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Basking sharks
\"This photo-illustrated book describes basking sharks, the second-largest sharks in the sea. Explains how they use gill-rakers to feed on zooplankton, their life cycle, and work being done to protect this species of huge but gentle sharks\"-- Provided by publisher.
Book
Basking shark sub-surface behaviour revealed by animal-towed cameras
While biologging tags have answered a wealth of ecological questions, the drivers and consequences of movement and activity often remain difficult to ascertain, particularly marine vertebrates which are difficult to observe directly. Basking sharks, the second largest shark species in the world, aggregate in the summer in key foraging sites but despite advances in biologging technologies, little is known about their breeding ecology and sub-surface behaviour. Advances in camera technologies holds potential for filling in these knowledge gaps by providing environmental context and validating behaviours recorded with conventional telemetry. Six basking sharks were tagged at their feeding site in the Sea of Hebrides, Scotland, with towed cameras combined with time-depth recorders and satellite telemetry. Cameras recorded a cumulative 123 hours of video data over an average 64-hour deployment and confirmed the position of the sharks within the water column. Feeding events only occurred within a metre depth and made up ¾ of the time spent swimming near the surface. Sharks maintained similar tail beat frequencies regardless of whether feeding, swimming near the surface or the seabed, where they spent surprisingly up to 88% of daylight hours. This study reported the first complete breaching event and the first sub-surface putative courtship display, with nose-to-tail chasing, parallel swimming as well as the first observation of grouping behaviour near the seabed. Social groups of sharks are thought to be very short term and sporadic, and may play a role in finding breeding partners, particularly in solitary sharks which may use aggregations as an opportunity to breed. In situ observation of basking sharks at their seasonal aggregation site through animal borne cameras revealed unprecedented insight into the social and environmental context of basking shark behaviour which were previously limited to surface observations.
Journal Article
Use of Photo-Identification and Mark-Recapture Methodology to Assess Basking Shark (Cetorhinus maximus) Populations
Following centuries of exploitation, basking sharks (Cetorhinus maximus) are considered by IUCN as Endangered in the Northeast Atlantic, where they have now been substantially protected for over two decades. However, the present size of this population remains unknown. We investigated the use of photo-identification of individuals' dorsal fins, combined with mark-recapture methodology, to investigate the size of populations of basking shark within the west coast of Scotland. From a total of 921 encounters photographed between 2004 and 2011, 710 sharks were found to be individually identifiable based on dorsal fin damage and natural features. Of these, only 41 individuals were re-sighted, most commonly both within days of, and close to the site of, the initial encounter. A smaller number were re-sighted after longer periods of up to two years. A comparison of the distinguishing features of individuals on first recording and subsequent re-sighting showed that in almost all cases these features remained little changed, suggesting the low re-sighting rate was not due to a loss of distinguishing features. Because of the low number of re-sighting we were not able to produce reliable estimates for the long-term regional population. However, for one 50 km diameter study area between the islands of Mull, Coll and Tiree, we were able to generate closed-population estimates for 6-9 day periods in 2010 of 985 (95% CI = 494-1683), and in 2011 of 201 (95% CI = 143-340). For the same 2011 period an open-population model generated a similar estimate of 213 (95% CI = 111-317). Otherwise the low rate and temporal patterning of re-sightings support the view that such local basking shark populations are temporary, dynamic groupings of individuals drawn from a much larger regional population than previously supposed. The study demonstrated the feasibility and limitations of photo-identification as a non-invasive technique for identifying individual basking sharks.
Journal Article
Behavioral response of megafauna to boat collision measured via animal-borne camera and IMU
Overlap between marine megafauna and maritime activities is a topic of global concern. Basking sharks ( Cetorhinus maximus ; CM) are listed as Globally Endangered under the IUCN, though reported sightings appear to be increasing in Ireland. While such trends in the region are welcome, increasing spatiotemporal overlap between CM and numerous water users poses an increased risk of boat strikes to the animals. To demonstrate the risk and impact of boat strikes on marine megafauna, we present camera-enabled animal-borne inertial measurement unit (IMU) data from a non-lethal boat strike on a CM within a proposed National Marine Park in Ireland. We tagged a ~7-m female CM in County Kerry, Ireland, which was struck by a boat ~6 h after tag deployment. Comparison of pre-strike data with 4 h of video and ~7.5 h of IMU data following the boat strike provides critical insight into the animal’s response. While the CM reacted momentarily with an increase in activity and swam to the seafloor, it quickly reduced its overall activity (i.e., overall dynamic body acceleration, tailbeat cycles, tailbeat amplitude, and vertical velocity) for the remainder of the deployment. Notably, the animal also ceased feeding for the duration of the video and headed towards deep offshore waters, which is in stark contrast to the pre-strike period where the animal was consistently observed feeding along the surface in shallow coastal water. This work provides insight into a CM’s response to acute injury and highlights the need for appropriate protections to mitigate risks for marine megafauna.
Journal Article
Seasonal changes in basking shark vertical space use in the north-east Atlantic
Mobile marine species can exhibit vast movements both horizontally and vertically. Spatial analysis of vertical movements may help improve an understanding of the processes that influence space use. Previously, vertical space use of basking sharks (Cetorhinus maximus) in the north-east Atlantic described movements largely within waters of the continental shelf during summer and autumn months, with few records of detailed vertical behaviour during winter. We use archival satellite telemetry data from 32 basking sharks (12 females, 6 males, and 14 of unknown sex measuring 4–5 m (n = 6), 5–6 m (n = 10), 6–7 m (n = 7), 7–8 m (n = 8), and 8–9 m (n = 1) estimated total length) tracked over 4 years (2012–2015). The satellite tags provided depth and temperature data for a cumulative 4489 days (mean 140 ± 97 days per shark, range 10–292 days) to describe vertical space use and thermal range of basking sharks in the north-east Atlantic. Basking sharks exhibit seasonality in vertical space use, revealing repeated ‘yo-yo’ movement behaviour with periods of occupancy at depths greater than 1000 m in late winter/early spring. Describing seasonal vertical space use in marine megavertebrates can increase knowledge of movements throughout their environment including physiological and morphological constraints to movement, nutrient transfer, and overlap with anthropogenic threats to inform future conservation strategies.
Journal Article
200 YEARS OF RECORDS OF THE BASKING SHARK, CETORHINUS MAXIMUS, IN THE EASTERN ADRIATIC
Lo squalo elefante era relativamente raro nell'Adriatico, ma dall'inizio del XXI secolo la sua presenza e aumentata in modo sostanziale. E stato ipotizzato che gli squali elefante migrino dal Mediterraneo verso l'Adriatico settentrionale, seguendo masse d'acqua che trasportano specifiche specie di copepodi sufficientemente abbondanti per la loro alimentazione. In questo lavoro vengono raccolti dati recenti e storici per riesaminare le tendenze spaziali e temporali della presenza dello squalo elefante nell'Adriatico. Nel corso degli ultimi 200 anni sono state raccolte in totale 75 segnalazioni, a partire dalla prima del 1822. La maggior parte degli avvistamenti risale alla stagione primaverile, quando ľabbondanza dei copepodi e piu elevata. Dopo la primavera, l'inverno, soprattutto la seconda meta, e il periodo dell'anno in cui e stata avvistata la maggior parte degli squali elefante, mentre durante l'autunno e l'estate sono stati registrati solo, rispettivamente, 7 e 6 esemplari di squalo elefante.
Journal Article
Basking sharks of the Arctic Circle: year-long, high-resolution tracking data reveal wide thermal range and prey-driven vertical movements across habitats
Background
Understanding the movement ecology of marine megaplanktivores is essential for conserving these ecologically significant species and managing their responses to environmental change. While telemetry has advanced our knowledge of filter-feeding mammal migrations, the annual movement patterns of large filter-feeding sharks, such as basking sharks (
Cetorhinus maximus
), remain poorly understood. This is particularly the case near their high latitude range limits where climate impacts are intensifying. In this study, we deployed pop-up satellite archival tags (PSATs) on
C. maximus
in northern Norway to investigate individual movement patterns and possible environmental drivers over an entire annual cycle.
Results
Geolocated tracks from two females revealed contrasting migration strategies: one shark performed a return migration spending boreal winter close to the Azores, while the other resided north of the Arctic Circle until January before moving to the North Sea in spring. Across these diverse habitats, both sharks utilized a wide thermal range. This included previously unrecorded short-term exposures to sub-zero temperatures, extending the known thermal tolerance of the species. High-resolution time series data from recovered PSATs enabled the use of advanced signal processing and gradient-based filtering techniques to investigate vertical movement patterns in relation to the physical and biological environment. In oceanic habitats, elevated use of the mesopelagic was observed together with diel vertical migration, whereas in shelf areas depth-use patterns were confined by topography and more variable, reflective of more dynamic hydrographic conditions and prey distributions. With zooplankton distributions being structured by ambient light, density gradients, and local topography, the alignment of frequented depths with isolumes, mixed layer depths, bathymetric contours, and bioluminescence events suggests these sharks actively track prey layers across diverse habitats.
Conclusions
Recorded eurythermy and behavioural plasticity suggest
C. maximus
to be well-adapted to dynamic ocean conditions. These traits may be critical for responding to the rapid climate-driven changes in the abiotic and biotic environments in high-latitudes, providing insights into how these endangered filter-feeders might navigate shifting ecosystems.
Graphical Abstract
Journal Article
Basking sharks in the northeast Atlantic
Basking sharksCetorhinus maximushave undergone widespread historic exploitation in the northeast Atlantic and are of conservation concern. A greater knowledge of their spatial and temporal habitat use is required to better inform subsequent monitoring and management strategies. Techniques such as light-based geolocation have provided great insights into individual movements, but currently available data do not permit extrapolation to the population level. Public recording schemes may, however, help to fill shortfalls in data gathering, especially when analysed in conjunction with data from these other techniques. We analysed 11 781 records (from 1988 to 2008) from 2 public recording databases operating in the UK. We describe 3 sightings hotspots: western Scotland, Isle of Man and southwest England, and highlight the marked seasonality of basking shark sightings, which were at their greatest during the northeast Atlantic summer (June to August). We further highlight a significant correlation between the duration of the sightings season in each year and the North Atlantic Oscillation, an atmosphere−ocean climate oscillation that has been linked to forcing of marine ecosystems. We augment patterns from public sightings records with effort-related data collected by boat-based transects at 2 regional sightings hotspots (western Scotland and southwest England). Analysis of reported body size data indicated that the annual proportion of small sharks (<4 m length) sighted by the public decreased, the proportion of medium-sized sharks sighted (4−6 m) increased, and the proportion of large sharks sighted (>6 m) remained constant. These patterns may be indicative of a population recovery following systematic harvesting in the 20th century.
Journal Article