Opportunities to enhance conservation success for sharks

High topic diversity and a critical nexus within the interconnected field of shark conservation

We quantitatively analysed the abstracts of 4401 peer-reviewed scientific papers and identified 29 topics in the shark conservation literature (Fig. 1a, Table S1). Some topics, specifically Geography & Species, Habitats, Monitoring & Methodology, and Research Type were deemed to be purely contextual topics and were therefore excluded from further analyses. The number of topics identified here is generally much greater than seen in previous studies that have quantitatively examined trends and priorities within shark research^4,7,8. This disparity demonstrates the complexity that may be lost when topics are selected a priori by the authors, rather than being identified directly from the data. The network of identified topics shows the field of shark conservation to be well interconnected with coherent structuring (Fig. 1a).

a Structural network of shark conservation and its constituent topics based on the strength of positive correlations in the scientific literature. b Change in the structural network of shark conservation topics over time. c Overall topic correlation, topic clusters are delineated by black lines, small points indicate correlations greater than 0.05 or less than −0.05, large points indicate correlations greater than 0.1 or less than −0.1. d Interdisciplinarity (± SE), proxied by change in mean primary topic gamma value over time (note inverted y-axis). e Variance of topic correlation strengths (± SE) within the network over time. Node diameter indicates relative topic frequency, node colour indicates topic clusters.

Four primary clusters of topics were quantitatively identified, each loosely corresponding to a different aspect of shark research:

1. (1)

   Life history – Growth, Reproduction, Sexual Dimorphism;

2. (2)

   Ecology and biology – Early Life, Ecosystem Role, Ecotoxicology, Movement Ecology, Spatial Distribution & Environmental Change, Spatial Management, Stress & Physiology, Trophic Ecology;

3. (3)

   Conservation science – Extinction Risk & Protections, Fisheries & Bycatch, Genetics, Markets & Fisheries Management, Population Structure, Taxonomy & Phylogeny; and

4. (4)

   Human-wildlife interaction – Behaviour & Deterrence, Public Perceptions, Shark Attack & Stingray Envenomation, Tourism.

A fifth evolutionary cluster was also identified but was largely not pertinent to shark conservation, and so was not considered further – Anatomy & Central Nervous System, Evolution, Immunology, and Proteomics. The cluster arises largely from the use of sharks as an evolutionary model system through which to understand key innovations in vertebrate evolution such as the development of the jaw, the appearance of the segmented organised brain, and the evolution of live-bearing young and maternal investment^11,12. Although considered an out-group for the purposes of our conservation-focused analysis, the evolutionary innovations and novelty of sharks has been used as a driver of conservation actions, such as through the EDGE (Evolutionary Distinct and Globally Endangered) paradigm¹³. The evolutionary cluster is connected to the shark conservation literature through the topics of Stress & Physiology, Taxonomy & Phylogeny, and Genetics. Additionally, the evolutionary cluster forms a bridge between shark conservation and broader biological inquiries, enriching our comprehension of shark biology and conservation while also contributing to the wider field of evolutionary biology, offering perspectives that are crucial for understanding the past, present, and future of vertebrates on Earth.

Central to the network are three topics that form the key nexus of shark conservation as a field of study: Public Perception, Markets & Fisheries Management, and Extinction Risk & Protections (Fig. 1a). This nexus bridges the gap between human-wildlife interactions and conservation, encapsulating what could be considered the most critical relationships in conservation science. This connection highlights how human interactions with nature shape the utilisation and management of natural resources, ultimately influencing their degradation and recovery^14,15. The nexus can also be thought of in economic terms. The market for sharks sits at the centre, being controlled through the balance of supply (Management and Extinction Risk & Protections) and demand (Public Perceptions, Shark Attack & Stingray Envenomation), where the goal of conservation is to adjust supply and demand in such a way as to improve conservation outcomes. Regardless, it is around this nexus that much of the shark conservation literature appears to revolve and to which most other topics can be traced.

The first set of key connections is apparent in two fully interconnected groups of topics associated with Public Perceptions and Extinction Risk & Protections, respectively. Public Perceptions of sharks, as a topic, is closely tied to fear in many areas of the world, particularly in the “west” (i.e. USA, Europe, South Africa and Australia), which dominates the research landscape⁸. This fear of sharks largely stems from deeply rooted cultural attitudes and is exacerbated in popular media^16,17. However, negative perceptions are slowly shifting through awareness campaigns and by positive exposures to sharks via ecotourism and recreational activities^18,19. This dynamic is reflected in the network by complete interconnectivity among Shark Attack & Stingray Envenomation, Tourism, Behaviour & Deterrence, and Public Perceptions, which also serve to link the ecology and human-wildlife interaction clusters (Fig. 1a). Public Perceptions also influence pro-conservation attitudes and norms^3,19 and have the potential to influence behaviour regarding consumptive use of sharks.

Similarly, full interconnectivity of Extinction Risk & Protections, Genetics, Taxonomy & Phylogeny, and Population Structure is observed within the network. An example of this connectivity is the control of international trade of shark products through the Convention on International Trade in Species (CITES). To-date more than 150 shark and ray species are listed on the appendices of CITES. CITES listings are a trade-focused measure which do not directly reduce the demand or supply of shark products but do attempt to ensure that international trade does not threaten the survival of species in the wild. One of the major challenges in enforcing trade restrictions and management efforts has been reliable species identification of shark products. Indeed, cases exist where improper species identification has led to entire fisheries statistics and subsequent trade records being rendered useless²⁰. Identification tasks rely on the development of dependable taxonomic keys, and genetic barcoding has become increasingly valuable for identifying cryptic species, fins, and body parts that have otherwise been difficult to monitor in global trade and markets^21,22. Similarly, with the advent of affordable and high-resolution genetic markers in the early 2000s, molecular tools have been increasingly used to reveal population structure and connectivity, as well as to address evolutionary, taxonomic, and phylogenetic issues in sharks^23,24,25. Both the Public Perceptions and Extinction Risk & Protections groups of topics are isolated from other topics in the conservation nexus, but are not disconnected from the wider network, where we also find groups of topics that share links between multiple components of the topic nexus.

The second set of connections can be seen in those groups of topics that associate with more than one part of the topic nexus. Fisheries & Bycatch forms a secondary link between Markets & Fisheries Management and Extinction Risk & Protections, with fisheries being the largest source of shark mortality and primary driver of shark extinction risk^1,5. Fisheries & Bycatch is subsequently linked closely with the key life-history topics, including Growth, Reproduction, Early Life, and Sexual Dimorphism, which are the underpinnings of fisheries stock assessments and management^26,27. Fisheries & Bycatch also links to Spatial Distribution & Environmental Change, which is key in defining the spatial risk to sharks posed by fisheries^28,29,30 and mortality due to ship strikes³¹, as well as the future of fisheries and sharks in a changing climate¹. Similarly, Spatial Management forms a secondary link between Markets & Fisheries Management and Public Perceptions and likely reflects the role that public attitudes and engagement play in both the establishment and ongoing effectiveness of management strategies for sharks^32,33. Like Fisheries & Bycatch, Spatial Management also acts as a link between other topics and the conservation nexus, specifically ecological topics like Movement Ecology, Spatial Distribution & Environmental Change, Ecosystem Role, and Trophic Ecology, topics that are critical to justifying and designing marine protected areas and other spatial protections³⁴.

The remaining topics, Ecotoxicology and Stress & Physiology, appear to be more peripheral to the conservation nexus, with Ecotoxicology only sharing a very weak link to Extinction Risk & Protections and Stress & Physiology sharing no link at all (Fig. 1). This indicates that both topics are not well integrated into the conservation literature, yet their inclusion is crucial for a comprehensive understanding of conservation science and future threats^35,36,37. Physiological research in sharks generally lags behind their teleost counterparts. Firstly, these types of research are typically carried out on only a handful of well-studied, small species that are amenable to captivity, which limits knowledge to certain taxa (e.g., Squalidae, Rajidae, Hemiscylliidae, Scyliorhinidae). Additionally, sharks have unique respiratory, osmoregulatory, and energy mobilisation pathways that are still being untangled, and these limit the current understanding of ecotoxicological and stressor impacts^38,39. Whether effects seen in well studied species can be directly extrapolated to other taxonomic groups is still uncertain. Ecotoxicology plays a pivotal role in identifying the impacts of pollutants on marine life, offering insights into lethal and sub-lethal stressors, including reproductive success, which may impact species survival⁴⁰. Similarly, Stress & Physiology provides critical insights into how environmental stressors, both natural and anthropogenic, affect the health and viability of individuals and potentially of populations⁴¹.

Negative associations expose existing weaknesses in shark conservation research

Despite the encouragingly strong interconnectivity generally found within the shark conservation literature, the analysis also reveals clear gaps that highlight potential opportunities and under-explored areas of study. Negative correlations observed between topics often simply reflect fundamental incompatibilities (Fig. 1c). For instance, it is unsurprising that Ecotoxicology and Shark Tourism rarely intersect. More concerning are those cases where clearly related topics, sometimes even those within the same topic cluster, appear to be disconnected. Here we discuss some of the prominent examples of negative correlations revealing what we consider to be key weaknesses and research gaps.

Spatial Management was negatively correlated with Trophic Ecology, despite being in the same topic cluster, and Behaviour & Deterrence. Spatial management approaches are commonly cited as potentially key conservation tools in the protection of sharks, particularly for species with relatively small movement ranges and/or where critical habitats such as foraging grounds can be identified for protection^34,42. The effect of protected areas on the behaviour and trophic interactions of sharks within them will likely be key to the success of conservation measures and the effects sharks might have within protected areas. The decoupling of these topics could, therefore, weaken prospective designs and assessments of success of spatial management measures for sharks. Similarly, both Stress & Physiology and Ecotoxicology were negatively correlated with Growth, and Stress & Physiology was also negatively correlated with Sexual Dimorphism. This decoupling was found despite the known potential for toxins and other stressors to negatively affect growth and developmental patterns, which may result in impacts on individual fitness, reproduction, population dynamics, and species’ recovery potential^35,36,43.

Arguably the most prominent example of decoupling between topics was observed for Movement Ecology and Population Structure. Our analysis suggests that papers addressing these two topics were least commonly contextualised by or within other topics (Fig. 2c). This apparent weakness is exacerbated by negative correlations, both between these two topics and with other topics that would be expected to be closely related. For example, demographics of species sub-populations are often related to ontogenetic or sexual segregation of habitat preferences⁴⁴, genetic connectivity is often a reflection of a species’ dispersal behaviour⁴⁵, and spatial management is less effective for species that can migrate beyond the borders of a protected area⁴⁶. Despite these clear ecological connections, Population Structure, Genetics, and Markets and Management were all negatively correlated with Movement Ecology. Similarly, Population Structure was negatively correlated with Spatial Distribution & Climate Change and Reproduction, regardless of the demographic differences in spatial distribution of many species^44,47 and the clear link between sexual demographics and reproductive ecology.

a Topic frequency based on gamma (the modelled proportional contribution of each topic to a given paper) and primary topic, grouped by topic cluster. b Primary topic to gamma ratio, higher values indicate topics more likely to be the primary topic within the papers in which they occur. c Percentage of threatened species by threat category and percentage of species impacted by overlapping threats according to the IUCN Red List.

It is important to be clear that negative correlations between topics do not mean that they are never discussed together within the shark conservation literature. Rather, it indicates where topics have rarely been brought together specifically for targeted analyses. A more comprehensive overview of potential weaknesses can be found in Table 1. Addressing these key weaknesses would represent progress toward a more comprehensive understanding of sharks as it relates to their conservation, presenting opportunities for researchers to specifically target key gaps and design their research accordingly.

An increasingly interdisciplinary science

There is broad consensus that interdisciplinary conservation research is critical in achieving conservation success across marine and terrestrial environments because human dimensions are critical to the design and implementation of effective management^9,10. This is no different for shark conservation. While the call for interdisciplinarity is often explicitly associated with bridging the gaps between the biological, social, and economic sciences, the sentiment also applies to the integration of topics within each of these disciplines. The number of positive correlations (edges) remained relatively stable over time (Fig. 1b) but variance in the strength of these correlations did not (F = 93.0, p < 0.001). Variance in topic correlation strength has declined (Fig. 1e) because of reductions in the extreme correlation strengths seen between some topic pairs in earlier works (Fig. 1b). At first glance, this decline in extreme correlation strengths might appear as a weakening of the network structure but, when paired with the simultaneous declines seen in gamma values for primary topics (F = 46.2, p < 0.001, Fig. 1d), it instead suggests that shark conservation research has become increasingly interdisciplinary (Fig. 1d). Further, there are several popular research topics closely associated with the boundary between the social and economic sciences and the biological sciences, including Markets & Fisheries Management, Public Perceptions, and Tourism (Fig. 1a, Fig. 2a). This trend of increasing interdisciplinarity in shark conservation must be considered a success for the field, reflecting an acknowledgement of its importance for effective conservation. Researchers should continue to strive for and promote interdisciplinary approaches going forward, helping to build meaningful linkages between research topics.

Key threats receive highest scientific effort, but combined threats are rarely considered

Examination of topic frequency metrics shows some divergence, but several topics rank consistently high and low across measures (Fig. 2a, b). Specifically, the topics of Markets & Fisheries Management, Fisheries & Bycatch, Movement Ecology, Taxonomy & Phylogeny, and Public Perceptions are popular in the scientific literature. It is also notable that specific components of Fisheries & Bycatch, such as species stock assessments, exist largely outside of the peer-reviewed literature in scientific journals. Instead, such work is typically published as peer-reviewed reports or grey literature, and so the relative popularity of this research topic is likely much greater than represented here. Conversely, Reproduction, Early Life, and Ecotoxicology receive relatively little attention, as do Behaviour & Deterrence and Shark Attack & Stingray Envenomation when considering only the primary topic of papers. Behaviour & Deterrence and Shark Attack & Stingray Envenomation are both closely linked to and discussed within the context of Public Perceptions, which likely explains their lower occurrence as primary topics relative to their overall contribution to the literature.

The primary direct threats to sharks are from exploitation in fisheries, habitat loss and degradation, and climate change, with pollution contributing relatively little to the overall extinction risk of species¹. The limited importance of pollution as a threat may result from of the relative underdevelopment of Ecotoxicology as a research field in shark conservation, its primary focus on individual organism survival, and as-yet limited evidence for population level effects. The current prioritisation of threats is reflected in the relative popularity of topics, with Fisheries & Bycatch and Markets & Fisheries Management being among the most popular topics, Spatial Distribution & Environmental Change a topic of moderate popularity, and Ecotoxicology being among the least common topics (Fig. 2a, b). Approximately 33% of threatened species are impacted by combinations of these threats (Fig. 2c); yet, we found only a weak positive correlation between the topics Fisheries & Bycatch and Spatial Distribution & Environmental Change and there was no correlation between Ecotoxicology and any other key threat related topic. Further, there was evidence for a negative correlation between Markets & Fisheries Management and Spatial Distribution & Environmental Change (Fig. 1c). Taken together this suggests that there is relatively little research that integrates the combined effects of multiple and potentially synergistic threats, highlighting this as a clear priority for future shark conservation research.

A rapidly growing field with some emergent topics but others falling by the wayside

The shark conservation literature has existed since at least the 1930s but its growth rapidly increased in the 1990s (Fig. 3a, b). The cumulative number of publications grew by 258% between 1990 and 2000, by a further 225% between 2000 and 2010, and by another 250% between 2010 and 2020, reflecting rapid exponential growth in the field. This rapid growth is reflected in the continuous exponential growth of almost all topics within the shark conservation scientific literature (Fig. 3c).

a Annual number of publications in shark conservation by year. b Cumulative number of publications in shark conservation by year. c Trends in shark conservation topics by year, note logarithmic scale on the y-axis.

While all topics have shown rapid (p < 0.001) and often exponential growth, some have experienced a “boom and bust” cycle with specific periods of rapid growth followed by a relative deceleration (Fig. 3c). The most prominent example is the topic Growth, which expanded particularly quickly between the late 1990s and early to mid-2000s, due to pioneering work in this time by a core group of researchers publishing Growth studies for numerous shark species. As a result, Growth became a core research topic in shark science, leading to a relative stabilisation in the topic’s growth rate. To a lesser extent, Sexual Dimorphism, Ecosystem Role, Tourism, Markets & Fisheries Managements, Fisheries & Bycatch, and Taxonomy & Phylogeny showed similar patterns, with particularly high relative growth during the 2000s and Fisheries & Bycatch sustaining this into the 2010s. These trends are likely driven by a combination of factors, including heightened global conservation awareness, the implementation of international agreements and national regulations aimed at protecting marine biodiversity, increased ocean exploration, and significant scientific discoveries that underscored the ecological importance of sharks^48,49. The sustained growth rate of Fisheries & Bycatch into the 2010s can be attributed to ongoing concerns over the impacts of commercial fishing practices on shark populations and the broader marine ecosystem, as well as improvements in monitoring technologies⁵⁰. Indeed, the rate of change for Fisheries & Bycatch may be even greater than it appears here, given that much of technical work in this area exists outside of the peer-reviewed literature published in scientific journals.

Movement Ecology, Population Structure and Genetics all showed the greatest relative increases in growth rates during the early 2010s. Advancements in, and accessibility of, biotelemetry and genetic sequencing technologies are likely a major driver of this change, dramatically enhancing our ability to study the movement patterns, population genetics, and genomic diversity of sharks^23,50. These technological breakthroughs have provided unprecedented insights into shark behaviour, migration, and population connectivity, informing conservation strategies and management practices. Additionally, the early 2010s saw an increased emphasis on the role of genetic diversity and ecosystem dynamics in the resilience of shark populations to environmental changes and human pressures^51,52. Since then, the growth rate of publications in Genetics has slowed substantially, suggesting the field has reached a phase of maturation where initial rapid advancements and discoveries (e.g., genetic barcoding, eDNA, genome sequencing etc.) have given way to a phase of consolidation and deeper exploration, and findings for new molecular markers have become more difficult to publish on their own. Additionally, the high costs and technical expertise required for genomic research may limit the initiation and success rate of new studies. As the field of Genetics becomes more integrated with other areas of shark research, such as ecology and conservation strategies, the focus may shift from pure genetics to interdisciplinary studies that incorporate genomic data and more deeply explore its implications for shark conservation and management.

The one exception to these general patterns is Ecotoxicology, which is a recent emergent topic in the shark and ray conservation literature. Since the late 2010s, Ecotoxicology has shown rapid growth, reflecting increased awareness of the potential impacts on human health from the consumption of shark meat and the realisation that pollutants may contribute to elevated extinction risk^53,54,55.