Search

Stranded marine mammals provide valuable insight into population health of free‐ranging conspecifics; however, the likelihood of carcass detection by the public or trained observers is not well known. To better understand carcass detection rates (CDR), we placed twelve decoy dolphin carcasses around Dauphin Island, Alabama, for 2 weeks, one during peak tourist season and one during the off season. Decoys were placed in regions representing different habitat types (marsh or beach) and levels of human use (low or high). Calls from the public were recorded, and trained observers actively searched for decoys via drone and visual observation either by vessel or UTV and walking. There were 2.5 times more public reports during the peak (n = 38) compared to off season (n = 15), with most reports being from the high‐traffic beach site during peak season (n = 27). Trained observers found more decoys (CDR = 0.88) than the public (CDR = 0.58), however, the public found two decoys that observers did not. Drone searches were slightly more successful (CDR = 0.83) than other methods (CDR = 0.79). Our results indicate that a combination of surveillance methods will enhance carcass detection, and our novel methods can be used across habitat types to improve stranding surveillance, better estimate stranding rates, and inform mortality estimates of many species. [ABSTRACT FROM AUTHOR]

Understanding reproductive output and success is integral to the conservation and management of threatened species and populations. Little is known about the reproductive parameters of Australian humpback dolphins (Sousa sahulensis), a species subject to cumulative pressures from anthropogenic threats due to their coastal distribution and strong site fidelity. This study assesses several reproductive parameters, including crude birth, recruitment, fecundity, and calf survival rates, in addition to interbirth intervals of Australian humpback dolphins inhabiting the near‐urban embayment of Moreton Bay, Queensland. Between 2014 and 2020, 106 adult females were photographically identified during 222 boat surveys. Of these, 75.5% (n = 80) were observed with calves. Birth seasonality was apparent and coincided with austral autumn and winter months. Interbirth intervals ranged between 1 and >6 years, with an average of 3.1 (SD = 1.1) years if offspring survived. Findings indicate declining fecundity rates (p <.05, M = 0.12, SD = 0.02), relatively low recruitment rates (M = 0.04, SD = 0.01) and calf survival rates to 1 year of age (M = 0.63, SD = 0.15). This study provides a useful baseline that can inform viability assessments and conservation measures, for both this population and others throughout the species range. [ABSTRACT FROM AUTHOR]

Long‐term distribution data for cetaceans are lacking, inhibiting the ability of management bodies to assess trends and react appropriately. Such is true even along the US Atlantic Outer Continental Shelf (OCS) where previous passive acoustic monitoring programs have laid the groundwork for monitoring cetacean occurrence over a multidecadal scale. Here, we continue and expand the scope of previous acoustic programs, providing a synopsis of the monthly cetacean acoustic occurrence from late 2017 to late 2020. Acoustic data were collected using bottom‐mounted autonomous recorders located at seven stations along the OCS in depths of 212–900 m. Automated cetacean vocalization detector‐classifiers were applied, and the resulting automated detections directed the manual review of a subset of the data by analysts. Only manual detections informed the occurrence results. Six baleen whale species and at least eight toothed whale species occurred in the region with diversity increasing in winter. In considering previous monitoring program results, we found evidence that some mysticete whales are spending less time in the region annually, confirmed that some species occur farther offshore than previously reported, and identified two previously unreported areas utilized by beaked whales. For effective species management, these findings must be considered, and monitoring programs continued. [ABSTRACT FROM AUTHOR]

Coexistence among sympatric species requires a certain degree of resource partitioning. In the Mexican South Pacific, information regarding the coexistence of Stenella attenuata (SA) and Stenella longirostris (SL) is lacking. Stable isotope analyses (δ15N and δ13C) were conducted to assess the differences in feeding habits to infer trophic position and amplitude as well as habitat use, based on Bayesian inference and a random forest (RF) classifier. Potential trophic relationships with other species were assessed by Bayesian mixing models. Feeding segregation between species was mainly based on carbon sources (δ13C: p(SA > SL) = 100%, RF Gini Impurity = 80%). Moreover, SA (n = 22) presented a broader isotopic niche than that of SL (n = 25; SIBER Bayesian Standard Ellipse areas = 0.91‰2 vs. 0.77‰2) with a 33% overlap, suggesting that SA uses more coastal habitats than SL. The most relevant prey species were the mesopelagic fish Benthosema panamense and the epipelagic fish Hyporhamphus naos (~50%), although B. panamense was more related to SL than SA: p(%SL > %SA) = 76.6%. The trophic positions were 4.0 (SA) and 3.8 (SL). Our results provide evidence of resource partitioning within a potential foraging ground for both dolphin species. [ABSTRACT FROM AUTHOR]

Photographic identification is an essential research and management tool for marine mammal scientists. However, manual identification of individuals is time‐consuming. To shorten processing times, we developed finFindR, an open‐source application that uses a series of neural networks to autonomously locate dorsal fins in unedited field images, quantify an individual's unique fin characteristics, and match them to an existing photograph catalog. During a blind test comparing manual searching to finFindR for common bottlenose dolphin (Tursiops Tursiops truncatus) photographs, experienced photo‐identification technicians achieved similar match rates but examined an order of magnitude fewer photographs using finFindR (an average of 10 required with finFindR versus 124 with manual search). In those tests, the correct identity was ranked in the first position in 88% of cases and was within the top 50 ranked positions in 97% of cases. Our observations suggest that finFindR's matching capabilities are robust to moderate variation in image quality and fin distinctiveness. Importantly, finFindR allows users to build a catalog of known individuals through time and match an unlimited number of individuals instead of being restricted to a predefined set. finFindR's convolutional neural networks could be re‐trained to identify members of many marine mammal species without altering finFindR's inherent structure. [ABSTRACT FROM AUTHOR]

Bony labyrinth morphology varies across marine mammals and contains key information regarding hearing sensitivity and ecology. The hearing ranges of globicephaline (Delphinidae: Globicephalinae) or melon‐headed dolphins, known as "Blackfish," have been extensively studied using acoustic technologies, but clade‐wide morphological analysis of the bony labyrinth is lacking. In this study, we investigate the variation in hearing‐relevant bony labyrinth morphology within globicephalines using μCT scans of isolated petrosals and digitally isolating the bony labyrinth of all species. Principal components analysis (PCA) of nine hearing‐relevant measurements of the cochlea alongside a broader sampling of terrestrial and aquatic artiodactyls shows Orcaella brevirostris and other globicephalines with higher levels of facial asymmetry and potentially more specialized echolocation abilities plotting near Monodon monoceros, Delphinapterus leucas, and Orcinus orca. The remaining globicephalines, which have more symmetrical skulls and other unique and acoustically relevant attributes, plotted towards the middle of the echolocating odontocete portion of the PCA. Our analysis thus reveals that inner ear morphology may correlate with both facial skull morphology and echolocation specializations, as these are intertwined. Furthermore, this study illustrates how morphological analyses, especially those centered on hearing, may provide critical conservation‐relevant information as direct access to audiograms becomes less tenable. [ABSTRACT FROM AUTHOR]

Understanding the social organization of coastal dolphins, particularly those that reside adjacent to major cities and urbanized regions, is important for understanding the vulnerability of populations to human impacts. We investigated the sexual segregation and social dynamics in a population of Australian humpback dolphins (Sousa sahulensis) inhabiting the near‐urban embayment of Moreton Bay, Queensland. Over 160 vessel‐based surveys, 148 adults were photographically identified. This included 116 assumed females and 32 assumed males. The population was sexually segregated (SSAS = 0.52, p <.001) and individuals preferred to associate with others of the same sex occupying the same spatial area (p <.025). Association indices between paired males were almost double those of females (x¯HWI=0.09, SD = 0.04 and x¯HWI=0.05, SD = 0.02, respectively). However, females were more gregarious than males and maintained a larger network of associates based on preferred companionships. The social network was also modular with five clusters identified, and a limited number of individuals had a disproportionate influence on connectivity between clusters. This highly fragmented social network implies that the population may be vulnerable to social disruptions from human impacts. [ABSTRACT FROM AUTHOR]

Marine mammal populations often have "hotspots" of distribution. Understanding what drives these is important for understanding relationships with habitat and evaluating exposure to threats. Few studies investigate the stability of hotspots, yet this information is vital in assessing their importance. In this study, over 9,000 sightings made during systematic surveys over 29 yr are used to establish the existence, locations, and temporal dynamics of hotspots for Hector's dolphins at Banks Peninsula, New Zealand. Sightings were divided into four seasons and three time periods to assess temporal trends in habitat use. Kernel density analysis was performed on sightings, weighted by survey effort. Density values at hotspots and reference areas were modeled according to season and time period using linear mixed models. Fifty percent of weighted sightings (n = 4,513) occurred within 21% of the study area. Hotspots had significantly higher densities during summer and these high‐density areas have remained consistent over time. Such consistency implies importance of these areas to the dolphins' ecology. This information adds to our knowledge of how this endangered species uses its habitat, suggests candidate areas for protection from threats, and provides a baseline for assessing habitat related impacts on Hector's dolphins at Banks Peninsula.© 2018 Society for Marine Mammalogy [ABSTRACT FROM AUTHOR]

Research on cetacean foraging ecology is central to our understanding of their spatial and behavioral ecology. Yet, functional mechanisms by which cetaceans detect prey across different scales remain unclear. Here, I postulate that cetaceans utilize a scale-dependent, multimodal sensory system to assess and increase prey encounters. I review the literature on cetacean sensory systems related to foraging ecology, and hypothesize the effective scales of each sensory modality to inform foraging opportunities. Next, I build two 'scale-of-senses' schematics for the general groups of dolphins and baleen whales. These schematics illustrate the hypothetical interchange of sensory modalities used to locate and discriminate prey at spatial scales ranging from 0 m to 1,000 km: (1) vision, (2) audition (sound production and sound reception), (3) chemoreception, (4) magnetoreception, and somatosensory perception of (5) prey, or (6) oceanographic stimuli. The schematics illustrate how a cetacean may integrate sensory modalities to form an adaptive foraging landscape as a function of distance to prey. The scale-of-senses schematic is flexible, allowing for case-specific application and enhancement with improved cetacean sensory data. The framework serves to improve our understanding of functional cetacean foraging ecology, and to develop new hypotheses, methods, and results regarding how cetaceans forage at multiple scales. [ABSTRACT FROM AUTHOR]

Seamounts are considered hot spots of biodiversity and can aggregate pelagic predators and their prey. Passive acoustic monitoring was conducted over 3 mo in 2012 to document the occurrence of odontocetes near a seamount chain in the central equatorial Pacific in relation to oceanographic changes over time. Beaked whale echolocation signals were most frequently encountered. The main beaked whale signal was an unknown type, BW38, which resembled signals produced by Blainville's beaked whales. It had high occurrence during high sea surface temperature and low sea surface salinity. Cuvier's beaked whales were the second most detected. They had an opposite pattern and were encountered more often when sea surface temperature was low and net primary productivity was high. Risso's dolphins and short-finned pilot whales had high acoustic densities, and echolocated predominantly at night. Risso's dolphins occurred more often during low sea surface height deviation. False killer whales were less frequently detected and mostly occurred during the day. Sperm whale detections were fewer than expected and associated with high chlorophyll a. Short duration Kogiidae encounters occurred on average every third day. These types of long-term site studies are an informative tool to comparatively assess species composition, relative abundance, and relationship to oceanographic changes. [ABSTRACT FROM AUTHOR]

Acoustic recordings of two closely related species, spinner dolphin ( Stenella longirostris) and pantropical spotted dolphin ( Stenella attenuata), were investigated from four different geographic locations: two in the Central Tropical Pacific, one in the Eastern Tropical Pacific and one in the Indian Ocean. The two delphinid species occur in tropical and warm temperate waters, with overlapping ranges. They produce very similar vocalizations, but at the same time their calls exhibit a certain degree of intraspecific variation among different geographic locations as has been observed in other delphinid species. Oscillatory whistles (whistles with at least two oscillations in their frequency contours) were identified and manually extracted from the recordings. Whistles with four or more maxima (oscillations) occurred only in spinner dolphins and they were present in all geographic regions investigated. In addition, the oscillatory whistles with two and three maxima were significantly more frequent in spinner than in spotted dolphins. The differences in oscillatory whistles for these two species seem to be consistent across study areas and therefore, could be used in addition to other whistle features to help distinguish between them. [ABSTRACT FROM AUTHOR]

The viability of healthy single stranded dolphins as immediate release candidates has received little attention. Responders have been reluctant to release lone delphinids due to their social needs, even when they pass the same health evaluations as mass stranded animals. This study tracked postrelease success of 34 relocated and released satellite tagged delphinids from single and mass strandings. Three postrelease survival parameters (transmission duration, swim speed, and daily distance) were examined to evaluate whether they differed among single stranded/single released (SS/SR), mass stranded/single released (MS/SR), or mass stranded/mass released (MS/MR) dolphin groups. Comparisons were also made between healthy and borderline release candidates. Satellite tags transmitted for a mean of 21.2 d (SD = 19.2, range = 1-79), daily distance traveled was 42.0 km/d (11.25, 20.96-70.72), and swim speed was 4.3 km/h (1.1, 2.15-8.54). Postrelease parameters did not differ between health status groups, however, SS/SR dolphins transmitted for a shorter mean duration than MS/ MR and MS/SR groups. Postrelease vessel-based surveys confirmed conspecific group location for two healthy,MS/SR dolphins. Overall, these results support the potential to release healthy stranded single delphinids; however, further refinement of health assessment protocols for these challenging cases is needed. [ABSTRACT FROM AUTHOR]

Visual line-transect surveys are commonly used to estimate cetacean abundance. A key parameter in such studies is g(0), the probability of detecting an animal that is directly on the transect line. This is typically considered to be constant for a species across survey conditions. A method is developed to estimate the relative values of g(0) in different survey conditions (Beaufort state) by comparing Beaufort-specific density estimates. The approach is based on fitting generalized additive models, with the presence of a sighting on a survey segment as the dependent variable, Beaufort state as the key explanatory variable, and year, latitude, and longitude as nuisance variables to control for real differences in density over time and space. Values of relative g(0) are estimated for 20 cetacean taxa using 175,000 km of line-transect survey data from the eastern and central Pacific Ocean from 1986 to 2010. Results show that g(0) decreases as Beaufort state increases, even for visually conspicuous species. This effect is greatest for the least conspicuous species (rough-toothed dolphins, beaked whales, minke whales, and dwarf and pygmy sperm whales). Ignoring these large effects results in a nontrivial bias in cetacean abundance estimates. [ABSTRACT FROM AUTHOR]

The current paucity of published blood values and other clinically relevant data for short-beaked common dolphins, Delphinus delphis, hinders the ability of veterinarians and responders to make well-informed diagnoses and disposition decisions regarding live strandings of this species. This study examined hematologic, clinical chemistry, and physical parameters from 26 stranded common dolphins on Cape Cod, Massachusetts, in light of their postrelease survival data to evaluate each parameter's efficacy as a prognostic indicator. Statistically and clinically significant differences were found between failed and survived dolphins, including lower hematocrit, hemoglobin, TCO2, and bicarbonate and higher blood urea nitrogen, uric acid, and length-to-girth ratios in animals that failed. In general when compared to survivors, failed dolphins exhibited acidosis, dehydration, lower PCVs, and decreased body condition. Additionally, failed dolphins had the highest ALT, AST, CK, LDH, GGT, and lactate values. These blood values combined with necropsy findings indicate that there are likely a variety of factors affecting postrelease survival, including both preexisting illness and stranding-induced conditions such as capture myopathy. Closer evaluation of these parameters for stranded common dolphins on point of care analyzers in the field may allow stranding personnel to make better disposition decisions in the future. [ABSTRACT FROM AUTHOR]

The health, postrelease movements, and behavior of mass stranded Atlantic white-sided dolphins ( Lagenorhynchus acutus) and short-beaked common dolphins ( Delphinus delphis) from Cape Cod, Massachusetts, were evaluated. Health was assessed through physical examination and blood analysis. Eleven dolphins (eight white-sided dolphins and three common dolphins) were relocated, outfitted with satellite transmitters, and released during seven mass stranding events. Five transmitters recorded only location, and six also included a time-depth recorder. Transmission duration ranged from 8 h to 218 d, with a mean of 117 d (median = 118 d, SD = 82 d), after release. All dolphins demonstrated extensive movement throughout the Gulf of Maine. The distribution of tagged dolphins was considered normal based on comparisons with published data for these species. Excluding the dolphin that transmitted for only 8 h, mean minimum speeds for individual dolphins ranged from 3.4 to 6.6 km/h; overall mean for all dolphins was 5.4 km/h (SD = 0.9 km/h). The five dolphins with time-depth recorders had mean dive depths of 8.6-40.3 m and mean dive durations of 46-296 s. Hematologic and biochemical data revealed only minor abnormalities. Data suggest that at least 10 of the 11 dolphins were likely successfully reintroduced into the wild. [ABSTRACT FROM AUTHOR]

The Yangtze finless porpoise ( Neophocaena asiaeorientalis asiaeorientalis) is an endangered freshwater porpoise subspecies unique to the Yangtze River basin. Seasonal variations in local distribution of the animal, as well as fish presence, sand dredging, ship navigation, and bridges were examined as potential factors affecting the occurrence of the animals. Passive acoustic surveys were performed regularly from May 2007 to August 2010, near the conjunction of the Yangtze River and Poyang Lake. The distribution of the porpoises was seasonally site-specific. In May and August, the animals were detected more often at river junctions than in the lake, but vice versa from November to February. The rate of the porpoise detection was significantly higher in areas of fish presence than in areas of absence. The number of porpoises detected did not differ significantly between the sand dredging operation and the prohibition period (in 2008), although the number of vessels obviously declined in 2008. Ship traffic and bridges also did not appear to affect the presence of porpoises. These results showed the relative importance of the various environmental factors, which is important for conservation of not only Yangtze finless porpoise but also endangered isolated cetaceans. [ABSTRACT FROM AUTHOR]

Management agencies often use geopolitical boundaries as proxies for biological boundaries. In Hawaiian waters a single stock is recognized of common bottlenose dolphins, Tursiops truncatus, a species that is found both in open water and near-shore among the main Hawaiian Islands. To assess population structure, we photo-identified 336 distinctive individuals from the main Hawaiian Islands, from 2000 to 2006. Their generally shallow-water distribution, and numerous within-year and between-year resightings within island areas suggest that individuals are resident to the islands, rather than part of an offshore population moving through the area. Comparisons of identifications obtained from Kaua‘i/Ni‘ihau, O‘ahu, the “4-island area,” and the island of Hawai‘i showed no evidence of movements among these island groups, although movements from Kaua‘i to Ni‘ihau and among the “4-islands” were documented. A Bayesian analysis examining the probability of missing movements among island groups, given our sample sizes for different areas, indicates that interisland movement rates are less than 1% per year with 95% probability. Our results suggest the existence of multiple demographically independent populations of island-associated common bottlenose dolphins around the main Hawaiian islands. [ABSTRACT FROM AUTHOR]

Tuna purse-seiners in the eastern tropical Pacific (ETP) capture yellowfin tuna by chasing and encircling herds of associated dolphins. This fishery has caused mortality in 14 dolphin species (20 stocks) and has led to significant depletions of at least three stocks. Although observed dolphin mortality is currently low, set frequency remains high and dolphin stocks are not recovering at expected rates. Mortality of nursing calves permanently separated from their mothers during fishery operations may be an important factor in the lack of population recovery, based on the recent discovery that calves do not accompany 75%–95% of lactating females killed in the purse-seine nets. We assessed age-specific potential for mother–calf separations and subsequent mortality of calves by reviewing and synthesizing published data on physiological and behavioral development in delphinids from birth through 3 yr postpartum. Results indicate that evasive behavior of mothers, coupled with the developmental state of calves, provides a plausible mechanism for set-related mother–calf separations and subsequent mortality of calves. Potential for set-related separation and subsequent mortality is highest for 0–12-mo-old dolphins and becomes progressively lower with age as immature dolphins approach adult stamina and attain independence. [ABSTRACT FROM AUTHOR]

Cetacean abundance is estimated for the U.S. Exclusive Economic Zone (EEZ) around the Hawaiian Islands based on a ship line-transect survey from August to November, 2002. Sighting detection functions are estimated from this and other NOAA research surveys from 1986 to 2002 using a new, multiple-covariate approach. Twenty-four species were seen on this survey, including two species (Fraser's dolphin [ Lagenodelphis hosei] and sei whale [ Balaenoptera borealis]) that had not been previously documented to occur in Hawaiian waters. The most abundant large whales are sperm whales ( Physeter macrocephalus) and Bryde's whales ( Balaenoptera edeni). The most abundant delphinids are pilot whales ( Globicephala macrorhynchus), rough-toothed dolphins ( Steno bredanensis), Fraser's dolphins, spotted dolphins ( Stenella attenuata), and striped dolphins ( Stenella coeruleoalba). Dwarf and pygmy sperm whales ( Kogia sima and Kogia breviceps) and Cuvier's beaked whales ( Ziphius cavirostris) are also estimated to be quite abundant. Some of the migratory baleen whales (fin whales [ Balaenoptera physalus], sei whales, minke whales [ B. acutorostrata], and humpback whales [ Megaptera novaeangliae]) were seen only late in the survey. Abundance is estimated for 19 cetacean species. The overall density of cetaceans is low in the study area, especially for delphinids. The precision of density and abundance estimates is generally low for all species because of the low number of sightings. [ABSTRACT FROM AUTHOR]

A long-term study of botos (inia geoffrensis) in the Brazilian Amazon permitted the comparison of survival and reproduction between 51 adults fitted with radio transmitters and an equal number that were captured and handled in the same way bitt released without a transmitter. For both sexes combined, 47 radio tagged botos (92.2%) survived at least three years after release compared with 42 (82.4%) without radios, equating to annual survival of 97.3% and 93.6% respectively. The difference was not statistically significant. Eight of 15 closely monitored radio tagged females were lactating at capture, and all their calves weaned successfully. Two that were pregnant at capture subsequently gave birth. The mean number of calves per year born to these 15 females after first release was 0.172 (SD=0.107) and to 17 non-tagged was 0.174 (SD = 0.095), again a non-significant difference. These results indicate that the anchoring of packages to the dorsal fin of dolphins can be accomplished with no measurable impact on their subsequent survival or reproductive output. However, botos may be unusually robust to handling, and this study should not be used to justify using similar techniques on other species without customary caution, diligence, and expert guidance. [ABSTRACT FROM AUTHOR]

In odontocetes the mandibular bone serves two functions: to capture prey, and as a means of the reception and transmission of sound waves through a fat body in the mandibular canal, which opens posteriorly as the mandibular foramen. The posterior part of the lateral wall of the odontocete mandible is thin, and appears to represent a compromise between a strong mandible for prey capture and a thin vibrating plate for hearing. We studied the intraspecific variation of minimum thickness of the lateral mandibular wall along four transects (T1-T4) at the area of the mandibular foramen, in relation to the skull size and the mandibular size in different-aged bottlenose dolphins, Tursiops truncatus (18 females between 1 and 42 yr, 17 males between 1 and 32 yr). The minimum thickness was absolutely at its lowest at the most posterior transect T1, but did not vary significantly between the sexes or between the ages. The minimum thickness varied significantly at the two most anterior transects, T3 and T4, both between the sexes and among the ages. The thickness increased throughout life among males, whereas in females it first increased and then starts to decrease around the age of 20. [ABSTRACT FROM AUTHOR]

Analysis of little-known manuscripts revealed that there have been at least two pre-Linnean descriptions of the South American freshwaterdolphin Inia geoffrensis (Blainville, 1817). The earliest one that we found was made by Frei Cristovao de Lisboa in a manuscript written around 1627. The second one was by Pehr Lofling, a disciple of Linnaeus, who wrote a very detailed and accurate description of this mammal in 1755. He used the binomial system to designate this species, andhis description was much more complete and sophisticated than the ones used by Linnaeus in the 10th edition of Systema Naturae for other cetaceans. This and other zoological work by Lofling remains almost completely unexamined to date. Like the outcome of other field work carried out by many Spanish scientists in America, failure to publish the findings of the expeditions resulted in scientific information being largely lost. [ABSTRACT FROM AUTHOR]

A boat survey was conducted from 5 to 26 June 1993 to estimate the abundance of the Amazon river dolphin (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) along ca. 120 km of the Amazon River bordering Colombia, Peru, and Brazil. Two survey methods were used: line transects during 5 d and strip transects during 15 d. The line transects were used to estimate the abundance of both species in the main channels of the Amazon at distances greater than 200 m from river banksand islands, and strip transects were used to estimate abundance in the remainder of the habitat. A total of 29 sightings was obtained using line transects, including 8 of Inia, 15 of Sotalia, and 6 with both species present. The total number of sightings made while using strip transects was 143, including 78 of Inia, 51 of Sotalia, and 14 with both species present. The distributions of sightings with respect to distance from the nearest bank were not significantly differentbetween the two species. Based on the results from the two methods,we estimate that there are 346 (CV = 0.12) Inia and 409 (CV = 0.13)Sotalia in the study area. Overall, the mean group size for Inia was2.9 individuals and for Sotalia was 3.9 individuals. Inia density (dolphin/km2) was highest in tributaries (4.8), followed byareas around islands (2.7) and along main banks (2.0); while Sotalia density was highest in lakes (8.6), followed by areas along main banks (2.8) and around islands (2.0). These are among the highest densities measured to date for any cetacean. [ABSTRACT FROM AUTHOR]