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

Patterns of overlapping habitat use of juvenile white shark and human recreational water users along southern California beaches

Juvenile white sharks (JWS) of the Northeastern Pacific population are present in nearshore southern California waters and form mixed size class (~1.5-3 m) aggregations for weeks to months, often within 500 m of shore. These nearshore beach habitats are heavily used for human recreation (e.g., surfing, swimming, body boarding, wading, and standup paddleboarding) and the amount of spatio-temporal overlap between JWS and humans is currently unknown. Increases in human population and the Northeastern Pacific population of white sharks have raised concern over human beach safety. To determine spatio-temporal JWS-human overlap at various spatial scales (e.g., across the entire southern California coastline, across different distances from shore, and within specific beach locations), 26 beach locations across southern California were surveyed monthly resulting in 1644 aerial drone surveys between January 2019 to March 2021. Thirteen environmental variables were assessed to predict when spatio-temporal overlap between JWS and water users was highest. Coast-wide distribution of JWS was clumped, limiting human-shark co-occurrence to specific locations, with 1096 of 1204 JWS observations occurring at Carpinteria and Del Mar Beach locations. Nearshore distribution indicated JWS are often close enough to the wave break to interact with some water users (median = 101 m, range = 2-702 m), although JWS had the most spatial overlap with stand-up paddlers. Daily human-shark co-occurrence was 97% at beaches where JWS aggregations had formed, and human activity showed high spatial overlap at shark aggregation sites. Although there is higher seasonal human-shark spatio-temporal overlap where aggregations form in southern California, the number of unprovoked shark bites across southern California is extremely low. This study provides evidence that high human-shark spatio-temporal overlap does not lead to an increased bite frequency in southern California, and there are a number of possible explanations as to why JWS are not biting water users despite daily encounters.

Quantifying Catch Rates, Shark Abundance and Depredation Rate at a Spearfishing Competition on the Great Barrier Reef, Australia

We developed and applied a method to quantify spearfisher effort and catch, shark interactions and shark depredation in a boat-based recreational spearfishing competition in the Great Barrier Reef Marine Park in Queensland. Survey questions were designed to collect targeted quantitative data whilst minimising the survey burden of spearfishers. We provide the first known scientific study of shark depredation during a recreational spearfishing competition and the first scientific study of shark depredation in the Great Barrier Reef region. During the two-day spearfishing competition, nine vessels with a total of 33 spearfishers reported a catch of 144 fish for 115 h of effort (1.25 fish per hour). A subset of the catch comprised nine eligible species under competition rules, of which 47 pelagic fish were weighed. The largest fish captured was a 34.4 kg Sailfish (Istiophorus platypterus). The most common species captured and weighed was Spanish Mackerel (Scomberomorus commerson). The total weight of eligible fish was 332 kg and the average weight of each fish was 7.1 kg. During the two-day event, spearfishers functioned as citizen scientists and counted 358 sharks (115 h effort), averaging 3.11 sharks per hour. Grey Reef Sharks (Carcharhinus amblyrhynchos) comprised 64% of sightings. Nine speared fish were fully depredated by sharks as spearfishers attempted to retrieve their catch, which equates to a depredation rate of 5.9%. The depredated fish included four pelagic fish and five reef fish. The shark species responsible were Grey Reef Shark (C. amblyrhynchos) (66%), Bull Shark (Carcharhinus leucas) (11%), Whitetip Reef Shark (Triaenodon obesus) (11%) and Great Hammerhead (Sphyrna mokarran) (11%). There were spatial differences in fish catch, shark sightings and rates of depredation. We developed a report card that compared average catch of fish, sightings of sharks per hour and depredation rate by survey area, which assists recreational fishers and marine park managers to assess spatio-temporal changes. The participating spearfishers can be regarded as experienced (average 18 days a year for average 13.4 years). Sixty percent of interviewees perceived that shark numbers have increased in the past 10 years, 33% indicated no change and 7% indicated shark numbers had decreased. Total fuel use of all vessels was 2819 L and was equivalent to 6.48 tons of greenhouse gas emissions for the competition.

How continuing mortality affects recovery potential for prohibited sharks: The case of white sharks in South Africa

It can be difficult to determine whether a prohibition to exploitation ensures effective conservation or recovery for species that remain exposed to fishing effort and other sources of mortality throughout their range. Here we used simulation modeling of four life history scenarios (different productivity and population size) to contextualize potential population response to multiple levels of mortality, using white sharks (Carcharodon carcharias) in South Africa as a case study. The species has been protected since 1991, yet substantial uncertainty about population dynamics persists and recent declines at two aggregation sites have renewed conservation concern. All scenarios indicated that annual removals in the 10s of individuals would substantially limit the potential for and magnitude of any abundance increase following prohibition. Because average known removals from the KwaZulu-Natal Sharks Board’s Bather Protection Program have typically remained higher than these thresholds, they likely eliminated much of the conservation benefit derived from prohibition. The only life history scenario to achieve appreciable increase when simulated removals were similar to published averages assumed maturation occurred at a much younger age than currently understood. Our results demonstrate why general application of life history-based simulations can provide a useful mechanism to evaluate the biological plausibility of life history information and abundance trends, and to explore the scope for population response to recovery actions. For South Africa, our results suggest that even known levels of white shark removals, which likely underestimate total removals within their range, may be sufficient to drive abundance decline and new mitigation measures may be required to ensure population recovery.

Current and future considerations for shark conservation in the Northeast and Eastern Central Pacific Ocean

Sharks are iconic and ecologically important predators found in every ocean. Because of their ecological role as predators, some considered apex predators, and concern over the stability of their populations due to direct and indirect overfishing, there has been an increasing amount of work focussed on shark conservation, and other elasmobranchs such as skates and rays, around the world. Here we discuss many aspects of current shark science and conservation and the path to the future of shark conservation in the Northeastern and Eastern Central Pacific. We explore their roles in ecosystems as keystone species; the conservation measures and laws in place at the international, national, regional and local level; the conservation status of sharks and rays in the region, fisheries for sharks in the Northcentral Pacific specifically those that target juveniles and the implications to shark conservation; a conservation success story: the recovery of Great White Sharks in the Northeast Pacific; public perceptions of sharks and the roles zoos and aquariums play in shark conservation; and the path to the future of shark conservation that requires bold partnerships, local stakeholders and innovative measures.

Effective number of white shark (Carcharodon carcharias, Linnaeus) breeders is stable over four successive years in the population adjacent to eastern Australia and New Zealand

Population size is a central parameter for conservation; however, monitoring abundance is often problematic for threatened marine species. Despite substantial investment in research, many marine species remain data-poor presenting barriers to the evaluation of conservation management outcomes and the modeling of future solutions. Such is the case for the white shark (Carcharodon carcharias), a highly mobile apex predator for whom recent and substantial population declines have been recorded in many globally distributed populations. Here, we estimate the effective number of breeders that successfully contribute offspring in one reproductive cycle (Nb) to provide a snapshot of recent reproductive effort in an east Australian-New Zealand population of white shark. Nb was estimated over four consecutive age cohorts (2010, 2011, 2012, and 2013) using two genetic estimators (linkage disequilibrium; LD and sibship assignment; SA) based on genetic data derived from two types of genetic markers (single nucleotide polymorphisms; SNPs and microsatellite loci). While estimates of Nb using different marker types produced comparable estimates, microsatellite loci were the least precise. The LD and SA estimates of Nb within cohorts using SNPs were comparable; for example, the 2013 age cohort Nb(SA) was 289 (95% CI 200-461) and Nb(LD) was 208.5 (95% CI 116.4-712.7). We show that over the time period studied, Nb was stable and ranged between 206.1 (SD ± 45.9) and 252.0 (SD ± 46.7) per year using a combined estimate of Nb(LD+SA) from SNP loci. In addition, a simulation approach showed that in this population the effective population size (Ne) per generation can be expected to be larger than Nb per reproductive cycle. This study demonstrates how breeding population size can be monitored over time to provide insight into the effectiveness of recovery and conservation measures for the white shark, where the methods described here may be applicable to other data-poor species of conservation concern.

An estimate of the number of white sharks Carcharodon carcharias interacting with ecotourism in Guadalupe Island

The aim of the present study is to provide an estimate of the number of white sharks Carcharodon carcharias that seasonally interact with ecotourism boats in Guadalupe Island using Schnabel's mark-recapture method and 6316 records of white sharks during 2012-2014. The results of the estimation highlight an abundance of 78 white sharks 95% C.I. (62.1, 105.6) interacting with ecotourism. The regulations regarding the number of tourists, boats and the monitoring of white sharks should be assessed to improve management decisions regarding the conservation and sustainable use of this threatened species.

Implications of life history uncertainty when evaluating status in the Northwest Atlantic population of white shark (Carcharodon carcharias)

To effectively protect at-risk sharks, resource managers and conservation practitioners must have a good understanding of how fisheries removals contribute to changes in abundance and how regulatory restrictions may impact a population trajectory. This means they need to know the number of animals being removed from a population and whether a given number of removals will lead to population increases or declines. For white shark (Carcharodon carcharias), theoretical quantities like the intrinsic rate of population increase or rebound potential (ability to increase in size following decline) are difficult to conceptualize in terms of real-world abundance changes, which limits our ability to answer practical management questions. To address this shortfall, we designed a simulation model to evaluate how our understanding of longevity and life history variability of white shark affects our understanding of population trends in the Northwest Atlantic. Then, we quantified the magnitude of removals that could have caused historical population declines, compared these to biologically based reference points, and explored the removal scenarios which would result in population increase. Our results suggest that removals on the order of 100s of juveniles per year could have resulted in population-level declines in excess of 60% during the 1970s and 1980s. Conservation actions implemented since the 1990s would have needed to be nearly 100% effective at preventing fishing mortality in order for the population to double in abundance over the last 30 years. Total removals from all fleets needed to be exceptionally small to keep them below biological reference points for white shark in the Northwest Atlantic. The population's inherent vulnerability to fishing pressure reaffirms the need for restrictive national and international conservation measures, even under a situation of abundance increase.

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.

Non-trophic impacts from white sharks complicate population recovery for sea otters

Complex interactions between protected populations may challenge the recovery of whole ecosystems. In California, white sharks (Carcharodon carcharias) mistargeting southern sea otters (Enhydra lutris nereis) are an emergent impact to sea otter recovery, inhibiting the broader ecosystem restoration sea otters might provide. Here, we integrate and analyze tracking and stranding data to compare the phenology of interactions between white sharks and their targeted prey (elephant seals, Mirounga angustirostris) with those of mistargeted prey (sea otters, humans). Pronounced seasonal peaks in shark bites to otters and humans overlap in the late boreal summer, immediately before the annual adult white shark migration to elephant seal rookeries. From 1997 to 2017, the seasonal period when sharks bite otters expanded from 2 to 8 months of the year and occurred primarily in regions where kelp cover declined. Immature and male otters, demographics most associated with range expansion, were disproportionately impacted. While sea otters are understood to play a keystone role in kelp forests, recent ecosystem shifts are revealing unprecedented bottom-up and top-down interactions. Such shifts challenge ecosystem management programs that rely on static models of species interactions.

Static Dental Disparity and Morphological Turnover in Sharks across the End-Cretaceous Mass Extinction

The Cretaceous-Palaeogene (K-Pg) mass extinction profoundly altered vertebrate ecosystems and prompted the radiation of many extant clades [1, 2]. Sharks (Selachimorpha) were one of the few larger-bodied marine predators that survived the K-Pg event and are represented by an almost-continuous dental fossil record. However, the precise dynamics of their transition through this interval remain uncertain [3]. Here, we apply 2D geometric morphometrics to reconstruct global and regional dental morphospace variation among Lamniformes (Mackerel sharks) and Carcharhiniformes (Ground sharks). These clades are prevalent predators in today's oceans, and were geographically widespread during the late Cretaceous-early Palaeogene. Our results reveal a decoupling of morphological disparity and taxonomic richness. Indeed, shark disparity was nearly static across the K-Pg extinction, in contrast to abrupt declines among other higher-trophic-level marine predators [4, 5]. Nevertheless, specific patterns indicate that an asymmetric extinction occurred among lamniforms possessing low-crowned/triangular teeth and that a subsequent proliferation of carcharhiniforms with similar tooth morphologies took place during the early Paleocene. This compositional shift in post-Mesozoic shark lineages hints at a profound and persistent K-Pg signature evident in the heterogeneity of modern shark communities. Moreover, such wholesale lineage turnover coincided with the loss of many cephalopod [6] and pelagic amniote [5] groups, as well as the explosive radiation of middle trophic-level teleost fishes [1]. We hypothesize that a combination of prey availability and post-extinction trophic cascades favored extant shark antecedents and laid the foundation for their extensive diversification later in the Cenozoic [7-10].

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.

Genetic relatedness reveals total population size of white sharks in eastern Australia and New Zealand

Conservation concerns exist for many sharks but robust estimates of abundance are often lacking. Improving population status is a performance measure for species under conservation or recovery plans, yet the lack of data permitting estimation of population size means the efficacy of management actions can be difficult to assess, and achieving the goal of removing species from conservation listing challenging. For potentially dangerous species, like the white shark, balancing conservation and public safety demands is politically and socially complex, often leading to vigorous debate about their population status. This increases the need for robust information to inform policy decisions. We developed a novel method for estimating the total abundance of white sharks in eastern Australia and New Zealand using the genetic-relatedness of juveniles and applying a close-kin mark-recapture framework and demographic model. Estimated numbers of adults are small (ca. 280-650), as is total population size (ca. 2,500-6,750). However, estimates of survival probability are high for adults (over 90%), and fairly high for juveniles (around 73%). This represents the first direct estimate of total white shark abundance and survival calculated from data across both the spatial and temporal life-history of the animal and provides a pathway to estimate population trend.

Diet Composition and Trophic Ecology of Northeast Pacific Ocean Sharks

Although there is a general perception of sharks as large pelagic, apex predators, most sharks are smaller, meso- and upper-trophic level predators that are associated with the seafloor. Among 73 shark species documented in the eastern North Pacific (ENP), less than half reach maximum lengths >200cm, and 78% occur in demersal or benthic regions of the continental shelf or slope. Most small (≤200cm) species (e.g., houndsharks) and demersal, nearshore juveniles of larger species (e.g., requiem sharks) consume small teleosts and decapod crustaceans, whereas large species in pelagic coastal and oceanic environments feed on large teleosts and squids. Several large, pelagic apex predator species occur in the ENP, but the largest species (i.e., Basking Shark, Whale Shark) consume zooplankton or small nekton. Size-based dietary variability is substantial for many species, and segregation of juvenile and adult foraging habitats also is common (e.g., Horn Shark, Shortfin Mako). Temporal dietary differences are most pronounced for temperate, nearshore species with wide size ranges, and least pronounced for smaller species in extreme latitudes and deep-water regions. Sympatric sharks often occupy various trophic positions, with resource overlap differing by space and time and some sharks serving as prey to other species. Most coastal species remain in the same general region over time and feed opportunistically on variable prey inputs (e.g., season migrations, spawning, or recruitment events), whereas pelagic, oceanic species actively seek hot spots of prey abundance that are spatiotemporally variable. The influence of sharks on ecosystem structure and regulation has been downplayed compared to that of large teleosts species with higher per capita consumption rates (e.g., tunas, billfishes). However, sharks also exert indirect influences on prey populations by causing behavioural changes that may result in restricted ranges and reduced fitness. Except for food web modelling efforts in Alaskan waters, the trophic impacts of sharks are poorly incorporated into current ecosystem approaches to fisheries management in the NEP.

Review of Current Conservation Genetic Analyses of Northeast Pacific Sharks

Conservation genetics is an applied science that utilizes molecular tools to help solve problems in species conservation and management. It is an interdisciplinary specialty in which scientists apply the study of genetics in conjunction with traditional ecological fieldwork and other techniques to explore molecular variation, population boundaries, and evolutionary relationships with the goal of enabling resource managers to better protect biodiversity and identify unique populations. Several shark species in the northeast Pacific (NEP) have been studied using conservation genetics techniques, which are discussed here. The primary methods employed to study population genetics of sharks have historically been nuclear microsatellites and mitochondrial (mt) DNA. These markers have been used to assess genetic diversity, mating systems, parentage, relatedness, and genetically distinct populations to inform management decisions. Novel approaches in conservation genetics, including next-generation DNA and RNA sequencing, environmental DNA (eDNA), and epigenetics are just beginning to be applied to elasmobranch evolution, physiology, and ecology. Here, we review the methods and results of past studies, explore future directions for shark conservation genetics, and discuss the implications of molecular research and techniques for the long-term management of shark populations in the NEP.

Genetic Diversity of White Sharks, Carcharodon carcharias, in the Northwest Atlantic and Southern Africa

The white shark, Carcharodon carcharias, is both one of the largest apex predators in the world and among the most heavily protected marine fish. Population genetic diversity is in part shaped by recent demographic history and can thus provide information complementary to more traditional population assessments, which are difficult to obtain for white sharks and have at times been controversial. Here, we use the mitochondrial control region and 14 nuclear-encoded microsatellite loci to assess white shark genetic diversity in 2 regions: the Northwest Atlantic (NWA, N = 35) and southern Africa (SA, N = 131). We find that these 2 regions harbor genetically distinct white shark populations (Φ ST = 0.10, P < 0.00001; microsatellite F ST = 0.1057, P < 0.021). M-ratios were low and indicative of a genetic bottleneck in the NWA (M-ratio = 0.71, P < 0.004) but not SA (M-ratio = 0.85, P = 0.39). This is consistent with other evidence showing a steep population decline occurring in the mid to late 20th century in the NWA, whereas the SA population appears to have been relatively stable. Estimates of effective population size ranged from 22.6 to 66.3 (NWA) and 188 to 1998.3 (SA) and evidence of inbreeding was found (primarily in NWA). Overall, our findings indicate that white population dynamics within NWA and SA are determined more by intrinsic reproduction than immigration and there is genetic evidence of a population decline in the NWA, further justifying the strong domestic protective measures that have been taken for this species in this region. Our study also highlights how assessment of genetic diversity can complement other sources of information to better understand the status of threatened marine fish populations.