Hexanchus Research Articles

Trophic ecology of a large-bodied marine predator, bluntnose sixgill shark Hexanchus griseus, inferred using stable isotope analysis

Identifying feeding patterns of large-bodied predators is necessary for predicting their potential effects on food web dynamics. However, diet information from stomach contents can be impractical to obtain because required sample sizes can be prohibitively large. In contrast, diet estimates obtained using Bayesian stable isotope mixing models require less sampling effort and can also reveal both population- and individual-level variation in diet. Here, we used an extensive stable isotope data set to evaluate the trophic role of bluntnose sixgill shark (Hexanchus griseus), a globally distributed species and among the largest sharks in the North Pacific. In total, 43 subadult sixgill sharks were sampled from Puget Sound, Washington, USA. Mixing model results indicated that the population feeds primarily on benthic fish and invertebrates (estimated median diet percentages: 33 and 35%, respectively). Further, the model indicated low individual variation in diets and that the feeding behavior of both individuals and the population as a whole tended towards generalism. Specifically, sixgill sharks appear to feed on prey groups approximately in proportion to their average biomass densities in the Puget Sound food web. As generalists, sixgill sharks are less likely to be affected by changes in the abundance of any single prey resource, and our results suggest they are unlikely to be important predators to at least some species of management concern. In addition, stable isotope data obtained opportunistically from an adult sixgill shark supports previously suggested ontogenetic movement patterns, whereby some adults make brief migrations into Puget Sound from outer coastal habitats, likely to birth, and pups feed, grow, and remain resident in Puget Sound for several years. Our findings provide insights into the trophic role of this important but understudied species and demonstrate how stable isotope analyses can further understanding of shark ecology.

Evidence of a predation event on a tagged Mediterranean spearfish (Tetrapturus belone; Pisces, Istiophoridae), inferred from pop-up satellite tagging data

The Strait of Messina is located at the centre of the Mediterranean Sea and is considered a biodiversity hotspot and an obligatory seasonal passage for different pelagic species such as sharks, marine mammals, and billfishes. For the first time, in the Strait of Messina, our research group tagged a Mediterranean spearfish (Tetrapturus belone) using a pop-up satellite archival tag (PSAT). The observation of abiotic parameters (depth, light, and temperature) recorded by the PSAT confirmed that the tagged specimen was predated after about nine hours. The tag was then regurgitated 14 days after the tag deployment date. The analysis of collected data seems to indicate that the predator may be an ectothermic shark, most likely the bluntnose sixgill shark (Hexanchus griseus).

NISHAH KELAMIN, HUBUNGAN PANJANG-BERAT DAN UKURAN REPRODUKSI HIU Hexanchus spp. DI PERAIRAN SELATAN NUSA TENGGARA

Hexanchus spp. are shark species that are often caught in the Indian Ocean southern Nusa Tenggara and its status is Near Threatened, but the information is still relatively limited. This study aimed to sex ratio, length-weight relationship and reproductive size of sixgill shark, Hexanchus spp. caught from southern Nusa Tenggara waters. This study had been carried out during 2016 at the Fishing Port of Tanjung Luar, East Lombok, West Nusa Tenggara. Shark data collection was done by measuring the length, weight, maturity and fishing data. Data were analyzed by formula of sex ratio, length-weight relationship, length at first maturity (Lm) and length at first catch (Lc). The results showed that Hexanchus spp. caught in the southern Nusa Tenggara waters consisted of H. nakamurai (81.82%) and H. griseus (18.18%). The sex ratio of both sharks was dominated by the female. The average of the total length of H. griseus (218.3 ± 96.08 cm). It was longer than H. nakamurai (127.6 ± 30.35 cm). The length-weight relationship of H. griseus was formulated as W = 0.0195 LT 2,6344 (R² = 0,9265) and H. nakamurai with the formula W = 0.002 LT 3,1098 (R² = 0.902). Both species have the size of Lm was bigger than the size of Lc which indicated to their population vulnerability.

Decrease in Sightings of Sixgill Sharks, Hexanchus griseus, in Elliott Bay, Seattle, WA, United States, a Comparison Between 2003–2005 and 2008–2015

The Bluntnose Sixgill Shark, Hexanchus griseus, is a large predatory shark, has a worldwide distribution and is listed as near-threatened by the International Union of Conservation of Nature (IUCN). The Seattle Aquarium collected observations of free-swimming Sixgill Sharks in Elliott Bay, Washington, under the aquarium’s pier in 20 m of water from 2003 to 2005 and again from 2008 to 2015 using the same methodology. Compared to total Sixgill sightings between 2003-2005 (273) fewer total Sixgills were sighted at the aquarium’s research station between 2008-2015 (33). The reason for the observed decline in sightings in unknown but based on data from other studies on Sixgills in Puget Sound during the same timeperiod the authors hypothesize the decrease may be due to natural variability of juvenile Sixgill recruitment to Elliott Bay.

Occurrence of Bluntnose Sixgill Shark Hexanchus griseus (Bonnaterre, 1788) in Yeşilovacık Bay, Northeastern Mediterranean

Aims: In this study, an individual of H. griseus was caught from the Mersin Bay and this ichthyologic note presents a new female record of H. griseus for Mersin Bay, Mediterranean coast of Turkey. This study aims to contribute to the chronological records of the species in the Mediterranean Sea.
 Place and Duration of Study: Yeşilovacık bay is a small bay in the west of Mersin Bay. Yeşilovacık bay which lies at approximately 36°07’n latitude, 33°37’e, longitude and about 143 km Western of in Mersin province, Turkey. Sample: On 19 March 2018 blunthose sixgill shark specimen was measured and then deposited in the Museum of the Systematic, Faculty of Fisheries, University of Mersin.
 Methodology: A female specimen of H. griseus with a total length (TL) of 350 cm (400 kg) was captured by a commercial trawl at a depth around of 280 m of the Yeşilovacık Bay (Northeastern Mediterranean Sea), Turkey. Total length was measured to the nearest 1 mm and the weight of the specimen was determined to the nearest kilogram.
 Results: Measurements of the specimen are presented and compared with the previous records of H. griseus in the Eastern Mediterranean coast of Turkey (Iskenderun Bay), which are given in Table. The identification was carried out according to the previous report.
 Conclusion: To date specific conservation measures are not known for this species for the Northeastern Mediterranean coast of Turkey. Thus, the effect of coastal fishery for sharks should be regularly monitored and protection measures should be taken in this region in order for this shark species to continue their conservation for the next generation.

First report of four deep-sea chondrichthyans (Elasmobranchii and Holocephali) from Andaman waters, India with an updated checklist from the region

This paper confirms the new distributional records of four rare deep-water chondrichthyans viz. sharpnose sevengill shark, Heptranchias perlo (Bonnaterre, 1788); bluntnose sixgill shark, Hexanchus griseus (Bonnaterre, 1788); bramble shark, Echinorhinus brucus (Bonnaterre, 1788); and sicklefin chimaera, Neoharriotta pinnata (Schnakenbeck, 1931) from the Andaman waters in the Indian EEZ. The above-mentioned four fish species have not been reported earlier from this region. More exploratory surveys in the region are essential to understand the diversity and distribution pattern which is essential for critical management actions. An updated checklist of chondrichthyans known from Andaman and Nicobar waters of India is also presented.

Resurrection of the sixgill shark Hexanchus vitulus Springer & Waller, 1969 (Hexanchiformes, Hexanchidae), with comments on its distribution in the northwest Atlantic Ocean

The sixgill sharks of the genus Hexanchus (Hexanchiformes, Hexanchidae) are large, rarely encountered deep-sea sharks, thought to comprise just two species: the bluntnose sixgill Hexanchus griseus (Bonaterre, 1788) and the bigeye sixgill Hexanchus nakamurai (Teng, 1962). Their distribution is putatively worldwide in tropical and temperate waters, but many verified records for these species are lacking, and misidentification is common. Taxonomic uncertainty has long surrounded H. nakamurai in particular, with debate as to whether individuals from the Atlantic constitute a separate species. Using 1,310 base pairs of two mitochondrial genes, COI and ND2, we confirm that bigeye sixgill sharks from the Atlantic Ocean (Belize, Gulf of Mexico, and Bahamas) diverge from those in the Pacific and Indian Oceans (Japan, La Reunion, and Madagascar) with 7.037% sequence divergence. This difference is similar to the genetic distance between both Atlantic and Indo-Pacific bigeye sixgill sharks and the bluntnose sixgill shark (7.965% and 8.200%, respectively), and between the entire genus Hexanchus and its sister genus Heptranchias (8.308%). Such variation far exceeds previous measures of species-level genetic divergence in elasmobranchs, even among slowly-evolving deep-water taxa. Given the high degree of morphological similarity within Hexanchus, and the fact that cryptic diversity is common even among frequently observed shark species, we conclude that these results support the resurrection of the name Hexanchus vitulus Springer and Waller, 1969 for bigeye sixgill sharks in the northwest Atlantic Ocean. We propose the common name “Atlantic sixgill shark” for H. vitulus, and provide new locality records from Belize, as well as comments on its overall distribution.

Insights about depth distribution, occurrence and swimming behavior of Hexanchus griseus in the Cantabrian Sea (NE Atlantic)

The bluntnose sixgill shark, Hexanchus griseus, is a relatively common, wide-spread and deepwater shark that occurs along outer continental shelves and upper slopes. However, very little information exists about its distribution patterns, migrations and behavior. Data on occurrence and behavior of sixgill sharks inhabiting waters north of Spain (Galicia and Cantabrian Sea, NE Atlantic) were obtained from yearly oceanographic trawl surveys, commercial fishing boat records, and a pop-up satellite archival tag that was deployed for 75 days on a juvenile specimen. Most of the specimens were caught in Galician waters, and nearly all of those from surveys were juveniles (length range, 60–121 cm). Catch depth from surveys and commercial vessels ranged from 90 to 800 m and 90 to 1750 m respectively. Combining all data, a moderate relationship was observed between catch depth and size of the sharks (r2=0.5001). The sex ratio was 1:1.86 in favor of females. September and February were the months with more catches reported from commercial vessels. Data obtained from one electronic pop-up tag (Mini PAT), provided information about depth and temperature preferences over 75 days. Mean depth obtained during that period was 913 m (depth range 727–1247 m), and the mean temperature was 10.3 °C, (range 8.0–10.8 ° C). Movements up and down in the water column within a single day ranged from 50 to 385 m. No cyclic diel vertical migration was however observed, the shark moved smoothly without a defined pattern. Vertical movements at semidiurnal frequency were intermittently observed. The thereby differences observed in the swimming behavior of H. griseus compared to other areas could be attributed to size, foraging activity or atypical behavior.

Shark length–length relationships: Studying morphology allows the detection of bias in routine fisheries sampling

Reliable information on length–length relationships is required for standardising and comparing size measures and for improving the use of available data and biological information collected as a function of different size measures. Here, we used biological information on sharks and rays collected by on-board observers in Western Australia since the 1990s to estimate the total length–fork length relationships for 31 species. For an additional five species, the estimated model coefficients were unreliable. Further analysis for four wobbegong species indicated that some records could have been misidentified. For sixgill sharks, the linear model fitted the data well but visual exploration showed that two observers systematically measured this species differently. Our study shows that length–length relationships can be used to detect biases in biological data sets.

New record and morphometry of the bluntnose sixgill shark Hexanchus griseus (Chondrichthyes: Hexanchidae) in Baja California Sur, Mexico

We report the first record and morphometry of a complete juvenile Hexanchus griseus female shark, caught in Punta Lobos, Baja California Sur, Mexico. The oceanographic conditions of the area, where the specimen was caught, coincide with the habitat previously described for this shark species, with low temperature, stable salinity and high hydrostatic pressure. There is an overall lack of information about H. griseus in Mexico, with only eight records of its presence in the Mexican Pacific. Moreover, none of these previous records provided detailed information on the morphometry of a whole bluntnose sixgill shark. Here, we present the morphometric measures of a complete individual caught in shallow waters. We expect this information to be useful for future comparisons of specimens caught in other coastal regions.

Seasonal and daily movements of the bluntnose sixgill shark (Hexanchus griseus) in the strait of Georgia from satellite tag data

The bluntnose sixgill shark (Hexanchus griseus Bonnaterre 1788) is a large, deepwater apex predator that, in a few unique locations such as the Strait of Georgia, Canada, occupies shallow, inshore waters as juveniles. The occurrence of pregnant females in the Strait of Georgia suggests that this is an important area for parturition and juvenile rearing. Bluntnose sixgill sharks are listed under the Canadian Species at Risk Act, and an understanding of their behaviour in habitat selection during important life stages is an important component for conservation measures. We employed satellite tags to identify the daily and seasonal depth and thermal habitats of juvenile bluntnose sixgill sharks in the Strait of Georgia. Tag release and depth information suggests that juvenile bluntnose sixgill sharks within the Strait of Georgia remain there until they mature and migrate to offshore deepwaters. The dominant depth habitat utilized by the juvenile sharks in our study was deepwater (>200 m), with only occasional occupation of depths <100 m, indicating that even within coastal areas, juveniles still prefer deepwater. Overall the sharks occupied shallower depths at night than during the day with increased vertical activity in both the day and night compared to crepuscular periods. Seasonally, the sharks occupied shallower depths in the summer than in winter, but inconsistency in the temperatures with which those depths were associated suggests that their vertical behaviour is influenced by local foraging opportunities and not by thermoregulation.

Shark Interactions With Directed and Incidental Fisheries in the Northeast Pacific Ocean: Historic and Current Encounters, and Challenges for Shark Conservation.

For over 100 years, sharks have been encountered, as either directed catch or incidental catch, in commercial fisheries throughout the Northeast Pacific Ocean. A long-standing directed fishery for North Pacific Spiny Dogfish (Squalus suckleyi) has occurred and dominated shark landings and discards. Other fisheries, mainly for shark livers, have historically targeted species including Bluntnose Sixgill Shark (Hexanchus griseus) and Tope Shark (Galeorhinus galeus). While incidental catches of numerous species have occurred historically, only recently have these encounters been reliably enumerated in commercial and recreational fisheries. In this chapter we present shark catch statistics (directed and incidental) for commercial and recreational fisheries from Canadian waters (off British Columbia), southern US waters (off California, Oregon, and Washington), and northern US waters (off Alaska). In total, 17 species of sharks have collectively been encountered in these waters. Fishery encounters present conservation challenges for shark management, namely, the need for accurate catch statistics, stock delineation, life history parameter estimates, and improved assessments methods for population status and trends. Improvements in management and conservation of shark populations will only come with the further development of sound science-based fishery management practices for both targeted and incidental shark fisheries.

NUEVOS REGISTROS DE PECES DE ESQUELETO CARTILAGINOSO PARA EL CARIBE COLOMBIANO Y UNO COMO AMPLIACIÓN DE SU DISTRIBUCIÓN GEOGRÁFICA EN EL CARIBE COLOMBIANO PARA BOCAS DE CENIZA, DEPARTAMENTO DE ATLÁNTICO, COLOMBIA

Three new records of cartilaginous fishes are added and the area of distribution of one species is expanded for the Colombian Caribbean. These are: the bramble shark Echinorhinus brucus (Bonaterre), the sand tiger shark Odontaspis ferox (Risso), the roughtail stingray Dasyatis centroura (Mitchill) and the bluntnose sixgill shark Hexanchus griseus (Bonaterre). The animals were captured by artisanal fishermen from Las Flores, Barranquilla, using long-lines, at Bocas de Ceniza, next to the mouth of the Magdalena river (Atlántico Department), Colombian Caribbean

Feeding Behavior of Subadult Sixgill Sharks (Hexanchus griseus) at a Bait Station

This is the first in-situ study of feeding behaviors exhibited by bluntnose sixgill sharks. Bait was placed beneath the Seattle Aquarium pier situated on the waterfront in Elliott Bay, Puget Sound, Washington at 20m of water depth. Cameras and lights were placed around the bait box to record sixgill shark presence and behavior while feeding. Analysis of feeding behavior revealed that sixgills utilize a bite comparable to many other elasmobranchs and aquatic vertebrates, have the ability to protrude their upper jaw, change their feeding behavior based on the situation, and employ sawing and lateral tearing during manipulation. The versatility of their feeding mechanism and the ability of sixgills to change their capture and food manipulation behaviors may have contributed to the species’ worldwide distribution and evolutionary success.

Impact of Marine Submergence and Season on Faunal Colonization and Decomposition of Pig Carcasses in the Salish Sea.

Pig carcasses, as human proxies, were placed on the seabed at a depth of 300 m, in the Strait of Georgia and observed continuously by a remotely operated camera and instruments. Two carcasses were deployed in spring and two in fall utilizing Ocean Network Canada’s Victoria Experimental Network under the Sea (formerly VENUS) observatory. A trial experiment showed that bluntnose sixgill sharks could rapidly devour a carcass so a platform was designed which held two matched carcasses, one fully exposed, the other covered in a barred cage to protect it from sharks, while still allowing invertebrates and smaller vertebrates access. The carcasses were deployed under a frame which supported a video camera, and instruments which recorded oxygen, temperature, salinity, density, pressure, conductivity, sound speed and turbidity at per minute intervals. The spring exposed carcass was briefly fed upon by sharks, but they were inefficient feeders and lost interest after a few bites. Immediately after deployment, all carcasses, in both spring and fall, were very rapidly covered in vast numbers of lyssianassid amphipods. These skeletonized the carcasses by Day 3 in fall and Day 4 in spring. A dramatic, very localized drop in dissolved oxygen levels occurred in fall, exactly coinciding with the presence of the amphipods. Oxygen levels returned to normal once the amphipods dispersed. Either the physical presence of the amphipods or the sudden draw down of oxygen during their tenure, excluded other fauna. The amphipods fed from the inside out, removing the skin last. After the amphipods had receded, other fauna colonized such as spot shrimp and a few Dungeness crabs but by this time, all soft tissue had been removed. The amphipod activity caused major bioturbation in the local area and possible oxygen depletion. The spring deployment carcasses became covered in silt and a black film formed on them and on the silt above them whereas the fall bones remained uncovered and hence continued to be attractive to large numbers of spot shrimp. The carcass remains were recovered after 166 and 134 days respectively for further study.

First autonomous recording of in situ dissolved oxygen from free-ranging fish

Biologging technology has enhanced our understanding of the ecology of marine animals and has been central to identifying how oceanographic conditions drive patterns in their distribution and behavior. Among these environmental influences, there is increasing recognition of the impact of dissolved oxygen on the distribution of marine animals. Understanding of the impact of oxygen on vertical and horizontal movements would be advanced by contemporaneous in situ measurements of dissolved oxygen from animal-borne sensors instead of relying on environmental data that may not have appropriate spatial or temporal resolution. Here, we demonstrate the capabilities of dissolved oxygen pop-up satellite archival tags (DO-PATs) by presenting the results from calibration experiments and trial deployments of two prototype tags on bluntnose sixgill sharks (Hexanchus griseus). The DO-PATs provided fast, accurate, and stable measurements in calibration trials and demonstrated high correlation with vertical profiles obtained via traditional ship-borne oceanographic instruments. Deployments on bluntnose sixgill sharks recorded oxygen saturations as low as 9.4 % and effectively captured the oceanography of the region when compared with World Ocean Atlas 2013 values. This is the first study to use an animal-borne device to autonomously measure and record in situ dissolved oxygen saturation from non-air-breathing marine animals. The DO-PATs maintained consistency over time and yielded measurements equivalent to industry standards for environmental sampling. Acquiring contemporaneous in situ measurements of dissolved oxygen saturation alongside temperature and depth data will greatly improve our ability to investigate the spatial ecology of marine animals and make informed predictions of the impacts of global climate change. The information returned from DO-PATs is relevant not only to the study of the ecology of marine animals but will also become a useful new tool for investigating the physical structure of the oceans.

Unexpected Positive Buoyancy in Deep Sea Sharks, Hexanchus griseus, and a Echinorhinus cookei.

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°C at 200–300 m but below 7°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.

Preliminary Observations of Population Genetics and Relatedness of the Broadnose Sevengill Shark, Notorynchus cepedianus, in Two Northeast Pacific Estuaries.

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.

First description of deep-water elasmobranch assemblages in the Exuma Sound, The Bahamas

Deep-sea chondrichthyans, like many deep-water fishes, are very poorly understood at the most fundamental biological, ecological and taxonomic levels. Our study represents the first ecological investigation of deep-water elasmobranch assemblages in The Bahamas, and the first assessment of species-specific resilience to capture for all of the species captured. Standardised deep-water longline surveys (n=69) were conducted September to December 2010 and 2011 between 472m and 1024m deep, resulting in the capture of 144 sharks from 8 different species. These included the Cuban dogfish, Squalus cubensis, the bigeye sixgill shark, Hexanchus nakamurai, the bluntnose sixgill shark, Hexanchus griseus, the smooth dogfish, Mustelus canis insularis, the roughskin dogfish, Centroscymnus owstoni, Springer׳s sawtail catshark, Galeus springeri and the false catshark, Pseudotriakis microdon. Preliminary genetic analysis indicated two or more species of gulper sharks, Centrophorus spp.; however, for the present study they were treated as a single species complex. Water depth and distance from the rocky structure of the Exuma Sound wall were inversely correlated with species richness, whereas seabed temperature was directly correlated with species richness. These variables also had a significant influence on the abundance and distribution of many species. Expanded depth ranges were established for S. cubensis and H. nakamurai, which, in the case of S. cubensis, is thought to be driven by thermal preferences. At-vessel mortality rates increased significantly with depth, and post-release mortality was thought to be high for some species, in part due to high post-release predation. This study highlights the importance of utilising strategic geographic locations that provide easy access to deep water, in combination with traditional expedition-based deep-ocean science, to accelerate the acquisition of fundamental ecological and biological insights into deep-sea elasmobranchs.

Vertical habitat and behaviour of the bluntnose sixgill shark in Hawaii

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.