Dolphin Sightings Research Articles

The Effects of Vessel Traffic on the Behavior Patterns of Common Dolphins in the Tagus Estuary (Portugal).

The impact of vessels on dolphin populations has been extensively studied worldwide. The common dolphin, Delphinus delphis, has been observed in the Tagus estuary for the past two centuries, and during the last several years, these sightings seem to have increased. This area has high levels of maritime traffic throughout the year, both commercial and recreational. To understand the possible effects of vessel traffic on dolphins' behavior, land-based observations were carried out from March 2022 to March 2023. For a total of 67 events (48.9 h of dolphin sightings), differences in behavioral budgets were noted. Although "neutral reaction" was the most observed response when vessels were in the vicinity of dolphins, "negative reaction" was also common and five times more abundant than "positive reaction". The GEE model showed statistical differences between these reaction types (positive, neutral, and negative). Markov chains' analysis revealed distinct patterns in the behavioral transition probabilities, as dolphins were more likely to switch to a traveling state when vessels were nearby. This study is the first step towards understanding a potential impact source in the area since it is expected that tourism companies expand due to the increase in dolphin sightings in the estuary.

Population distribution and drivers of habitat use for the Burrunan dolphins, Port Phillip Bay, Australia.

Bottlenose dolphin (Tursiops) populations, also described as the Burrunan dolphins, consist of a resident population of approximately 150 individuals in Port Phillip Bay (PPB), Victoria. Previous reports indicate distribution across a small southern region of PPB; however, little is known about their full distribution patterns across the entire PPB region. Here, we investigate the spatiotemporal distribution of the Burrunan dolphins across four zones representative of PPB benthic habitats and bathymetry to gain a better understanding of the potential drivers of the population's habitat use. Port Phillip Bay, Victoria, Australia. One hundred and twenty-nine boat-based surveys were undertaken between March 2015 and August 2021, encompassing 181 sightings. Generalised linear models (GLMs) were used to investigate annual, seasonal and zonal variation. We found no variation in sighting frequencies between years. Austral summer and winter had a significantly higher sighting frequency than autumn. We found that Burrunan dolphins utilise the entire bay, further extending the species range, and show a significantly higher number of sightings in the southern zone than in any other zones. Overlaying dolphin sightings with known oceanographic characteristics within PPB, we found bathymetry and benthic habitats were potential drivers for the Burrunan dolphins distribution and habitat use within the bay, with the dolphins significantly favouring the 5-10 and 10-15 m contour depths. These results show a more widespread distribution across the bay than previously documented. We recommend expansion of the current marine protected areas in the north and south of the bay. This study has increased our understanding of the vital habitat for the Burrunan dolphin populations. By providing evidence-based conservation recommendations, we hope to improve and contribute to future research, conservation management plans and effective marine protected areas across PPB for the resident Burrunan dolphin population.

Exploring mtDNA Databases to Evaluate the Population Structure and Genetic Diversity of Tursiops truncatus in the Atlantic Ocean: Implications for the Conservation of a Small, Offshore Population

Inshore and offshore bottlenose dolphin, Tursiops truncatus, ecotypes were distinguished through genetics, distribution, diet, morphology, diversity, and social behaviors. Although T. truncatus is a widely studied species, few studies have focused on offshore populations. Offshore biodiversity is frequently neglected due to the difficulty of data collection, and therefore, it is challenging to assess how threatened these populations are. A small, offshore population of dolphins residing around the Saint Peter Saint Paul Archipelago (SPSPA) in the middle of the Atlantic Ocean has been monitored for several years, and a decrease in the number of dolphin sightings has recently been noticed. We analyzed a comprehensive mtDNA control-region sequence dataset for this species to infer the conservation status and better understand the relationships between the SPSPA population and other offshore populations. We assessed the genetic diversity and population structure of the bottlenose dolphin from inshore and offshore populations of the Atlantic Ocean. Offshore populations are more genetically diverse and have less variation between populations than inshore populations. The offshore populations share haplotypes, indicating potential gene flow. However, the SPSPA population presented the lowest levels of genetic diversity between populations. The conservation status of the SPSPA population is concerning, and it is necessary to apply effective management strategies to guarantee its protection.

Environmental drivers of odontocete occurrence in a nearshore temperate habitat

Nearshore coastal waters off Northumberland, UK, are important habitats for marine megafauna including marine mammals and seabirds. The area also features extensive anthropogenic activities including shipping, marine renewables development, fisheries, and tourism. Meanwhile there is lack of baseline data on odontocete occurrence to allow assessment of the potential impact from human disturbance. A recent increase in the number of sightings of common bottlenose dolphins (Tursiops truncatus) in the area has provided impetus for new data to inform conservation managers and policy makers. To provide this information, acoustic data were collected using broadband passive acoustic recorders at three sites (Druridge Bay, Newbiggin-by-the-Sea, and St Mary's Island) off the Northumberland coast and analyzed to identify species-specific click trains of common bottlenose dolphin, white-beaked dolphin (Lagenorhynchus albirostris) and harbor porpoise (Phocoena phocoena). Detection data were modelled for each species as detection-positive hours (DPH) using Generalized Additive Models with Generalized Estimating Equations (GAM-GEE) to investigate the effects of environmental covariates, including day of year, diel phase, lunar phase, salinity, year, month and sea surface temperature, and their interaction with location. A total of 20,845 h were recorded during the study resulting in a total of 1660, 71, and 1111 DPH identified for bottlenose dolphin, white-beaked dolphin and porpoise, respectively. The final model for bottlenose dolphins retained all covariates except tidal index and an interaction between location and day of year after model selection and all covariates in the final model were significant. The final model for white-beaked dolphin retained month, year, location, diel index, temperature, lunar index and interactions between location and diel index and all covariates except lunar index were significant. The final model for porpoise retained all covariates and interactions between location and diel phase, day of year, lunar index, and salinity, however, salinity and the interaction between location and lunar index were not significant. Bottlenose dolphin clicks were detected in the area year-round but with two peak periods in occurrence coinciding with May and late September; white-beaked dolphin clicks were detected predominantly in July and August; and porpoise clicks were present year-round but with much greater probability of detection at the Druridge Bay site, where there were peaks in winter and in September. The results of this study provide important information for conservation and management actions that may be needed to reduce anthropogenic pressures on odontocetes in UK waters.

Enhancing Interpretation of Cetacean Acoustic Monitoring: Investigating Factors that Influence Vocalization Patterns of Atlantic Bottlenose Dolphins in an Urbanized Estuary, Charleston Harbor, South Carolina, USA

The Charleston Harbor in South Carolina (SC)is a major port that experiences high levels of vessel traffic. Historical analyses of coastal bottlenose dolphin (Tursiops truncatus, now Tursiops erebennus) sightings identified multiple core use areas in the harbor that overlap with these anthropogenic activities. Informed by these long-term spatial data, passive acoustic monitoring, visual surveys, and prey sampling were conducted from December 2017 to June 2019 to assess the relationships and multivariate interactions that may influence dolphin vocalization patterns. Vocalizations varied spatially and temporally, peaking in fall and winter months coinciding with decreases in water temperature and daylight hours, following patterns previously reported in other SC estuaries. Dolphin prey and total fish abundance decreased with water temperature, which may indicate that dolphins echolocate and whistle more frequently in the winter months when prey are scarce and sound-producing species are less soniferous. Dolphin sightings and vocalizations were highly correlated. Dolphin occurrence was highest in the areas surrounding the confluence of the Cooper and Wando Rivers, along the shipping channel, where vessel and sound-producing fish detections were greatest. When vessel noise occurred, dolphins increased their vocalizations, which suggests that this population may be modifying its acoustic repertoire in response to increased noise levels. Multivariate interactions indicate strong spatial and seasonal patterns in vocalization rates that may be associated with dolphin and prey abundance as well as noise-induced redundancy.

Cetacean and Sea Turtle Observations in the Remote Mid-Atlantic (NW) Ocean

Due to its remoteness, little is known about the occurrence and abundance of cetaceans and sea turtles in the pelagic mid-Atlantic Ocean. Data on cetacean and sea turtle occurrence and distribution were collected by dedicated biological observers to address U.S. monitoring and mitigation requirements associated with a vessel-based academic geophysical survey in international waters in the northwestern and mid-Atlantic Ocean from 14 June through 16 July 2018. A total of 6,949 km (503 h) of visual observations occurred while surveying north from Bermuda and ending in Woods Hole, Massachusetts. A total of 928 cetaceans representing at least 14 species and 15 sea turtles representing three species were observed. The most frequently observed cetacean species was the Atlantic white-sided dolphin (Lagenorhynchus acutus) (340 individuals; 37%) followed by the short-beaked common dolphin (Delphinus delphis) (286 individuals; 31%) and pilot whales (Globicephala spp.) (95 individuals; 10%). These sighting data also included an extralimital sighting of a sperm whale (Physeter macrocephalus) calf at 43.44° N latitude and 36.85° W longitude, and extralimital sightings of Atlantic white-sided dolphins below 38° N latitude. Loggerhead sea turtles (Caretta caretta) were seen most frequently (6 individuals). This study addresses a data gap in documented occurrence and lack of occurrence of cetaceans and sea turtles over a large pelagic area in the northwestern Atlantic Ocean during summer.

Distribution and behaviour of striped dolphins in the southwestern Mediterranean Sea based on whale-watching data

The striped dolphin (Stenella coeruleoalba) is a cosmopolitan cetacean and the most commonly sighted dolphin in the Mediterranean Sea. It usually appears in groups of very different sizes, ranging from less than ten to more than 500 individuals, although it is usually found in groups of between 21 and 50 individuals. In the western Mediterranean, and more specifically in the Gulf of Mazarrón, S. coeruleoalba was the most frequently sighted cetacean during the 1042 whale-watching trips. The goal of this study was to establish the spatial and temporal distribution of striped dolphin sightings along the Gulf of Mazarrón between 2004 and 2014. Spatial patterns were analysed using a Random Forest based Species Distribution Model to estimate the presence of the species. Twentythree variables (three geographic, one temporal, eight geomorphometric and twelve oceanographic) were used as predictors. Out of the 1042 cruises, 872 records of striped dolphins were obtained. Some variations in the grouping patterns of these mammals were observed during the years 2006–2007, with an average shift in the size of the groups to fewer individuals (3−10). This variation is probably related to an epizootic event of morbillivirus occurring during those years, which was responsible for an abnormal rate of strandings of striped dolphins and long-finned pilot whales (Globicephala melas). The Random Forest model allowed to select 6 predictors related to morphometry and sea currents, suggesting the importance of specific habitat in offshore areas between 1000 and 3000 m depth in the continental slope.

Submarine canyons as key habitats to preserve Risso’s dolphin (Grampus griseus) populations in the northwestern Mediterranean Sea

This paper summarises the occurrence of Risso’s dolphin (Grampus griseus) over 12 years (2009 to 2021) in the northwestern Mediterranean. The study was conducted off the central coast of Catalonia (NE Spain) in an area covering 8,026 km2 and featuring a system of submarine canyons. The habitat is suitable for a wide diversity of species, including deep-diving cetaceans. In addition, a different dataset from other projects was included to compare distances to the coast from different periods. A visual effort of 8,756 km was carried out with the recording of 17 Risso’s dolphin sightings. The relative mean density of Risso’s dolphin was 0.0078 individuals/km2 (SD 0.1, n = 17), and the densities were higher in the southern canyons than in the northern canyons. Furthermore, the distance to the coast and the depth of any sighting were compared by gathering data from different periods and surveys (first period 1985–2014; second period 2016–2021) with 34 sightings analysed. Significant differences were found related to the distance to the coast, showing a displacement of the animals to more pelagic areas between periods. The spatial distribution of Risso’s dolphins was investigated by applying a generalized additive model based on sighting data collected during standardized vessel surveys. Four predictive variables were considered, taking into account the local physiographical features. The model showed that the spatial distribution of Risso’s dolphin in the area was related to the slope and slope variation. The species showed preferences for offshore areas as significant differences were obtained in terms of the distance of sightings to the coast. This paper provides new insights into the distribution of Risso’s dolphin in the central western Mediterranean Sea and identifies the submarine canyons of northern Catalonia as an essential habitat for the species.

Multiple social benefits drive the formation of mixed-species groups of Australian humpback and Indo-Pacific bottlenose dolphins

Mixed-species groups are common amongst diverse taxa including fishes, birds, and mammals. Antipredator, foraging, and social benefits have been proposed as functional explanations for mixed-species group formation. Amongst delphinids, mixed-species groups are widespread, but little is known about their function. To investigate the potential benefits of delphinid mixed-species groups, we compared the number of individuals, the age composition, and the behaviour of single- and mixed-species sightings of Australian humpback (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) observed around the North West Cape, Western Australia. We found no difference in the number of individuals or the age composition of humpback dolphins present in single- and mixed-species sightings, whereas bottlenose dolphins were present in larger numbers in single-species sightings than in mixed-species sightings due to a higher number of adults. Socialising was the initial observed behavioural state of 36.1% of mixed-species sightings, compared to only 5.1% and 10.3% of humpback and bottlenose dolphin single-species sightings, respectively. Furthermore, both species travelled and foraged less frequently when in mixed-species groups. Of 93 mixed-species groups observed during a focal follow of ≥ 10 min, 32 (34.4%) involved aggressive and/or sexual behaviours typically initiated by bottlenose dolphins towards humpback dolphins while the remaining 61 (65.6%) involved only neutral and affiliative behaviours. The results of this study suggest that the observed mixed-species groups provide multiple social benefits, particularly those pertaining to socio-sexual behaviours and the development and care of young.Significance statementNumerous species of dolphin are known to form mixed-species groups. Few studies have, however, investigated the antipredator, foraging, and/or social benefits that these species may gain by grouping with other species. Investigating these benefits is key to understanding the impacts of mixed-species groups on the species and individuals involved. We evaluated the potential functions of mixed-species groups of two sympatric, inshore dolphin species — the Australian humpback dolphin and the Indo-Pacific bottlenose dolphin. We found that both species engage in socialising behaviour more frequently when in mixed-species groups and that they engage in a diverse variety of interspecific behavioural interactions. Our results not only indicate that social benefits are the key drivers of these mixed-species groups, but they also highlight the complexity of social interactions between dolphin species.

Status, ecology and conservation of Irrawaddy dolphins (Orcaella brevirostris) in Malampaya Sound, Palawan, Philippines

A geographically isolated population of Irrawaddy dolphins was recently discovered in Malampaya Sound, Palawan, Philippines. Line-transect surveys conducted in April-November 2001 covered 884km of trackline in the entire Sound and resulted in a total population estimate of 77 individuals (CV = 27.4%), confined to the inner portion (133.7km2 ). For all Irrawaddy dolphin sightings, where ecological data were collected (n = 48), the mean temperature was 30.2°C, depth 6.5m, salinity 28.3ppt and turbidity 2.2NTUs. Significantly higher turbidity, lower salinity and shallower depth were recorded in the inner Sound compared to adjacent waters. Bottlenose dolphins Tursiops sp. (probably truncatus) were observed in waters just outside of where Irrawaddy dolphins were recorded. During the study, at least two Irrawaddy dolphins were accidentally killed in bottom-set nylon gillnets used to catch crabs, locally called matang quatro. Reports from local fishermen also indicated that as many as three additional animals may have been killed in these nets during the same period. These findings strongly suggest that the Irrawaddy dolphin population in Malampaya Sound is in immediate danger of extirpation due to low numbers, limited range and high mortality. This is the only known population of the species in the Philippines and the nearest known other population is in northern Borneo, some 550km to the south. Recommendations for conserving the population include that: (1) socioeconomic alternatives be developed to promote the conservation goal of reducing the incidence of dolphin entanglement in matang quatro gillnets; (2) gillnet free zones be established in core areas of dolphin distribution; (3) Irrawaddy dolphins be promoted as a flagship species of environmental health in the Sound; (4) a long-term programme be established to monitor the dolphin population; and (5) additional investigations be conducted to determine if Irrawaddy dolphins occur in other areas of the Philippines.

The estimation of the detection function and g(0) for shortbeaked common dolphins (Delphinus delphis), using doubleplatform data collected during the NASS-95 Faroese survey

This paper examines the data for common dolphins collected during a general double-platform line transect cetacean survey carried out in waters around the Faroe Islands in 1995 (from southeastern Iceland to western Ireland) in order to determine the extent to which a correction factor can be estimated to account for animals missed on the trackline and for responsive movement towards the vessel. A major assumption of conventional distance-based methods is that all objects at zero distance from the line are detected (i.e. g(0)=1). If this assumption is violated the estimated density and hence abundance will be negatively biased. It also assumes that animals do not respond to the survey vessel before they are detected by the observers. If the animals are attracted to the vessel, for example, this will result in a positively biassed estimate. The g(0) estimate was obtained using the method of Borchers et al. (1998). Visual inspection of the data suggested that the dolphins were attracted to the vessel and this was accounted for following the Buckland and Turnock (1992) approach. Coefficients of variation (CVs) and confidence intervals (CIs) were estimated using a non-parametric bootstrap procedure. During the survey, almost 1,700 n.miles were sailed on primary research effort. There were 153 common dolphin sightings including 52 duplicates. The chosen model for the detection function incorporated perpendicular distance, group size and Beaufort sea state. The resulting estimate of g(0) was 0.7961 (CV=0.14). Density estimates obtained under an assumption of no responsive movement are almost six times higher than when it is taken into account, highlighting the importance of collecting appropriate data to allow analysis of this potential problem in cetacean surveys.

Northern Range Expansion of California Coastal Bottlenose Dolphins (Tursiops truncatus)

The California coastal stock of bottlenose dolphins (Tursiops truncatus) expanded its range north from the Southern California Bight, its historical range, into Central California coincident with the 1982-1983 El Niño event. Since the late 1980s, bottlenose dolphin sightings north of Central California have been increasingly reported. To determine the present-day northern range limit for these dolphins, photo-identification efforts were carried out from 2007 to 2018 in San Francisco Bay and nearby coastal waters during which 84 individuals were identified. The results demonstrate a significant range expansion along the Northern California coast at least as far as Sonoma County (38.7º N). Comparisons with photo-identification catalogs compiled south of San Francisco from 1981 to 2015 revealed that 92% of the 84 dolphins were matched to Monterey Bay (n = 77), Santa Barbara (n = 27), Santa Monica Bay (n = 29), Orange County (n = 9), Corona Del Mar (n = 2), San Diego (n = 31), and Ensenada, Mexico (n = 1). Many of the 84 dolphins (54%) showed long-range movements across the stock’s range between the Southern California Bight and the San Francisco Bay Area. The greatest movement distance recorded was by two individuals first observed in San Diego, California, in the 1980s and subsequently in Puget Sound, Washington (47º N), in 2017, setting a coastal bottlenose dolphin long-distance movement record of at least 2,500 km.

Territory and status of dolphins in Nepalese rivers: A review

Abstract Nepal is endowed with vast water resources in form of glaciers, lakes, streams and rivers. All the rivers in Nepal are connected to the Ganges River system of India. The major rivers are reliable sources of water and provide habitats for aquatic animals, opportunities for hydropower, and irrigation development in downstream regions. The major rivers of Nepal include the Koshi, Gandaki (Narayani), Karnali, and Mahakali which all drain from north to south. These rivers support abundant wildlife species as well as a number of fishes, amphibians and reptiles. Most relevant to this paper is the Ganges River Dolphin (Platanista gangetica gangetica Roxburgh, 1801). The Ganges River Dolphin preys on fish, mollusks and other aquatic animals. Being a migratory species, the dolphins move from larger rivers to their tributaries, seasonally. This paper is intended to review work on the status of Ganges River Dolphin in Nepalese rivers based on secondary data obtained from different published surveys targeting the dolphin populations. The records show the Karnali River has the largest population, followed by the Koshi and Narayani rivers, with no record of dolphins occupying the Mahakali River in recent years. Recent surveys estimate about 100 dolphins living in Nepal, with 80 estimated in the Karnali River. Different anthropogenic activities including construction work, intensive fishing, pollution and the lack of conservation planning for dolphins have resulted in a decline in number from historic levels. The presence of this aquatic mammal is considered as an indicator of the overall health of the river system. The COVID-19 pandemic and lock down might have caused increased dolphin sightings. Additionally, high pre-monsoon precipitation in recent years have caused higher water levels in smaller rivers, possibly causing the dolphins to return earlier than usual to the major river systems. They were recorded to be present in the Mohana and other small tributaries of the Karnali River as early as first week of June.

Influences of natural and anthropogenic habitat variables on Indo-Pacific humpback dolphins Sousa chinensis in Hong Kong

Indo-Pacific humpback dolphinsSousa chinensisin Hong Kong (HK) waters are part of a large (ca. 2000-2500), but declining, population that occurs in the Pearl River Estuary of southern China. To understand the factors that may influence dolphin densities in 4 different parts of HK, a 25 yr (1996-2020) database containing 66439 km of line-transect survey effort and 4052 dolphin sightings was used. Seventeen variables representing natural and anthropogenic habitat factors were evaluated using generalized additive models (GAMs) to identify the preferred habitat of humpback dolphins in HK waters. In the environmental GAMs, rainfall, salinity, and river discharge were significant factors related to dolphin density, consistent with their observed strong preference for estuarine habitat. Sea surface temperature was also important for one of the 4 subareas. In the anthropogenic GAMs, the only significant variable was the amount of habitat area lost to land reclamation in North Lantau. This finding is consistent with previous hypotheses that heavy coastal engineering in North Lantau has contributed to observed declines in dolphin abundance during the study period. This study provides some of the first quantitative indications of factors that are potentially influencing the density of the animals in various parts of HK. As such, it will be a valuable tool for evaluating and mitigating potential impacts of both natural and anthropogenic factors on the population in the future.

Identifying the hotspot area of Indo-Pacific humpback (Sousa chinensis) and Irrawaddy (Orcaella brevirostris) dolphins in Matang waters, Perak, Malaysia

Abstract. Sobri MF, Jaaman SA, Mohamed Z, Muda AM, Rashid MASA, Zhang X. 2022. Identifying the hotspot area of Indo Pacific humpback (Sousa chinensis) and Irrawaddy (Orcaella brevirostris) dolphins in Matang waters, Perak, Malaysia. Biodiversitas 23: 6106-6113. The Indo-Pacific humpback (Sousa chinensis) and Irrawaddy (Orcaella brevirostris) dolphins are commonly found in Matang waters, Perak, Malaysia. The area, filled with healthy mangrove forest cover and many estuaries, is not only frequented by the dolphins for its daily activities but also extensively used by locals for artisanal fishing, charcoal production and organizing ecotourism activities. There could be a high risk of human-dolphin interactions, hence determining habitat use and identifying critical habitats or "hot spots" for the dolphins in the area is important. In this study, a series of dedicated boat sighting surveys were conducted from March 2019 to January 2021 between Kuala Gula and Kuala Jarum Mas. The total survey effort was 3325.76 km, 44 days and 239.38 hours. A total of 69 dolphin sightings were made, consisting of 40 Indo-Pacific humpback dolphins and 29 Irrawaddy dolphin observations, respectively. During each sighting, every dolphin's behaviors were categorized as: foraging, socializing, evasive, and travelling, and the survey area was classified into four blocks. The results indicated that different dolphin species have different hotspot areas. For Indo-Pacific humpback dolphins, the hotspot area was Kuala Sangga, while the Irrawaddy dolphin's hotspot area was outside Kuala Larut, and there was a small overlap between the two dolphins' hotspots. Among the recorded behaviors (n: 281) for both dolphins, the most dominant was foraging (72.24%), followed by socializing (17%), evasive (7%), and travelling (4%). The sighted foraging behavior for both dolphin populations was dominant, indicating the hot spots were major foraging grounds for the two dolphin populations. This study shows the importance of Matang waters as a conservation area for the species and warrants further studies of dolphin population ecology to be conducted.

Occurrence, site fidelity, and abundance of bottlenose dolphins (Tursiops truncatus) in the Western Ligurian Sea

This study represents the first complete assessment of the population dynamics and residency of common bottlenose dolphins in the Western Ligurian Sea, an area where bottlenose dolphin sightings have been rare in the past. Despite being greatly affected by human pressure, information about this population is poor and sporadic. The study took place from April 2018 to December 2020. One hundred and seventy‐one dedicated boat surveys were performed year‐round resulting in 95 sightings, and 123 individuals were identified through mark‐recapture photo‐identification techniques. Dolphins were found to be present year‐round. Most of the animals (79) used the area sporadically and only 19.5% of the dolphins (24) were seen regularly. Bottlenose dolphin abundance was estimated at 248 using open POPAN population models (CI 95% = 217–284). These results help fill a knowledge gap by highlighting the regular presence of bottlenose dolphins in the study area. Moreover, they represent a baseline for future studies investigating abundance trends and the conservation status of this population

Prioritising river stretches using multi-modelling habitat suitability of Gangetic dolphin (Platanista gangetica) as a flagship species for aquatic biodiversity conservation in the Ganga River Basin, India

Globally, the river ecosystems are threatened due to human-driven exploitation and indiscriminate resource use. The rate of species loss is a magnitude higher in these ecosystems, hence, identifying conservation priority areas as refugia, using the flagship-cum-indicator species approach can aid in long-term conservation of multiple species and ensure uninterrupted functioning of ecological processes. For effective conservation planning, we derived the site occupancy and abundance of Gangetic dolphin (Platanista gangetica) as a flagship species in the Ganga River Basin, and modelled their distribution vis-à-vis river conditions for identifying Conservation Priority Stretches (CPS). The study incorporates the first-ever basin-wide (4635 km river) Gangetic dolphin (GD) sightings to estimate range decline, abundance, and identify CPS of select rivers in the Basin. A total of 2151 sightings of surfacing dolphins with mean encounter rate of 0.55 ± 0.09 sightings/km of the river was observed from the surveyed stretch. The GD encounter rate varied significantly across the surveyed rivers (Analysis of Variance, F = 3.08, p < 0.001). We estimated 24.37 % decline in the dolphin distribution range in the basin since 19th Century. The estimated population size of the dolphin in the Basin was 3330 individuals ± 620 individuals (Confidence Interval 95 % = 2304–4668; Coefficient of Variance = 18.61) which varied across the river. The site occupancy and abundance were best predicted by channel depth (β = 0.82 ± 0.46), meanders (β = 02.56 ± 0.87) and individual rivers, whereas channel width (β = 0.11 ± 0.08) and survey efforts influenced detection probabilities. Further, we identified 610 km (12.2 %) of river stretches as high CPS in the Basin based on the prediction probability (≥0.70) of GD. Protection of these stretches is likely to ensure sustained reproduction of GD and provide refugia for other threatened species of the Ganga River and its tributaries, which is under increasing anthropogenic pressure.

Distribution, abundance and sighting patterns of multiple stocks of bottlenose dolphins (Tursiops truncatus) in coastal Virginia waters

Three stocks of bottlenose dolphins (Tursiops truncatus) are currently known to co‐occur in the coastal waters off Virginia. A combination of visual line‐transect survey and photo‐identification (photo‐ID) methods was used to gather information on the occurrence, distribution and density of bottlenose dolphins in the coastal areas of Norfolk and Virginia Beach. Sixty‐one line‐transect surveys were completed in two zones (inshore and offshore) between August 2012 and August 2015, with on‐effort coverage of 6,550 kilometres (km) of trackline over 349.6 hours. Conventional line‐transect analysis of bottlenose dolphin sightings showed both spatial and temporal variation in density (D) and abundance (N), with highest density in the inshore zone during warm‐water months (June–November). Inshore densities were calculated as 3.51 individuals per square kilometre (km2) (N = 1,090) in the warm‐water season and 0.73 individuals/km2 (N = 225) in cool‐water months (December–May). Densities in the offshore zone were 1.24 individuals/km2 (N = 741) in the warm‐water season and 0.61 individuals/km2 (N = 366) in the cool‐water season. Warm‐water density peaks support previous research; however, this study documents a cold‐season presence of dolphins in both zones not previously reported. Twenty‐seven photo‐ID surveys were completed and a photo‐ID catalogue was created using photos taken during both dedicated photo‐ID and line‐transect surveys, containing 1,335 unique individuals. Re‐sighting numbers were low, with less than 17% of individuals documented on more than one occasion, and only 4% sighted on three or more occasions, with a maximum of 4 re‐sightings. Days between re‐sightings ranged from 5 to 970, though distance between re‐sightings spanning greater time was minimal (less than 25km). Sighting locations of photo‐IDed dolphins were broken into three regions to investigate seasonal and spatial patterns of individual distribution; dolphins sighted only in the Cape Henry (CH) or Inner Bay (IB) region were observed mostly during summer and fall seasons (93% CH, 100% IB) with none occurring during winter. For the Outer Coastline (OC) region, the majority of sightings (88%) occurred during fall and winter. Additionally, there were two individuals sighted during winter months also sighted during summer more than 130km to the north inside Chesapeake Bay (matched to a different catalogue). These sightings support the possibility that the current stock structure does not account for all dolphins in the region. Stock structure as currently understood is complicated, with overlapping spatial distribution of three stocks that separate seasonally. Our study provides information that may be useful for management, but also additional findings that could impact future stock designations

Influence of environmental variability on the distribution and abundance of the pantropical spotted dolphin (Stenella attenuata) in the Mexican Central Pacific

The Mexican Central Pacific (MCP) has complex oceanographic dynamics, with a well-defined seasonal pattern and influence of interannual sea surface temperature anomalies. The present study investigated the relationship between the distribution/abundance of spotted dolphins and the environmental conditions in the MCP. Dolphin sighting data were collected during January 2010–December 2015; distance to coast, sea surface temperature, and chlorophyll a (Chla) were obtained for each sighting location through georeferenced maps and satellite images. A total of 746 spotted dolphin sightings were obtained with a survey effort of 31,695 km. Spotted dolphins in the MCP showed a predominantly coastal distribution regardless of environmental conditions. Significant differences were detected concerning regional seasonal variation, with higher mean sighting rate during the stratified periods and higher density/abundance estimates during the mixed periods. Spotted dolphins showed preference for warmer conditions, with higher mean sighting rate, abundance, and distance to coast during the neutral and warm interannual periods. During the coolest conditions (cold periods), mean sighting rate and abundance decreased and spotted dolphins were located closer to the coast, where Chla concentration increased. These results suggest that changes in the distribution/abundance of these dolphins could be associated with habitat quality related to tropical (El Niño/Southern Oscillation) and extratropical (The Blob) oceanographic phenomena, which could be modulating their foraging activities.

Mapping the Exposure of Pantropical Spotted Dolphins and Common Bottlenose Dolphins to Different Categories of Vessel Traffic in Maui Nui, Hawai‘i

The increase and diversification of vessel traffic worldwide has resulted in a variety of known disturbances to dolphins. As a remote island chain, the Hawaiian Islands rely heavily on the marine environment for transportation, recreation, and fishing that aggregates into significant levels of vessel traffic. Given the known presence of dolphins in this region, there is a gap in knowledge regarding the relative exposure risk that vessel traffic poses to island-associated dolphin populations in Hawai‘i. This research identified the spatial distribution of pantropical spotted dolphins (Stenella attenuata) and common bottlenose dolphins (Tursiops truncatus) and determined the extent of their overlap with various categories of vessel traffic in Maui Nui, Hawai‘i. Species distribution was determined using kernel density estimates based on sighting data from 50 spotted dolphin sightings and 75 bottlenose dolphin sightings collected from 2013 to 2018. A combination of vessel GPS and Automatic Identification Systems (AIS) vessel tracks were used to quantify vessel traffic within the study area, resulting in 22,464 vessel tracks that were subsequently compiled into categories for analysis from 2014 to 2017. Risk of vessel exposure between vessels and dolphins, or co-occurrence, was determined by calculating the product of predicted dolphin density and predicted vessel density. Both species were exposed to vessel traffic risk over 100% of their distribution, but their highest exposure risk varied spatially, with high risk defined as &gt; 0.25 on a normalized scale of low (0) to high (1) risk. The highest vessel exposure risk for spotted dolphins occurred in deeper, offshore waters, while the highest risk for bottlenose dolphins was in shallow, coastal waters. The vessel categories with the largest area of high risk to spotted dolphins were tour vessel and commercial fishing vessel traffic, while commercial transportation traffic had the highest exposure risk for bottlenose dolphins. This article provides baseline information for understanding the vulnerability of two island-associated dolphin populations to the threat of vessel traffic and highlights the differences in relative exposure risk based on species and vessel categories.