Dusky Dolphins Research Articles

Drag reduction and locomotory power in dolphins: Gray's paradox revealed.

For 88 years, biologists and engineers have sought to understand the hydrodynamics enabling dolphins to swim at speeds seemingly beyond their energetic capabilities, a phenomenon known as Gray's paradox. Hydromechanical models calculating the drag of swimming dolphins estimated power requirements for sustained high-speed swimming, which were physiologically impossible. Using an uncrewed aerial vehicle, we calculated the total power of free-ranging dusky dolphins (Lagenorhynchus obscurus) at speeds from 0.9 to 6.9 m s-1, deriving drag coefficients (Cd) and drag. Our results showed that the Cd decreased exponentially with speed, reducing drag by up to 89% at speeds >2 m s-1, with an additional 17% reduction during porpoising (>4.0 m s-1). At 6.9 m s-1, drag was 32 N, with a power of 15.8 W kg-1, nearly identical to the mass-specific allometric prediction for the maximum aerobic capacity of other mammals and physiologically possible. The Cd at speeds >2.5 m s-1 indicated reduced turbulence in the boundary layer around the dolphin's body, thereby reducing drag. The ability of dusky dolphins to swim at sustained high speeds resulted from an exponential decrease in Cd, which was further reduced by porpoising, thereby explaining the low drag and locomotory power that resolved Gray's paradox.

Dusky dolphin (Lagenorhynchus obscurus) habitat use in a mussel farming region and changes over time

ABSTRACT Information on marine mammal habitat use in coastal areas can provide a better understanding of anthropogenic effects on species. Admiralty Bay, New Zealand has extensive near-shore mussel farms and is an important dusky dolphin (Lagenorhynchus obscurus) foraging habitat. Prior dusky dolphin research indicated a declining trend in regional abundance from the late 1990s to mid-2000s. We describe dusky dolphin behaviour, habitat patch use and movement patterns in Admiralty Bay in 2011–2012 and compare findings to previous research to capture long-term trends. Sampling methods included small-boat group follows and photo-identification and shore-based theodolite tracking. Dolphin encounter rate, mean group size, and individual resighting rate showed a negative trend from previous years. Coordinated prey ball herding, a foraging technique documented in previous years, was rarely observed during this study. Dolphins were more likely to forage near farms (excluding specialised prey herding), though they seldom entered farms. Near shore, swimming speed was slower, reorientation rate was higher, and linearity was lower in the presence of farms than in their absence. This research builds upon prior studies, suggesting a continued decline in dusky dolphin presence, and highlights differences in habitat patch use in a human-altered ecosystem.

Anomalous pigmentation in a dusky dolphin Lagenorhynchus obscurus from Golfo Nuevo, Argentinian Patagonia

Anomalous pigmentation conditions occur with low frequency in marine mammals. Here is presented the first known record and resighting of a hypo-pigmented all-white dusky dolphin from Golfo Nuevo off Península Valdés, Argentinian Patagonia. The anomalously pigmented Lagenorhynchus obscurus was observed three times between 2016 and 2018. Images suggest that this could be a case of a leucistic dusky dolphin, apparently a single individual. The hypo-pigmented individual was estimated to have reached adult body size, and was probably sexually mature. More research is required for a proper understanding of the effect on behavioral, ecological and physiological conditions.

Marine megafauna interactions with the Peruvian artisanal purse-seine fleet

Bycatch is a global problem for marine megafauna. Peru is a major fishing nation known in particular for its industrial purse-seine fishery for the Peruvian anchovy (Engraulis ringens). However, Peru also has an artisanal purse-seine fishery for this same resource and other species such as Pacific bonito (Sarda chiliensis), Pacific chub mackerel (Scomber japonicus) and Chilean jack mackerel (Trachurus murphyi) used for human consumption. While there is limited documentation of bycatch of megafauna by the industrial purse-seine fishery, there is no similar information for the artisanal purse-seine fishery. Therefore, this study aimed to assess the marine megafauna (small-cetaceans, seabirds, sea turtles, and elasmobranchs) bycatch interactions of the Peruvian artisanal purse-seine fishery for consumption for the year 2019 (before the COVID-19 pandemic) through interviews with artisanal fishers in 5 landing ports (San Jose, Santa Rosa, Callao, Pucusana, and San Andres). We found that these fisheries had bycatch with all taxa groups, with dusky dolphins (Lagenorhynchus obscurus), guanay cormorants (Leucocarbo bougainvilliorum) and eagle rays (Myliobatis spp.) having the highest quantity and most frequently reported bycatch. Multiple correspondence analysis (MCA) indicated an association between the bycatch of 1–3 common dolphins (Delphinus spp.) per set from the northern ports (San Jose and Santa Rosa) and vessels of 20–36 MT. Likewise, a relationship was found between smooth hammerhead sharks (Sphyrna zygaena) and vessel storage capacity, with a similar result reported for the bycatch of humpback smoothhounds (Mustelus whitneyi). In addition, a relationship of eagle ray (Myliobatis spp.) bycatch with a landing port was identified, and several respondents mentioned catching more than 1 MT per set. Fifty-three percent of fishers considered bycatch to be a problem, mainly because it wastes their time during fishing operations. This preliminary study is the first research that demonstrates bycatch in artisanal purse-seine in Peru and can serve as a baseline for future research. We also highlight the need to quantify bycatch rates of marine megafauna using additional methodologies, such as monitoring with onboard observers.

What do small cetaceans tell us about trace elements pollution on the Argentinean coast? Franciscana dolphin as a biomonitor

Trace elements (TEs) constitute the oldest emerging pollutants globally, most occur from natural sources, but a few are derived from anthropogenic sources. Marine mammals are considered bioindicators of ecosystem contamination. The aims of this review is compile reports on essential and nonessential TEs occurrence in small cetaceans from Argentinean waters; and to review the existing information on the concentration of TEs in the Franciscana dolphin, a biomonitor species of the Argentine coastal marine ecosystem. We searched reports where levels of TEs were present in small cetaceans from and eight species were analysed: Pontoporia blainvillei, Tursiops truncatus gephyreus, Kogia breviceps, Delphinus delphis, Lagenorhynchus obscurus, Lagenodelphis hasei, Cephaloryhchus commersonii and Ziphius cavirostris. Essential TEs like Zn, Cu, Mn, Cr, Fe, Co, Ni, Mo, Se, As, Au, Ag, Sn, and nonessential TE as Pb, Cd, Hg, As was considered. The reports compiled in this article analysed kidney, liver, muscle and occasionally brain, skin, lung and spleen, covering a temporal range of 30 years, from 1982 to 2016. Of data analysis, we identify knowledge gaps, species of small cetaceans for which the concentration of trace metals is not yet known and areas on the Argentine coast where there are no reports that analyse them. The most recent information corresponds to the 2010 decade, and in those subsequent publications, the samples were taken at that time. This emphasizes the importance of reviewing this data, in order to compare old and new datasets, create contamination timelines and evaluate possible increases or decreases of contaminants in different study areas. The information recopilated will serve as valuable baselines to detect the future impact of increasing human, even natural, activities on marine ecosystems in the South Atlantic Ocean.

Group dynamics and activity patterns of two dolphin species, the Common Dolphin (Delphinus delphis) and Dusky Dolphin (Lagenorhynchus obscurus) in direct sympatry

Abstract Co-occurrence of species with similar ecological requirements could bring individual advantages, but when resources are scarce, co-occurring species would segregate to reduce competition and thus enable coexistence. We studied co-occurrence, group dynamics, and activity budgets, along with the seasonal and diel variation, of two sympatric species: the Common Dolphin (Delphinus delphis) and Dusky Dolphin (Lagenorhynchus obscurus), in Northern Patagonia, Argentina. Dolphin groups were recorded by nautical surveys between 2006 and 2018, and classified the groups by their constituent species (single- or multispecies groups), group size, age/sex composition, predominant activity, season, and time block. Frequencies were arranged in contingency tables and analyzed by log-linear models. Activity bout duration also was recorded by focal-group follows and compared using nonparametric tests. Single- and multispecies groups’ occurrence varied throughout the year, with a period of high co-occurrence in spring and summer, when both species and most of multispecies groups were present, and a period of low co-occurrence, when mostly common dolphins remained in the area. Common dolphins occur in groups of more than a hundred while dusky dolphins do so in groups of 50 animals at most. Feeding time budget was higher in Dusky Dolphin single- and multispecies groups. Common dolphins showed a seasonal and diurnal pattern in activity budget. During the period of high co-occurrence, dusky dolphins’ group size showed a diurnal pattern, and both species increased their feeding time budgets, a putative indication of more favorable foraging conditions. During this period, dusky dolphins associate occasionally in multispecies groups, increasing feeding time budget and feeding bouts duration, suggesting that the association between the two species is beneficial.

Potential impact of unregulated dolphin watching activities in Patagonia

Since 1997, dolphin watching activities have increased in Patagonia, with dusky and Commerson’s dolphins as the target species. To assess the impact of this activity, dolphin watching tours were monitored. For dusky dolphins, the number of tourists increased from 1,393 in 1997 to 1,840 in 2000. The encounter rate grew from 25% during 1999 to 90% in 2001. Most of the groups observed ranged from 50-100 animals. Data were recorded from both a commercial and a research boat. Dusky dolphins showed a short-term reaction to boats and feeding was the most affected behaviour. For Commerson’s dolphins, the number of tourists increased from 532 in 1999 to 2,113 in 2001. The encounter rate averaged 95.58%. There is seasonality in the abundance of Commerson’s dolphins in the area during the colder months (May-December), when schools are larger, than in the warmer months. Dolphins showed a short-term reaction to the presence of the boat, performing aerial displays which are otherwise rarely seen. The direct gross estimated income for the companies carrying out dolphin watching is around US$60,000 per year. The activity has great potential to become an industry of its own. Nevertheless, the activity is at present undertaken on an irregular basis.

Management of gillnet bycatch of cetaceans in New Zealand

Bycatch of dolphins in inshore gillnets first attracted scientific and management attention in New Zealand in the 1980s. During 1984-88, 50-150 dusky dolphins were killed each year at Kaikoura in gillnets set at the surface to catch bait for rock lobster. At the same time, annual catches of 20-100 Hector’s dolphins occurred in Canterbury waters in bottom-set commercial and recreational gillnets. These catches resulted in the banning of surface-set gillnetting at Kaikoura in 1989 and in the creation of the Banks Peninsula Marine Mammal Sanctuary in 1988 to protect Hector’s dolphin. An additional gillnet closure was established to protect North Island Hector’s dolphin in 2003. A key problem is that current information on catches in these and other areas is scant. One observer programme has been successfully implemented in a commercial gillnet fishery (Canterbury area, 1997/98 fishing season). Its estimate of Hector’s dolphin bycatch (17) is clearly unsustainable by the local population. Pingers have been voluntarily used in these fisheries, but there are no data establishing their effectiveness, and it has not been possible to ensure consistency of pinger use. There are no reliable estimates of numbers taken in recreational fisheries. Area closures are used to mitigate gillnet bycatch of Hector’s dolphin, however it appears that the Banks Peninsula Marine Mammal Sanctuary is not large enough to ensure the persistence of the Canterbury population. There is a bycatch limit in place for this population, although it is unenforced. We argue that management of this species via bycatch limits is not practical, however. Hector’s dolphin’s low abundance and separation into several distinct populations means that appropriate bycatch limits would be very small, and this necessitates very comprehensive observer coverage to be confident they are not exceeded. We propose that increasing the size of protected areas is the most reliable option for conservation.

Odontocetes of the Southern Ocean Sanctuary

Twenty-eight odontocete species were identified as occupying sub-Antarctic and Antarctic habitat covered by the 1994 IWC-established Southern Ocean Sanctuary. Toothed whales evidently play an important part in the Antarctic polar ecosystem. Twenty-two species are autochthonous in showing a regular, apparently year-round, presence in the Sanctuary: Physeter macrocephalus, Kogia breviceps, Orcinus orca, Globicephala melas edwardii, Pseudorca crassidens, Lagenorhynchus cruciger, Lagenorhynchus obscurus, Lissodelphis peronii, Cephalorhynchus commersonii, Cephalorhynchus hectori, Tursiops truncatus, Delphinus delphis, Phocoena dioptrica, Hyperoodon planifrons, Berardius arnuxii, Ziphius cavirostris, Tasmacetus shepherdi, Mesoplodon layardii, Mesoplodon traversii, Mesoplodon grayi, Mesoplodon bowdoini and Mesoplodon hectori. Six species are considered vagrants into the Sanctuary: Kogia sima, Grampus griseus, Steno bredanensis, Mesoplodon peruvianus, Mesoplodon densirostris and Mesoplodon mirus. However, vagrant status of these three mesoplodonts is only provisionally assigned, considering that improved knowledge of diagnostic features of beaked whales should, as in recent years, continue to facilitate at-sea identification. Two species are considered as having a ‘contiguous’ range (records less than 2° north of Sanctuary boundaries): Mesoplodon ginkgodens (at 39°S) and Mesoplodon mirus (at 38°24’S). The habitual southern range of at least four odontocetes extends significantly farther poleward than expected. G. melas edwardii is regularly encountered south of the Antarctic Polar Front, much like M. grayi which is known to reach the Ross Sea ice edge (ca. 67°S). Z. cavirostris and L. obscurus cross the Polar Front occasionally. The distribution of M. peruvianus and M. traversii and their relation to SST are unclear. Their southernmost records, 42°31’S and 44°17’S respectively, may either be extralimital or, more likely, reflect ordinary austral range. Temporally non-aligned distribution patterns of Hyperoodon planifrons in Antarctic and South African waters may suggest stock segregation.

The oxygen minimum zone relative depth is a key driver of dolphin habitats in the northern Humboldt Current System

Highly mobile odontocetes need habitats with environmental conditions with the potential of aggregating enough and high-quality prey, to maximize foraging success. Until now, the characterization of those habitats was in terms of physical and biological indicators of high production, capable of attracting and sustaining prey. Nevertheless, there has been no approach to quantifying the effects of a biophysical characteristic of the ocean with proven effects on the vertical distribution of prey for cetaceans: The oxygen minimum zone (OMZ) depth. In the northern branch of the Humboldt Current System off Peru (~6-18° S), a shallow OMZ (30-50 m) affects the distribution of the Peruvian anchovy (Engraulis ringens), main prey for several marine predators, including dolphins. We hypothesized these predators would aggregate in productive areas, but with preference for places where the relative OMZ depth can constrain prey vertically, making it more accessible and maximizing foraging success. We fitted Bayesian habitat models for three dominant odontocete species in this region, with multiple combinations of environmental covariates, smoothing techniques, and temporal and spatial random effects. Cetacean data came from 23 dedicated surveys spanning 2001-2019. Habitat predictors included the spatial anomalies of sea surface temperature, surface chlorophyl-a, pycnocline depth and OMZ depth. Dusky (Lagenorhynchus obscurus) and common dolphins (Delphinus delphis) preferred productive, cold areas with a very shallow OMZ, regardless of the season, while bottlenose dolphins (Tursiops truncatus) aggregated in both cold and warm waters, also with shallow OMZ. The former two species of higher metabolic demands would maximize energy intake by selecting areas with highly aggregated prey, while the latter, of more moderate metabolic needs and more diverse prey, would exploit less restricted habitats.

Seasonal variation and group size affect movement patterns of two pelagic dolphin species (Lagenorhynchus obscurus and Delphinus delphis).

Movement is a key factor in the survival and reproduction of most organisms with important links to bioenergetics and population dynamics. Animals use movement strategies that minimize the costs of locating resources, maximizing energy gains. Effectiveness of these strategies depends on the spatial distribution, variability and predictability of resources. The study of fine-scale movement of small cetaceans in the pelagic domain is limited, in part because of the logistical difficulties associated with tagging and tracking them. Here we describe and model the fine-scale movement patterns of two pelagic dolphin species using georeferenced movement and behavioral data obtained by tracking dolphin groups on board small vessels. Movement patterns differed by species, group sizes and seasons. Dusky dolphin groups moved shorter distances when feeding and longer distances when traveling whereas the common dolphin did the same only when they moved in large groups. In summer, both dolphins cover longer distances in a more linear path, while in winter the movement is more erratic and moving shorter distances. Both species of dolphins prey on small pelagic fishes, which are patchily distributed and show seasonal variability in school sizes and distribution. However, dusky dolphins rely on anchovy to a larger extent than common dolphins. In Nuevo Gulf, anchovy shoals are smaller and separated by shorter distances in winter and dusky dolphins´ movement pattern is consistent with this. Dusky and common dolphins are impacted by tourism and fisheries. Further modelling of movement could be inform spatial based management tools.

Evidence of Type A-Like Killer Whales (Orcinus orca) Predating on Marine Mammals Along the Atacama Desert Coast, Chile

Killer whales (Orcinus orca) are marine apex predators distributed across the world’s oceans. In the last 40 years, researchers have increasingly differentiated killer whales into ecotypes based on genetics, morphology, behaviour, acoustic repertoire, habitat, and trophic ecology. While killer whale ecotypes in the Northern Hemisphere are well studied, the recognition of distinct killer whale forms in the Southern Hemisphere is mainly limited to Antarctic waters. Although present in less studied regions, such as along the Atacama Desert coast in the Southeast Pacific Ocean, limited information is available regarding their biology or trophic ecology. Herein, multiple lines of evidence are presented for killer whale predation on marine mammals in northern Chile. Using information from systematic boat-based surveys, whale-watching tour surveys, and reports from fishermen/citizen scientists, 19 killer whale sightings are reported along the coast of northern Chile (from the Arica and Parinacota region in the north to the Atacama region in the south). Killer whales were photo-identified as corresponding to the Southern Hemisphere Type A-like ecotype according to their dorsal fin shape and white eye patch. One killer whale pod, which included two males, one female, one juvenile, and one calf, was resighted six times from 2016 to 2021 in northern Chile and was observed hunting South American sea lions (Otaria flavescens), dusky dolphins (Lagenorhynchus obscurus), and long-beaked common dolphins (Delphinus cf. capensis). Killer whales were observed taking advantage of large aggregations of sea lions associated with the offshore purse-seine fishery. Adult killer whales used the hulls of fishing vessels to prevent sea lions from escaping. Photo-identification analysis of a fin whale (Balaenoptera physalus) catalogue from Mejillones Peninsula showed that 2.6% of all identified whales had apparent rake marks from killer whale teeth on their dorsal fins. These data show that Type A-like killer whales in northern Chile prey on several species of marine mammals.

Abundance and distribution patterns of cetaceans and their overlap with vessel traffic in the Humboldt Current Ecosystem, Chile

The Humboldt Current Ecosystem (HCE) is one of the most productive marine ecosystems, sustaining one of the largest fishing industries in the world. Although several species of cetaceans are known to inhabit these productive waters, quantitative assessments of their abundance and distribution patterns are scarce and patchy. Here, we present the first abundance and distribution estimates for fin whale (Balaenoptera physalus), southeast Pacific blue whales (Balaenoptera musculus), sperm whale (Physeter macrocephalus), dusky dolphin (Lagenorhynchus obscurus), and common dolphin (Delphinus spp.) in the entire Chilean portion of the HCE. Line transect surveys were conducted during 2016–2021 between 18° S and 41° S and up to ~ 200 km offshore, and data were analyzed using distance sampling methods. Group counts were modelled as a function of environmental variables using single step Bayesian Binomial N-mixture model (BNMM), which allows full uncertainty propagation between model components. By using spatially explicit predictions of cetacean densities and observed vessel densities in the HCE, we provide quantitative assessments on the relative probability of cetaceans encountering vessels (RPCEV). Dusky dolphin and fin whale showed the largest distribution overlap with industrial and artisanal fishery fleets. Our results highlight areas where effort should be prioritized to address the extant but unquantified negative interactions between vessels and cetaceans in Chilean HCE.

Predatory species left stranded following the collapse of the sardine Sardinops sagax (Pappe, 1854) stock off the northern Benguela upwelling system: A review

Sardine Sardinops sagax (Pappe, 1854) is an important forage species, in the northern Benguela Upwelling System (nBUS). Sardine was a key forage species of the pelagic component in the nBUS, but the population collapsed due to a combination of overfishing and ecosystem change and variability in the nBUS. Multiple predators depended on sardine as high-quality food at the time when it was abundant. This study reviewed the current knowledge of predators' diet pre- and post-collapse of the sardine population stock in the nBUS. Data were sourced from previous studies on the trophic interaction in the nBUS. The historical and contemporary diets of commercially exploited species that prey on sardine were found to be relatively well documented while there are data deficiencies for non-commercially exploited species including cetaceans, sharks and some non-commercially exploited fish species. Amongst the species most reliant on sardine as prey are African penguin Spheniscus demersus, Cape gannet Morus capensis, Cape cormorant Phalacrocorax capensis, Cape monkfish Lophius vomerinus, shallow-water hake Merluccius capensis, deep-water hake M. paradoxus, snoek Thyrsites atun, Albacore tuna Thunnus alalunga, Cape fur seal Arctocephalus pusillus, Common dolphins Delphinus delphis, Dusky dolphins Lagenorhynchus obscurus, Heaviside's dolphins Cephalorhynchus heavisidii, six gills shark Hexanchus griseus, copper sharks Carcharhinus brachyurus, school shark Galeorhinus galeus and pelagic sharks Prionace glauca. Some predatory species such as the African penguin, Cape cormorant, Cape gannet, and school shark, are considered endangered as per the International Union for the Conservation of Nature (IUCN) Red List criteria. Populations of Albacore tuna and Cape monkfish are near threatened, while Cape fur seals, common dolphins, and shallow-water hake are considered the least concerned on the IUCN list. Although the collapse of sardine has affected the food webs it has not resulted in collapse, rather there have been changes. Three prey species; bearded goby, Cape horse mackerel, and jellyfish, were identified as alternative prey for some of the predators that previously fed on sardine. This review showed how overexploitation of sardine could have had far-reaching effects on the nBUS through feeding linkages. Goby, although a low-quality prey, is a successful substitute for sardine probably mostly due to its high abundance, tolerance to low oxygen and ability to thrive in warm waters. However, further studies are needed to establish the strength of the ecological linkages of the prey species that replaced sardine in the diets of the predators.

Citizen science as a tool to assess cetacean diversity in the Atacama Desert coast

An understanding of the fine-scale distribution, diversity, and population trends of endangered cetacean species threatened by anthropogenic activities is key for their conservation. In remote places such as the coastline of the Atacama Desert in Northern Chile, research focused on cetacean species is often limited by funding, and personnel. However, scientific interest from local citizens could be an alternative and useful tool to obtain valuable information on cetaceans. Here, we assess the power of citizen science as a tool to determine cetacean species richness, group size, behaviour, seasonality and conservation threats in Northern Chile by undertaking training workshops for local citizens on cetacean species identification and data gathering. A total of 24 citizen scientists reported 495 cetacean sightings during the period between 2013 and 2020, with a total of 19 cetacean species observed (6 mysticetes and 13 odontocetes). One bay (Mejillones Bay) dominated most cetacean sightings (73%) in the study area (Antofagasta region). Seven cetacean species were more frequently observed year-round: Balaenoptera physalus, Megaptera novaeangliae, Grampus griseus, Tursiops truncatus, Lagenorhynchus obscurus, Delphinus capensis and Phocoena spinipinnis. Other species were either opportunistically observed or were rare stranded species. Four species of cetaceans, B. physalus, D. capensis, L. obscurus, and P. spinipinnis were observed mostly traveling (43%) and feeding (37%), indicating that the study region is an important feeding area. Twelve cetacean species were reported stranded, and citizen scientists identified 3 species of cetaceans entangled in fishing nets. This study shows the potential of citizen science for providing information on cetacean richness and threats in remote locations such as the South East Pacific. This approach may improve knowledge on fine-scale cetacean distribution for regions that are sparsely populated and have limited research activity.

Dietary plasticity of two coastal dolphin species in the Benguela upwelling ecosystem

Defining the trophic relationships of marine predators and their dietary preferences is essential in understanding their role and importance in ecosystems. Here we used stable isotope analysis of skin samples (δ15N values reflecting trophic level and δ13C values reflecting foraging habitat) to investigate resource partitioning and spatial differences of the feeding ecology of dusky dolphins Lagenorhynchus obscurus and Heaviside’s dolphins Cephalorhynchus heavisidii from 2 coastal study sites separated by 400 km along the coast of central (Walvis Bay) and southern (Lüderitz) Namibia in the Benguela upwelling ecosystem. Overall, isotopic niches of both predators were significantly different, indicating partitioning of resources and foraging habitats. Despite their smaller body size, Heaviside’s dolphins fed at a significantly higher trophic level than dusky dolphins. Stable isotope mixing models revealed that both species fed on high trophic level prey (i.e. large Merluccius spp., large Sufflogobius bibarbatus, and Trachurus t. capensis) at Walvis Bay. The diet of both dolphin species included smaller pelagic fish and squid at Lüderitz. Spatial differences highlight that Heaviside’s and dusky dolphins may exhibit dietary plasticity driven by prey availability, and that they likely form distinct population segments. Important prey for both dolphin species, specifically Merluccius spp. and T. t. capensis, are the main target of trawl fisheries in the Benguela upwelling ecosystem, highlighting potential resource overlap between dolphins and fisheries.

Vertebral Morphology in Partially Sympatric Dolphins: A 3D Approach

In cetaceans, increased body flexibility is associated with increased maneuverability, this affects the animal’s swimming speed and foraging behavior. A more stable body form is associated with fast swimming and wide turns. One factor that affects the flexibility of a cetacean’s body is the structure and interference of its vertebrae. Differences in vertebral morphology confer different muscular insertion sites and a potential mechanical advantage for swimming muscles. We studied vertebral morphology in four closely related and partially sympatric dolphin species from the Southern Hemisphere: Commerson’s dolphin (Cephalorhynchus commersonii), Peale’s dolphin (Lagenorhynchus australis), the dusky dolphin (Lagenorhynchus obscurus) and the hourglass dolphin (Lagenorhynchus cruciger). The former two species are usually considered coastal, associated with complex habitats where foraging strategies require greater maneuverability; they also show plasticity in their prey preferences. The latter two species are considered fast-swimming cooperative feeders, with long distance movements reflecting prey availability in pelagic habitats. We employed three-dimensional (3D) geometric morphometric techniques and multivariate analyses to evaluate differences in vertebral morphology. Our analyses tested whether particular morphologies that limit or enhance flexibility were associated with preferred habitats and feeding strategies. We established links between morphology and behavioral patterns based on the biomechanical significance of specific vertebral morphological features. Principal Component Analyses (PCA) showed great differentiation between species in all the studied regions along the vertebral column. This was especially evident in the middle area, except in the case of dusky and hourglass dolphins which showed no discernible morphological difference in their mid-column vertebrae. PCA results were supported by significant Mahalanobis Distances between groups. Species associated with complex habitats and behaviors possessed morphological features associated with greater flexibility of the column (i.e. spool-shaped vertebrae with short erect processes), whereas cooperative-feeder species possessed features associated with greater stability (i.e. disk-shaped vertebrae with long strongly bent processes). Even though these species are closely related and partially sympatric, vertebral morphology is distinctive and varies with the differential foraging strategies and habitat of each species. These findings reveal high morphological plasticity among these dolphin species, highlighting the importance of behavioral complexity and of habitat use in the evolutionary development of morphological adaptations.

First Abundance Estimates of Heaviside’s (Cephalorhynchus heavisidii) and Dusky (Lagenorhynchus obscurus) Dolphins Off Namibia Using a Novel Visual and Acoustic Line Transect Survey

Knowledge of a population’s abundance is of primary importance for conservation management. However, robust estimates of abundance are often difficult to obtain, especially for cetaceans which spend most of their lives submerged. Cetacean abundance is commonly estimated using aerial or vessel-based line transect surveys and distance sampling methods. During 2012–2014, the first line transect surveys to estimate cetacean abundance were conducted in Namibian waters. Surveys took place in the Namibian Islands’ Marine Protected Area (NIMPA), a large MPA located along the southern Namibian coastline. A combined visual and acoustic double-platform survey configuration was used to investigate the factors affecting detectability of the endemic Heaviside’s dolphin (Cephalorhynchus heavisidii) and dusky dolphin (Lagenorhynchus obscurus obscurus). The present analysis estimates the probability of detection on the transect line (g(0)) for these two species and generates density and abundance estimates which incorporate a correction for both animals missed on the transect line and attractive responsive movement. The average annual baseline density and abundance estimates for Heaviside’s dolphins in the NIMPA region during 2012–2014 were 0.08 individuals/km2 (CV = 28.6%, 95% CI = 0.04–0.15 individuals/km2) and 1594 individuals (CV = 28.6%, 95% CI =776–3275), respectively. The average annual baseline density and abundance estimates for dusky dolphins in the NIMPA region during 2012–2014 were 0.16 individuals/km2 (CV = 26.2%, 95% CI = 0.10–0.28 individuals/km2) and 3493 individuals (CV = 26.2%, 95% CI: 2015–6052), respectively. A discussion on the distribution of Heaviside’s and dusky dolphins is provided for this region where such information are urgently needed. Based on existing knowledge of the species and area, these estimates are regarded as reasonable. They indicate moderate sized populations of animals within the NIMPA and provide an important first baseline on which future estimates can build.

Pathological Findings in Cetaceans Sporadically Stranded Along the Chilean Coast

Chile has one of the largest coastlines in the world with at least 50% of the world cetacean species occurring within its jurisdictional waters. However, little is known regarding the health status and main causes of death in cetaceans off continental Chile. In this report, we summarize the major pathological findings and most likely causes of death of 15 cetaceans stranded along the Chilean coast between 2010 and 2019. Drowning, due to fishing gear entanglement, was the most likely cause of death in 3 Burmeister’s porpoises (Phocoena spinipinnis), a Risso’s dolphin (Grampus griseus) and a short-beaked common dolphin (Delphinus delphis). Additionally, the 3 Burmeister’s porpoises had mild to moderate eosinophilic and histiocytic pneumonia with pulmonary vasculitis most likely associated with the nematode Pseudalius inflexus. A fourth Burmeister’s porpoise died of drowning after stranding alive at a sandy beach. Two fin whales (Balaenoptera physalus) died most likely of trauma associated with large vessel collision. A long-finned pilot whale (Globicephala melas) and an Orca (Orcinus orca) stranded most likely due to traumatic intra/interspecific interaction with other odontocete although for the pilot whale, osteoporosis and edentulism could have played a role. For a Strap-toothed beaked whale (Mesoplodon layardi), Dwarf sperm whale (Kogia sima), Southern right-whale dolphin (Lissodelphis peronii), Peale’s dolphin (Lagenorhynchus australis) and a dusky dolphin (Lagenorhynchus obscurus), the cause of stranding could not be determined. This study shows, despite the small number of examined carcasses that in Chile, human related trauma is an important cause of single cetacean stranding events.

Allometry and Ontogeny in the Vertebral Column of Southern Hemisphere Dolphins: a 3D Morphofunctional Approach

Commerson’s dolphin (Cephalorhynchus commersonii), Peale’s dolphin (Lagenorhynchus australis), dusky dolphin (Lagenorhynchus obscurus), and hourglass dolphin (Lagenorhynchus cruciger) are closely related species occurring in the Southern Hemisphere. Even though they are partially sympatric, they have evolved towards diverse foraging strategies, prey, and habitat preferences. Size variation occurs both among and within species. Our goal was to study the allometric patterns and ontogenetic trajectories in the vertebral column of these dolphins related to their ecology. We choose nine functionally homologous vertebrae in each column and digitized five different tridimensional configurations, containing 28 to 41 landmarks depending on the functional region under analysis. We explored different levels of allometry (static, ontogenetic, and evolutionary) using multivariate regressions of shape vs. log-transformed centroid size. We compared ontogenetic allometry regression vectors to evidence differences in growth patterns. Finally, we calculated Procrustes distances of shape configurations to determine the timing of shape shifts. Of the studied species, When compared to the Lagenorhynchus species, Commerson’s dolphin showed the highest degrees of allometry, both static and ontogenetic. The macroevolutive trends indicate a high degree of evolutionary morphological change, which could be associated with the rapid evolutionary radiation proposed for the group. This is the first time that allometric changes are studied in small cetaceans by means of 3D geometric morphometrics.