From little things big things grow: enhancement of an acoustic telemetry network to monitor broad-scale movements of marine species along Australia's east coast.

BACKGROUND: Acoustic telemetry has become a fundamental tool to monitor the movement of aquatic species. Advances in technology, in particular the development of batteries with lives of > 10 years, have increased our ability to track the long-term movement patterns of many species. However, logistics and financial constraints often dictate the locations and deployment duration of acoustic receivers. Consequently, there is often a compromise between optimal array design and affordability. Such constraints can hinder the ability to track marine animals over large spatial and temporal scales. Continental-scale receiver networks have increased the ability to study large-scale movements, but significant gaps in coverage often remain. METHODS: Since 2007, the Integrated Marine Observing System's Animal Tracking Facility (IMOS ATF) has maintained permanent receiver installations on the eastern Australian seaboard. In this study, we present the recent enhancement of the IMOS ATF acoustic tracking infrastructure in Queensland to collect data on large-scale movements of marine species in the northeast extent of the national array. Securing a relatively small initial investment for expanding receiver deployment and tagging activities in Queensland served as a catalyst, bringing together a diverse group of stakeholders (research institutes, universities, government departments, port corporations, industries, Indigenous ranger groups and tourism operators) to create an extensive collaborative network that could sustain the extended receiver coverage into the future. To fill gaps between existing installations and maximise the monitoring footprint, the new initiative has an atypical design, deploying many single receivers spread across 2,100 km of Queensland waters. RESULTS: The approach revealed previously unknown broad-scale movements for some species and highlights that clusters of receivers are not always required to enhance data collection. However, array designs using predominantly single receiver deployments are more vulnerable to data gaps when receivers are lost or fail, and therefore "redundancy" is a critical consideration when designing this type of array. CONCLUSION: Initial results suggest that our array enhancement, if sustained over many years, will uncover a range of previously unknown movements that will assist in addressing ecological, fisheries, and conservation questions for multiple species.

Capture Response and Long-Term Fate of White Sharks (Carcharodon carcharias) after Release from SMART Drumlines.

Human-shark conflict has been managed through catch-and-kill policies in most parts of the world. More recently, there has been a greater demand for shark bite mitigation measures to improve protection for water users whilst minimizing harm to non-target and target species, particularly White Sharks (Carcharodon carcharias), given their status as a Threatened, Endangered, or Protected (TEP) species. A new non-lethal shark bite mitigation method, known as the Shark-Management-Alert-in-Real-Time (SMART) drumline, alerts responders when an animal takes the bait and thereby provides an opportunity for rapid response to the catch and potentially to relocate, tag, and release sharks. Thirty-six White Sharks were caught on SMART drumlines in New South Wales, Australia, and tagged with dorsal fin-mounted satellite-linked radio transmitters (SLRTs) and acoustic tags before release. Thirty-one sharks were located within 10 days, 22 of which provided high-quality locations (classes 1 to 3) suitable for analysis. Twenty-seven percent and 59% of these sharks were first detected within 10 and 50 h of release, respectively. For the first three days post-release, sharks moved and mostly remained offshore (>3.5 km from the coast), irrespective of shark sex and length. Thereafter, tagged sharks progressively moved inshore; however, 77% remained more than 1.9 km off the coast and an average of 5 km away from the tagging location, 10 days post-release. Sharks were acoustically detected for an average of 591 days post-release (ranging from 45 to 1075 days). Although five of the 36 sharks were not detected on acoustic receivers, SLRT detections for these five sharks ranged between 43 and 639 days post-release, indicating zero mortality associated with capture. These results highlight the suitability of SMART drumlines as a potential non-lethal shark bite mitigation tool for TEP species such as White Sharks, as they initially move away from the capture site, and thereby this bather protection tool diminishes the immediate risk of shark interactions at that site.

Bull Shark (Carcharhinus leucas) Occurrence along Beaches of South-Eastern Australia: Understanding Where, When and Why.

Unprovoked shark bites have increased over the last three decades, yet they are still relatively rare. Bull sharks are globally distributed throughout rivers, estuaries, nearshore areas and continental shelf waters, and are capable of making long distance movements between tropical and temperate regions. As this species is implicated in shark bites throughout their range, knowledge of the environmental drivers of bull shark movements are important for better predicting the likelihood of their occurrence at ocean beaches and potentially assist in reducing shark bites. Using the largest dataset of acoustically tagged bull sharks in the world, we examined the spatial ecology of 233 juvenile and large (including sub-adult and adult) bull sharks acoustically tagged and monitored over a 5.5-year period (2017-2023) using an array of real-time acoustic listening stations off 21 beaches along the coast of New South Wales, Australia. Bull sharks were detected more in coastal areas of northern NSW (<32° S) but they travelled southwards during the austral summer and autumn. Juveniles were not detected on shark listening stations until they reached 157 cm and stayed north of 31.98° S (Old Bar). Intra-specific diel patterns of occurrence were observed, with juveniles exhibiting higher nearshore presence between 20:00 and 03:00, whilst the presence of large sharks was greatest from midday through to 04:00. The results of generalised additive models revealed that large sharks were more often found when water temperatures were higher than 20 °C, after >45 mm of rain and when swell heights were between 1.8 and 2.8 m. Understanding the influence that environmental variables have on the occurrence of bull sharks in the coastal areas of NSW will facilitate better education and could drive shark smart behaviour amongst coastal water users.

Scaling of Activity Space in Marine Organisms across Latitudinal Gradients.

AbstractUnifying models have shown that the amount of space used by animals (e.g., activity space, home range) scales allometrically with body mass for terrestrial taxa; however, such relationships are far less clear for marine species. We compiled movement data from 1,596 individuals across 79 taxa collected using a continental passive acoustic telemetry network of acoustic receivers to assess allometric scaling of activity space. We found that ectothermic marine taxa do exhibit allometric scaling for activity space, with an overall scaling exponent of 0.64. However, body mass alone explained only 35% of the variation, with the remaining variation best explained by trophic position for teleosts and latitude for sharks, rays, and marine reptiles. Taxon-specific allometric relationships highlighted weaker scaling exponents among teleost fish species (0.07) than sharks (0.96), rays (0.55), and marine reptiles (0.57). The allometric scaling relationship and scaling exponents for the marine taxonomic groups examined were lower than those reported from studies that had collated both marine and terrestrial species data derived using various tracking methods. We propose that these disparities arise because previous work integrated summarized data across many studies that used differing methods for collecting and quantifying activity space, introducing considerable uncertainty into slope estimates. Our findings highlight the benefit of using large-scale, coordinated animal biotelemetry networks to address cross-taxa evolutionary and ecological questions.

From rivers to ocean basins: The role of ocean barriers and philopatry in the genetic structuring of a cosmopolitan coastal predator.

The Bull Shark (Carcharhinus leucas) faces varying levels of exploitation around the world due to its coastal distribution. Information regarding population connectivity is crucial to evaluate its conservation status and local fishing impacts. In this study, we sampled 922 putative Bull Sharks from 19 locations in the first global assessment of population structure of this cosmopolitan species. Using a recently developed DNA-capture approach (DArTcap), samples were genotyped for 3400 nuclear markers. Additionally, full mitochondrial genomes of 384 Indo-Pacific samples were sequenced. Reproductive isolation was found between and across ocean basins (eastern Pacific, western Atlantic, eastern Atlantic, Indo-West Pacific) with distinct island populations in Japan and Fiji. Bull Sharks appear to maintain gene flow using shallow coastal waters as dispersal corridors, whereas large oceanic distances and historical land-bridges act as barriers. Females tend to return to the same area for reproduction, making them more susceptible to local threats and an important focus for management actions. Given these behaviors, the exploitation of Bull Sharks from insular populations, such as Japan and Fiji, may instigate local decline that cannot readily be replenished by immigration, which can in turn affect ecosystem dynamics and functions. These data also supported the development of a genetic panel to ascertain the population of origin, which will be useful in monitoring the trade of fisheries products and assessing population-level impacts of this harvest.

Ontogenetic changes in the tooth morphology of bull sharks (Carcharhinus leucas).

Teeth are an integral component of feeding ecology, with a clear link between tooth morphology and diet, as without suitable dentition prey cannot be captured nor broken down for consumption. Bull sharks, Carcharhinus leucas, undergo an ontogenetic niche shift from freshwater to marine habitats, which raises the question: does tooth morphology change with ontogeny? Tooth shape, surface area and thickness were measured using both morphometrics and elliptic Fourier analysis to determine if morphology varied with position in the jaw and if there was an ontogenetic change concordant with this niche shift. Significant ontogenetic differences in tooth morphology as a function of position in the jaw and shark total length were found, with upper and lower jaws of bull sharks presenting two different tooth morphologies. Tooth shape and thickness fell into two groupings, anterior and posterior, in both the upper and lower jaws. Tooth surface area, however, indicated three groupings, mesial, intermediate and distal, in both the upper and lower jaws. While tooth morphology changed significantly with size, showing an inflection at sharks of 135 cm total length, each morphological aspect retained the same tooth groupings throughout. These ontogenetic differences in tooth morphologies reflect tooth strength, prey handling and heterodonty.

Overcoming multi-year impacts of maternal isotope signatures using multi-tracers and fast turnover tissues in juvenile sharks.

Stable isotopes are often used to determine the ecological role of different age classes of animals, but particularly for young animals this approach may be compromised. During gestation and or incubation body tissues of the young are derived directly from the mother. In neonates or post hatching, there is a period of transformation as the young grow and forage independently, but during this period different organs will continue to reflect the maternal isotopic signature as a function of their turnover rate. How long this maternal hangover persists remains poorly understood. We applied a multi-tracer approach (δ15N, δ13C and δ34S) to stable isotope signatures in juvenile bull sharks (Carcharhinus leucas) up to 6.5 years post parturition. We found that maternal provisioning was detectable for up to 3.5 years after birth in muscle but only detectable in young-of-the-year for liver. Inclusion of sulphur revealed when maternal signatures disappeared from low-turnover tissue, while also identifying the spatial and trophic ecology patterns from fast-turnover tissue. These results reveal the importance of sampling fast turnover tissues to study the trophic ecology of juvenile elasmobranchs, and how the use of only δ15N and δ13C isotopes is likely to make maternal patterns more difficult to detect.

Population structure, connectivity, and demographic history of an apex marine predator, the bull shark Carcharhinus leucas.

Knowledge of population structure, connectivity, and effective population size remains limited for many marine apex predators, including the bull shark Carcharhinus leucas. This large-bodied coastal shark is distributed worldwide in warm temperate and tropical waters, and uses estuaries and rivers as nurseries. As an apex predator, the bull shark likely plays a vital ecological role within marine food webs, but is at risk due to inshore habitat degradation and various fishing pressures. We investigated the bull shark's global population structure and demographic history by analyzing the genetic diversity of 370 individuals from 11 different locations using 25 microsatellite loci and three mitochondrial genes (CR, nd4, and cytb). Both types of markers revealed clustering between sharks from the Western Atlantic and those from the Western Pacific and the Western Indian Ocean, with no contemporary gene flow. Microsatellite data suggested low differentiation between the Western Indian Ocean and the Western Pacific, but substantial differentiation was found using mitochondrial DNA. Integrating information from both types of markers and using Bayesian computation with a random forest procedure (ABC-RF), this discordance was found to be due to a complete lack of contemporary gene flow. High genetic connectivity was found both within the Western Indian Ocean and within the Western Pacific. In conclusion, these results suggest important structuring of bull shark populations globally with important gene flow occurring along coastlines, highlighting the need for management and conservation plans on regional scales rather than oceanic basin scale.

Long-term patterns of abundance, residency and movements of bull sharks (Carcharhinus leucas) in Sydney Harbour, Australia.

Bull sharks (Carcharhinus leucas) are known to frequent nearshore environments, particularly estuaries, resulting in interactions with humans. Knowledge of the behaviour of large individuals in temperate, estuarine environments is limited. This acoustic telemetry study reports on residency and movement patterns of 40 sub-adult and adult bull sharks in Sydney Harbour, a large temperate estuary, over seven years. Bull sharks exhibited clear seasonal patterns in their occurrence during the austral summer and autumn, with abundance peaking in January and February. This pattern was consistent between sexes and across all sizes. Bull sharks displayed weak diel differences in their spatial distribution, with individuals using areas further from the Harbour entrance more frequently during the day and at low tides. A diel pattern in depth use was apparent, with sharks utilising deeper water during daytime and moving shallower at night. Bull sharks had high individual inter-annual variability in their spatial distribution, however, when data were aggregated among all individuals and years, two locations of increased use were identified. Water temperature was the key predictor for seasonal movements and return behaviour to this estuary, suggesting that increasing water temperatures as a result of climate change may lead to higher shark abundance and possibly longer periods of residency in Sydney Harbour. Understanding the drivers for bull shark abundance and distribution will hopefully facilitate better education and shark smart behaviour by estuarine water-users, especially during summer and autumn months.

Patterns of Occurrence of Sharks in Sydney Harbour, a Large Urbanised Estuary.

Information about spatial and temporal variability in the distribution and abundance of shark-populations are required for their conservation, management and to update measures designed to mitigate human-shark interactions. However, because some species of sharks are mobile, migratory and occur in relatively small numbers, estimating their patterns of distribution and abundance can be very difficult. In this study, we used a hierarchical sampling design to examine differences in the composition of species, size- and sex-structures of sharks sampled with bottom-set longlines in three different areas with increasing distance from the entrance of Sydney Harbour, a large urbanised estuary. During two years of sampling, we obtained data for four species of sharks (Port Jackson, Heterodontus portusjacksoni; wobbegong, Orectolobus maculatus; dusky whaler, Carcharhinus obscurus and bull shark, Carcharhinus leucas). Only a few O. maculatus and C. obscurus were caught, all in the area closest to the entrance of the Harbour. O. maculatus were caught in all seasons, except summer, while C. obscurus was only caught in summer. Heterodontus portusjacksoni were the most abundant species, caught in the entrance location mostly between July to November, when water temperature was below 21.5°C. This pattern was consistent across both years. C. leucas, the second most abundant species, were captured in all areas of Sydney Harbour but only in summer and autumn when water temperatures were above 23°C. This study quantified, for this first time, how different species utilise different areas of Sydney Harbour, at different times of the year. This information has implications for the management of human-shark interactions, by enabling creation of education programs to modify human behaviour in times of increased risk of potentially dangerous sharks.

Habitat-associations of turban snails on intertidal and subtidal rocky reefs.

Patchiness of habitat has important influences on distributions and abundances of organisms. Given the increasing threat of loss and alteration of habitats due to pressures associated with humans, there is a need for ecologists to understand species' requirements for habitat and to predict changes to taxa under various future environmental conditions. This study tested hypotheses about the generality of patterns described for one species of marine intertidal turban snail for a different, yet closely-related species in subtidal habitats along the coast of New South Wales, Australia. These two closely-related species live in similar habitats, yet under quite different conditions, which provided an opportunity to investigate how similar types of habitats influence patterns of distribution, abundance and size-structure in intertidal versus subtidal environments. For each species, there were similar associations between biogenically structured habitat and densities. The intertidal species, Turbo undulates, were more abundant, with greater proportions of small individuals in habitats formed by the canopy-forming alga, Hormosira banksii, the solitary ascidian, Pyura stolonifera or the turfing red alga, Corallina officinalis compared to simple habitat (bare rock). Similarly, more Turbo torquatus were found in biogenically structured subtidal habitat, i.e. canopy-forming algae, Ecklonia radiata, mixed algal communities ('fringe'), or turfing red algae (Corallina officinalis and Amphiroa aniceps) than where habitat is simple (barrens). Small T. torquatus were more abundant in areas of turf and 'fringe', while large snails were more abundant in areas of kelp and barrens. These patterns were found at each location sampled (i.e. eight intertidal and two subtidal rocky reefs) and at all times of sampling, across each environment. This study highlighted the consistent influence of biogenically structured habitats on the distribution, abundance and size-structure of intertidal and subtidal turban snails and forms a basis for increasing the understanding of the potential underlying processes causing such patterns.