Residency, habitat use and sexual segregation of white sharks, Carcharodon carcharias in False Bay, South Africa

Individual residency behaviours and seasonal long-distance movements in acoustically tagged Caribbean reef sharks in the Cayman Islands

Understanding how reef-associated sharks use coastal waters through their ontogeny is important for their effective conservation and management. This study used the horizontal movements of acoustically tagged Caribbean reef sharks (Carcharhinus perezi) to examine their use of coastal space around the Cayman Islands between 2009 and 2019. A total of 39 (59.1%) tagged sharks (male = 22, female = 17, immature = 18, mature = 21) were detected on the islands wide network of acoustic receivers. The detection data were used to calculate values of Residency Index (RI), Site-Fidelity Index (SFI) and minimum linear displacement (MLD), as well as for network analysis of individual shark movements to test for differences between demographics, seasons, and diel periods. Sharks were detected for up to 1,598 days post-tagging and some individuals showed resident behaviour but the majority of tagged individuals appear to have been one-off or only occasional transient visitors to the area. Generally, individuals showed strong site-fidelity to different areas displaying linear home ranges of < 20 km. The evidence indicates that there was no pattern of diel behaviour. Tagged sharks generally showed increased movements within and between islands during the summer (April-September), which may be related to breeding activity. Some individuals even made occasional excursions across 110 km of open water > 2,000 m deep between Grand Cayman and Little Cayman. One mature female shark showed a displacement of 148.21 km, the greatest distance reported for this species. The data shows that the distances over which some sharks moved, greatly exceeded the extent of any one of the islands' marine protected areas indicating that this species may be more mobile and dispersive than previously thought. This study provides support for the blanket protection to all sharks throughout Cayman waters, which was incorporated within the National Conservation Act in 2015.

The biology and ecology of the basking shark: A review

Here we review the literature on the basking shark (Cetorhinus maximus, Gunnerus, 1765), well known as the second largest extant shark (and fish) species globally. Previous reviews were published by Kunzlik in 1988 and Sims in 2008, but in the last 15 years modern electronic and DNA sequencing technologies have resulted in considerable advances in our knowledge of the species' behaviour and ecology. Basking sharks are planktivores and under appropriate conditions spend prolonged periods at the ocean surface feeding on copepod prey that primarily make up their diet, the behaviour that gave rise to their common name. In general, they are migratory and move into higher latitude waters during the summer months, when loose surface-feeding aggregations may form at favoured sites, the best known of which at present occur at hotspots on the west coasts of Britain and Ireland. The species is found circumglobally in temperate waters, but they are also now known on occasion to migrate at depth between northern and southern hemispheres, as well as across oceans within the northern hemisphere. In the past basking shark were more abundant across much of their range, but, consequent on targeted fisheries and in some places intentional eradication, became everywhere scarce, with recent population recovery in the north-east Atlantic being the result of protective measures initiated in the 1990s. Despite their charismatic nature, some of their most fundamental biological processes including copulation, gestation and birth remain largely unknown, due to their migratory and often deep-water lifestyle. In contrast, the deployment of small-scale archival and satellite tags has revealed the details of both broadscale migratory movements and horizontal and vertical foraging behaviours. Recent genetic studies support evidence suggesting a degree of site fidelity in relation to seasonal feeding grounds, which likely explains why in the past local populations have collapsed following periods of intensive fishing. Other recent research using aerial drones and towed cameras has revealed within loose feeding aggregations elements of social behaviour that may have a courtship function as well as enhance feeding efficiency.

What acoustic telemetry can and cannot tell us about fish biology

Acoustic telemetry (AT) has become ubiquitous in aquatic monitoring and fish biology, conservation, and management. Since the early use of active ultrasonic tracking that required researchers to follow at a distance their species of interest, the field has diversified considerably, with exciting advances in both hydrophone and transmitter technology. Once a highly specialized methodology, however, AT is fast becoming a generalist tool for those wishing to study or conserve fishes, leading to diversifying application by non-specialists. With this transition in mind, we evaluate exactly what AT has become useful for, discussing how the technological and analytical advances around AT can address important questions within fish biology. In doing so, we highlight the key ecological and applied research areas where AT continues to reveal crucial new insights and, in particular, when combined with complimentary research approaches. We provide a comprehensive breakdown of the state of the art for applications of AT, discussing the ongoing challenges, where its strengths lie, and how future developments may revolutionize fisheries management, behavioral ecology and species protection. Through selected papers we illustrate specific applications across the broad spectrum of fish biology. By bringing together the recent and future developments in this field under categories designed to broadly capture many aspects of fish biology, we hope to offer a useful guide for the non-specialist practitioner as they attempt to navigate the dizzying array of considerations and ongoing developments within this diverse toolkit.

The essential habitat role of a unique coastal inlet for a widely distributed apex predator

Essential habitats support specific functions for species, such as reproduction, feeding or refuge. For highly mobile aquatic species, identifying essential habitats within the wider distribution range is central to understanding species ecology, and underpinning effective management plans. This study examined the movement and space use patterns of sevengill sharks (Notorynchus cepedianus) in Caleta Valdés (CV), a unique coastal habitat in northern Patagonia, Argentina. Seasonal residency patterns of sharks were evident, with higher detectability in late spring and early summer and lower during autumn and winter. The overlap between the residency patterns of sharks and their prey, elephant seals, suggests that CV functions as a seasonal feeding aggregation site for N. cepedianus. The study also found sexual differences in movement behaviour, with males performing abrupt departures from CV and showing increased roaming with the presence of more sharks, and maximum detection probability at high tide. These movements could be related to different feeding strategies between sexes or mate-searching behaviour, suggesting that CV may also be essential for reproduction. Overall, this study highlights the importance of coastal sites as essential habitats for N. cepedianus and deepens our understanding of the ecological role of this apex predator in marine ecosystems.

Evidence of Non-Random Social Interactions between Pairs of Bait-Attracted White Sharks in Gansbaai (South Africa)

Knowledge about the social behavior of sharks is a growing research field, but not many observations are available on the social interactions between pairs of sharks in the presence of passive surface bait and mainly related to aggregations. Between 2009 and 2018, in Gansbaai, South Africa, 415 white sharks were sighted, and 525 surface-generated social interactions were identified, exhibited by 169 different white sharks. The mean sighting rate was 0.91 (range 0.18–1.53) white sharks per hour. Eight patterns of social interaction were exhibited: swim by, parallel swim, follow/give way, follow, give way, stand back, splash fights, and piggyback. Non-random interactions occurred when pairs of specimens approached the passive surface bait, confirming that the white sharks made a real choice, showing a dominance hierarchy during the ten years of data collection. Evidence of non-random social interactions in the surface behavior of bait-attracted white sharks Carcharodon carcharias in Gansbaai’s transient population was the goal of this research.

Larvae of Clistobothrium grimaldii (Cestoda: Phyllobothriidea) from a Cape fur seal (Arctocephalus pusillus pusillus) kept in a zoo in Japan

The larval form of the Phyllobothriidea cestode was found in the blubber of a Cape fur seal (Arctocephalus pusillus pusillus) from a zoo in Japan. Bladder-bearing larval cestodes with a scolex have been occasionally reported from blubbers of pinnipeds and morphologically identified as Clistobothrium delphini (formerly known as Phyllobothrium delphini) or rarely Clistobothrium grimaldii (Monorygma grimaldii). Although the larvae here morphologically resembled C. delphini, the 28S rDNA sequence was 100% (1,430/1,430 bp) homologous to the registered sequence of C. grimaldii (GenBank Accession No. KU724058). This discrepancy between morphological and molecular analyses confirms the difficulty of identifying C. delphini and C. grimaldii larvae based solely on morphology, and the need for molecular data to elucidate the morphological variations in Clistobothrium parasites.

Sex Differences in the Individual Behaviour of Bait-Attracted White Sharks (Carcharodon carcharias, Linnaeus, 1758) Are Linked to Different Environmental Factors in South Africa

The white shark (Carcharodon carcharias) is a charismatic species and, consequently, one of the most studied and protected sharks. This species can be found in a wide range of temperatures and depths, showing site fidelity and migrating across the oceans. This offers a challenge to understanding the processes influencing their lifecycle and, more importantly, assessing anthropogenic disturbances to their populations. These predators' behaviour has been linked to diverse abiotic factors. Here, an ethological approach was used to understand the influence of environmental variables on white shark behaviour. A different environmental impact was found between the activity of females and males toward the bait. Females performed a higher number of behaviours under daylight, lower sea surface temperatures, short wavelets, clear and cloudy skies, under La Niña events, elevated moonlight and high tides. Males behaved with more complexity at dawn, medium sea surface temperatures, large wavelets, few clouds, high tides, and elevated moonlight. The world's aquatic habitats are experiencing significant physiochemical shifts due to human-induced climate change. Knowledge about how white sharks respond to environmental factors is essential to guide management and conservation actions.

Site fidelity and shallow genetic structure in the common smooth-hound shark Mustelus mustelus confirmed by tag-recapture and genetic data

The common smooth-hound shark, Mustelus mustelus, is a widely distributed demersal shark under heavy exploitation from various fisheries throughout its distribution range. To assist in the development of appropriate management strategies, the authors evaluate stock structure, site fidelity and movement patterns along the species' distribution in southern Africa based on a combination of molecular and long-term tag-recapture data. Eight species-specific microsatellite markers (N = 73) and two mitochondrial genes, nicotinamide adenine dehydrogenase subunit 4 and control region (N = 45), did not reveal any significant genetic structure among neighbouring sites. Nonetheless, tagging data demonstrate a remarkable degree of site fidelity with 76% of sharks recaptured within 50 km of the original tagging location. On a larger geographic scale, dispersal is governed by oceanographic features as demonstrated by the lack of movements across the Benguela-Agulhas transition zone separating the South-East Atlantic Ocean (SEAO) and South-West Indian Ocean (SWIO) populations. Microsatellite data supported very shallow ocean-based structure (SEAO and SWIO) and historical southward gene flow following the Agulhas Current, corroborating the influence of this dynamic oceanographic system on gene flow. Moreover, no movements between Namibia and South Africa were observed, indicating that the Lüderitz upwelling formation off the Namibian coast acts as another barrier to dispersal and gene flow. Overall, these results show that dispersal and stock structure of M. mustelus are governed by a combination of behavioural traits and oceanographic features such as steep temperature gradients, currents and upwelling systems.

Loss of an apex predator in the wild induces physiological and behavioural changes in prey

Predators can impact prey via predation or risk effects, which can initiate trophic cascades. Given widespread population declines of apex predators, understanding and predicting the associated ecological consequences is a priority. When predation risk is relatively unpredictable or uncontrollable by prey, the loss of predators is hypothesized to release prey from stress; however, there are few tests of this hypothesis in the wild. A well-studied predator-prey system between white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) in False Bay, South Africa, has previously demonstrated elevated faecal glucocorticoid metabolite concentrations (fGCMs) in seals exposed to high levels of predation risk from white sharks. A recent decline and disappearance of white sharks from the system has coincided with a pronounced decrease in seal fGCM concentrations. Seals have concurrently been rafting further from shore and over deeper water, a behaviour that would have previously rendered them vulnerable to attack. These results show rapid physiological and behavioural responses by seals to release from predation stress. To our knowledge, this represents the first demonstration in the wild of physiological changes in prey from predator decline, and such responses are likely to increase given the scale and pace of apex predator declines globally.

Possible causes of a substantial decline in sightings in South Africa of an ecologically important apex predator, the white shark

A decline in sightings of a top predator, the white shark (Carcharadon carcharias), in South Africa was quantified in order to identify possible causes for this decline. White shark sightings data across 8 years (2011–2018), recorded from a cage-diving vessel in Gansbaai are reported. A significant decline in mean total white shark sightings per boat trip (>6 in 2011 to <1 in 2018) and a 69% reduction in the probability of a sighting were found. Correlating with this decline in sightings is a rise in sightings of sevengill sharks (Notorynchus cepedianus) in False Bay and copper sharks (Carcharhinus brachyurus) in Gansbaai, as well as substantial ecosystem changes. The effects of lethal conservation measures such as the use of shark nets in KwaZulu-Natal; the direct and indirect effects of overfishing including a reduction in smoothhound (Mustelus mustelus) and soupfin (Galeorhinus galeus) sharks; and novel predation on white sharks are discussed as possible causative factors for this decline in white shark sightings.

Agrochemicals in freshwater systems and their potential as endocrine disrupting chemicals: A South African context

South Africa is the largest agrochemical user in sub-Saharan Africa, with over 3000 registered pesticide products. Although they reduce crop losses, these chemicals reach non-target aquatic environments via leaching, spray drift or run-off. In this review, attention is paid to legacy and current-use pesticides reported in literature for the freshwater environment of South Africa and to the extent these are linked to endocrine disruption. Although banned, residues of many legacy organochlorine pesticides (endosulfan and dichlorodiphenyltrichloroethane (DDT)) are still detected in South African watercourses and wildlife. Several current-use pesticides (triazine herbicides, glyphosate-based herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and chlorpyrifos) have also been reported. Agrochemicals can interfere with normal hormone function of non-target organism leading to various endocrine disrupting (ED) effects: intersex, reduced spermatogenesis, asymmetric urogenital papillae, testicular lesions and infertile eggs. Although studies investigating the occurrence of agrochemicals and/or ED effects in freshwater aquatic environments in South Africa have increased, few studies determined both the levels of agricultural pesticides present and associated ED effects. The majority of studies conducted are either laboratory-based employing in vitro or in vivo bioassays to determine ED effects of agrochemicals or studies that investigate environmental concentrations of pesticides. However, a combined approach of bioassays and chemical screening will provide a more comprehensive overview of agrochemical pollution of water systems in South Africa and the risks associated with long-term chronic exposure.

Spatiotemporal distribution patterns of immature Australasian white sharks (Carcharodon carcharias)

In Australian and New Zealand waters, current knowledge on white shark (Carcharodon carcharias) movement ecology is based on individual tracking studies using relatively small numbers of tags. These studies describe a species that occupies highly variable and complex habitats. However, uncertainty remains as to whether the proposed movement patterns are representative of the wider population. Here, we tagged 103 immature Australasian white sharks (147-350 cm fork length) with both acoustic and satellite transmitters to expand our current knowledge of population linkages, spatiotemporal dynamics and coastal habitats. Eighty-three sharks provided useable data. Based on individual tracking periods of up to 5 years and a total of 2,865 days of tracking data, we were able to characterise complex movement patterns over ~45° of latitude and ~72° of longitude and distinguish regular/recurrent patterns from occasional/exceptional migration events. Shark movements ranged from Papua New Guinea to sub-Antarctic waters and to Western Australia, highlighting connectivity across their entire Australasian range. Results over the 12-year study period yielded a comprehensive characterisation of the movement ecology of immature Australasian white sharks across multiple spatial scales and substantially expanded the body of knowledge available for population assessment and management.

Transcriptome-Derived Microsatellites Demonstrate Strong Genetic Differentiation in Pacific White Sharks

Recent advances in genome-scale sequencing technology have allowed the development of high resolution genetic markers for the study of nonmodel taxa. In particular, transcriptome sequencing has proven to be highly useful in generating genomic markers for use in population genetic studies, allowing for insight into species connectivity, as well as local adaptive processes as many transcriptome-derived markers are found within or associated with functional genes. Herein, we developed a set of 30 microsatellite markers from a heart transcriptome for the white shark (Carcharodon carcharias), a widely distributed and globally vulnerable marine predator. Using these markers as well as 10 published anonymous genomic microsatellite loci, we provide 1) the first nuclear genetic assessment of the cross-Pacific connectivity of white sharks, and 2) a comparison of the levels of inferred differentiation across microsatellite marker sets (i.e., transcriptome vs. anonymous) to assess their respective utility to elucidate the population genetic dynamics of white sharks. Significant (FST = 0.083, P = 0.05; G″ST = 0.200; P = 0.001) genetic differentiation was found between Southwestern Pacific (n = 19) and Northeastern Pacific (n = 20) white sharks, indicating restricted, cross Pacific gene flow in this species. Transcriptome-derived microsatellite marker sets identified much higher (up to 2×) levels of genetic differentiation than anonymous genomic markers, underscoring potential utility of transcriptome markers in identifying subtle population genetic differences within highly vagile, globally distributed marine species.Subject areas: Population structure and phylogeography; Conservation genetics and biodiversity.

Eyes on the size: accuracy of visual length estimates of white sharks, Carcharodon carcharias

Visual estimates have been used extensively to determine the length of large organisms that are logistically challenging to measure. However, there has been little effort to quantify the accuracy or validity of this technique despite inaccurate size estimates leading to incorrect population assessments and misinformed management strategies. Here, we compared visually estimated total length measurements of white sharks, Carcharodon carcharias, during cage-diving operations with measurements obtained from stereo-video cameras and assessed the accuracy of those estimates in relation to suspected biases (shark size, and observer experience and gender) using generalized linear mixed-models and linear regressions. Observer experience on board cage-diving vessels had the greatest effect on the accuracy of visual length estimates, with scientists being more accurate (mean accuracy ± standard error: 23.0 ± 16.5 cm) than crew (39.9 ± 33.8 cm) and passengers (49.4 ± 38.5 cm). Observer gender and shark size had no impact on the overall accuracy of visual length estimates, but passengers overestimated sharks less than 3 m and underestimated sharks greater than 3 m. Our findings show that experience measuring animals is the most substantial driver of accurate visual length estimates regardless of the amount of exposure to the species being measured. Scientists were most accurate, even though crew observe white sharks more frequently. Our results show that visual length estimates are not impacted by shark size and are a valid measurement tool for many aspects of C. carcharias research, provided they come from people who have previously been involved in measuring animals, i.e. scientists.

Blacktip shark Carcharhinus limbatus presence at fishing piers in South Carolina: association and environmental drivers

We tagged 12 Carcharhinus limbatus with acoustic transmitters and monitored their presence at five piers along the north-east coast of South Carolina, USA in 2016 and four piers in 2017 using acoustic receivers. Data were analysed with pier association indices (PAI), mixed models and fast Fourier transformation analyses to identify potential factors related to residence time and presence at piers and any cyclical patterns in visits to piers. While the majority of monitored C. limbatus were infrequently detected at piers, three (25.0%) were highly associated with piers (PAI ≥ 0.50). Of the C. limbatus that were detected after initial capture, three (25.0%) recorded detection events only at the pier where they were tagged and two individuals (16.7%) recorded at least one detection event at all monitored piers. The best-fit model explaining C. limbatus residence time at piers included terms for pier location and diel cycle (w_i = 0.88), whereas the best fit model explaining presence-absence of C. limbatus at piers included terms for tidal height, diel cycle, barometric pressure and angler count (w_i = 0.98). Carcharhinus limbatus did not appear to display cyclical patterns in their visits to piers. Along the north-east coast of South Carolina, association of C. limbatus with piers is a phenomenon for a proportion of mature individuals, but continued research is necessary to understand if this behaviour is driven by attraction to and feeding on angler discards or increased foraging opportunities resulting from the attraction of potential prey to the physical structure provided by piers.

Disappearance of white sharks leads to the novel emergence of an allopatric apex predator, the sevengill shark

Despite global declines of apex predatory sharks, evidence for ecosystem consequences remains limited and debated. This is likely a result of both the logistical difficulties of measuring such processes in marine systems and also due to shifting baselines, whereby the ecosystem changes have occurred prior to monitoring. Between 2000–2018, we conducted standardized monitoring of white shark (Carcharodon carcharias) abundance patterns (N = 6,333 shark sightings) and predatory activity (N = 8,076 attacks on seals) at Seal Island, a Cape fur seal (Arctocephalus pusillus pusillus) colony in False Bay, South Africa. Over the 18-year study, declines in white shark abundance and attack rates were documented between 2015–2018, with anomalous lows occurring in 2017 and 2018. This included prolonged periods of complete white shark absence from Seal Island. The disappearance of white sharks from Seal Island coincided with the unprecedented appearance of sevengill sharks (Notorynchus cepedianus; N = 120 sightings), an otherwise allopatric kelp-associated apex predator in False Bay. We also recorded a sevengill shark attacking a live seal in the absence of white sharks. These data provide empirical evidence for behavioral shifts in an allopatric marine predator following the decline and disappearance of white sharks from a foraging site. This study demonstrates the importance of historical data and long-term monitoring for disentangling ecological consequences of apex predator declines.

Summer at the beach: spatio-temporal patterns of white shark occurrence along the inshore areas of False Bay, South Africa

BACKGROUND

Understanding white shark (Carcharodon carcharias) habitat use in coastal areas adjacent to large cities, is an important step when formulating potential solutions to the conservation conflict that exists between humans and large predatory sharks. In this study, we present the findings of a 2.5-year study of white shark occurrence and movement patterns adjacent to the City of Cape Town in False Bay, South Africa, with a focus on spring and summer months. Fifty-one white sharks were monitored annually at three offshore and twelve inshore sites by VR2 acoustic receivers, over 975 days from 1 May 2005 to 31 December 2007.

RESULTS

Occurrence patterns at inshore sites during spring and summer were analysed using a generalized additive mixed model (GAMM) with a spatial term (longitude, latitude), time of day and year included as explanatory variables for site use. We found that sharks occurred more frequently at inshore sites along the northern and northwestern shores, compared to the rest of the bay, and they transitioned most frequently between four adjacent beach sites that encompass the most popular recreational water use areas in Cape Town. There was significant diel variation, with higher shark occurrence around midday, and a peak in shark occurrence in 2005, when human-shark interactions also peaked. However, we found no effect of shark size on occurrence patterns at inshore sites.

CONCLUSIONS

White sharks showed the highest levels of occurrence at specific inshore sites between Muizenberg and Strandfontein beach, and thus inclusion of these sites within False Bay's marine protected area (MPA) network or recognition as Ecological or Biological Significant Areas (EBSAs) should be a future consideration. These insights into white shark habitat use at inshore sites in False Bay are important for successfully applying the principles of marine spatial planning (MSP) and for making science-based policy decisions. Furthermore, this information can be used to reduce potential shark-human conflict by incorporating it into future shark safety education campaigns.

Sex, size and isotopes: cryptic trophic ecology of an apex predator, the white shark Carcharodon carcharias

Demographic differences in resource use are key components of population and species ecology across the animal kingdom. White sharks (Carcharodon carcharias) are migratory, apex predators, which have undergone significant population declines across their range. Understanding their ecology is key to ensuring that management strategies are effective. Here, we carry out the first stable isotope analyses of free-swimming white sharks in South Africa. Biopsies were collected in Gansbaai (34.5805°S, 19.3518°E) between February and July 2015. We used Stable Isotope Bayesian Ellipsis in R and traditional statistical analyses to quantify and compare isotopic niches of male and female sharks of two size classes, and analyse relationships between isotopic values and shark length. Our results reveal cryptic trophic differences between the sexes and life stages. Males, but not females, were inferred to feed in more offshore or westerly habitats as they grow larger, and only males exhibited evidence of an ontogenetic niche shift. Lack of relationship between δ13C, δ15N and female shark length may be caused by females exhibiting multiple migration and foraging strategies, and a greater propensity to travel further north. Sharks < 3 m had much wider, and more diverse niches than sharks > 3 m, drivers of which may include individual dietary specialisation and temporal factors. The differences in migratory and foraging behaviour between sexes, life stages, and individuals will affect their exposure to anthropogenic threats, and should be considered in management strategies.

Our use, misuse, and abandonment of a concept: Whither habitat?

The foundational concept of habitat lies at the very root of the entire science of ecology, but inaccurate use of the term compromises scientific rigor and communication among scientists and nonscientists. In 1997, Hall, Krausman & Morrison showed that ‘habitat’ was used correctly in only 55% of articles. We ask whether use of the term has been more accurate since their plea for standardization and whether use varies across the broader range of journals and taxa in the contemporary literature (1998–2012). We searched contemporary literature for ‘habitat’ and habitat‐related terms, ranking usage as either correct or incorrect, following a simplified version of Hall et al.'s definitions. We used generalized linear models to compare use of the term in contemporary literature with the papers reviewed by Hall et al. and to test the effects of taxa, journal impact in the contemporary articles and effects due to authors that cited Hall et al. Use of the term ‘habitat’ has not improved; it was still only used correctly about 55% of the time in the contemporary data. Proportionately more correct uses occurred in articles that focused on animals compared to ones that included plants, and papers that cited Hall et al. did use the term correctly more often. However, journal impact had no effect. Some habitat terms are more likely to be misused than others, notably ‘habitat type’, usually used to refer to vegetation type, and ‘suitable habitat’ or ‘unsuitable habitat’, which are either redundant or nonsensical by definition. Inaccurate and inconsistent use of the term can lead to (1) misinterpretation of scientific findings; (2) inefficient use of conservation resources; (3) ineffective identification and prioritization of protected areas; (4) limited comparability among studies; and (5) miscommunication of science‐based findings. Correct usage would improve communication with scientists and nonscientists, thereby benefiting conservation efforts, and ecology as a science.

The tooth, the whole tooth and nothing but the tooth: tooth shape and ontogenetic shift dynamics in the white shark Carcharodon carcharias

Results from this study of the white shark Carcharodon carcharias include measurements obtained using a novel photographic method that reveal significant differences between the sexes in the relationship between tooth cuspidity and shark total length, and a novel ontogenetic change in male tooth shape. Males exhibit broader upper first teeth and increased distal inclination of upper third teeth with increasing length, while females do not present a consistent morphological change. Substantial individual variation, with implications for pace of life syndrome, was present in males and tooth polymorphism was suggested in females. Sexual differences and individual variation may play major roles in ontogenetic changes in tooth morphology in C. carcharias, with potential implications for their foraging biology. Such individual and sexual differences should be included in studies of ontogenetic shift dynamics in other species and systems.

Shark Spotters: Successfully reducing spatial overlap between white sharks (Carcharodon carcharias) and recreational water users in False Bay, South Africa

White sharks (Carcharodon carcharias) are apex predators that play an important role in the structure and stability of marine ecosystems. Despite their ecological importance and protected status, white sharks are still subject to lethal control to reduce the risk of shark bites for recreational water users. The Shark Spotters program, pioneered in Cape Town, South Africa, provides a non-lethal alternative for reducing the risk of human-shark conflict. In this study we assessed the efficacy of the Shark Spotters program in reducing overlap between water users and white sharks at two popular beaches in False Bay, South Africa. We investigated seasonal and diel patterns in water use and shark presence at each beach, and thereafter quantified the impact of different shark warnings from shark spotters on water user abundance. We also assessed the impact of a fatal shark incident on patterns of water use. Our results revealed striking diel and seasonal overlap between white sharks and water users at both beaches. Despite this, there was a low rate of shark-human incidents (0.5/annum) which we attribute partly to the success of the Shark Spotters program. Shark spotters use visual (coloured flags) and auditory (siren) cues to inform water users of risk associated with white shark presence in the surf zone. Our results showed that the highest risk category (denoted by a white flag and accompanying siren) caused a significant reduction in water user abundance; however the secondary risk category (denoted by a red flag with no siren) had no significant effect on water users. A fatal shark incident was shown to negatively impact the number of water users present for at least three months following the incident. Our results indicate that the Shark Spotters program effectively reduces spatial overlap between white sharks and water users when the risk of conflict is highest.

A novel application of multi-event modeling to estimate class segregation in a highly migratory oceanic vertebrate

Spatial segregation of animals by class (i.e., maturity or sex) within a population due to differential rates of temporary emigration (TE) from study sites can be an important life history feature to consider in population assessment and management. However, such rates are poorly known; new quantitative approaches to address these knowledge gaps are needed. We present a novel application of multi-event models that takes advantage of two sources of detections to differentiate temporary emigration from apparent absence to quantify class segregation within a study population of double-marked (photo-identified and tagged with coded acoustic transmitters) white sharks (Carcharodon carcharias) in central California. We use this model to test if sex-specific patterns in TE result in disparate apparent capture probabilities (p^o ) between male and female white sharks, which can affect the observed sex ratio. The best-supported model showed a contrasting pattern of Pr(TE) from coastal aggregation sites between sexes (for males Pr[TE] = 0.015 [95% CI = 0.00, 0.31] and Pr[TE]= 0.57 [0.40, 0.72] for females), but not maturity classes. Additionally, by accounting for Pr(TE) and imperfect detection, we were able to estimate class-specific values of true capture probability (p^* ) for tagged and untagged sharks. The best-supported model identified differences between maturity classes but no difference between sexes or tagging impacts (tagged mature sharks p^*  = 0.55 (0.46-0.63) and sub-adult sharks p*  = 0.36 (0.25, 0.50); and untagged mature sharks p^*  = 0.50 (0.39-0.61) and sub-adults p^*  = 0.18 (0.10, 0.31). Estimated sex-based differences in p^o were linked to sex-specific differences in Pr(TE) but not in p^* ; once the Pr(TE) is accounted for, the p^* between sexes was not different. These results indicate that the observed sex ratio is not a consequence of unequal detectability and sex-specific values of Pr(TE) are important drivers of the observed male-dominated sex ratio. Our modeling approach reveals complex class-specific patterns in Pr(TE) and p^* in a mark-recapture data set, and highlights challenges for the population modeling and conservation of white sharks in central California. The model we develop here can be used to estimate rates of temporary emigration and class segregation when two detection methods are used.

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.

Segregation or aggregation? Sex-specific patterns in the seasonal occurrence of white sharks Carcharodon carcharias at the Neptune Islands, South Australia

The seasonal patterns of occurrence of male and female white sharks Carcharodon carcharias at the Neptune Islands in South Australia were reviewed. Analyses of a 14 year data series indicate that females seasonally aggregate in late autumn and winter coinciding with the maximum in-water availability of lactating female long-nose fur seals and seal pups. During this period, observed male:female sex ratios were similar; whereas during late spring and summer, males continued to visit, but females were rarely recorded. There was no evidence for segregation by sex or size at the Neptunes, but the highly focused seasonal pattern of occurrence of females compared with the year-round records of males suggests that there are likely to be differences between the sexes in overall distribution and movement patterns across southern Australia. It is suggested that foraging strategies and prey selection differ between sexes in C. carcharias across the life-history stages represented and that sex-specific foraging strategies may play an important role in structuring movement patterns and the sex ratios observed at such aggregation sites. Differences between sexes in distribution, movement patterns and foraging strategies are likely to have implications for modelling the consequences of fisheries by-catch between regions or jurisdictions and other spatially or temporally discrete anthropogenic effects on C. carcharias populations. Such differences urge for caution when estimating the size of C. carcharias populations based on observations at pinniped colonies due to the likelihood of sex-specific differences in movements and patterns of residency. These differences also suggest a need to account for sex-specific movement patterns and distribution in population and movement models as well as under conservation actions.

Risks and advantages of using surface laser photogrammetry on free-ranging marine organisms: a case study on white sharks Carcharodon carcharias

This study employed a non-lethal measurement tool, which combined an existing photo-identification technique with a surface, parallel laser photogrammetry technique, to accurately estimate the size of free-ranging white sharks Carcharodon carcharias. Findings confirmed the hypothesis that surface laser photogrammetry is more accurate than crew-based estimations that utilized a shark cage of known size as a reference tool. Furthermore, field implementation also revealed that the photographer's angle of reference and the shark's body curvature could greatly influence technique accuracy, exposing two limitations. The findings showed minor inconsistencies with previous studies that examined pre-caudal to total length ratios of dead specimens. This study suggests that surface laser photogrammetry can successfully increase length estimation accuracy and illustrates the potential utility of this technique for growth and stock assessments on free-ranging marine organisms, which will lead to an improvement of the adaptive management of the species.

There and back again: a review of residency and return migrations in sharks, with implications for population structure and management

The overexploitation of sharks has become a global environmental issue in need of a comprehensive and multifaceted management response. Tracking studies are beginning to elucidate how shark movements shape the internal dynamics and structure of populations, which determine the most appropriate scale of these management efforts. Tracked sharks frequently either remain in a restricted geographic area for an extended period of time (residency) or return to a previously resided-in area after making long-distance movements (site fidelity). Genetic studies have shown that some individuals of certain species preferentially return to their exact birthplaces (natal philopatry) or birth regions (regional philopatry) for either parturition or mating, even though they make long-distance movements that would allow them to breed elsewhere. More than 80 peer-reviewed articles, constituting the majority of published shark tracking and population genetic studies, provide evidence of at least one of these behaviors in a combined 31 shark species from six of the eight extant orders. Residency, site fidelity, and philopatry can alone or in combination structure many coastal shark populations on finer geographic scales than expected based on their potential for dispersal. This information should therefore be used to scale and inform assessment, management, and conservation activities intended to restore depleted shark populations.

Residency patterns and migration dynamics of adult bull sharks (Carcharhinus leucas) on the east coast of southern Africa

Bull sharks (Carcharhinus leucas) are globally distributed top predators that play an important ecological role within coastal marine communities. However, little is known about the spatial and temporal scales of their habitat use and associated ecological role. In this study, we employed passive acoustic telemetry to investigate the residency patterns and migration dynamics of 18 adult bull sharks (195–283 cm total length) tagged in southern Mozambique for a period of between 10 and 22 months. The majority of sharks (n = 16) exhibited temporally and spatially variable residency patterns interspersed with migration events. Ten individuals undertook coastal migrations that ranged between 433 and 709 km (mean  = 533 km) with eight of these sharks returning to the study site. During migration, individuals exhibited rates of movement between 2 and 59 km.d⁻¹ (mean  = 17.58 km.d⁻¹) and were recorded travelling annual distances of between 450 and 3760 km (mean  = 1163 km). Migration towards lower latitudes primarily took place in austral spring and winter and there was a significant negative correlation between residency and mean monthly sea temperature at the study site. This suggested that seasonal change is the primary driver behind migration events but further investigation is required to assess how foraging and reproductive activity may influence residency patterns and migration. Results from this study highlight the need for further understanding of bull shark migration dynamics and suggest that effective conservation strategies for this vulnerable species necessitate the incorporation of congruent trans-boundary policies over large spatial scales.

A re-evaluation of the size of the white shark (Carcharodon carcharias) population off California, USA

White sharks are highly migratory and segregate by sex, age and size. Unlike marine mammals, they neither surface to breathe nor frequent haul-out sites, hindering generation of abundance data required to estimate population size. A recent tag-recapture study used photographic identifications of white sharks at two aggregation sites to estimate abundance in “central California” at 219 mature and sub-adult individuals. They concluded this represented approximately one-half of the total abundance of mature and sub-adult sharks in the entire eastern North Pacific Ocean (ENP). This low estimate generated great concern within the conservation community, prompting petitions for governmental endangered species designations. We critically examine that study and find violations of model assumptions that, when considered in total, lead to population underestimates. We also use a Bayesian mixture model to demonstrate that the inclusion of transient sharks, characteristic of white shark aggregation sites, would substantially increase abundance estimates for the adults and sub-adults in the surveyed sub-population. Using a dataset obtained from the same sampling locations and widely accepted demographic methodology, our analysis indicates a minimum all-life stages population size of >2000 individuals in the California subpopulation is required to account for the number and size range of individual sharks observed at the two sampled sites. Even accounting for methodological and conceptual biases, an extrapolation of these data to estimate the white shark population size throughout the ENP is inappropriate. The true ENP white shark population size is likely several-fold greater as both our study and the original published estimate exclude non-aggregating sharks and those that independently aggregate at other important ENP sites. Accurately estimating the central California and ENP white shark population size requires methodologies that account for biases introduced by sampling a limited number of sites and that account for all life history stages across the species' range of habitats.

Seasonal distribution and historic trends in abundance of white sharks, Carcharodon carcharias, in the western North Atlantic Ocean

Despite recent advances in field research on white sharks (Carcharodon carcharias) in several regions around the world, opportunistic capture and sighting records remain the primary source of information on this species in the northwest Atlantic Ocean (NWA). Previous studies using limited datasets have suggested a precipitous decline in the abundance of white sharks from this region, but considerable uncertainty in these studies warrants additional investigation. This study builds upon previously published data combined with recent unpublished records and presents a synthesis of 649 confirmed white shark records from the NWA compiled over a 210-year period (1800-2010), resulting in the largest white shark dataset yet compiled from this region. These comprehensive records were used to update our understanding of their seasonal distribution, relative abundance trends, habitat use, and fisheries interactions. All life stages were present in continental shelf waters year-round, but median latitude of white shark occurrence varied seasonally. White sharks primarily occurred between Massachusetts and New Jersey during summer and off Florida during winter, with broad distribution along the coast during spring and fall. The majority of fishing gear interactions occurred with rod and reel, longline, and gillnet gears. Historic abundance trends from multiple sources support a significant decline in white shark abundance in the 1970s and 1980s, but there have been apparent increases in abundance since the 1990s when a variety of conservation measures were implemented. Though the white shark's inherent vulnerability to exploitation warrants continued protections, our results suggest a more optimistic outlook for the recovery of this iconic predator in the Atlantic.

The last frontier: catch records of white sharks (Carcharodon carcharias) in the Northwest Pacific Ocean

White sharks are highly migratory apex predators, globally distributed in temperate, sub-tropical, and tropical waters. Knowledge of white shark biology and ecology has increased recently based on research at known aggregation sites in the Indian, Atlantic, and Northeast Pacific Oceans; however, few data are available for the Northwest Pacific Ocean. This study provides a meta-analysis of 240 observations of white sharks from the Northwest Pacific Ocean between 1951 and 2012. Records comprise reports of bycatch in commercial fisheries, media accounts, personal communications, and documentation of shark-human interactions from Russia (n = 8), Republic of Korea (22), Japan (129), China (32), Taiwan (45), Philippines (1) and Vietnam (3). Observations occurred in all months, excluding October-January in the north (Russia and Republic of Korea) and July-August in the south (China, Taiwan, Philippines, and Vietnam). Population trend analysis indicated that the relative abundance of white sharks in the region has remained relatively stable, but parameterization of a 75% increase in observer effort found evidence of a minor decline since 2002. Reliably measured sharks ranged from 126–602 cm total length (TL) and 16–2530 kg total weight. The largest shark in this study (602 cm TL) represents the largest measured shark on record worldwide. For all countries combined the sex ratio was non-significantly biased towards females (1∶1.1; n = 113). Of 60 females examined, 11 were confirmed pregnant ranging from the beginning stages of pregnancy (egg cases) to near term (140 cm TL embryos). On average, 6.0±2.2 embryos were found per litter (maximum of 10) and gestation period was estimated to be 20 months. These observations confirm that white sharks are present in the Northwest Pacific Ocean year-round. While acknowledging the difficulties of studying little known populations of a naturally low abundance species, these results highlight the need for dedicated research to inform regional conservation and management planning.

Environmental influences on the abundance and sexual composition of white sharks Carcharodon carcharias in Gansbaai, South Africa

The seasonal occurrence of white sharks visiting Gansbaai, South Africa was investigated from 2007 to 2011 using sightings from white shark cage diving boats. Generalized linear models were used to investigate the number of great white sharks sighted per trip in relation to sex, month, sea surface temperature and Multivariate El Niño/Southern Oscillation (ENSO) Indices (MEI). Water conditions are more variable in summer than winter due to wind-driven cold water upwelling and thermocline displacement, culminating in colder water temperatures, and shark sightings of both sexes were higher during the autumn and winter months (March-August). MEI, an index to quantify the strength of Southern Oscillation, differed in its effect on the recorded numbers of male and female white sharks, with highly significant interannual trends. This data suggests that water temperature and climatic phenomena influence the abundance of white sharks at this coastal site. In this study, more females were seen in Gansbaai overall in warmer water/positive MEI years. Conversely, the opposite trend was observed for males. In cool water years (2010 to 2011) sightings of male sharks were significantly higher than in previous years. The influence of environmental factors on the physiology of sharks in terms of their size and sex is discussed. The findings of this study could contribute to bather safety programmes because the incorporation of environmental parameters into predictive models may help identify times and localities of higher risk to bathers and help mitigate human-white shark interactions.

The influence of environmental variables on the presence of white sharks, Carcharodon carcharias at two popular Cape Town bathing beaches: a generalized additive mixed model

Shark attacks on humans are high profile events which can significantly influence policies related to the coastal zone. A shark warning system in South Africa, Shark Spotters, recorded 378 white shark (Carcharodon carcharias) sightings at two popular beaches, Fish Hoek and Muizenberg, during 3690 six-hour long spotting shifts, during the months September to May 2006 to 2011. The probabilities of shark sightings were related to environmental variables using Binomial Generalized Additive Mixed Models (GAMMs). Sea surface temperature was significant, with the probability of shark sightings increasing rapidly as SST exceeded 14 °C and approached a maximum at 18 °C, whereafter it remains high. An 8 times (Muizenberg) and 5 times (Fish Hoek) greater likelihood of sighting a shark was predicted at 18 °C than at 14 °C. Lunar phase was also significant with a prediction of 1.5 times (Muizenberg) and 4 times (Fish Hoek) greater likelihood of a shark sighting at new moon than at full moon. At Fish Hoek, the probability of sighting a shark was 1.6 times higher during the afternoon shift compared to the morning shift, but no diel effect was found at Muizenberg. A significant increase in the number of shark sightings was identified over the last three years, highlighting the need for ongoing research into shark attack mitigation. These patterns will be incorporated into shark awareness and bather safety campaigns in Cape Town.