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Network analysis of the endemic spotted gully shark Triakis megalopterus reveals spatial vulnerability to exploitation in the Western Cape, South Africa
Abstract
The spotted gully shark Triakis megalopterus (Triakidae) is a mesopredatory species endemic to southern
Africa. It is currently listed as Least Concern on the IUCN Red List in accordance with an estimated increase in
population size, general release by recreational linefishers and incidental catches in the commercial linefisheries.
Previous research suggests this species to be resident, and as such it is likely to receive protection in coastal
marine protected areas (MPAs). However, its ecology and movement behaviour remain poorly studied. This study
employed acoustic telemetry to provide information on the species’ movements along the coast of the Western
Cape Province, South Africa. We used network analyses to investigate movement randomness, associations
between individuals, sexual segregation, and the effectiveness of MPAs. Our findings reveal nonrandom
movements as well as patterns of co-occurrence between individuals. Spatial network analysis suggested
sexual segregation, because areas of high use (Walker Bay and De Hoop) differed between males and females.
Co-occurrences were observed exclusively in Walker Bay, chiefly between males, with no co-occurrence found
between females. The tagged spotted gully sharks were not detected extensively within existing MPA boundaries,
though there was no significant difference between their movements inside and outside protected areas for both
sexes.
... Despite the abundance of tagging information, surprisingly little research has been done on the movement behaviour of T. megalopterus, except for a comparative review of five species tagged in the ORI-CFTP, in which Maggs et al. (2019) described this shark as wide-ranging, with a higher probability of ranging in adults than in juveniles. More recently, a spatial network analysis using acoustic telemetry in the WC revealed non-random movements as well as patterns of co-occurrence between individuals (Cottrant et al. 2023). That study also suggested sexual segregation, because areas of high use differed between males and females. ...
... Further research using acoustic telemetry may elucidate this hypothesis, but we note that the current Acoustic Tracking Array Platform (ATAP) receivers may be moored too deep (most are diver-retrieved in 16-30 m) (see review by Murray et al. 2022) to effectively track T. megalopterus, which is largely found in the surf zone. The recent findings by Cottrant et al. (2023) may have been constrained by this limitation as well as by their small sample size of acoustically tagged animals (n = 25). Nevertheless, those authors did show clear evidence of non-random movement Figure 9: Movements of wide-ranging adult Triakis megalopterus that moved >50 km in less than 365 days in the current study (n = 17) with patterns of spatial segregation by sex, along with co-occurrence between individuals. ...
... Furthermore, females generally grow bigger and live longer than males and have a lower natural mortality rate (Wagstaff 2023). Therefore, it is likely that movement behaviour differs between males and females, as was recently shown by Cottrant et al. (2023). Such differences have also been observed for T. semifasciata, which show sex-specific aggregations at different times of year (Nosal et al. 2014). ...
The coastal waters of South Africa are habitat to a diverse composition of sharks that are vulnerable to exploitation, many of which are endemic and/or classified by the International Union for Conservation of Nature (IUCN) Red List as Threatened or Data Deficient. Accordingly, this region has been identified as a global research and conservation priority for elasmobranchs. The De Hoop Marine Protected Area (MPA), in the Western Cape Province of South Africa, provides 288 km² of no-take protection within its boundaries. However, the region experiences heavy commercial fishing, with two vessels actively operating as dedicated shark longliners (as of 2022). When crossing MPA boundaries, sharks are susceptible to capture by these vessels. Utilizing passive acoustic telemetry, the present study evaluated the movements of a threatened juvenile shark species, the smooth hammerhead (Sphyrna zygaena), both inside and adjacent to the De Hoop MPA, and along the greater coastline. Movement data from 20 tagged sharks were used to explore the effects of spatial, environmental, and management variables on their residency and movement patterns. Results indicate a high reliance of sharks on unprotected waters immediately adjacent to the MPA’s eastern boundary, an area of high biological productivity due to its proximity to the mouth of an estuary. Although some tagged sharks did move regionally along the South African coastline, individuals spent 95% of their days detected just outside the eastern boundary of the MPA, rendering them vulnerable to commercial shark longlining occurring there. These findings have conservation implications for smooth hammerhead sharks in South Africa and present an opportunity to revisit management practices that may optimize spatial protection for an important life stage of this threatened species.
The role dentition plays in shaping feeding patterns has significant consequences for understanding the evolution of shark feeding. Despite this, research has generally failed to compare tooth morphology and feeding ecology in elasmobranchs. For this reason, the diet and dentition of 120 Triakis megalopterus were examined from three study sites in southern Africa. Teleosts and crustaceans dominated the prey categories of T. megalopterus in Angola, and the Eastern Cape, respectively. Individuals from the Western Cape, however, fed almost exclusively on Jasus lalandii (%N > 98.1). This suggests that T. megalopterus may employ flexible foraging tactics to exploit abundant prey. Despite substantial dietary differences by region, the dentition of T. megalopterus remained remarkably similar amongst populations. This species possesses an anterior plate-like dentition with posterior teeth that rise into a short but sharp cusp, consistent with a trophic generalist. It is hypothesized that T. megalopterus employs a suction feeding mechanism while depressing and rotating these cusps, forming a plate-like grasping surface, thus avoiding tooth damage when feeding on crustaceans. Despite regional dental homology, a strong correlation exists between the feeding habits and dental divergence of this species throughout ontogeny. Small T. megalopterus have dorsoventrally flattened, more molariform lateral teeth which increase in area and width from small to large specimens, while the medial-inferior and medial-superior teeth lengthen producing a single cusp. The ontogenetic variation in dietary composition (increasing importance of cephalopods with size) appeared to be facilitated by this cusp development for grasping and/or manipulating soft-bodied prey.
The Blue Economy is a global initiative aimed at using marine resources to create economic viability and environmental sustainability. While successes have been reported, for example, in Europe and China, examples of African successes are notably missing. Abject poverty, unemployment and food insecurity are everyday concerns on the African continent; however, its large latitudinal coverage gives rise to extremely biodiverse marine fauna, which could promote socio-economic development of coastal communities through initiatives such as sustainably-managed fisheries. In order to improve sustainability via improved management, information on a species and its habitat is needed, particularly how it moves and in which areas it occurs. Acoustic telemetry is a powerful tool used to determine the movements of aquatic animals, the success of which has led to the development of several large-scale networks throughout the globe, including South Africa’s Acoustic Tracking Array Platform. This network, formally in place for the last decade, has now matured, and data are revealing insights into residency, habitat connectivity and transboundary movements of a multitude of animals, with some species having been continually detected for the past 10 years. These data are also actively being incorporated into marine spatial planning efforts, with the aim of protecting threatened and endemic species. Due to knowledge generation, successful benefit-sharing arrangements, and dedication to engage with the public and other stakeholder groups, the ATAP represents a highly successful example of ocean stewardship in Africa.
There are many syntheses on the role of animal behavior in understanding and mitigating conservation threats for wildlife. That body of work has inspired the development of a new discipline called conservation behavior. Yet, the majority of those synthetic papers focus on non-fish taxa such as birds and mammals. Many fish populations are subject to intensive exploitation and management and for decades researchers have used concepts and knowledge from animal behavior to support management and conservation actions. Dr. David L. G. Noakes is an influential ethologist who did much foundational work related to illustrating how behavior was relevant to the management and conservation of wild fish. We pay tribute to the late Dr. Noakes by summarizing the relevance of animal behavior to fisheries management and conservation. To do so, we first consider what behavior has revealed about how fish respond to key threats such as habitat alteration and loss, invasive species, climate change, pollution, and exploitation. We then consider how behavior has informed the application of common management interventions such as protected areas and spatial planning, stock enhancement, and restoration of habitat and connectivity. Our synthesis focuses on the totality of the field but includes reflections on the specific contributions of Dr. Noakes. Themes emerging from his approach include the value of fundamental research, management-scale experiments, and bridging behavior, physiology, and ecology. Animal behavior plays a key role in understanding and mitigating threats to wild fish populations and will become more important with the increasing pressures facing aquatic ecosystems. Fortunately, the toolbox for studying behavior is expanding, with technological and analytical advances revolutionizing our understanding of wild fish and generating new knowledge for fisheries managers and conservation practitioners.
Shark dive ecotourism is a lucrative industry in many regions around the globe. In some cases, sharks are provisioned using bait, prompting increased research on how baited dives influence shark behavior and yielding mixed results. Effects on patterns of habitat use and movement seemly vary across species and locations. It is unknown, however, whether wide-ranging, marine apex predators respond to provisioning by changing their patterns of grouping or social behavior. We applied a tiered analytical approach (aggregation-gregariousness-social preferences) examining the impact of provisioning on the putative social behavior of tiger sharks (Galeocerdo cuvier) at a dive tourism location in The Bahamas. Using network inference on three years of acoustic tracking data from 48 sharks, we tested for non-random social structure between non-provisioned and provisioned monitoring sites resulting in 12 distinct networks. Generally considered a solitary nomadic predator, we found evidence of sociality in tiger sharks, which varied spatiotemporally. We documented periods of both random (n = 7 networks) and non-random aggregation (n = 5 networks). Three of five non-random aggregations were at locations unimpacted by provisioning regardless of season, one occurred at an active provisioning site during the dry season and one at the same receivers during the wet season when provision activity is less prevalent. Aggregations lasted longer and occurred more frequently at provisioning sites, where gregariousness was also more variable. While differences in gregariousness among individuals was generally predictive of non-random network structure, individual site preferences, size and sex were not. Within five social preference networks, constructed using generalized affiliation indices, network density was lower at provisioning sites, indicating lower connectivity at these locations. We found no evidence of size assortment on preferences. Our data suggest that sociality may occur naturally within the Tiger Beach area, perhaps due to the unusually high density of individuals there. This study demonstrates the existence of periodic social behavior, but also considerable variation in association between tiger sharks, which we argue may help to mitigate any long-term impacts of provisioning on this population. Finally, we illustrate the utility of combining telemetry and social network approaches for assessing the impact of human disturbance on wildlife behavior.
Despite their potential to generate high-quality positioning data, the use of high-resolution acoustic telemetry systems (HRATS) has been neglected in coastal marine areas due to the limitations that these environments pose to the transmission of acoustic signals. In this work, we applied a HRATS and social network analysis (SNA) to study the social interactions of the pearly razorfish (Xyrichtys novacula), a small coastal wrasse, in a Mediterranean marine reserve. Our analysis was based on proximity measures estimated from high-resolution trajectories from 232 individuals tracked during 55 days within a marine protected area. Associations were defined as the proportion of 5-min intervals in which two individuals were observed within 1 m from each other, and social networks were generated for the overall tracking period and for each particular day. The obtained network parameters were contrasted against 1,000 null association models obtained by randomly redistributing individual trajectories within the study area. The obtained networks showed a harem-like social structure, with agonistic behavior between males and larger association indices between individuals of different sex. Occasionally, sporadic associations of large groups of females were observed conducting excursions along the study area. By providing a comprehensive view of the organizational structure of the pearly razorfish, our study demonstrates the potential of HRATS to efficiently produce high-throughput tracking data from large numbers of individuals and of proper null social model formulation to reconstruct the social networks in wild-living marine fish populations. The combination of HRATS and SNA represents a powerful tool to study key ecological processes regarding the social interactions of individuals, including social dynamics, collective movements, and the response to environmental perturbations.
Knowledge of the movement patterns of the lemon shark Negaprion acutidens is poor in contrast to the allopatric N. brevirostis. Using acoustic telemetry, we investigated daily (diel and tidal) and seasonal patterns in residency, fidelity, home range, habitat preference, and migratory patterns along the Ningaloo coast, Australia. Thirty eight adult N. acutidens were monitored for up 6.1 years (mean 2.5 y) with 19 animals detected for more than 3 years and 5 for more than 5 y. Approximately 50% of the tagged animals remained within 10 km of their tagging location for more than 12 months (average core home range of 1.7 km²). Surprisingly, residency of adults was greater than juveniles in this known nursery area. Adults showed a strong preference for lagoon habitats and moved into shallow nearshore habitats at high tide and at night. During winter months, female sharks shifted their core home range 0.45 km further offshore into deeper lagoon areas, a shift which is likely due to behavioral thermoregulation. Space use by males and females within core areas was asynchronous indicating sexual segregation by resident sharks. Both resident and non-resident sharks were detected up to 140 km away. These highly directional (southerly) and rapid movements (140 km in 2 days) were largely correlated with the parturition and mating periods with males departing 1–2 months earlier than females. In females, periodicity of migrations was variable with evidence of annual and biannual patterns. Negaprion acutidens are highly susceptible to over-fishing and movement data are essential to quantify spatio-temporal overlap with fisheries and assist with developing spatially explicit stock assessment models.
Information on the spatial ecology of reef sharks is critical to understanding life-history patterns, yet gaps remain in our knowledge of how these species move and occupy space. Previous studies have focused on offshore reefs and atolls with little information available on the movement and space use of sharks utilising reef habitats closer to shore. Cross-shelf differences in physical and biological properties of reefs can alter regional ecosystem processes resulting in different movement patterns for resident sharks. Passive acoustic telemetry was used to examine residency, space use and depth use of 40 blacktip reef sharks, Carcharhinus melanopterus, on an inshore reef in Queensland, Australia, and assess temporal or biological influences. All sharks showed strong site-attachment to inshore reefs with residency highest among adult females. Sharks exhibited a sex-based, seasonal pattern in space use where males moved more, occupied more space and explored new areas during the reproductive season, while females utilised the same amount of space throughout the year, but shifted the location of the space used. A positive relationship was also observed between space use and size. There was evidence of seasonal site fidelity and long-distance movement with the coordinated, annual migration of two adult males to the study site during the mating season. Depth use was segregated with some small sharks occupying shallower depths than adults throughout the day and year, most likely as refuge from predation. Results highlight the importance of inshore reef habitats to blacktip reef sharks and provide evidence of connectivity with offshore reefs, at least for adult males.
Threatened chondrichthyan diversity is high in developing countries where scarce resources, limited data, and minimal stakeholder support often render conservation efforts challenging. As such, data on many species, including many evolutionarily distinct endemics, is poor in these countries and their conservation status and habitat needs remain uncertain. Here, we used baited remote underwater videos (BRUVs; n = 419) conducted at 167 sites over two years to assess the frequency of occurrence (FO), relative abundance, diversity, and structure of chondrichthyan assemblages in one of the world’s chondrichthyan biodiversity and endemism hotspots, South Africa. We compared chondrichthyan assemblages across three habitat types, and between unprotected and protected areas (a small marine protected area [MPA] and a larger, seasonal whale sanctuary). Although in total we observed 18 chondrichthyan species (11 families), over half of all observations were of just two species from the same family of mesopredatory endemic catsharks; only 8.8% were larger shark species. These mesopredatory species do not appear to be threatened, but some skates and larger shark species, including some endemics, were much rarer. Overall chondrichthyan FO was high (81% of all BRUVs); FO was higher in kelp (100% of BRUVS) and reef (93%) sites than at sites in sandy habitat (63%), which had a distinct chondrichthyan community. Independent of habitat, the chondrichthyan community did not relate strongly to protection. Because sites with kelp and reef habitat were rare in the whale sanctuary, this protected area had a lower chondrichthyan FO (67% of BRUVs) than either unprotected sites (81%) or those in the small MPA (98%), as well as having lower chondrichthyan relative abundance and species richness. Our study provides evidence of the importance of distinct habitat types to different chondrichthyan species, and suggests that even small MPAs can protect critical habitats, such that they may provide safe refuge for endemic species as anthropogenic pressures increase.
We present spatsoc , an r package for conducting social network analysis with animal telemetry data.
Animal social network analysis is a method for measuring relationships between individuals to describe social structure. Proximity‐based social networks are generated from animal telemetry data by grouping relocations temporally and spatially, using thresholds that are informed by the characteristics of the species and study system.
spatsoc fills a gap in r packages by providing flexible functions, explicitly for animal telemetry data, to generate edge lists and gambit‐of‐the‐group data, perform data‐stream randomization, and generate group by individual matrices.
The implications of spatsoc are that current users of animal telemetry or otherwise georeferenced data for movement or spatial analyses will have access to efficient and intuitive functions to generate social networks.
In recent decades, network analyses have become ubiquitous in ecology, facilitating our understanding of linkages between paired entities, whether it be genes, proteins, individuals, species, or habitats (Blüthgen et al., 2008; Croft et al., 2008; Krause et al., 2007; Proulx et al., 2005; Wey et al., 2008). Network theory (also known as graph theory) originates from the mathematical and social sciences but has developed concurrently across many disciplines, including computational science, physics, management, genetics, and epidemiology (Newman, 2010), to name but a few.
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.
Electronic supplementary material
The online version of this article (10.1186/s40462-018-0125-5) contains supplementary material, which is available to authorized users.
Individuals can play different roles in maintaining connectivity and social cohesion in animal populations and thereby influence population robustness to perturbations. We performed a social network analysis in a reef shark population to assess the vulnerability of the global network to node removal under different scenarios. We found that the network was generally robust to the removal of nodes with high centrality. The network appeared also highly robust to experimental fishing. Individual shark catchability decreased as a function of experience, as revealed by comparing capture frequency and site presence. Altogether, these features suggest that individuals learnt to avoid capture, which ultimately increased network robustness to experimental catch-and-release. Our results also suggest that some caution must be taken when using capture-recapture models often used to assess population size as assumptions (such as equal probabilities of capture and recapture) may be violated by individual learning to escape recapture.
One goal of global marine protected areas (MPAs) is to ensure they represent a breadth of taxonomic biodiversity. Ensuring representation of species in MPAs, however, would require protecting vast areas of the global oceans and does not explicitly prioritize species of conservation concern. When threatened species are considered, a recent study found that only a small fraction of their geographic ranges are within the global MPA network. Which global marine areas, and what conservation actions beyond MPAs could be prioritized to prevent marine extinctions (Convention on Biological Diversity Aichi Target 12), remains unknown. Here, we use systematic conservation planning approaches to prioritize conservation actions for sharks, rays and chimaeras (class Chondrichthyes). We use chondrichthyans as they have the highest proportion of threatened species of any marine class. We find that expanding the MPA network by 3% in 70 nations would cover half of the geographic range of 99 imperilled endemic chondrichthyans. Our hotspot analysis reveals that just 12 nations harbour more than half (53) of the imperilled endemics. Four of these hotspot nations are within the top ten chondrichthyan fishing nations in the world, but are yet to implement basic chondrichthyan fisheries management. Given their geopolitical realities, conservation action for some countries will require relief and reorganization to enable sustainable fisheries and species protection.
The common smoothhound (Mustelus mustelus) is the topmost bio-economically and recreationally important shark species in southern Africa, western Africa and Mediterranean Sea. Here, we used the Illumina HiSeq™ 2000 next-generation sequencing (NGS) technology to develop novel microsatellite markers for Mustelus mustelus. Two microsatellite multiplex panels were constructed from 11 polymorphic loci and characterised in two populations of Mustelus mustelus representative of its South African distribution. The markers were then tested for cross-species utility in Galeorhinus galeus, Mustelus palumbes and Triakis megalopterus, three other demersal coastal sharks also subjected to recreational and/or commercial fishery pressures in South Africa. We assessed genetic diversity (NA, AR, HO, HE and PIC) and differentiation (FST and Dest) for each species and also examined the potential use of these markers in species assignment. In each of the four species, all 11 microsatellites were variable with up to a mean NA of 8, AR up to 7.5, HE and PIC as high as 0.842. We were able to reject genetic homogeneity for all species investigated here except for T. megalopterus. We found that the panel of the microsatellite markers developed in this study could discriminate between the study species, particularly for those that are morphologically very similar. Our study provides molecular tools to address ecological and evolutionary questions vital to the conservation and management of these locally and globally exploited shark species.
Analyses of animal social networks have frequently benefited from techniques derived from other disciplines. Recently, machine learning algorithms have been adopted to infer social associations from time-series data gathered using remote, telemetry systems situated at provisioning sites. We adapt and modify existing inference methods to reveal the underlying social structure of wide-ranging marine predators moving through spatial arrays of passive acoustic receivers. From six months of tracking data for grey reef sharks (Carcharhinus amblyrhynchos) at Palmyra atoll in the Pacific Ocean, we demonstrate that some individuals emerge as leaders within the population and that this behavioural coordination is predicted by both sex and the duration of co-occurrences between conspecifics. In doing so, we provide the first evidence of long-term, spatially extensive social processes in wild sharks. To achieve these results, we interrogate simulated and real tracking data with the explicit purpose of drawing attention to the key considerations in the use and interpretation of inference methods and their impact on resultant social structure. We provide a modified translation of the GMMEvents method for R, including new analyses quantifying the directionality and duration of social events with the aim of encouraging the careful use of these methods more widely in less tractable social animal systems but where passive telemetry is already widespread.
Background: False-positive data (better known as “false detections”) in VEMCO VR2 acoustic telemetry monitor-ing studies that use pulse position modulation coding can cause biased or erroneous outcomes in data analysis. To understand the occurrence of false detections in acoustic monitoring data sets, the results of a range test experiment using eight acoustic receivers and 12 transmitters were examined.
Results:One hundred and fifty one tag ID codes were recorded, 137 of which were identified as likely from false detections, 12 were from test tags, and two were from tagged sharks. False detections accounted for <0.05% of detections (918) in the experiment. False detection tag ID codes were not randomly distributed amongst the avail-able codes, being more likely to occur at IDs close to tags used in the experiment. Receivers located near the bottom recorded the most false detections and tag ID codes from false detections. Receivers at the same depth did not differ significantly in the mean number of daily false detections. The daily number of false detections recorded by a receiver did not conform to a random pattern, and was not strongly correlated with daily receiver performance.
Conclusions:In an era of increasing data sharing and public storage of scientific data, the occurrence of false detections is of significant concern and the results of this study demonstrate that while rare they do occur and can be identified and accounted for in analyses.
Understanding the efficacy of marine protected areas (MPAs) for wide-ranging predators is essential to designing effective management and conservation approaches. The use of acoustic monitoring and network analysis can improve our understanding of the spatial ecology and functional connectivity of reef-associated species, providing a useful approach for reef-based conservation planning. This study compared and contrasted the movement and connectivity of sharks with different degrees of reef association. We examined the residency, dispersal, degree of reef connectivity, and MPA use of grey reef (Carcharhinus amblyrhynchos), silvertip (C. albimarginatus), and bull (C. leucas) sharks monitored in the central Great Barrier Reef (GBR). An array of 56 acoustic receivers was used to monitor shark movements on 17 semi-isolated reefs. Carcharhinus amblyrhynchos and C. albimarginatus were detected most days at or near their tagging reef. However, while C. amblyrhynchos spent 80% of monitoring days in the array, C. albimarginatus was only detected 50% of the time. Despite both species moving similar distances (,50 km), a large portion of the population of C. albimarginatus (71%) was detected on multiple reefs and moved more frequently between reefs and management zones than C. amblyrhynchos. Carcharhinus leucas was detected less than 20% of the time within the tagging array, and 42% of the population undertook long-range migrations to other arrays in the GBR. Networks derived for C. leucas were larger and more complex than those for C. amblyrhynchos and C. albimarginatus. Our findings suggest that protecting specific reefs based on prior knowledge (e.g., healthier reefs with high fish biomass) and increasing the level of protection to include nearby, closely spaced reef habitats (,20 km) may perform better for species like C. albimarginatus than having either a single or a network of isolated MPAs. This design would also provide protection for larger male C. amblyrhynchos, which tend to disperse more and use larger areas than females. For wide-ranging sharks like C. leucas, a combination of spatial planning and other alternative measures is critical. Our findings demonstrate that acoustic monitoring can serve as a useful platform for designing more effective MPA networks for reef predators displaying a range of movement patterns.
Chondrichthyans (sharks, skates, rays and chimaeras) are captured in many marine fisheries. Management and research efforts directed at chondrichthyan fishing are often neglected because of low product value, taxonomic uncertainty, low capture rates, and harvesting by multiple fisheries. In South Africa's diverse fishery sectors, which include artisanal as well as highly industrialised fisheries, 99 (49%) of 204 chondrichthyan species that occur in southern Africa are targeted regularly or taken as bycatch. Total reported dressed catch for 2010, 2011 and 2012 was estimated to be 3 375 t, 3 241 t and 2 527 t, respectively. Two‑thirds of the reported catch was bycatch. Regulations aimed at limiting chondrichthyan catches, coupled with species‑specific permit conditions, currently exist in the following fisheries: demersal shark longline, pelagic longline, recreational line, and beach‑seine and gillnet. Limited management measures are currently in place for chondrichthyans captured in other South African fisheries. Catch and effort dataseries suitable for stock assessments exist for fewer than 10 species. Stock assessments have been attempted for five shark species: soupfin Galeorhinus galeus, smoothhound Mustelus mustelus, white Carcharodon carcharias, spotted ragged‑tooth Carcharias taurus, and spotted gully Triakis megalopterus. Fishery‑independent surveys and fishery observer data, which can be used as a measure of relative abundance, exist for 67 species. Compared with most developing countries, South African shark fishing is relatively well controlled and managed. As elsewhere, incidental capture and bycatch remain challenges to the appropriate management of shark species. In 2013, South Africa's National Plan of Action for the Conservation and Management of Sharks (NPOA‑Sharks) was published. Implementation of the NPOA‑Sharks should help to improve chondrichthyan management in the near future.
Tonic immobility is a widely used technique for the surgical implantation of acoustic tags in elasmobranchs (sharks, skates, and rays), yet it is still not broadly recognised as an acceptable procedure by many regulatory bodies, animal care committees, and even journal ethics standards. To highlight its regular use and applicability as a field procedure, a literature search was conducted on the anaesthetic technique adopted for all existing elasmobranch-focused acoustic telemetry papers, up to 31 December 2013. A total of 57 studies were identified that contained relevant details on surgical methodological procedures. Of these, the majority of studies (43, or 75.4%) employed tonic immobility, while 10 (17.6%) used general chemical anaesthetic and 4 (7%) used local chemical anaesthetic. These studies identify that tonic immobility provides an effective anaesthetic for surgical implantation in elasmobranchs, as it offers several benefits over chemical anaesthetics, both from a practical and from an animal welfare perspective. Practically, rapid induction and recovery optimizes the surgical procedure, desirable under often complex field conditions, where general chemical anaesthetics prolong duration and administration is often unfeasible because of the size of study animals. Benefits over chemical anaesthetic for animal welfare include no risk of overdose, no uptake of chemicals to body tissues, minimal disruption to respiration, thereby reducing potential for negative sublethal impacts that influence postrelease behaviour, and immediate and full recovery. Given these benefits and its long-standing use in field studies, it is recommended that tonic immobility be recognised as an acceptable anaesthetic technique for surgical procedures on elasmobranchs.
Understanding the links between external variables such as habitat and interactions with conspecifics and animal space‐use is fundamental to developing effective management measures. In the marine realm, automated acoustic tracking has become a widely used method for monitoring the movement of free‐ranging animals, yet researchers generally lack robust methods for analysing the resulting spatial‐usage data.
In this study, acoustic tracking data from male and female broadnose sevengill sharks Notorynchus cepedianus , collected in a system of coastal embayments in southeast Tasmania were analyzed to examine sex‐specific differences in the sharks’ coastal space‐use and test novel methods for the analysis of acoustic telemetry data.
Sex‐specific space‐use of the broadnose sevengill shark from acoustic telemetry data was analysed in two ways: The recently proposed spatial network analysis of between‐receiver movements was employed to identify sex‐specific space‐use patterns. To include the full breadth of temporal information held in the data, movements between receivers were furthermore considered as transitions between states of a Markov chain, with the resulting transition probability matrix allowing the ranking of the relative importance of different parts of the study area.
Both spatial network and Markov chain analysis revealed sex‐specific preferences of different sites within the study area. The identification of priority areas differed for the methods, due to the fact that in contrast to network analysis, our Markov chain approach preserves the chronological sequence of detections and accounts for both residency periods and movements.
In addition to adding to our knowledge of the ecology of a globally distributed apex predator, this study presents a promising new step towards condensing the vast amounts of information collected with acoustic tracking technology into straightforward results which are directly applicable to the management and conservation of any species that meet the assumptions of our model.
Coastal ecosystems, such as estuaries, salt marshes, mangroves and seagrass meadows, comprise some of theworld’s most productive and ecologically significant ecosystems. Currently, the predominant factor considered in valuing coastal wetlands as fish habitats is the contribution they make to offshore, adult fish stocks via ontogenetic migrations. However, the true value of coastal nurseries for fish is much more extensive, involving several additional, fundamentally important ecosystem processes. Overlooking these broader aspects when identifying and valuing habitats risks suboptimal conservation outcomes, especially given the intense competing human pressures on coastlines and the likelihood that protection will have to be focussed on specific locations rather than across broad sweeps of individual habitat types. We describe 10 key components of nursery habitat value grouped into three types: (1) connectivity and population dynamics (includes connectivity, ontogenetic migration and seascape migration), (2) ecological and ecophysiological factors (includes ecotone effects, ecophysiological factors, food/predation trade-offs and food webs) and (3) resource
dynamics (includes resource availability, ontogenetic diet shifts and allochthonous inputs). By accounting for ecosystem complexities and spatial and temporal variation, these additional components offer a more comprehensive account of habitat value. We explicitly identify research needs and methods to
support a broader assessment of nursery habitat value. We also explain how, by better synthesising results from existing research, some of the seemingly complex aspects of this broader view can be addressed efficiently.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
The use of marine protected areas (MPAs) in South Africa should be revised in the light of growing problems related to the over-use of marine resources. No consistent policy has been applied to the establishment and management of MPAs. Existing MPAs include marine reserves, restricted areas, single-species restricted areas, National Parks, estuarine protected areas, trawling reserves and offshore islands, declared under a variety of legislation. Marine reserves and restricted areas have been declared under the Sea Fishery Act, but are managed by provincial authorities. The provincial authorities in the Northern, Western and Eastern Cape lack the necessary resources for marine management. By contrast, the KwaZulu-Natal Nature Conservation Service and the National Parks Board are adequately staffed and equipped to provide all the necessary managerial functions in their MPAs. The effectiveness of most MPAs is not assessed and they do not have clearly stated objectives or management plans. MPA boundaries are inappropriately demarcated at sea. Two marine biogeographic zones and two marine habitat types are poorly represented in MPAs. Ecologically and economically important species are well represented in MPAs, but effective protection is less satisfactory. Some MPAs are playing an important role in fisheries management. Community resource-use programmes in KwaZulu-Natal have helped to control poaching to some extent. It is recommended that South Africa establish a MPA Programme. MPA objectives should be clearly stated and communicated to the public through education programmes. Research, monitoring and enforcement in MPAs should be improved.
Commercial fishing in False Bay, South Africa, began in the 1600s. Today chondrichthyans are regularly taken in fisheries throughout the bay. Using a combination of catch, survey and life history data, the occurrence and long-term changes in populations of chondrichthyans in False Bay are described. Analyses of time series data from five fishing methods between 1897 and 2011 provided trends in relative abundance. Of the 37 species found in False Bay, 25 showed no significant trends for any fishing method. Of the 12 species and two genera with catch trends, four showed a common trend across methods, two increasing (Mustelus mustelus and Carcharhinus brachyurus) and two decreasing (Galeorhinus galeus and Raja spp.), while another two species (Triakis megalopterus and Isurus oxyrinchus) showed highly significant declines in one method only. A productivity index was used in conjunction with information on species distribution, catch frequency and trends in abundance to assess the vulnerability of False Bay chondrichthyans. Among the various habitats and depth strata, the surf-zone has been most impacted by exploitation, whereas the conservation of deep-water, soft-sediment species has benefited from the ban on trawling in False Bay implemented in 1928. Continued monitoring using catch data from beach-seine, recreational shore-angling and the commercial linefishery is recommended, along with improved species identification.
For marine predators there is a paucity of studies on social behaviour, and even fewer
studies have quantified interactions between individuals. In the present study, we looked at the social
structure and leadership of free-ranging juvenile lemon shark Negaprion brevirostris in a known
aggregation site, Bimini, the Bahamas. Observations of these sharks were made from towers placed
in a mangrove inlet, where clear, shallow, protected waters made it possible to record group compositions
of externally colour-code tagged wild juvenile lemon sharks. Thirty-eight different individual
sharks were observed to use the area over a 2 yr period. Results show repeated social interactions
suggestive of active partner preference. In addition, we found that group structure was mostly
explained by body length, and possibly by preference for relatives but not by sex. Finally, we
observed that some sharks led more groups than others and that those lead individuals were usually
larger than those following them. This study quantifies the social structure of a free-ranging shark
population and provides novel insights into the social behaviour of juvenile sharks.
Large, solitary, marine predators such as sharks have been observed to aggregate at specific areas. Such aggregations are almost certainly driven by foraging and behavioural strategies making space for diverse spatial organizations. Reef-associated shark species often show strong patterns of site fidelity that could be viewed as a prerequisite for sociality. However, there is limited empirical evidence that such aggregations are driven by intrinsic social factors. Association data for blacktip reef sharks, Carcharhinus melanopterus, were obtained from photoidentification surveys conducted in Moorea coral reefs (French Polynesia). We adapted a social network approach to demonstrate evidence of four main communities and two subcommunities within the population. We confronted the resulting structure with candidate explanatory variables. Sharks formed spatial groups characterized by nonrandom and long-term associations, despite opportunities for social relationships to develop between communities. Sex and length of sharks tended to influence assortment at the population and community levels. Individual space use also explained community structure, although spatial assortment was globally weaker than random expectations, suggesting that observed associations were not an artefact of the sampling design or spatial distribution of individuals. We conclude that the observed grouping patterns not only resulted from passive aggregations for specific resources, but rather the communities developed from an active choice of individuals as a sign of sociability. Individual preferences and adaptation to local conditions, as well as demographic, ecological and anthropogenic factors, may explain the social variability between communities. This suggests that a stable grouping strategy may confer substantial benefits in this marine predator.
Protected areas for the South African rock lobster, Jasus lalandii, were sampled by using divers, traps and ringnets at sites within and adjacent to four protected areas (St Helena Bay, Saldanha Bay and Table Bay rock lobster sanctuaries and the Betty's Bay marine reserve), over two years. Virtually no rock lobsters were found in St Helena Bay sanctuary, probably because of periodic harmful algal blooms. Abundance was greater in Saldanha Bay sanctuary than in adjacent fished areas, but only once in two years. Sizes were, however, larger in this sanctuary than the fished areas. By an order of magnitude, fewer and smaller rock lobsters were caught within the Table Bay sanctuary than in adjacent areas. Only at Betty's Bay were rock lobster sizes and abundance consistently greater inside than outside the reserve. Fecundity was similar among sites, with females in protected areas contributing no more to egg production than would be expected by the proportional area occupied by protected areas. Rock lobsters do benefit from protection in Betty's Bay reserve, but the west coast sanctuaries appear poorly located and seemingly contain large areas of unsuitable substrate. They clearly need relocation to be effective.
1. Determining how animals move within their environment is a fundamental knowledge that contributes to effective management and conservation. Continuous ‘round‐the‐clock’ animal movement data are frequently gathered using biotelemetry technology, providing discrete data packages on the presence–absence of animals at known locations through time. Current analyses of such data do not generally account for the interconnectivity of locations as animals move between them and consequently do not integrate graphically or statistically a temporal component to spatial changes.
2. Here, we describe the novel application of network analyses to electronic tag data whereby nodes represent locations and edges the movements of individuals. We demonstrate some of the descriptive and quantitative approaches for determining how an animal’s movement interconnects home range habitats. Telemetry data from arrays of recorders provide movement data of individual animals, and as examples of the method proposed, we examine the movements of two distinct shark species, the small‐spotted catshark ( Scyliorhinus canicula ) and the Caribbean reef shark ( Carcharhinus perezi ). In doing so, we consider both local and global network properties from an animal movement perspective and simulate the effects of node disruption as a proxy for habitat disturbance.
3. Comparative visual representations of two catshark movement networks suggest, for example, potential differences in space use. Multiple regression quadratic assignment procedure shows that habitat is a significant predictor of movement behaviour.
4. Null modelling of C. perezi movement data, corrected for the spatial restriction of static nodes, demonstrated a significant, non‐random distribution of directed movements among sites. Additionally, the connectivity of this animal’s movement network is significantly reduced through targeted disruption of a site of high centrality but not through disruption of a randomly selected site.
5. Network theory is a well‐established theoretical framework and its integration into the fast developing field of animal movement and telemetry might improve significantly how we interpret animal space use from electronically recorded data. This technique has potentially wide application in animal behaviour but may also inform the management of habitat harbouring threatened or endangered species via the simulation, modelling and intuitive visualisation of animal movement interactions.
Despite our critical dependence on aquatic wildlife, we lack a complete understanding of the drivers of population stability and structure for most fish species. Social network analysis has been increasingly used to investigate animal societies as it explicitly links individual decision-making to population-level processes and demography. While the study of social structure is of great ecological interest, it is also potentially important for species of economic value or of conservation concern. To date however, there has been little focus on how social processes are likely to influence the conservation of fish populations. Here we identify applications for how a social network approach can help address broad fish conservation themes such as population structure, biological invasions or fisheries management. We discuss the burgeoning opportunities offered and challenges still faced by current technologies to integrate social network approaches within fish conservation. The study of animal societies has been increasingly explored with social network analysis. In this Perspective, the importance of these networks in directing conservation efforts of fish is highlighted.
Sharks are among the most threatened vertebrates on the planet. Marine protected areas (MPAs) have been widely established and promoted as a shark conservation tool. However, the geographic ranges of most imperiled shark species (endemic and threatened) fall outside the current global networks of MPAs, leaving the protective benefits of this tool questionable for the shark species of highest conservation concern. The Western Cape of South Africa is a hotspot for endemic and threatened shark species. Here, we examined the potential protective benefit of a no-take marine reserve (the De Hoop MPA) for imperiled shark species using baited remote underwater video stations (BRUVS). Eleven shark species were documented, with six of 11 species (55%) classified as threatened with extinction by the International Union for the Conservation of Nature. The composition of the shark assemblage was dominated by small to mid-sized species, including small endemics. Species-specific habitat preferences were identified, with all these habitats represented in the MPA. Frequency of occurrence and relative abundance of sharks on BRUVS were significantly higher inside the De Hoop MPA than outside. Both protected and commercially exploited sharks species exhibited higher relative abundance inside the MPA. Relative abundance also increased inside the MPA with increasing distance from the reserve boundaries. Our findings suggest that no-take MPAs can be an effective tool for protecting shark species of conservation concern, including threatened endemics, particularly if the MPA adequately incorporates their preferred habitats.
• Knowledge about spatial and temporal variability in the distribution and abundance of predators is necessary to adapt measures to mitigate human–wildlife interactions.
• Acoustic telemetry and network analyses were used to investigate the spatial ecology of bull sharks, the species responsible for most shark bites in Reunion Island, one of the world's shark bite hotspots.
• The west coast of the island was not used uniformly by every individual, with size predicting the movements of sharks along the coast.
• Node-based metrics – closeness, node strength, and cumulated continuous residency times – derived from up to 181 monthly movement networks from 20 individuals, revealed that smaller sharks (<250 cm total length) primarily used the south-west coast while larger individuals spent most of their time in the northern region with regular visits to multiple areas along the coast.
• This study provides essential knowledge on bull shark behaviour and central areas used at different periods of the year, which correlates well with the dynamics of observed shark bites. Our approach provides a non-invasive alternative to help predicting and anticipating human–shark conflicts and avoid shark culling programmes detrimental to the conservation of large predators such as sharks.
Genetic relatedness in animal societies is often a factor that drives the structure of social groups. In the marine world, most studies which have investigated this question have focused on marine mammals such as whales and dolphins. For sharks, recent studies have demonstrated preferential associations among individuals from which social communities emerge. Assortment patterns have been found according to phenotypic or behavioral traits, but the role of genetic relatedness in shaping the social structure of adult shark populations has, to the best of our knowledge, never been investigated. Here, we used a social network analysis crossed with DNA microsatellite genotyping to investigate the role of the genetic relatedness in the social structure of a blacktip reef shark (Carcharhinus melanopterus) population. Based on the data from 156 groups of sharks, we used generalized affiliation indices to isolate social preferences from nonsocial associations, controlling for the contribution of sex, size, gregariousness, spatial, and temporal overlap on social associations, to test for the influence of genetic relatedness on social structure. A double-permutation procedure was employed to confirm our results and account for issues arising from potentially elevated type I and type II error rates. Kinship was not a predictor of associations and affiliations among sharks at the dyad or community levels as individuals tended to associate independently of the genetic relatedness among them. The lack of parental care in this species may contribute to the breakdown of family links in the population early in life, thereby preventing the formation of kin-based social networks.
Knowledge of the diel spatial ecology of wild animals is of great interest to ecologists and relevant to resource management and conservation. Sharks are generally considered to be more active during nocturnal periods than during the day; however, few studies have empirically evaluated diel variation in shark habitat use and how anthropogenic disturbances may influence these patterns. In the western central Atlantic Ocean, tiger sharks (Galeocerdo cuvier) are highly abundant in the shallow waters of the Little Bahama Bank, Bahamas. Within the northwest edge of the Bank, there is an area nicknamed “Tiger Beach,” where tiger sharks are provisioned year-round at spatially discrete ecotourism dive sites spanning ~ 1.5 km². In this study, we used an array of acoustic receivers encircling an area of 102.4 km² to evaluate for potential differences in diel spatial habitat use patterns for 42 tagged tiger sharks at Tiger Beach and the surrounding area. Using tracking data from 24 June 2014 to 13 May 2015, we evaluated spatial and diel patterns of shark activity space, centers of activity, residency and the daily proportion of sharks detected within the array. Sharks were detected during both day and night with no significant diel differences in habitat use metrics across the array, although spatial differences in residency existed. Four sharks accounted for 53.8% of residency data throughout the tracking period, with the majority of sharks primarily entering and exiting the array, except during summer months when the most of the tagged tiger sharks were absent from the array. We also found limited empirical support for hypothesized effects of provisioning tourism on tiger shark habitat use. However, additional research at finer, individual scales, may be needed to better resolve the potential influence of provisioning on tiger sharks at Tiger Beach.
New technologies have vastly increased the available data on animal movement and behaviour. Consequently, new methods deciphering the spatial and temporal interactions between individuals and their environments are vital. Network analyses offer a powerful suite of tools to disentangle the complexity within these dynamic systems, and we review these tools, their application, and how they have generated new ecological and behavioural insights. We suggest that network theory can be used to model and predict the influence of ecological and environmental parameters on animal movement, focusing on spatial and social connectivity, with fundamental implications for conservation. Refining how we construct and randomise spatial networks at different temporal scales will help to establish network theory as a prominent, hypothesis-generating tool in movement ecology.
Commercial fishing in False Bay, South Africa, began in the 1600s. Today chondrichthyans are regularly taken in fisheries throughout the bay. Using a combination of catch, survey and life history data, the occurrence and long-term changes in populations of chondrichthyans in False Bay are described. Analyses of time series data from five fishing methods between 1897 and 2011 provided trends in relative abundance. Of the 37 species found in False Bay, 25 showed no significant trends for any fishing method. Of the 12 species and two genera with catch trends, four showed a common trend across methods, two increasing (Mustelus mustelus and Carcharhinus brachyurus) and two decreasing (Galeorhinus galeus and Raja spp.), while another two species (Triakis megalopterus and Isurus oxyrinchus) showed highly significant declines in one method only. A productivity index was used in conjunction with information on species distribution, catch frequency and trends in abundance to assess the vulnerability of False Bay chondrichthyans. Among the various habitats and depth strata, the surf-zone has been most impacted by exploitation, whereas the conservation of deep-water, soft-sediment species has benefited from the ban on trawling in False Bay implemented in 1928. Continued monitoring using catch data from beach-seine, recreational shore-angling and the commercial linefishery is recommended, along with improved species identification.
New technologies have vastly increased the available data on animal movement and behaviour. Consequently, new methods deciphering the spatial and temporal interactions between individuals and their environments are vital. Network analyses offer a powerful suite of tools to disentangle the complexity within these dynamic systems and we review these tools, their application, and how they have generated new ecological and behavioural insights. We suggest that network theory can be used to model and predict the influence of ecological and environmental parameters on animal movement, focusing on spatial and social connectivity, with fundamental implications for conservation. Refining how we construct and randomise spatial networks at different temporal scales will help establish network theory as a prominent, hypothesis-generating tool in movement ecology.
Acoustic telemetry is being increasingly used to study the ecology of many aquatic organisms. This widespread use has been advanced by national and international tracking programs that coordinate deployment of passive acoustic telemetry networks on a regional and continental scale to detect tagged animals. While it is well‐known that environmental conditions can affect the performance of acoustic receivers, these effects are rarely quantified despite the profound implications for tag detection and hence the ecological inferences.
Here, we deployed eight receivers at different depths within the water column and at different orientations (hydrophone up or down) and 12 tags 200–800 m from the receivers for 234 days to investigate how the tag detection range of acoustic receivers varied through time and under different meteorologic and oceanographic conditions.
The study showed that receiver depth and orientation, and time since deployment had the largest effect on the detection range. Thermocline gradient and depth, and wind speed were the environmental factors most affecting detection range, while wind direction, precipitation and atmospheric pressure had negligible or no effect. Comparison of results to a proposed general acoustic theory model and previous studies showed that findings from specific habitat types cannot be generalised and applied across other habitats or environments.
A good understanding of the acoustic coverage and temporal variations in relation to environmental conditions are crucial to accurate interpretation of results, and ensuing management recommendations. We recommend that each study include stationary reference tags to measure changes in detection probability with time, help refine detection range, and be used to improve confidence in the reporting and interpretation of the data.
The sampling of animals for the purpose of measuring associations and interactions between individuals has led to the development of several statistical methods to deal with biases inherent in these data. However, these methods are typically computationally intensive and complex to implement.
Here, I provide a software package that supports a range of these analyses in the R statistical computing environment. This package includes a novel approach to estimating re‐association rates of time between frequently sampled individuals.
I include extended demonstration of the syntax and examples of the ability for this software to interface with existing network analysis packages in R.
This bridges a gap in the tools that are available to biologists wishing to analyse animal social networks in R.
This study documents validation of vertebral band-pair formation in spotted gully shark (Triakis megalopterus) with the use of f luorochrome injection and tagging of captive and wild sharks over a 21-year period. Growth and mortality rates of T. megalopterus were also estimated and a demographic analysis of the species was conducted. Of the 23 OTC (oxytetracycline) -marked vertebrae examined (12 from captive and 11 from wild sharks), seven vertebrae (three from captive and four from wild sharks) exhibited chelation of the OTC and f luoresced under ultraviolet light. It was concluded that a single opaque and translucent band pair was deposited annually up to at least 25 years of age, the maximum age recorded. Reader precision was assessed by using an index of average percent error calculated at 5%. No significant differences were found between male and female growth patterns (P>0.05), and von Bertalanffy growth model parameters for combined sexes were estimated to be L∞=1711.07 mm TL, k=0.11/yr and t0= -2.43 yr (n=86). Natural mortality was estimated at 0.17/yr. Age at maturity was estimated at 11 years for males and 15 years for females. Results of the demographic analysis showed that the population, in the absence of fishing mortality, was stable and not significantly different from zero and particularly sensitive to overfishing. At the current age at first capture and natural mortality rate, the fishing mortality rate required to result in negative population growth was low at F>0.004/ yr. Elasticity analysis revealed that juvenile survival was the principal factor in explaining variability in population growth rate.
Aggregations in the distribution of individuals are an almost universal phenomenon in living organisms. Groups of animals that display collective coordinated movement without forming stable social bonds such as fish schools are a special type of aggregation. In tropical tuna fisheries, aggregating behaviour is directly exploited through the use of artificial fish aggregating devices (FADs). Hence, understanding the dynamics of schooling behaviour and the potential impacts of FADs upon it may have ramifications for tuna management. As a novel way of quantifying spatiotemporal co-occurrences of animals, we applied network statistics to acoustic tracking data to identify the co-occurrences of individual yellowfin tuna, Thunnus albacares, in an array of FADs and determine the frequency and temporal dynamics of these co-occurrences. We observed large interannual variation in movement rates of tuna between FADs, and corresponding interannual variability in the mean number of spatiotemporal associates for each individual as well as the temporal stability of associations. When movement rates were high, associations within FAD aggregations decayed to randomness three times faster than when movement rates were lower. This raises the possibility that if FADs are sufficiently close for fish to perform frequent between-FAD movements, school mixing may be increased and cohesion reduced.
The concept of elasmobranch species using nursery areas was introduced in the early 1900s and has been an accepted aspect of shark biology and behavior for several decades. Despite several descriptions of how shark species use nursery areas and what types of regions nurseries may be found in, no explicit definition of what constitutes a shark nursery area has been presented. Here we evaluate the assumptions of the current shark nursery paradigm in light of available data. Based on examination of these assumptions and available methods of quantifying and accurately describing shark nursery areas, a new more quantitative definition of shark nursery areas is proposed. This definition requires 3 criteria to be met for an area to be identified as a nursery: (1) sharks are more commonly encountered in the area than other areas; (2) sharks have a tendency to remain or return for extended periods; and (3) the area or habitat is repeatedly used across years. These criteria make the definition of shark nursery areas more compatible with those for other aquatic species. The improved definition of this concept will provide more valuable information for fisheries managers and shark biologists.
Abstract Catch-and-release angling is a well-established practice in recreational angler behaviour and fisheries management. Accompanying this is a growing body of catch-and-release research that can be applied to reduce injury, mortality and sublethal alterations in behaviour and physiology. Here, the status of catch-and-release research from a symposium on the topic is summarised. Several general themes emerged including the need to: (1) better connect sublethal assessments to population-level processes; (2) enhance understanding of the variation in fish, fishing practices and gear and their role in catch and release; (3) better understand animal welfare issues related to catch and release; (4) increase the exchange of information on fishing-induced stress, injury and mortality between the recreational and commercial fishing sectors; and (5) improve procedures for measuring and understanding the effect of catch-and-release angling. Through design of better catch-and-release studies, strategies could be developed to further minimise stress, injury and mortality arising from catch-and-release angling. These strategies, when integrated with other fish population and fishery characteristics, can be used by anglers and managers to sustain or enhance recreational fishing resources.
Indices used to measure the frequency of association between individuals in fission/fusion societies are frequently borrowed from ecological studies of species association without adequate justification. In this paper several such indices, under specific conditions likely to be encountered in field studies of animal behaviour, are examined. Each of three indices commonly found in the literature is shown through simulations to be accurate over only a narrow range of possible sampling biases. As an alternative approach, examples are given of the derivation of a maximum-likelihood estimator based on two simple models. Both models assume that a constant proportion of existing subgroups is located. In the first model all combinations of subgroups are equally likely to be located, while in the second model the probability of locating one subgroup is allowed to differ from that of the others. The maximum-likelihood estimators are shown to be less biased and to have lower variance than the other three indices under the assumptions of the models.