A specimen of a sixgill bluntnose shark, Hexanchus griseus (Bonnaterre, 1788), was caught 1.5 NM north of Cape Ronek (Izola, Slovenia) in a fishing net for large-sized flatfish (such as turbot) on 28 ...January 2018. Three other older cases of catch of sixgill bluntnose sharks were recorded in Slovenia and the Gulf of Trieste. Among these, the finding of the specimen in the Lagoon of Marano and Grado is unusual although there are reported cases of sixgill bluntnose sharks in rivers. An analysis of the available data on the bluntnose sixgill shark in the Adriatic Sea, obtained from different published papers, social media and other sources, was done to understand whether the occurrence of H. griseus in the northern Adriatic differs from other parts. A generalised linear model (GLM) approach revealed that larger specimens are more frequently sighted across the Adriatic Sea, while in the Northern Adriatic part, significantly smaller specimens (juveniles) were recorded in comparison to the Central and Southern parts. It seems that the bluntnose sixgill shark is not in conjunction with a common large shark decreasing trend across the whole Mediterranean Sea.
Primjerak volonje Hexanchus griseus (Bonnaterre, 1788), ulovljen je 28. siječnja 2018. 1,5 NM sjeverno od rta Ronek (Izola, Slovenija) u mrežu za lov na velike plosnate ribe (kao što su plosnatice). Još dva prethodna slučaja ulova volonje zabilježena su u Sloveniji i Tršćanskom zaljevu. Među njima, neobičan je pronalazak primjerka u lagunama Marano i Grado, iako postoje prijavljeni ulovi volonje u rijekama. Učinjena je analiza dostupnih podataka o volonji u Jadranskom moru, dobivenih iz različitih objavljenih radova, društvenih mreža i drugih izvora, kako bi se ustanovilo razlikuje li se pojava H. griseusa u sjevernom Jadranu od ostalih dijelova. Pristup generaliziranog linearnog modela (GLM) otkrio je da se veći primjerci češće viđaju diljem Jadranskog mora, dok su u sjevernom dijelu Jadrana zabilježeni znatno manji primjerci (mladi) u odnosu na srednji i južni dio. Čini se da volonja nije povezan s uobičajenim trendom smanjenja broja velikih morskih pasa u cijelom Sredozemnom moru.
Bait-based remote underwater video (BRUV) systems are effective devices for remotely observing fish and other marine organisms in challenging environments. The development of a long duration (24 h) ...surface BRUV observation surveys allowed the monitoring of scarce and elusive pelagic sharks and the direct impact on non-targeted species of longline fishing in the Western Mediterranean. Technological limitations, such as the limited storage capacity and a single surface camera, were improved by (i) adding a deep camera equipped with light (below 80 m depth) and (ii) replacing Gopros with a multi-camera video surveillance system (surface and depth) with a storage capacity of several days and access to real-time observation. Based on a deployment effort of 1884 h video data, we identified 11 blue sharks (Prionace glauca) and one bluntnose sixgill shark (Hexanchus griseus), a deep-sea species that scarcely swims at the surface. The real-time observation capability was a powerful tool for reducing logistical costs and for raising environmental awareness in educational and outreach programmes.
The broadnose sevengill shark, Notorynchus cepedianus, a common coastal species in the eastern North Pacific, was sampled during routine capture and tagging operations conducted from 2005-2012. One ...hundred and thirty three biopsy samples were taken during these research operations in Willapa Bay, Washington and in San Francisco Bay, California. Genotypic data from seven polymorphic microsatellites (derived from the related sixgill shark, Hexanchus griseus) were used to describe N. cepedianus genetic diversity, population structure and relatedness. Diversity within N. cepedianus was found to be low to moderate with an average observed heterozygosity of 0.41, expected heterozygosity of 0.53, and an average of 5.1 alleles per microsatellite locus. There was no evidence of a recent population bottleneck based on genetic data. Analyses of genetic differences between the two sampled estuaries suggest two distinct populations with some genetic mixing of sharks sampled during 2005-2006. Relatedness within sampled populations was high, with percent relatedness among sharks caught in the same area indicating 42.30% first-order relative relationships (full or half siblings). Estuary-specific familial relationships suggest that management of N. cepedianus on the U.S. West Coast should incorporate stock-specific management goals to conserve this ecologically important predator.
We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are ...nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two species of deep-sea sharks (bluntnose sixgill sharks, Hexanchus griseus, and a prickly shark, Echinorhinus cookei) in their natural habitat, we used accelerometer-magnetometer data loggers to measure their swimming performance. Both species of deep-sea sharks showed similar diel vertical migrations: they swam at depths of 200-300 m at night and deeper than 500 m during the day. Ambient water temperature was around 15 degree C at 200-300 m but below 7 degree C at depths greater than 500 m. During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats. This positive buoyancy may adaptive for stealthy hunting (i.e. upward gliding to surprise prey from underneath) or may facilitate evening upward migrations when muscle temperatures are coolest, and swimming most sluggish, after spending the day in deep, cold water. Positive buoyancy could potentially be widespread in fish conducting daily vertical migration in deep-sea habitats.
Stable nitrogen (δ15N) and carbon (δ13C) isotope ratios from muscle, liver and yolk were analysed from the mother and embryos of an ovoviviparous shark, Hexanchus griseus. Embryonic liver and muscle ...had similar δ15N and δ13C ratios or were depleted in heavy isotopes, compared to the same maternal somatic and reproductive yolk tissues, but no relationship existed between δ15N or δ13C and embryo length, as expected, because a switch to placental nourishment is lacking in this species. This study expands the understanding of maternal nourishment and embryonic stable isotope differences in ovoviviparous sharks.
Bluntnose sixgill sharks are apex predators and scavengers that are near-globally distributed in slope and shelf habitats, but many aspects of their behaviour and ecology are poorly understood. A ...better understanding of how oceanographic variables influence sixgill shark behaviour may help predict their distribution, response to increasing anthropogenic stressors including climate change, and role in ecosystems throughout their geographic range. We used satellite telemetry to observe the vertical behaviour of four bluntnose sixgill sharks in the subtropical oligotrophic waters of Hawaii. A strong diel vertical movement cycle was observed, with sharks spending nighttime in thermocline waters and descending into the oxygen minimum zone in daytime. Depth changes generally occurred between nautical twilight and sunrise/sunset. Dive initiation and dive completion were significantly correlated with nautical dawn and sunrise, respectively. A stepwise generalised estimating equations model was used to investigate vertical speeds in the daytime and nighttime depth habitats, and this analysis revealed that photic zone light level was the primary factor correlated with vertical speed. Outside of the depth transitions, higher vertical speeds were observed when photic zone light was low, suggesting more active foraging in the shallow nighttime habitat than the deep daytime habitat.
Diel vertical migration is a widespread behavioral phenomenon where organisms migrate through the water column and may modify behavior relative to changing environmental conditions based on ...physiological tolerances. Here, we combined a novel suite of biologging technologies to examine the thermal physiology (intramuscular temperature), fine-scale swimming behavior and activity (overall dynamic body acceleration as a proxy for energy expenditure) of bluntnose sixgill sharks (Hexanchus griseus) in response to environmental changes (depth, water temperature, dissolved oxygen) experienced during diel vertical migrations. In the subtropical waters off Hawai'i, sixgill sharks undertook pronounced diel vertical migrations and spent considerable amounts of time in cold (5-7°C), low oxygen conditions (10-25% saturation) during their deeper daytime distribution. Further, sixgill sharks spent the majority of their deeper daytime distribution with intramuscular temperatures warmer than ambient water temperatures, thereby providing them with a significant thermal advantage over non-vertically migrating and smaller-sized prey. Sixgill sharks exhibited relatively high rates of activity during both shallow (night) and deep (day) phases and contrary to our predictions, did not reduce activity levels during their deeper daytime distribution while experiencing low temperature and dissolved oxygen levels. This demonstrates an ability to tolerate the low oxygen conditions occurring within the local oxygen minimum zone. The novel combination of biologging technologies used here enabled innovative in situ deep-sea natural experiments and provided significant insight into the behavioral and physiological ecology of an ecologically important deepwater species.
Historically, the Mediterranean Sea supported a rich shark fauna. Presently, however, populations of most shark species have significantly declined, largely due to intense fishing pressure.
...Interviews with crew members of bottom trawlers, drifting longliners and bottom longliners operating off the Costa Brava (Catalonia, NE Spain) were conducted between October 2016 and July 2017 in order to gather information on the current bycatch rate of several shark species.
Interviews covered 41.2% of the fleet and respondents were asked for the bycatch of selected shark species—Alopias vulpinus, Cetorhinus maximus, Galeorhinus galeus, Hexanchus griseus, Isurus oxyrinchus, Mustelus spp., Prionace glauca, and Squalus acanthias—in two distinct time periods.
Bottom trawlers captured the highest diversity of species (eight) followed by bottom longliners (seven), and drifting longliners (three). Most respondents (89.7%) declared having captured at least one shark from 2006 to 2016 but only 56.4% declared having captured at least one shark from 2016 to 2017.
From 2016 to 2017, the whole fleet captured 89 specimens of H. griseus (95% confidence interval (CI) = 145, 34), 14 of G. galeus (95% CI = 30, 0), 3 of A. vulpinus (95% CI = 8, 0), 3 of I. oxyrinchus (95% CI = 8, 0), 3 of C. maximus (95% CI = 6, 0), and no Mustelus spp. The total bycatch of P. glauca and S. acanthias was uncertain due to extremely loose confidence intervals.
A significant decline was perceived by fishermen in the bycatch of C. maximus and S. acanthias, whereas the bycatch of H. griseus was considered to have remained stable.
This study suggests a dramatic reduction in the abundance of most of the medium‐sized and large sharks of the Costa Brava and the likely disappearance of Mustelus spp. from the area. Only H. griseus, S. acanthias, and P. glauca are still being bycaught frequently.
In the last 50 yr, shark populations showed steep declines in the Mediterranean Sea. The IUCN lists most Mediterranean species as threatened (55%), while considering 27.5% of them Data Deficient. ...Here, sharks are currently one of the rarest and more elusive groups of animals, and data from fisheries and scientific monitoring still insufficiently support robust abundance and distribution assessments. New technologies can fill this data gap by linking people and scientists through new monitoring strategies. SharkPulse, an international collaborative project, aims at creating a large world database of shark occurrence records by mining images on the web, social networks, and private archives. Here we analyzed 1186 sharkPulse records from the Mediterranean Sea. We collected records to characterize spatio-temporal patterns on 37 species, highlighting distribution changes for 5, and, by using generalized linear models, estimating trends in sighting for the most abundant species. With 273 records,
Hexanchus griseus
had the most sighting records since the beginning of the series. We identified pupping areas and aggregation sites for immature
Prionace glauca
and
Isurus oxyrinchu
s
; pinpointed strongholds of the Critically Endangered
Squatina squatina
to focus conservation efforts; and identified broader than previously reported regional distribution ranges for
Alopias superciliosus
,
Dalatias licha
,
Heptranchias perlo
,
H. griseus
,
Oxynotus centrina
, and
P. glauca
. We confirmed that fishing is still the major threat for Mediterranean sharks and call for a greater effort in controlling the emerging patterns with efficient conservation effort indexes. If properly standardized, opportunistic data can efficiently and cost-effectively advance our understanding of shark abundance, distribution, and conservation status.
The Guatemalan Caribbean has a deepwater fishing area close to the shore around the Cayman Trench. This study reports the first record of the bluntnose sixgill shark (Hexanchus griseus) from this ...fishing area. Fishery-independent surveys using longlines at ~430-465 m depth, ~11 km northeast of El Quetzalito fishing village, were conducted in 2022 and 2023. Two bluntnose sixgill sharks were captured during these surveys. The sharks were females with total lengths of 300 and 310 cm, with morphological characteristics consistent with this species. These are the first confirmed records of bluntnose sixgill sharks in the western Caribbean Sea. Expanding coastal fisheries to deeper waters presents an emerging threat to deep-sea chondrichthyans in the region. Therefore, periodic fisheries monitoring is needed to estimate their vulnerability to fishing pressure.
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