 and 473 recaptures) were recaptured in SA (Bartes et al., unpublished data). The low percentage of WA-tagged dusky sharks moving into SA, higher percentage of SA-tagged sharks moving into WA and the species’ preference for tropical to warm temperate habitats (Last and Stevens, 2009; Rigby et al., 2019) suggest that dusky shark distribution in this region is likely centered around WA. Small numbers of individuals may occasionally move toward SA when conditions are suitable, such as increased water temperatures during the austral summer (Rogers et al., 2013b). Dusky sharks tagged in NSW showed inter-state movements, but were constrained to eastern Australia, similar to the movements of dusky sharks tagged with pop-up satellite tags in northern NSW (Barnes et al., 2016).

Our findings showing dusky shark movements within, but not between the southwest and east coasts support the separated population structure suggested by Geraghty et al. (2014), with two separated populations in the east and west coasts. These findings challenge the hypothesis of panmixia within Australia proposed by other genetic studies (Ovenden et al., 2009; Benavides et al., 2011a; Junge et al., 2019). While it is possible that some connectivity occurs through northern Australia, this could not be determined in our study due to the limited number of receivers deployed in far north WA and Northern Territory. Bass Strait has previously been identified as a provincial zoogeographic boundary and a region of significant clustering of breaks (Dawson, 2005), with several marine species showing genetic divergences in the vicinity of this region (e.g., white shark, Carcharodon carcharias; Blower et al., 2012; sawsharks, Pristiophorus spp.; Nevatte et al., 2021). Such divergence is likely related to the historical total barrier to gene flow during the late Pliocene, when periods of cold climate and low sea-level segregated warm temperate organisms east or west of the emergent Bassian Isthmus resulting in population divergence and speciation (Waters, 2008). During subsequent periods of warmer and higher seas, sister taxa expanded into the Bass Strait region leading to weakly correlated phylogeographic and biogeographic patterns, with gene flow across Bass Strait hindered by modern oceanographic conditions creating a barrier to dispersal (Dawson, 2005). The panmixia reported in previous studies might have been facilitated by the movements of a small number of individuals (Lowe and Allendorf, 2010) and reflects the broad connectivity of populations that might occur over multiple generations (i.e., at the evolutionary scale). While this information is valuable for understanding species behavior and evolution, it is less relevant for stock assessment and management advice (Kerr et al., 2016). In situations when genetic analyses suggest panmixia, demographic independence can still occur, leading to discrete stocks that should be modeled and assessed separately (Braccini et al., 2016).

In the case of bronze whalers, acoustic tracking suggests that bronze whalers in southwest Australia represent a single population with complex movements; a WA component that frequents SA and sometimes venture into NSW and a SA component more likely to move eastwards to NSW through VIC. The SA-tagged sharks were also less likely to spend time in WA. This movement from WA and SA to eastern Australia somewhat contrasts with the movement of dusky shark and other large marine predators, i.e., white sharks, where Bass Strait delineates east and west coast stocks. A separation of the WA bronze whalers from the rest of Australia was suggested by Junge et al. (2019), but is not supported by our findings as 18 of the WA-tagged sharks (23%) moved inter-state, including seven bronze whalers detected in VIC, TAS, and NSW. Overall, bronze whalers show significant connectivity between WA, SA, and NSW. As SA is at the center of the bronze whaler distribution, it is likely playing a central part in this single Australia-wide population.

Seasonal movements varied between species, with dusky sharks showing considerably more defined seasonal inter-state movement patterns than bronze whalers. Dusky shark movements matched the previously identified migration westward and across the Great Australian Bight to WA during autumn (Rogers et al., 2013b). In WA, adult dusky sharks occurring north of Perth have a high chance of moving south during the Austral summer, while dusky sharks south of Perth are more likely to move north during the Austral winter (Braccini et al., 2018a). Similarly, dusky shark movements on the east coast support previously reported short-term movements obtained via pop-up satellite tags (Barnes et al., 2016). Large-scale seasonal migrations of dusky sharks are common globally, including in South Africa (Hussey et al., 2009), Western Atlantic Ocean (Kohler et al., 1998; Bangley et al., 2020), and Gulf of Mexico (Hoffmayer et al., 2014). The long battery life of acoustic tags lasting multiple years, however, enabled us to identify consistent occurrence and timing of inter-state movements between SA and WA from individual dusky sharks not previously recorded. Previous studies have also highlighted a strong seasonal occurrence of bronze whalers in inshore waters and migrations likely driven by water temperature and availability of resources (Lucifora et al., 2005; Dudley and Cliff, 2010; Drew et al., 2019). In southern Australia, bronze whalers migrate inside the South Australian gulfs in spring to early autumn (September–April), which coincides with seasonally warm gulf water temperatures (Drew et al., 2019). Additionally, departure from the gulfs in late autumn (May–June) was observed by all bronze whalers tagged with pop-up tags and tracked for more than 60 days (five out of 10 tagged sharks), coinciding with the cooling off gulf and inshore coastal water temperatures (Drew unpublished data). Yet, bronze whalers can be found in the South Australian gulfs throughout winter (Drew et al., 2019), showing that they can withstand the 11°C water temperature during mid-winter (July–August) (Petrusevics, 1993). The lack of consistency in bronze whaler seasonal migrations between SA and WA observed in this study does not support the seasonal migration of bronze whalers outside the SA gulfs identified previously (Drew et al., 2019). However, the same study also shows that some bronze whalers remain in the gulfs throughout winter, highlighting a considerable degree of individual variation in movement patterns (as shown by Shark ID explaining more deviance than month).

Bronze whalers were previously known to occur throughout southern Australia from Geraldton (WA) across to Coffs Harbour (NSW) (Last and Stevens, 2009; Huveneers et al., 2020). The IMOS receivers detected six bronze whalers in Ningaloo Reef, representing an extension of the known western Australian distribution toward tropical waters and to lower latitudes compared to eastern Australia. While the species is considered to be cosmopolitan in warm temperate and some tropical areas, these detections off Ningaloo Reef are the most northern records of a bronze whaler within tropical waters in Australia.

Publicly available datasets were analyzed in this study. This data can be found here: https://animaltracking.aodn.org.au/.

The animal study was reviewed and approved by the Flinders University Animal Ethics Committee (ethics approval #E360) and NSW DPI Animal Care and Ethics Committee (Ref. 12/19).

CH, MD, RM, and MB conceived the study. YN, MB, and CH analyzed the data and wrote the manuscript. All authors contributed to the fieldwork and data management, and edited and approved the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.