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Possible collapse of reef shark populations in remote coral reef ecosystems in the Coral Sea (Western Pacific)
Abstract
The decline of meso-predators such as reef sharks is a concern as such species can have important
ecological roles in maintaining reef ecosystem resilience. Two field trips conducted in August 2010 and November
2011 to the Chesterfield archipelago (Coral Sea) allowed us to assess the abundances and average sizes of
medium-bodied Carcharhinidae with a specific focus on grey reef shark (Carcharhinus amblyrhynchos), through
fishing (46 hours of accumulated effort) and underwater visual censuses (25 hours of accumulated effort). We
found low abundance and small average total length (TL) for all reef shark species, and in the case of the grey
reef shark, an average abundance of 2.1 individuals/dive with the majority of animals less than 110 cm TL. We
compared our findings with historical data and, given our low sampling effort, we so far hypothesise that a general
strong decline in the reef shark populations may have occurred in this area, probably due to recent overfishing.
The enforcement of conservation measures is strongly recommended among these remote reefs as well as
complementary studies for confirming this hypothesis
... Based on new results acquired since then, we believe that this region will have the potential to be reconsidered as an IMMA in the future. Humpback whales will then fully play their role of an umbrella species of conservation, whose protection will be beneficial to other marine species that use the Chesterfield-Bellona archipelago, and specifically to lesser-known megafauna species (Borsa et al. 2010, Read et al. 2015, Clua & Vignaud 2016, Juhel et al. 2018). ...
Humpback whales Megaptera novaeangliae were severely depleted by commercial whaling. Understanding key factors in their recovery is a crucial step for their conservation world- wide. In Oceania, the Chesterfield-Bellona archipelago was a primary whaling site in the 19th cen- tury, yet has been left almost unaffected by anthropogenic activities since. We present the results of the first multidisciplinary dedicated surveys in the archipelago assessing humpback whale pop- ulations 2 centuries post-whaling. We encountered 57 groups during 24 survey days (2016−2017), among which 35 whales were identified using photographs of natural markings (photo-ID), 38 using genotyping and 22 using both. Humpback whales were sparsely distributed (0.041 whales km−1): most sightings concentrated in shallow inner-reef waters and neighbouring offshore shal- low banks. The recently created marine protected area covers most of the areas of high predicted habitat suitability and high residence time from satellite-tracked whales. Surprisingly for a breed- ing area, sex ratios skewed towards females (1:2.4), and 45% of females were with calf. Connec- tivity was established with the New Caledonia breeding area to the east (mtDNA FST = 0.001, p > 0.05, 12 photo-ID and 10 genotype matches) and with the Australian Great Barrier Reef breeding area to the west (mtDNA FST = 0.006, p > 0.05). Movement of satellite-tracked whales and photo- ID matches also suggest connections with the east Australian migratory corridor. This study con- firms that humpback whales still inhabit the Chesterfield-Bellona archipelago 2 centuries post whaling, and that this pristine area potentially plays a role in facilitating migratory interchange among breeding grounds of the western South Pacific.
... Gray reef sharks are among the most abundant reef sharks in the Indo-Pacific and can comprise up to 50% of the upper trophic level biomass on coral reefs in some areas 19 . Yet possible collapses of gray reef shark populations have been documented, even on reefs with relatively low human impacts 20 . Without a pelagic larval phase in their life cycle, population connectivity in gray reef sharks occurs exclusively through movements of adults and juveniles [21][22][23][24] . ...
Human activities have resulted in the loss of over 90% of sharks in most ocean basins and one in four species of elasmobranch are now listed at risk of extinction by the IUCN. How this collapse will affect the ability of populations to recover in the face of continued exploitation and global climate change remains unknown. Indeed, important ecological and biological information are lacking for most shark species, particularly estimates of genetic diversity and population structure over a range of spatial scales. Using 15 microsatellite markers, we investigated genetic diversity and population structure in gray reef sharks over their Indo-Pacific range (407 specimens from 9 localities). Clear genetic differentiation was observed between the Indian and the Pacific Ocean specimens (FST = 0.145***). Further differentiation within the Pacific included a West and East cleavage as well as North-Central and South-Central Pacific clusters. No genetic differentiation was detected within archipelagos. These results highlight the legacy of past climate changes and the effects of large ocean expanses and circulation patterns on contrasting levels of connectivity at global, regional and local scales. Our results indicate a need for regional conservation units for gray reef sharks and pinpoint the isolation and vulnerability of their French Polynesian population.
... The growing demand for marine protein and ongoing demand for shark fins have increasingly made reef sharks the target of commercial as well as traditional subsistence fisheries [24,25]. Consequently, reef-based fishing for these species by both legal and illegal fishing operations is widespread and is thought to have driven sharp declines in abundance in affected areas [26,27]. Marine protected areas and shark sanctuaries have been declared around the world to attempt to relieve the fishing pressure on sharks, but deterring illegal fishing remains challenging given the paucity of alternatives for many fishers, the incentive provided by the relatively lucrative fin trade, and the logistics of enforcement [28,29]. ...
Abstract Acoustic tagging is typically used to gather data on the spatial ecology of diverse marine taxa, informing questions about spatio-temporal attributes such as residency and home range, but detection data may also reveal unanticipated insights. Many species demonstrate predictable site fidelity, and so a sudden cessation of detections for multiple individuals may be evidence of an atypical event. During 2013 and 2014, we acoustically tagged 47 grey reef sharks (Carcharhinus amblyrhynchos) and 48 silvertip sharks (Carcharhinus albimarginatus) near reefs in the British Indian Ocean Territory (BIOT) Marine Protected Area (MPA). From March 2013 to November 2014 inclusive, tags were ‘lost’, i.e. permanently ceased to be detected within the monitoring area, at an average rate of 2.6 ± 1.0 tags per month. Between 1 and 10 December 2014, detection data suggest the near-simultaneous loss of 15 of the remaining 43 active tagged sharks, a monthly loss rate over five times higher than during the previous 21 months. Between 4 and 14 December of 2014, the BIOT patrol vessel encountered 17 vessels engaged in suspected illegal fishing in the northern BIOT MPA; such sightings averaged one per month during the previous 8 months. Two of these vessels were arrested with a total of 359 sharks on board, of which grey reef and silvertip sharks constituted 47% by number. The unusual and coincident peaks in tag loss and vessel sightings, and the catch composition of the arrested vessels, suggest illegal fishing as a plausible explanation for the unusual pattern in our detection data. A Cox proportional hazards model found that the presence of fishing vessels increased the risk of tag loss by a factor of 6.0 (95% CI 2.6–14.0, p
... Despite its wide distribution and its occurrence in fisheries, its life history traits have only been studied in Australia, Indonesia, and Hawaii (18,19). This species is recorded in the IUCN red list with an "NT" (Near Threatened) status since 2009 (20), as it has been intensively overfished in many areas, even remote ones (21). This is a medium size shark, with TL between 60 and 70 up to 200 and 220 cm, incidentally larger as a maximum size of 250 cm has been observed (19). ...
An accurate assessment of the biting shark size is paramount for better understanding the agonistic behavior of a species toward humans. The gray reef shark is involved in many accidental bites. Based on the capture of 35 gray sharks, we calculated the algorithm that allows the assessment of the shark size, through the interdental distance (IDD) inferred from teeth marks on human wounds. Our results show a negative allometric relationship and that IDD calculated from imprints perpetrated by the upper jaw are globally similar with those from the lower jaw, in spite of heterodonty. We applied our findings to two cases of accidental bites by this species. Both of the victims had declared that the shark length was “at least 2 m”. Based on our algorithm, the assessment of the shark TL was approx. 180 and 160 cm, respectively, which correspond to an overestimation of ≥20% by the victims.
... Clipperton, loin d'avoir été protégé par son isolement, semble en avoir fait les frais avec des processus de pillage des ressources tels qu'ils ont récemment été décrits sur les récifs des Chesterfield, d'autres archipels éloignés du Pacifique occidental, eux aussi sous tutelle de la France(Clua and Vignaud 2016). Cette situation n'est pas sans poser un problème de résilience locale avec la disparition de ces prédateurs et de leur rôle critique dans les écosystèmes marins, mais aussi à une échelle régionale, s'agissant pour certaines espèces d'animaux migrateurs (voir section suivante).Figure 2-37. ...
Une expédition de nature éco-touristique privilégiant la plongée et une démarche de «science participative» s’est déroulée du 28 janvier au 11 février 2016 autour de l’atoll de Clipperton et dans l’archipel mexicain des Revillagigedos. Organisée par une société canadienne, la logistique a été assurée par le navire Quino El Guardian, un troller américain de 28 m basé au port de San José del Cabo (Basse-Californie mexicaine) et géré par un équipage mexicain de neuf personnes (incluant deux moniteurs de plongée). Le groupe de 11 touristes relevant de sept nationalités (Mexique, USA, Pologne, Puerrto Rico, Canada, Grèce et Angleterre) était géré par les deux chefs d’expédition canadiens (Michel Labrecque et Julie Ouimet). Ils ont bénéficié de l’encadrement technique de trois scientifiques respectivement francocolombien (Sandra BesSudo), mexicain (Mauricio Hoyos) et français (Eric CLUA).Après un voyage de 120 h, trois jours de mouillage à l’ouest de l’atoll de Clipperton (en face de Port Jaouen) ont permis aux 16 plongeurs accompagnés des deux moniteurs de débarquer à terre le 02 février, entre 11 :00 et 17 :15. Une cérémonie de levée des couleurs françaises a permis de remplacer le drapeau qui avait disparu et de vérifier le bon état de la stèle déposée par le député Folliot en avril 2015. Trois personnes (E. Clua, S. Bessudo et M. Labrecque) ont éffectué à pied le tour de l’atoll pendant que les autres personnes se contentaient de visiter le rocher avant de ré-embarquer. Le tour d’ilôt a permis de constater la vivacité de la végétation rampante et des cocotiers, une baisse potentielle de la densité des crabes et des oiseaux et la présence de rats nuisant probablement aux autres animaux. L’atoll est toujours jonché de nombreux déchets abandonnés par les missions précédentes et apportés par la mer, en grande majorité depuis les pays d’Amérique centrale (Nicaragua, Costa Rica, etc.). Deux épaves légères (cannots en fibre autour de 5 m de long) se sont échouées à proximité du rocher, au sud de l’atoll. La mesure du cordon littoral sur les deux sites correspondant à l’emplacement des anciennes passes ont débouché sur des largeurs de 40 m au nord-est et seulement 14 m au sud (au lieu de 20 m en avril 2015). Cette dernière mesure montre une érosion très agressive laissant présager une ré-ouverture naturelle prochaine.
Les journées du 03 et 04 février ont été dédiées à six plongées sur la pente récifale à l’ouest (cinq) et à l’est (une) de l’atoll, entre 12 et 30 m de profondeurs. Sur le plan scientifique, ces plongées ont permis de remplacer les batteries de récepteurs acoustiques qui avaient été installés -de même que des requins marqués- autour de Clipperton entre 2010 et 2013 par l’équipe mexicaine, sans autorisation de la France ni partenariat avec des chercheurs français. Outre cet aspect logistique, ce début de collaboration a permis de capturer et marquer trois autres requins dont deux adultes de requin des Galapagos (autour de 2 m) avec des marques acoustiques et satellitaires, et un juvénile de requin à pointe blanche (80 cm de long) seulement avec une marque acoustique. Sous l’eau, la densité des requins est apparue extrêmement faible, de même que la longueur moyenne correspondant à des individus juvéniles pour la grande majorité. Il semblerait que ces populations de requins ont subi une surpêche importante dans les décennies précédentes.
Concernant les récifs autour de l’atoll, ils semblent en excellente santé avec un taux de recouvrement en corail vivant souvent >80%. Bien que peu variées en espèces, les populations de poissons de récifs sont en densités importantes. Les plongées ont permis l’identification d’une nouvelle espèce, jamais observée à ce jour autour de l’atoll, à savoir la carangue géante Caranx ignobilis. Certaines espèces endémiques comme la demoiselle de Clipperton Stegastes baldwini pourraient faire l’objet d’une utilisation commerciale sur le marché de l’aquariophilie, point qui ne semble pas acquis pour le poisson ange de Clipperton Holacanthus limbaughi, dont les densités semblent insuffisantes de prime abord. Une étude plus approfondie est néanmoins en cours de réalisation sur la base d’une vingtaine de comptages sous-marins effectués au cours des six plongées.
Le 03 février au matin vers 07 :00, un thonnier senneur mexicain d’une soixantaine de mètres, dénommé «Conquista» (photo ci-dessus), est venu s’ancrer à l’ouest de l’atoll, à environ 2 miles au large. Un hélicoptère a été déployé de même que deux cannots avec une dizaine de marins en tout, dont huit se sont mis à l’eau avec palmes-masque-tuba sur le platier en face du camp Bougainville, probablement pour pêcher des langoustes. Deux marins ont débarqué à terre où ils sont restés deux heures (entre 9 et 11 :00) avant d’être récupérés par les cannots et sans que leur activité à terre n’ait pu être déterminée. Il convient de noter que ce débarquement sans autorisation du haussariat de Polynésie française est illicite. Par ailleurs, le capitaine du Conquista n’a répondu à aucune des trois sollicitations radio du capitaine du Quino El Guardian et les réponses des marins des cannots, approchés pour connaître leurs intentions sur la zone, sont restées très évasives. Le navire a quitté les lieux le jour même vers 13 :00.
Sur le chemin du retour, deux jours entiers ont été dédiés à six plongées sans portée scientifique sur les îles de Socorro et Benedicto (archipel des Revillagigedo).
Cette expédition, première à imposer la prise en charge intégrale d’un ressortissant français contre l’autoraisation de débarquement et de travail scientifique autour de l’atoll, a permis de ré-affirmer la souveraineté française en présence de mexicains parfois ambigüs sur le sujet au cours de l’expédition. Elle a posé les jalons d’une implication de scientifiques français, à travers le CRIOBE, dans les recherches qui s’effectuent à une échelle régionale au sein du «Corridor du Pacifique Tropical Est», dénommé CMAR et créé en 2004 par les quatre pays fondateurs que sont le Costa Rica, Panama, Colombie et Equateur, chacun détenant une île océanique à savoir respectivement Cocos island, Coïba island, Malpelo et l’Archipel des Galapagos. Ce réseau est en passe de s’étendre au Nord en incluant le Mexique, via l’archipel des Revillagigedos. Il apparaît urgent que la France mette en œuvre des mesures de protection effective des ressources -a minima dans les eaux mitoyennes de l’atoll (12 miles)-, avec probablement un statut d’Aire Marine Protégée comme il existe dans toutes les autres îles du CMAR, condition sine qua non pour pouvoir officiellement et dès que possible rejoindre ce réseau, comme les logiques politique et régionale le suggèrent.
Synthèse des opérations technico-scientifiques menées par la mission Mexicano-française sur Clipperton en Oct. 2019
INforme tecnico de las operaciones tecnico-cientifica que se desarollaron durante la mision en Clipperton en octubre 2019
Synthèse des opérations technico-scientifiques menées par la mission Mexicano-française sur Clipperton en Oct. 2019
Une mission à caractère scientifique s’est effectuée sur l’atoll de Clipperton-La Passion du 6 au 13 août 2018 dans le cadre de l’expédition Tara Pacific qui a débuté en mai 2016 et qui se terminait par ces récifs sous juridiction française avant un 32ième site sur l’île de Coiba au Panama. Tara Pacifique, dont la direction scientifique est assurée par le CRIOBE USR3278
EPHE-CNRS-UPVD et le Centre Scientifique de Monaco, se focalise sur la compréhension de la résilience des récifs coralliens face aux changements globaux actuels et opère sur la base de prélèvements de coraux et de poissons, de même que la collecte d’eau océanique et côtière, pour étudier la flore microbienne de ces différents compartiments. A ces opérations de routine menées par sept scientifiques appuyés par un équipage de six personnes et deux opérateurs média, se sont rajoutés deux volets traitant respectivement des requins et des poissons, co-financés par le Ministère de de la transition écologique et solidaire. Servie par une fenêtre météo de 48h, la goélette Tara a quitté Cabo San Lucas au Mexique le 2 août pour atteindre l’atoll de Clipperton le 6 août, soit deux jours avant la coupure décalée de sa trajectoire par l’ouragan John. Sur place, les conditions de mer, dominées par une houle de Sud-Ouest, ont été très favorables aux débarquements à partir du mouillage situé à l’Est de
l’atoll, en face de l’ex-camp américain. En mer, trois sites principaux d’investigation ont été mis en oeuvre, respectivement à l’Est, au Sud et à l’Ouest de l’atoll, sur lesquels s’est opérée la méthodologie classique de Tara Pacifique. Les résultats préliminaires démontrent un état de santé du corail exceptionnellement bon à l’échelle du Pacifique. La diversité corallienne est certes faible avec trois espèces (massives, encroûtantes et à branches très courtes) très dominantes, à savoir des Porites, Pavona et Pocillopora, mais la couverture moyenne en corail vivant est autour de 70%, et les signes de maladies ou de blanchissement relativement faibles. La présence de l’espèce Millepora platyphylla a été mise en évidence pour la première fois sur l’atoll, et une carotte d’une longueur exceptionnelle de 133 cm a été
prélevée dans un des Porites géants de la zone. Conformément au protocole Tara Pacific, dix spécimens de chirurgien bagnard Acanthurus triostegus ont été prélevés, de même que cinq indivdus de poisson cocher Zanclus cornutus, afin d’étudier leur microbiome
associé. A ces deux espèces clefs, se sont rajoutés le prélèvement de plus de 200 échantillons d’ADN parmi 38 espèces de poissons de récifs différentes, en se focalisant sur les espèces endémiques telles que le poisson ange de Limbaugh Holacanthus limbaughi
dont une courbe de croissance pourra être calculée afin de mieux appréhender sa résilience. Deux nouvelles espèces de poissons (connues mais jamais observées sur Clipperton) ont été identifiées, à savoir le poisson ange royal Holacanthus passer, et le poisson perroquet étoilé Calotomus carolinensis. Les peuplements ichtyologiques sont apparus en très bonne santé, avec une biomasse en meso-prédateurs (notamment les gros mérous et murènes) extrêmement importante. Concernant les requins, en comparaison avec les observations alarmantes de 2016, les observations convergent vers une augmentation de la densité et de la taille des individus, en particulier sur l’espèce dominante qui est le requin à pointes blanches Carcharhinus albimarginatus. D’une moyenne de 4,2 requins observés par plongée en 2016, la moyenne est passée à environ 11 requins par plongée, soit quasiment trois fois plus. La deuxième espèce en densités observées est le requin des Galapagos C. galapagensis, au sein de laquelle dominent aussi les individus d’une longueur inférieure à 90 cm (64%), mais les adultes de plus de 120 cm de long sont aussi présents. Le requin corail Trianodon obesus, qui n’avait pas été obervé en 2016, a lui aussi été observé à plusieurs reprises, dont certains individus > 150 cm LT. idem pour le requin marteau à festons Sphyrna lewinii, dont une demi-douzaine d’individus entre 250 et 400 cm LT ont été observés en plongée, en deçà de 20 m de profondeur. Outre les observations, de la pêche à partir des bateaux et des biopsies sous-marines (à l’aide d’une arbalète) ont permis de collecter une vingtaine d’échantillons de requins à pointes blanches et de requins des Galapagos, de même que quatre échantillons de requin corail. Dans le cadre d’une étude sur la connectivité spatiale des requins à l’échelle régionale, trois récepteurs acoustiques ont été installés sur Clipperton en 2010. Des résultats préliminaires montrent des déplacements des requins entre l’archipel des Revillagigedo (Mexique) au Nord et l‘archipel des Galapagos (Equateur) au Sud et ce, via Clipperton (France) au sein de ce que l’on considère comme le ‘corridor du Pacifique Tropical Est’. Les batteries de ces récepteurs ont été changées et les données
récupérées. Elles montrent la présence d’une vingtaine de requins différents, dont certains sont potentiellement extérieurs à Clipperton, ce qui confirmerait de nouveau la nécessité d’envisager la gestion de ces espèces migratrices à une échelle internationale. A terre, plusieurs débarquements ont permis de remettre en place un drapeau français sur la stèle à l’ouest de l’atoll, et de constater que la plaque commémorative posée en 2015 par le député Folliot avait disparue, probablement victime d’un acte de vandalisme. L’atoll est toujours jonché d’une quantité impressionnante de déchets, essentiellement en plastiques et venus par la mer. Subsistent néanmoins aussi les traces encore très visibles de la présnece humaine dans le camps de Bougainville et l’ancien camps américain au sein duquel
de nombreux obus désamorcés ont été empilés dans des sacs à gravas pour apparement être exportés de l’atoll, opération qui semble ne pas avoir aboutie, ce qui donne une piètre image de l’endroit. Il n’y a pas de nouvelle épave conséquente de bateau à déplorer, et les anciennes s’érodent inexorablement. Deux tombes datées de 2016,
apparemment humaines, ont été observées au Nord-Est de l’atoll. La mesure des cordons littoraux au niveau des anciennes passes naturelles, montre une stabilité depuis 2016,
malgré des signes d’érosion omniprésents autour de l’atoll. La largeur la plus étroite mesurée au niveau du rocher au Sud débouche sur 15 m (14 m mesurée en 2016) et 35 m au Nord-Est, alors qu’un passage fréquent d’eau semble exister dans une dépression du socle
phosphato-calcaire, sur une longueur de 47 m. La végétation rampante semble en extension, de même que les cocotiers (>1400 de plus de 1 m de haut contre 800 en 2016). En revanche, les crabes Johngarcia planatus, les rats Rattus rattus et les populations d’oiseaux, notamment de fous masqués Sula dactylactra (<30 000 individus au sol entre 7h et midi) semblent en baisse. Les frégates Fregata minor (>500) et nodis bruns Anous stolidus (>800) semblent en revanche en hausse.
The degree of reef isolation is thought to limit the frequency of long-range dispersals in reef-associated sharks. Therefore, understanding how the behaviour and spatial ecology of a species differ across reef habitats is essential for developing sound conservation approaches. The present study examined the residency, movement and activity space of grey reef sharks (Carcharhinus amblyrhynchos) in the central Great Barrier Reef (GBR). An array of 56 acoustic receivers covering 17 semi-isolated coral reefs across 150 km was used to monitor shark movements. Forty C. amblyrhynchos were tagged with acoustic transmitters and monitored from 251-821 d. Most sharks were detected on a single reef; however, some individuals (4 females; 10 males) moved to up to five reefs. Residency index ranged from 0.02-1.0, with a mean ± SD of 0.78 ± 0.26. Mixed effect models showed that weekly and monthly residency was mainly influenced by shark size, with little or no effect of environmental parameters. Although C. amblyrhynchos were present year-round, juvenile sharks had lower residency to their tagging reef than adults. In addition, mature females were detected less between November and mid-February, which coincides with reported parturition in the central GBR. Long-term monitoring data revealed that C. amblyrhynchos exhibited high residency to their tagging reef, and therefore even in systems with semi-isolated reefs such as the GBR this species may benefit from spatial management approaches at the reef level. However, behavioural differences between sexes and life-stages of C. amblyrhynchos reported in this study suggest marine reserves may provide lower protection relative to remote and isolated coral reefs.
A comparison between the northwestern Hawaiian islands (NWHI), a large, remote, and lightly fished area, and the main Hawaiian islands (MHI), an urbanized, heavily fished area, revealed dramatic differences in the numerical density, size, and biomass of the shallow reef fish assemblages. Grand mean fish standing stock in the NWHI was more than 260% greater than in the MHI. The most striking difference was the abundance and size of large apex predators (primarily sharks and jacks) in the NWHI compared to the MHI. More than 54% of the total fish biomass in the NWHI consisted of apex predators, whereas this trophic level accounted for less than 3% of the fish biomass in the MHI. In contrast, fish biomass in the MHI was dominated by herbivores (55%) and small-bodied lower-level carnivores (42%). Most of the dominant species by weight in the NWHI were either rare or absent in the MHI and the target species that were present, regardless of trophic level, were nearly always larger in the NWHI, These differences represent both near-extirpation of apex predators and heavy exploitation of lower trophic levels in the MHI compared to the largely unfished NWHI. The reefs in the NWHI are among the few remaining large-scale, intact, predator-dominated reef ecosystems left in the world and offer an opportunity to understand how unaltered ecosystems are structured, how they function, and how they can most effectively be preserved. The differences in fish assemblage structure in this study are evidence of the high level of exploitation in the MHI and the pressing need for ecosystem-level management of reef systems in the MHI as well as the NWHI.
This study confirms reports by fishermen of a large and predictable aggregation of whale sharks Rhincodon typus along the Belize Barrier Reef. Although whale sharks are rarely sighted at this location during most of the year, we counted as many as 25 whale sharks in a 50 m diameter area on 1 occasion and tagged 6 sharks during a 22 min period on another. The whale shark aggregation coincides seasonally and temporally with a multispecies reef-fish spawning aggregation at a reef promontory, Gladden Spit, at sunset, during the full and last- quarter moon periods of April and May each year. We report here, for the first time, that whale sharks feed on the freshly released spawn of cubera snappers Lutjanus cyanopterus and dog snappers L, jocu (Lutjanidae), and have documented the phenomenon with still and digital video photography of hundreds of feeding events. There is consensus locally that this remarkable interaction is in need of immediate protection from overfishing of snappers and unregulated tourism development. Our continued investigations are providing management recommendations for a new marine reserve at the site.
The decline of large‐bodied predatory species in the oceans is a concern both from a sustainability perspective and because such species can have important ecological roles. Sharks are particularly vulnerable to fishing as their life histories are characterized by late age at maturity, large body size, and low fecundity.
Substantial shark population declines have been documented for a number of coastal and pelagic systems, with high population abundance limited to a few remote locations. The relative abundance and composition of reef shark populations are assessed from 1975 to 2006 at a remote, largely uninhabited, group of atolls in the central Indian Ocean; the Chagos Archipelago.
Number of sharks observed per scientific dive declined from a mean of 4.2 in the 1970s to 0.4 in 2006, representing a decline of over 90%. Silvertip sharks displayed an increase in abundance from 1996, whereas blacktip and whitetip reef sharks were rarely encountered in 2006.
Poaching in the archipelago, is the most likely cause of these declines, highlighted by a number of illegal vessels containing large numbers of sharks arrested since 1996. The data highlight that shark populations, even in remote, otherwise pristine, marine areas, are vulnerable to distant fishing fleets, and a range of strategies will need to be used in concert for their conservation. Copyright © 2010 John Wiley & Sons, Ltd.
The diversity of oceanic and continental reef structures of New Caledonia is reviewed, taking into account their geological
history and in particular, that since the last interglacial period. To guide this review, a new path is provided by following
the diversity of units that have been mapped and characterized using high spatial resolution optical remote sensing data for
the main New Caledonian coral reef complexes (banks, atolls, uplifted reefs, drowned reefs, fringing reefs, barrier reefs,
patch reefs) and their individual reef-forming units. This interpretation, based on geomorphology, depth, and exposure has
provided 161unit types distributed across 4,537km2 of reef area and 31,336km2 of non-reef area. In addition to shallow reefs (0–30m) described by optical remote sensing, the bathymetry of deep slopes
between −20 to −1,000m were recently mapped using multibeam acoustic data providing additional data to explain the morphological
diversity. With the detailed three-dimensional topographic information acquired, hitherto unrecognized marine terraces and
faulting became visible, indicating different episodes of formation of the barrier reef and of sea level variations. Finally,
dating and coring corals provided a more accurate understanding of the genesis of the present reef structures. In contrast
with the synoptic remote sensing data, cores provided only point data, but allowed the addition of a precise temporal dimension
to the description of New Caledonian reefs. Cores provided a significant body of the information necessary for the establishment
of models of reef settlement and development during the last interglacial ages in the New Caledonian region. The combined
examination of the different sources of data, and the exhaustive description of remotely sensed reef units, allow a qualitative
synoptic parallel to be drawn between the morphology of modern reefs and the contrasting patterns of reef growth, subsidence,
and uplift rates occurring around New Caledonia.
Although marine protected areas (MPAs) are a common conservation strategy, these areas are often designed with little prior knowledge of the spatial behaviour of the species they are designed to protect. Currently, the Coral Sea area and its seamounts (north-east Australia) are under review to determine if MPAs are warranted. The protection of sharks at these seamounts should be an integral component of conservation plans. Therefore, knowledge on the spatial ecology of sharks at the Coral Sea seamounts is essential for the appropriate implementation of management and conservation plans. Acoustic telemetry was used to determine residency, site fidelity and spatial use of three shark species at Osprey Reef: whitetip reef sharks Triaenodon obesus, grey reef sharks Carcharhinus amblyrhynchos and silvertip sharks Carcharhinus albimarginatus. Most individuals showed year round residency at Osprey Reef, although five of the 49 individuals tagged moved to the neighbouring Shark Reef (~14 km away) and one grey reef shark completed a round trip of ~250 km to the Great Barrier Reef. Additionally, individuals of white tip and grey reef sharks showed strong site fidelity to the areas they were tagged, and there was low spatial overlap between groups of sharks tagged at different locations. Spatial use at Osprey Reef by adult sharks is generally restricted to the north-west corner. The high residency and limited spatial use of Osprey Reef suggests that reef sharks would be highly vulnerable to targeted fishing pressure and that MPAs incorporating no-take of sharks would be effective in protecting reef shark populations at Osprey and Shark Reef.
In recent decades, large pelagic and coastal shark populations have declined dramatically with increased fishing; however, the status of sharks in other systems such as coral reefs remains largely unassessed despite a long history of exploitation. Here we explore the contemporary distribution and sighting frequency of sharks on reefs in the greater-Caribbean and assess the possible role of human pressures on observed patterns.
We analyzed 76,340 underwater surveys carried out by trained volunteer divers between 1993 and 2008. Surveys were grouped within one km2 cells, which allowed us to determine the contemporary geographical distribution and sighting frequency of sharks. Sighting frequency was calculated as the ratio of surveys with sharks to the total number of surveys in each cell. We compared sighting frequency to the number of people in the cell vicinity and used population viability analyses to assess the effects of exploitation on population trends. Sharks, with the exception of nurse sharks occurred mainly in areas with very low human population or strong fishing regulations and marine conservation. Population viability analysis suggests that exploitation alone could explain the large-scale absence; however, this pattern is likely to be exacerbated by additional anthropogenic stressors, such as pollution and habitat degradation, that also correlate with human population.
Human pressures in coastal zones have lead to the broad-scale absence of sharks on reefs in the greater-Caribbean. Preventing further loss of sharks requires urgent management measures to curb fishing mortality and to mitigate other anthropogenic stressors to protect sites where sharks still exist. The fact that sharks still occur in some densely populated areas where strong fishing regulations are in place indicates the possibility of success and encourages the implementation of conservation measures.
Despite an Indo-Pacific wide distribution, the movement patterns of grey reef sharks (Carcharhinus amblyrhynchos) and fidelity to individual reef platforms has gone largely unstudied. Their wide distribution implies that some individuals have dispersed throughout tropical waters of the Indo-Pacific, but data on large-scale movements do not exist. We present data from nine C. amblyrhynchos monitored within the Great Barrier Reef and Coral Sea off the coast of Australia. Shark presence and movements were monitored via an array of acoustic receivers for a period of six months in 2008. During the course of this monitoring few individuals showed fidelity to an individual reef suggesting that current protective areas have limited utility for this species. One individual undertook a large-scale movement (134 km) between the Coral Sea and Great Barrier Reef, providing the first evidence of direct linkage of C. amblyrhynchos populations between these two regions. Results indicate limited reef fidelity and evidence of large-scale movements within northern Australian waters.
L'archipel des îles Chesterfield, situé dans la mer du Corail entre la Grande Barrière de Corail et la Nouvelle-Calédonie, a fait l'objet de deux campagnes océanographiques, l'une concernant l'ichtyofaune (CORAIL 1) et l'autre le benthos (CORAIL 2). La campagne CORAIL 2 a échantillonné les fonds meubles du lagon de Chesterfield et les bancs Lansdowne et Fairway avec 149 dragages et 23 traits de chalut à perche. Après un exposé de l'intérêt biogéographique de l'archipel de Chesterfield replacé dans le contexte géologique régional, une description des objectifs des campagnes CORAIL est présentée. Les premières observations concernant la nature des fonds, les sédiments et la répartition de certains taxa sont données en annexe sous forme de cartes. (Résumé d'auteur)
Top‐down control can be an important determinant of ecosystem structure and function, but in oceanic ecosystems, where cascading effects of predator depletions, recoveries, and invasions could be significant, such effects had rarely been demonstrated until recently.
Here we synthesize the evidence for oceanic top‐down control that has emerged over the last decade, focusing on large, high trophic‐level predators inhabiting continental shelves, seas, and the open ocean.
In these ecosystems, where controlled manipulations are largely infeasible, ‘pseudo‐experimental’ analyses of predator–prey interactions that treat independent predator populations as ‘replicates’, and temporal or spatial contrasts in predator populations and climate as ‘treatments’, are increasingly employed to help disentangle predator effects from environmental variation and noise.
Substantial reductions in marine mammals, sharks, and piscivorous fishes have led to mesopredator and invertebrate predator increases. Conversely, abundant oceanic predators have suppressed prey abundances. Predation has also inhibited recovery of depleted species, sometimes through predator–prey role reversals. Trophic cascades have been initiated by oceanic predators linking to neritic food webs, but seem inconsistent in the pelagic realm with effects often attenuating at plankton.
Top‐down control is not uniformly strong in the ocean, and appears contingent on the intensity and nature of perturbations to predator abundances. Predator diversity may dampen cascading effects except where nonselective fisheries deplete entire predator functional groups. In other cases, simultaneous exploitation of predator and prey can inhibit prey responses. Explicit consideration of anthropogenic modifications to oceanic foodwebs should help inform predictions about trophic control.
Synthesis and applications . Oceanic top‐down control can have important socio‐economic, conservation, and management implications as mesopredators and invertebrates assume dominance, and recovery of overexploited predators is impaired. Continued research aimed at integrating across trophic levels is needed to understand and forecast the ecosystem effects of changing oceanic predator abundances, the relative strength of top‐down and bottom‐up control, and interactions with intensifying anthropogenic stressors such as climate change.
Large predatory fishes have long played an important role in marine ecosystems and fisheries. Overexploitation, however, is gradually diminishing this role. Recent estimates indicate that exploitation has depleted large predatory fish communities worldwide by at least 90% over the past 50-100 years. We demonstrate that these declines are general, independent of methodology, and even higher for sensitive species such as sharks. We also attempt to predict the future prospects of large predatory fishes. (i) An analysis of maximum reproductive rates predicts the collapse and extinction of sensitive species under current levels of fishing mortality. Sensitive species occur in marine habitats worldwide and have to be considered in most management situations. (ii) We show that to ensure the survival of sensitive species in the northwest Atlantic fishing mortality has to be reduced by 40-80%. (iii) We show that rapid recovery of community biomass and diversity usually occurs when fishing mortality is reduced. However, recovery is more variable for single species, often because of the influence of species interactions. We conclude that management of multi-species fisheries needs to be tailored to the most sensitive, rather than the more robust species. This requires reductions in fishing effort, reduction in bycatch mortality and protection of key areas to initiate recovery of severely depleted communities.
Effective conservation requires rigorous baselines of pristine conditions to assess the impacts of human activities and to evaluate the efficacy of management. Most coral reefs are moderately to severely degraded by local human activities such as fishing and pollution as well as global change, hence it is difficult to separate local from global effects. To this end, we surveyed coral reefs on uninhabited atolls in the northern Line Islands to provide a baseline of reef community structure, and on increasingly populated atolls to document changes associated with human activities. We found that top predators and reef-building organisms dominated unpopulated Kingman and Palmyra, while small planktivorous fishes and fleshy algae dominated the populated atolls of Tabuaeran and Kiritimati. Sharks and other top predators overwhelmed the fish assemblages on Kingman and Palmyra so that the biomass pyramid was inverted (top-heavy). In contrast, the biomass pyramid at Tabuaeran and Kiritimati exhibited the typical bottom-heavy pattern. Reefs without people exhibited less coral disease and greater coral recruitment relative to more inhabited reefs. Thus, protection from overfishing and pollution appears to increase the resilience of reef ecosystems to the effects of global warming.
Intra-guild predation (IGP), depredation of hooked sharks and cannibalism by large individuals on smaller conspecifics have been documented for both tiger shark Galeocerdo cuvier and bull shark Carcharhinus leucas. In this paper we report a case study of cumulative inter and intra-depredation from the Indian and the Pacific Ocean involving large G. cuvier and C. leucas adults at the final stage. These findings further the hypothesis that IGP and cannibalism among adult sharks occurs in the wild and potentially influences predator-prey relationships in tropical marine ecosystems.
sampling includes 4 widely separated locations in the Indo-Pacific and 11 islands in French Polynesia with different levels of coastal development. 14 microsatellite loci were analysed for samples from all locations and 2 mitochondrial DNA fragments, the control region and cytochrome b, were examined for 10 locations. For microsatellites, genetic diversity is higher for the locations in the large open systems of the Red Sea and Australia than for the fragmented habitat of the smaller islands of French Polynesia. Strong significant structure was found for distant locations with FST values as high as ~0.3, and a smaller but still significant structure is found within French Polynesia. Both mitochondrial genes show only a few mutations across the sequences with a dominant shared haplotype in French Polynesia and New Caledonia suggesting a common lineage different to that of East Australia. Demographic history analyses indicate population expansions in the Red Sea and Australia that may coincide with sea level changes after climatic events. Expansions and flat signals are indicated for French Polynesia as well as a significant recent bottleneck for Moorea, the most human-impacted lagoon of the locations in French Polynesia.
Cited By (since 1996):23, Export Date: 26 November 2013, Source: Scopus
Long-line surveys, conducted over the past 24 years from Bimini, Bahamas reveal a decline of 83% in the number of lemon sharks caught annually compared with catches for the same period 20 years ago. We suggest that resident lemon sharks might have learned to avoid long-line equipment based on their ability to detect electric fields. We tested whether juvenile lemon sharks were able to learn to avoid baited metal hooks. Six sharks were individually presented with two visually similar, baited hooks: a metal circle hook and a plastic replica. If the sharks attempted to feed from the control hook (plastic), they were undisturbed and allowed to take the bait. If the sharks attempted to feed from the metal hook, they were mechanically disturbed by the observer to create a negative stimulus. To test for active learning, the correct response (taking the bait off the plastic hook) was correlated with the number of experimental sessions. Despite an average of 146 trials, statistical evidence for learning in any of the sharks could not be conclusively demonstrated. Thus it was not possible under the present experimental conditions to confirm whether lemon sharks could actively avoid long-line hooks using electroreception.
The quantification of spatial and temporal movement patterns of coral reef sharks is important to understand their role in reef communities and to aid the design of conservation strategies for this predatory guild. We observed 4 species of reef sharks aggregating in an inshore bay in the north of Western Australia for over 2 yr, using acoustic telemetry and visual censuses to examine how they partitioned this site in space and time. We fitted 58 sharks with acoustic transmitters: Carcharhinus melanopterus (36), C. amblyrhynchos (11), Negaprion acutidens (7) and Triaenodon obesus (4). Aggregations consisted primarily of C. melanopterus, although C. amblyrhynchos and N. acutidens were often present. We observed aggregations by visual census in summer (maximum of 44 sharks). Detections were highest during warmer months (Sep to Mar) for all species, although some individuals showed year-round residency. C. melanopterus, C. amblyrhynchos and N. acutidens had strong diel patterns of attendance at the aggregation site. Peak daily detections occurred from 13: 00 to 14: 00 h local time for C. melanopterus and C. amblyrhynchos; juvenile C. melanopterus and N. acutidens peaked at 05:00 and 10:00 h, respectively. There was considerable spatial overlap of core areas of use (50% kernel density estimates) at the northern end of the bay by all species; the southern end was used primarily by C. melanopterus and N. acutidens. Aggregations of C. melanopterus and C. amblyrhynchos consisted mainly of adult females, some of them pregnant. Courtship behaviour in C. melanopterus and T. obesus suggests that these aggregations are related to reproduction. All species displayed inter-annual site fidelity. The long-term presence of juvenile C. melanopterus and N. acutidens also suggests that this bay provides suitable conditions for younger age classes.
Tasman Sea guyots are former volcanic islands that have subsided an average of 166 fathoms since the Middle or Late Tertiary. These guyots have heights up to 2333 fathoms above the surrounding sea floor; their flat-topped platforms are at depths ranging from 50 to 500 fathoms below present sea level. Reef formations are found along the edges and on the tops of the guyots. Volcanic rocks dredged from these truncated undersea mountains are alkaline olivine basalts with large augite, olivine and plagioclase phenocrysts in a glassy matrix. Limestones dredged from the guyots are either hard, light brown, massive foraminiferal limestones or grey, porous limestones composed mainly of skeletal material. Using foraminiferal evidence we assign an age for the limestone accumulation of Late Pliocene or Early Pleistocene. Phosphorite is also present and is thought to be formed from the replacement of carbonate ions in calcareous material by phosphate ions. There is some evidence of direct precipitation of phosphatic material as coatings on many of the basalt samples.
There is growing recognition of the important roles played by predators in regulating ecosystems and sustaining biodiversity. Much attention has focused on the consequences of predator‐regulation of herbivore populations, and associated trophic cascades. However apex predators may also control smaller ‘mesopredators’ through intraguild interactions. Removal of apex predators can result in changes to intraguild interactions and outbreaks of mesopredators (‘mesopredator release’), leading in turn to increased predation on smaller prey. Here we provide a review and synthesis of studies of predator interactions, mesopredator release and their impacts on biodiversity. Mesopredator suppression by apex predators is widespread geographically and taxonomically. Apex predators suppress mesopredators both by killing them, or instilling fear, which motivates changes in behaviour and habitat use that limit mesopredator distribution and abundance. Changes in the abundance of apex predators may have disproportionate (up to fourfold) effects on mesopredator abundance. Outcomes of interactions between predators may however vary with resource availability, habitat complexity and the complexity of predator communities. There is potential for the restoration of apex predators to have benefits for biodiversity conservation through moderation of the impacts of mesopredators on their prey, but this requires a whole‐ecosystem view to avoid unforeseen negative effects.
‘Nothing has changed since I began.
My eye has permitted no change.
I am going to keep things like this.’
From ‘Hawk Roosting’, by Ted Hughes.
Marine ecosystems are suffering severe depletion of apex predators worldwide; shark declines are principally due to conservative life-histories and fisheries overexploitation. On coral reefs, sharks are strongly interacting apex predators and play a key role in maintaining healthy reef ecosystems. Despite increasing fishing pressure, reef shark catches are rarely subject to specific limits, with management approaches typically depending upon no-take marine reserves to maintain populations. Here, we reveal that this approach is failing by documenting an ongoing collapse in two of the most abundant reef shark species on the Great Barrier Reef (Australia). We find an order of magnitude fewer sharks on fished reefs compared to no-entry management zones that encompass only 1% of reefs. No-take zones, which are more difficult to enforce than no-entry zones, offer almost no protection for shark populations. Population viability models of whitetip and gray reef sharks project ongoing steep declines in abundance of 7% and 17% per annum, respectively. These findings indicate that current management of no-take areas is inadequate for protecting reef sharks, even in one of the world's most-well-managed reef ecosystems. Further steps are urgently required for protecting this critical functional group from ecological extinction.
Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how predators influence communities by inflicting mortality on prey and inducing behavioral modifications (risk effects). Both mechanisms are important in marine communities, and a sole focus on the effects of predator-inflicted mortality might severely underestimate the importance of predators. We outline direct and indirect consequences of marine predator declines and propose an integrated predictive framework that includes risk effects, which appear to be strongest for long-lived prey species and when resources are abundant. We conclude that marine predators should be managed for the maintenance of both density- and risk-driven ecological processes, and not demographic persistence alone.
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