White-sided Dolphins Research Articles

Mesopelagic predator-prey interactions revealed by joint passive and active acoustic observations

The ocean’s mesopelagic zone is one of the Earth’s largest habitats and contains large numbers of animals, playing important roles in aquatic food webs—notably as food for many marine mammal species. While these communities have been observed for decades on echosounders as sound scattering layers (SSLs), their ecological dynamics remain poorly understood. Using a broadband (0-128 kHz) hydrophone and an upward-looking echosounder (38 kHz) at a cabled observatory in Monterey Bay, CA in early 2019, we observed numerous occasions where SSLs abruptly increased their depth during bouts of echolocation clicks from odontocete whales. Clicking bouts occurred 6.1 times per day on average, mostly at night, lasting minutes to several hours. Pacific white-sided dolphins and Risso’s dolphins produced most of these clicks. Increases in clicking were significantly cross-correlated with increases in SSL depth. The deepest SSL, centered at 400–500 m depth, dove 15 m during an average clicking bout, with some excursions up to 100 m. Video surveys from a remotely operated vehicle identified juvenile Pacific hake, myctophids, and sergestid shrimp as likely constituents of these SSLs. Our results suggest these animals actively dive to avoid odontocetes, and that fear of predation can restructure mesopelagic ecosystems on short time scales.

Estimates of the Abundance of Cetaceans in the Central North Atlantic based on the NASS Icelandic and Faroese Shipboard Surveys Conducted in 2015

The North Atlantic Sightings Survey (NASS), the sixth in a series of surveys conducted between 1987 and 2015, was conducted in June/July 2015 and covered a large area of the northern North Atlantic. The Icelandic and Faroese ship survey component of the NASS covered the area between the Faroe Islands and East Greenland from latitude 52° to 72° N. The survey used 3 vessels and an independent double-platform configuration with each platform staffed by a minimum of 2 observers. Here we present both uncorrected abundance estimates derived using Multiple Covariates Distance Sampling, and corrected abundance estimates derived using Mark-Recapture Distance Sampling, for the following species: fin (Balaenoptera physalus), common minke (B. acutorstrata), humpback (Megaptera novaeangliae), blue (B. musculus), sei (B. borealis), sperm (Physeter macrocephalus), long-finned pilot (Globicephala melas) and northern bottlenose (Hyperoodon ampullatus) whales as well as white-beaked (Lagenorhynchus albirostris) and white-sided (L. acutus) dolphins. We then compare these estimates to those from previous NASS and put them into context with estimates from adjoining areas of the North Atlantic.

Pulsed call sequences as contact calls in Pacific white-sided dolphins (Lagenorhynchus obliquidens)

Pacific white-sided dolphins are a group-living species and appear to exchange "contact calls" to maintain group cohesion. The aim of this study was to find and characterize their contact calls. Calls were recorded from two females at Osaka Aquarium KAIYUKAN (OAK) and three females at Izu-Mito Sea Paradise (IMSP). Because they often produced pulsed calls consecutively, a "pulsed call sequence" was defined as three or more successive pulsed calls occurring within 325 ms, which was calculated using a bout analysis. The pulsed call sequences increased during separation periods and decreased during reunions and were used for vocal exchange, suggesting that the sequences are contact calls in Pacific white-sided dolphins. Most of the pulsed call sequences were classified into unique types; several stereotyped, repeated patterns were found. One sequence type was found at OAK and the two dolphins shared the type; they exchanged sequences with type matching. On the other hand, three sequence types were found in IMSP and the three dolphins shared all of the types; however, each dolphin preferentially used different types and frequently exchanged with their own favorite types but not with type matching. These results suggest that the sequence type may function as an individual and/or group identity.

Beyond Suction-Feeding Fishes: Identifying New Approaches to Performance Integration During Prey Capture in Aquatic Vertebrates

Organisms are composed of hierarchically arranged component parts that must work together to successfully achieve whole organism functions. In addition to integration among individual parts, some ecological demands require functional systems to work together in a type of inter-system performance integration. While performance can be measured by the ability to successfully accomplish ecologically relevant tasks, integration across performance traits can provide a deeper understanding of how these traits allow an organism to survive. The ability to move and the ability to consume food are essential to life, but during prey capture these two functions are typically integrated. Suction-feeding fishes have been used as a model of these interactions, but it is unclear how other ecologically relevant scenarios might reduce or change integration. To stimulate further research into these ideas, we highlight three contexts with the potential to result in changes in integration and underlying performance traits: (1) behavioral flexibility in aquatic feeding modes for capturing alternative prey types, (2) changes in the physical demands imposed by prey capture across environments, and (3) secondary adaptation for suction prey capture behaviors. These examples provide a broad scope of potential drivers of integration that are relevant to selection pressures experienced across vertebrate evolution. To demonstrate how these ideas can be applied and stimulate hypotheses, we provide observations from preliminary analyses of locally adapted populations of Trinidadian guppies (Poecilia reticulata) capturing prey using suction and biting feeding strategies and an Atlantic mudskipper (Periophthalmus barbarus) capturing prey above and below water. We also include a re-analysis of published data from two species of secondarily aquatic cetaceans, beluga whales (Delphinapterus leucas) and Pacific white-sided dolphins (Lagenorhynchus obliquidens), to examine the potential for secondary adaptation to affect integration in suction prey capture behaviors. Each of these examples support the broad importance of integration between locomotor and feeding performance but outline new ways that these relationships can be important when suction demands are reduced or altered. Future work in these areas will yield promising insights into vertebrate evolution and we hope to encourage further discussion on possible avenues of research on functional integration during prey capture.

Temporal patterns in Pacific white-sided dolphin communication at Barkley Canyon, with implications for multiple populations

Acoustic data collection is a cost-effective approach to evaluating activity patterns of otherwise challenging-to-study offshore cetaceans. However, manual analysis of acoustic data is time-consuming and impractical for large data sets. This study evaluates diel and seasonal patterns in Pacific white-sided dolphin communication through automated analysis of 20 months of continuous acoustic data collected from the Barkley Canyon node of Ocean Networks Canada’s NEPTUNE observatory, offshore of Vancouver Island, British Columbia, Canada. In this study, cetacean signals are manually annotated in a sub-set of the data, and 94 time and frequency features of these and other sounds are extracted and used to train random forest classifiers targeting Pacific white-sided dolphin pulsed calls. The performance of binary and multiclass models with various forest sizes, minimum leaf sizes, and confidence thresholds for Pacific white-sided dolphin classification are compared through nested 10-fold cross-validation to select the best model. Vocalizations are classified with the resultant classifier, manually verified, and examined for seasonal and diel patterns. Pacific white-sided dolphins are shown to be vocally active during dawn and day in spring and summer and at dusk and night year-round with reduced overall activity in fall and winter, suggesting both consistent nocturnal and migratory diurnal populations.

The Northern Extent of Pacific White-Sided Dolphin (Lagenorhynchus obliquidens) Distribution in the Eastern North Pacific

We present updated distribution information for the Pacific White-sided Dolphin (Lagenorhynchus obliquidens, PWSD) in the southeastern Bering Sea, including Bristol Bay. The northern extent of this dolphin's range currently includes Bristol Bay and a 150-km swath north of the Aleutian Island chain, and presence in the Bering Sea has been described as rare. Our review of National Marine Fisheries Service marine mammal survey data (1991 to 2010) and sighting reports from sources such as the Platforms of Opportunity Program, whaling and fisheries catch records, and naturalist reports (1975 to 2016) found numerous sightings (n = 191) and year-round presence within the current range. We also found 18 sightings, 2 of which were bycatch in commercial fisheries, that occurred north of the existing range and in all seasons. Based on these results, we suggest PWSD presence in the Bering Sea is not rare, occurs year-round, extends farther north than shown on current range maps, and that site-fidelity may occur in areas near and within Bristol Bay.

Bioaccumulation of methylmercury within the marine food web of the outer Bay of Fundy, Gulf of Maine.

Mercury and methylmercury were measured in seawater and biota collected from the outer Bay of Fundy to better document mercury bioaccumulation in a temperate marine food web. The size of an organism, together with δ13 C and δ15 N isotopes, were measured to interpret mercury levels in biota ranging in size from microplankton (25μm) to swordfish, dolphins and whales. Levels of mercury in seawater were no different with depth and not elevated relative to upstream sources. The δ13 C values of primary producers were found to be inadequate to specify the original energy source of various faunas, however, there was no reason to separate the food web into benthic, demersal and pelagic food chains because phytoplankton has been documented to almost exclusively fuel the ecosystem. The apparent abrupt increase in mercury content from “seawater” to phytoplankton, on a wet weight basis, can be explained from an environmental volume basis by the exponential increase in surface area of smaller particles included in “seawater” determinations. This physical sorption process may be important up to the macroplankton size category dominated by copepods according to the calculated biomagnification factors (BMF). The rapid increase in methylmercury concentration, relative to the total mercury, between the predominantly phytoplankton (<125μm) and the zooplankton categories is likely augmented by gut microbe methylation. Further up the food chain, trophic transfer of methylmercury dominates resulting in biomagnification factors greater than 10 in swordfish, Atlantic bluefin tuna, harbour porpoise, Atlantic white-sided dolphin and common thresher shark. The biomagnification power of the northern Gulf of Maine ecosystem is remarkably similar to that measured in tropical, subtropical, other temperate and arctic oceanic ecozones.

The cholinergic system in the basal forebrain of the Atlantic white-sided dolphin (Lagenorhynchus acutus).

The basal forebrain (BFB) cholinergic neurotransmitter system is important in a number of brain functions including attention, memory, and the sleep-wake cycle. The size of this region has been linked to the increase in encephalization of the brain in a number of species. Cetaceans, particularly those belonging to the family Delphinidae, have a relatively large brain compared to its body size and it is expected that the cholinergic BFB in the dolphin would be a prominent feature. However, this has not yet been explored in detail. This study examines and maps the neuroanatomy and cholinergic chemoarchitecture of the BFB in the Atlantic white-sided dolphin (Lagenorhynchus acutus). As in some other mammals, the BFB in this species is a prominent structure along the medioventral surface of the brain. The parcellation and distribution of cholinergic neural elements of the dolphin BFB was comparable to that observed in other mammals in that it has a medial septal nucleus, a nucleus of the vertical limb of the diagonal band of Broca, a nucleus of the horizontal limb of the diagonal band of Broca, and a nucleus basalis of Meynert. The observed BFB cholinergic system of this dolphin is consistent with evolutionarily conserved and important functions for survival.

Anatomical Studies of the Recurrent Laryngeal Nerve and the Subclavian Artery in Rough-Toothed Dolphins (Steno bredanensis) and Pacific White-Sided Dolphins (Lagenorhynchus obliquidens)

In mammals, the recurrent laryngeal nerve (RLN) loops around the ligamentum arteriosum on the left side and around the subclavian artery on the right side. It is generally considered that such an asymmetric arrangement of the right and left RLN is brought about by a transformation of the aortic arches at an early stage of embryonic development. We have dissected rough-toothed dolphins and Pacific white-sided dolphins in this study. In both species, the right RLN hooked around the costocervical trunk, while the left RLN hooked around the ligamentum arteriosum. The external carotid artery, the internal carotid artery, the subclavian artery, and the costocervical trunk arose radially from the brachiocephalic trunk on the right side. The left costocervical trunk arose directly from the aortic arch. The right and left subclavian arteries ran ventral to the subclavian vein and the vagus nerve, in a similar manner to the secondary subclavian arteries in birds. These findings suggest that the replacement of the primary (or dorsal) subclavian artery by the secondary (or ventral) subclavian artery might occur at a certain embryonic stage in dolphins as well.

A decade of marine mammal acoustical presence and habitat preference in the Bering Sea

As Arctic seas rapidly change with increased ocean temperatures and decreased sea ice extent, traditional marine mammal distributions may be altered, and non-traditional Arctic species may shift poleward. Extant and seasonal continental shelf odontocete species in the Arctic Ocean include sperm whales, killer whales, beluga whales, delphinids, harbour porpoises, and Dall’s porpoises. Until recently, recording constraints limited higher sampling rates, preventing detection of many of these high frequency-producing species in the Arctic seas. Using one of the first long-term data sets to record clicks, buzzes, and whistles, multiple species have been detected and classified to explore shifting distributions in the Arctic Corridor. The data were then paired with environmental variables for generalized linear and additive modeling. Pacific white-sided dolphins were detected for a few years during minimal ice coverage. Dall’s porpoises and Risso’s dolphins may have been acoustically detected in areas farther n...

Feasibility of using the National Marine Mammal Tissue Bank for retrospective exploratory studies of perfluorinated alkyl acids

Perfluorinated alkyl acids (PFAAs) have been used for 50+ years in materials such as stain-resistant treatments for paper and clothing, lubricants, and foam fire extinguishers. PFAAs are characterized by a fully fluorinated alkyl chain with a terminal acid group. Their long half-lives and ubiquitous environmental distribution create considerable concern for wildlife and human exposure. There is interest in examining temporal trends of PFAAs using the National Marine Mammal Tissue Bank (NMMTB), but NMMTB tissues are frozen and cryohomogenized in polytetrafluoroethylene (PTFE)-based materials. Because PTFE supplies may leach PFAAs into samples, this study mimicked collection, processing and storage steps of NMMTB samples and measured PFAA leaching to determine the feasibility of using this sample archive for PFAA temporal trends. We also explored concentrations in Atlantic white-sided dolphin (Lagenorhynchus acutus, WSDs) and rough-toothed dolphin (Steno bredanensis, RTDs) blubber (n=3 and 0) and liver (n=48 and 12, respectively). The materials used in NMMTB protocols may add up to 0.968ng/g perfluorooctanoic acid (PFOA), 0.090ng/g perfluorononanoic acid (PNFA), and 0.221ng/g perfluorooctane sulfonate (PFOS) to each archived sample. Leaching of PFNA and PFOS from supplies compared to dolphin levels was negligible, but PFOA contributions were substantially higher than levels found in most dolphin liver samples. Therefore, monitoring PFOA temporal trends from the NMMTB would require careful consideration. RTDs had significantly higher levels of PFOS and PFNA than WSDs. Both species have similar life history, trophic status, and foraging behaviors in deep pelagic waters, so differences could be from latitudinal variation in contamination. RTDs stranded in Florida; WSDs stranded farther north mostly in Massachusetts. Juveniles had significantly higher levels of PFOS and PFNA than adults in both species, suggesting growth dilution as they approach maturity. PFOS significantly decreased after 2001 in both species as expected based on changes in production.

Marine Mammal and Sea Turtle Sightings During a Survey of the Endeavour Segment of the Juan de Fuca Ridge, British Columbia

Marine mammals and sea turtles were documented as part of a monitoring and mitigation program during a seismic study offshore (~250 km) from Vancouver island, British Columbia, during August–September 2009. Forty-one marine mammals in nine groups were sighted. Dall’s Porpoise (Phocoenoides dalli) was the most frequently sighted species. A Sperm Whale (Physeter macrocephalus), a pod of Pacific White-sided Dolphins (Lagenorhynchus obliquidens), an unidentified toothed whale, a Northern Elephant Seal (Mirounga angustirostris), and a leatherback Turtle (Dermochelys coriacea) were also observed. These data augment current knowledge on the occurrence of marine mammals and sea turtles in the offshore waters of British Columbia.

Estimation of Environmental Factors That Influence Migration Timing and Distribution of Pacific White-Sided Dolphins Around Hokkaido, Japan

Abstract Hydrographic, hydroacoustic, and cetacean visual surveys were conducted using a quantitative echosounder to estimate environmental factors influencing migration timing and distribution of Pacific white-sided dolphins (Lagenorhynchus obliquidens) in the Tsugaru Strait (nonbreeding area) and Funka Bay (breeding area) near Hokkaido, Japan. Higher numbers of dolphins were observed in the Tsugaru Strait during May and June and in Funka Bay during June, July, and August. Potential prey were observed in the Tsugaru Strait in May, June, and August, but they may not have been of a suitable size in August, which may explain absence of the dolphins in that month. In Funka Bay, potential prey were abundant in May, but dolphins may have been absent because of small prey size and low water temperature. In a smaller-scale analysis, the relationship between dolphins and prey was different in the two areas, with dolphin distribution more closely associated with prey distribution in the Funka Bay breeding area. Th...

Complete mitochondrial genome of the Pacific white-sided dolphin Lagenorhynchus obliquidens (Cetacea: Delphinidae)

The Pacific white-sided dolphin (Lagenorhynchus obliquidens Gill, 1865) is one of the most abundant, widely distributed delphinids in the North Pacific Ocean, whose existence is threatened by fisheries and environmental contamination. Herein, we report the first complete mitochondrial genome of L. obliquidens. The 16,392-bp sequenced genome exhibited typical cetacean mitochondrial gene arrangement, consisted of the typical set of 37 genes, one replication origin, and a D-loop. As expected, the genome displayed the highest similarity with that of Cephalorhynchus heavisidii and was distinct from that of L. albirostris. Multigene phylogeny also revealed that L. obliquidens was closely related to C. heavisidii, thus suggesting that the genus Lagenorhynchus is polyphyletic, in accordance with the results of recent molecular phylogenetic studies. The results provide information fundamental for genetic and conservation studies for L. obliquidens.

Automated classification of Pacific white-sided dolphin (Lagenorhynchus obliquidens) pulsed calls for diel pattern assessment

Diel patterns in marine mammal activity can be difficult to assess visually due to the cost of ship time and limitations of daylight and weather during expeditions that can bias analysis based on sightings. Acoustic data can be a cost-effective approach to evaluate activity patterns in marine mammals. However, manual analysis of acoustic data is time consuming, and impractical for large data sets. This study seeks to evaluate diel patterns in Pacific white-sided dolphin communication through automated analysis of one year of continuous acoustic data collected from the Barkley Canyon node of Ocean Networks Canada’s NEPTUNE observatory, offshore of Vancouver Island, British Columbia, Canada. In this study, marine mammal acoustic signals are manually annotated in a sub-set of the data, and used to train a random forest classifier targeting Pacific white-sided dolphin pulsed calls. Binary and multiclass classifiers are compared, and the effects of different data-balancing methods are evaluated. The results from automated classification of the full data set are used to determine whether a diel pattern in Pacific white-sided dolphin communication exists in this region.

Do Belugas (Delphinapterus leucas), Bottlenose Dolphins (Tursiops truncatus), &amp; Pacific White-Sided Dolphins (Lagenorhynchus obliquidens) Display Lateralized Eye Preference when Presented with Familiar or Novel Objects?

Lateralization of behaviors and information processing are common across species. Hypothesized to be crucial for more efficient responding to environmental stimuli, lateralization has been investigated for a number of topics. Cetaceans are proposed to be hemispheric specialists, given a small corpus callosum, complete decussation of the optic nerve, and the ability to respond to a different visual stimulus presented to each eye simultaneously. Research with cetaceans has shown strong biases in a number of behaviors, including swimming, foraging, social interactions, and responses to myriad visual stimuli. Given similar evolutionary pressures, different species of cetaceans should display similar lateralized preferences. Previous research with bottlenose dolphins in managed care and wild striped dolphins indicated a right eye preference when viewing unfamiliar objects. The purpose of the current study was to evaluate the eye preference of belugas, bottlenose dolphins, and Pacific white-sided dolphins (lags) in managed care when viewing familiar and unfamiliar objects. The results from 11 belugas, 5 bottlenose dolphins, and 5 lags indicated that consistent group level eye preferences were not present. The belugas preferred to view both types of objects with both eyes, with the majority of the belugas showing a left-eye preference when a monocular gaze was used. Bottlenose dolphins tended to view both objects with their right eye while lags used their left eye when viewing objects. These results may have been affected by viewing objects below water versus above water. The belugas and the Pacific white-sided dolphins were able to view the objects below water, which may have elicited more naturalistic visual examinations of the objects (i.e., greater ecological validity). Viewing objects within one’s habitat may facilitate the discrimination of an object rather than simply its detection, which is may be more likely when encountering stimuli above the surface of the water as the bottlenose dolphins had to do in the present and past research. Future research should compare if presentation of the stimulus above water versus below water affects the eye preference displayed.

Temporal and spatial trends in stranding records of cetaceans on the Irish coast, 2002–2014

Using Irish strandings data collected between 2002 and 2014, seasonal and annual trends in the number of strandings for all strandings identified to species level (N = 1480), and for the five most frequently reported species: common dolphin (25.7% of records), harbour porpoise (22.2%), long-finned pilot whale (8.8%), striped dolphin (6.9%) and bottlenose dolphin (6.9%) were investigated. With the exception of bottlenose dolphins, there was a significant linear increase in the number of strandings across years for all species and for all strandings collectively, that were identified to species-level. Only common dolphins demonstrated a significant increase in the proportion of records relative to all other strandings, which may be indicative of a real rise in the number of strandings of this species. Common dolphins and harbour porpoises showed a similar significant difference in monthly strandings, with more strandings occurring during the earlier months of the year. Significant differences in the gender of stranded animals were found in common, striped, bottlenose and Atlantic white-sided dolphins and sperm and pygmy sperm whales. Live and mass stranding events were primarily comprised of pelagic species. Most strandings occurred on the south and west coasts, with two hotspots for live and mass strandings identified. The patterns and trends identified are discussed in relation to the caveats in interpreting strandings data. Specifically to Ireland, the findings highlight the urgent need to build on the current volunteer reporting network and augment this comprehensive dataset with post-mortem examinations to better understand the cause of the trends identified. The importance of strandings data in informing conservation and management guidelines of these species’ is discussed.

Modelling cetacean distribution and mapping overlap with fisheries in the northeast Atlantic

Improving our knowledge of cetacean distribution and habitat use are key if we are to effectively ensure good conservation status for these species. Often however, sufficient data are lacking, inhibiting conservation efforts of many species. This study aims to combine historical datasets to generate habitat suitability models and thus maps, for eight species of cetacean regularly sighted in the Irish portion of the northeast Atlantic; fin whale, minke whale, pilot whale, sperm whale, white-sided dolphin, bottlenose dolphin, Risso's dolphin and white-beaked dolphin. Habitat suitability models were developed using MaxEnt modelling software for a range of environmental factors; sea-surface temperature, mixed layer depth, depth, slope, chlorophyll a concentration, sea surface salinity, distance to the 200 m contour. We predicted species exposure to fishing gears by integrating habitat suitability models with information on fishing vessel activities within the study area. The main predictors of habitat suitability for all species were topographic variables, particularly depth and slope, highlighted by two areas of high species richness around areas of topographic heterogeneity, along the continental shelf and on the west coast. Combining habitat models with fishing activity, indicated areas of high exposure off the north and south coasts and in an area known as the Porcupine Bank off the west coast. These results are valuable for conservation and management of cetaceans and fisheries in the study area. Methods can be easily adjusted to allow replication for other species and other anthropogenic activities. We recommend future effort focuses on winter months to fill in the gaps on year round cetacean distribution.

A preliminary acoustic survey of echolocating marine mammals in the Bering Sea

As the Arctic seas rapidly change with increased ocean temperatures and decreased sea ice extent, traditional Arctic marine mammal distributions will be altered, and non-traditionally Arctic species may shift poleward. Arctic species typically include sperm, bowhead, humpback, right, gray, fin, and blue whales; odontocetes, specifically killer and beluga whales; and several pinnipeds species. Their acoustic presence has been documented because they produce relatively low-frequency sounds that are detectable by many common, remotely-deployed recording platforms. Until recently, however, recording constraints of power and storage limited higher sampling rates and prevented the detection of many high-frequency-producing species in the Arctic seas. Such species likely include Baird’s, Cuvier’s, and Stejneger’s beaked whales, as well as Northern right whale and Pacific white-sided dolphins. Using one of the first long-term data sets to record relatively high frequencies in the Bering Sea, signal types similar ...

Meals on Wheels? A Decade of Megafaunal Visual and Acoustic Observations from Offshore Oil & Gas Rigs and Platforms in the North and Irish Seas.

A decade of visual and acoustic detections of marine megafauna around offshore Oil & Gas (O&G) installations in the North and Irish Seas are presented. Marine megafauna activity was monitored visually and acoustically by Joint Nature Conservation Committee (JNCC) qualified and experienced Marine Mammal Observers (MMO) and Passive Acoustic Monitoring (PAM) Operators respectively, with real-time towed PAM in combination with industry standard software, PAMGuard. Monitoring was performed during routine O&G industrial operations for underwater noise mitigation purposes, and to ensure adherence to regulatory guidelines. Incidental sightings by off-effort MMOs and installation crew were also reported. Visual and acoustic monitoring spanned 55 non-consecutive days between 2004 and 2014. A total of 47 marine mammal sightings were recorded by MMOs on dedicated watch, and 10 incidental sightings of marine megafauna were reported over 10 years. Species included: harbour porpoise (Phocoena phocoena), Atlantic white-sided dolphin (Lagenorhynchus acutus), white beaked dolphin (Lagenorhynchus albirostris), common dolphin (Delphinus delphis), minke whale (Balaenoptera acutorostrata), common seal (Phoca vitulina), grey seal (Halichoerus grypus) and, basking shark (Cetorhinus maximus). Passive Acoustic Monitoring was conducted on two occasions in 2014; 160 PAM hours over 12 days recorded a total of 308 individual clicks identified as harbour porpoises. These appear to be the first such acoustic detections obtained from a North Sea drilling rig whilst using a typically configured hydrophone array designed for towing in combination with real-time PAMGuard software. This study provides evidence that marine megafauna are present around mobile and stationary offshore O&G installations during routine operational activities. On this basis, Environmental Impact Assessments (EIAs) for decommissioning O&G platforms should be carried-out on a case-by-case basis, and must include provisions for hitherto overlooked marine megafauna.