Accurate demographic data play a critical role in implementing conservation strategies and identifying key areas for species preservation. The population abundance and density of Sotalia guianensis were estimated in the estuarine-lagoon complex of Cananéia, Brazil, using distance sampling. The survey covered 1339.91 km with 83 h 05 min in effort. A half-normal model with cosine adjustments was evaluated as the best fit to estimate an abundance of 193 individuals (95% CI 158–237) and a density of 2.55 ind km−2. The majority of sightings occurred in the Baía de Trapandé (48.72%), followed by the Mar de Cananéia (32.72%) and Mar de Cubatão (18.56%), the three areas surrounding the Ilha de Cananéia. The study confirmed the heterogeneous distribution of the estuary and found that the Mar de Cananéia and the Baía de Trapandé were more densely populated, with 2.76 ind km−2 (95% CI 1.93–3.96) and 2.76 ind km−2 (95% CI 2.07–3.66), respectively, while the Mar de Cubatão was less densely populated, with 1.59 ind km−2 (95% CI 1.04–2.44). The findings support previous research indicating a stable population over the last few decades. The Cananéia estuary is an ecologically diverse region located between protected areas under different categories of environmental protection and harbours a significant population of S. guianensis, providing essential resources for feeding and breeding. Protected areas have proven to be effective tools for preserving both marine and terrestrial environments. Despite the close proximity to humans and constant threats, the study underscores the importance of the area for the conservation of the species.

This study investigated the emission of subaquatic noise from recreational tourism motorboats, schooners and a sea-bottom mounted water pump. Analyses demonstrated alterations in several whistle (IF: t = 2.42, P = 0.015; FF: t = −2.22, P = 0.025) and calls patterns (MIF: t = −3.13, P = 0.001; MAF: t = −3.49, P = 0.0005; FD: t = −2.21, P = 0.027; D: t = 2.89, P = 0.004), caused primarily by motorboats. Duration of clicks was also modified (D: t = −3.85, P = 0.0001), mainly by the water pump. The frequency range of all noises (0.43–35.8 kHz) overlaps that used by dolphins (1–48 kHz), causing sound emissions changes, with a considerable increase in number of whistles and a reduction in clicks trains. These changes may be a strategy developed by these dolphins to overcome the noise band. Mitigation measures, such as boating regulations and environmental education for the local community, boaters and tourists are needed to conserve the species. The Guiana dolphin population is apparently already suffering, evidenced by diminished residence time and reduced number of individuals entering the inlet during the presence of pleasure craft.

Niche differentiation is the process by which species evolve different forms of resource use, and is used to explain the co-occurrence in a variety of habitats. The Bayesian framework of isotopic niche through quantitative niche metrics was applied to estimate and compare the niche breadth of two sympatric coastal dolphins Pontoporia blainvillei and Sotalia guianensis in a tropical marine area. The standard ellipse areas (SEAs) based on species were quite similar, but the SEAs based on age class showed that the matures’ niche space is larger than the immatures’ for both dolphins. A probabilistic comparison of SEAs indicated that specific differences are negligible compared with age class differences. Trophic level measures (δ15N range) indicated that the dolphins are comparable as top predators, and that immature specimens have a lower range of trophic levels than mature ones. In terms of variability of food sources (δ13C range), S. guianensis showed a larger value than P. blainvillei and mature specimens had larger δ13C range than immatures for both species. In general, P. blainvillei and S. guianensis were similar in the niche metrics, with SEAs overlap of 52.1 and 39.7%. The immature specimens showed reduced isotopic niche overlap between species (<3%). In conclusion, Pontoporia blainvillei and S. guianensis specimens have similar isotopic niches, but pronounced differences between immature and mature specimens, both intraspecific and interspecific. Isotopic niche and quantitative metrics along with previous data on stomach contents provide a strong representation of species niche and their relationships.

Morphometric characteristics of the periotic–timpanic bone complex in the middle ear of cetaceans, are effective characteristics in evaluating systematics. However, they have not been used for studies of geographic variation regarding dolphins of the genus Sotalia. This study aimed to compare the periotic–timpanic of Sotalia guianensis from four distinct locations, considered here as different operational taxonomic units, Amapá/Pará (AM/PA), Maranhão/Piauí (MA/PI), Ceará (CE), and Rio de Janeiro (RJ), using 21 morphometric measurements. Multivariate analysis showed significant distinction mainly between the units of northern (AM/PA and MA/PI) and south-eastern (RJ) Brazilian coast. The timpanic bone showed variation, reaching larger sizes in the Brazilian south coast unit, corroborating current molecular data on the geographic variation of S. guianensis.

Two distinct species have been recently recognized for the genus Sotalia: S. fluviatilis, occurring in the Amazon River basin, and S. guianensis, from Honduras (15°58′N and 85°42′W) to Santa Catarina State (Florianópolis, southern Brazil—27°35′S and 48°34′W). For the first time the sternum and the appendicular skeleton of the two species of the genus Sotalia are compared. A comparative osteological work was performed with marine samples (from the States of Ceará, north-eastern and Santa Catarina, southern regions of Brazil) and riverine samples (Amazonas State) in relation to metric characters (scapula, flipper and sternum). There was a clear distinction of two species in relation to postcranial skeleton in the morphometric analysis (canonical variate analysis) presented. The flipper and the glenoid cavity of the scapula were proportionally wider in the fluvial species. The sternum, however, was smaller in this species in relation to the maximum width of the manubrium. Nevertheless, this structure still needs to be further studied.

This study presents new information on feeding habits of Guiana dolphins, Sotalia guianensis, in south-eastern Brazil, together with new regression equations to evaluate the weight and length of fish from otoliths, showing an overview on the knowledge about this species’ diet in this area. Eighteen stomach contents had been analysed and compared to 180 samples collected in another eight feeding studies. The analysed specimens were either incidentally caught in gillnets used in coastal waters by the fleet based in the Cananéia main harbour (25°00′S 47°55′W), south of São Paulo State, or found dead in inner waters of the Cananéia estuary between 2003 and 2009. Based on the index of relative importance analysis, the most important fish species were the banded croaker, Paralonchurus brasiliensis. Doryteuthis plei was the most representative cephalopod species. Stellifer rastrifer was the most important fish species observed in dolphins in inner estuarine waters and P. brasiliensis in recovered dolphins from coastal waters. Loliguncula brevis is the only cephalopod species reported from dolphins found in inner estuarine waters up to date. Doryteuthis plei was the most important cephalopod species observed in coastal dolphins. When considering other feeding studies, the most representative fish family in the diet of S. guianensis was Sciaenidae, which is mainly represented by demersal fishes. The main preys of S. guianensis are abundant in the studied areas, which may indicate an opportunistic feeding habit. The majority of them are not the most important target species by the commercial fishery in south-eastern Brazil.

Home ranges of seven Guiana dolphins (Sotalia guianensis) (Van Bénéden, 1864) were studied in the Cananéia estuary (~25°03′S 47°55′W), south-eastern Brazil. Boat-based observations were conducted from May 2000 to July 2003 in ~132 km2 of protected inner waters. The photo-identification technique was used to follow naturally marked individuals through time and space. From a total of 138 catalogued individuals, five males and two females presented 20+ sightings and were used for home range estimation. Sightings were plotted and analysed in a Geographic Information System (GIS). With the ‘Home Range Tools’ extension the fixed kernel density estimator with band width (smoothing parameter) chosen via least squares cross-validation was performed for each individual. The fixed kernel method was used to estimate the non-parametric utility distribution of each dolphin, keeping band width (h) constant for a data set. The first polygons created by these parameters had an amoeboid shape and in some cases more than one centre of activity. The 95% home range estimated outlines varied from 1.6 to 22.9 km2 (7.9 ± 8.3 km2). This large interval shows strong evidences on individual variation in S. guianensis' home ranges. Several individuals showed small home ranges when compared to other cetacean species. An overlap of home ranges of different sizes and shapes were observed for Guiana dolphins with large range movements. Centres of activity were concentrated in the main entrance of the Cananéia estuary. This was a first attempt to understand the way S. guianensis uses the Cananéia estuary and such data are essential for conservation and management purposes.

Between August 2005 and July 2007, 376 Guiana dolphin (Sotalia guianensis) encounters were documented in Sepetiba Bay. For the purpose of this study all dolphin sightings were classified as groups or aggregations. Groups (N = 268, x = 20, range = 1–90) consisted of dolphins in apparent association usually engaged in the same behaviour and moving in the same direction. Aggregations (N = 108, x = 204, range = 100–450) consisted of temporal association of numerous groups gathered in an area, totalling several hundred individuals usually engaged in the same behaviour but not necessarily moving in the same direction. Guiana dolphin groups and aggregations differed significantly in behavioural activity, spatial and temporal distribution and age composition. Most aggregated dolphins were engaged in foraging/feeding activities and primarily located at the interior of the bay, and also, aggregations were more likely recorded during morning hours and had more neonates and calves present when compared to groups. The plotted GPS records of aggregations formed two distinct clusters of distribution within the bay, indicating potential areas where conservation strategies could be implemented.

To determine the group structure of Sotalia guianensis on the coast of Paraná State, two bays, Baía de Paranaguá and Baía de Guaratuba, were surveyed between March 1999 and February 2000 and between July 2002 and June 2003 respectively. The heterogeneity of the studied areas was considered a relevant factor, therefore these areas were sectored and each sector was investigated independently. The surveys were carried out through boat line transects and all the animals were registered, even those localized between two transects. The records were taken for the quantity of groups as well as for the amount of individuals in each group and, whenever possible, the individuals were classified as calves or adults. The data were treated both separately and comparatively between the two bays and amongst the sectors of each bay. In Baía de Paranaguá, 735 individuals were registered, divided into 253 groups which varied from 2 to 37 individuals for an average of 2.9 individuals per group. Most of the groups presented a family formation (59.29%) and were observed during the morning. The individuals that were clearly identified as calves represented 24% of the total (176 individuals), adults 55% (405 individuals) and 21% (154 individuals) were impossible to determine. In Baía de Guaratuba, only 32 S. guianensis individuals were observed and distributed into 14 groups for an average of 2.13 individuals per group. These groups varied from 2 to 6 individuals mostly in family formations (50%). The same quantity of individuals and groups was observed during different periods of the day. Of the 32 sampled individuals, 25 (78%) were adults, only one was a calf (3%) and 6 (19%) were undetermined.

The present study demonstrates the influence of the time of the day and tide on the frequency of individuals and on the social and foraging behaviour of an equatorial population of Sotalia guianensis. A population of S. guianensis from enseada do Curral (6°13′00″S 35°3′36″W), north-eastern Brazil, was investigated from October 1999 to September 2003. Social and foraging activities oscillated significantly, being less intense at high tide in comparison to other analysed tides. Socialization was more intense between 1000 and 1300 hours and foraging was more intense in between tides, than at high tide. No diurnal or tidal fluctuation was found on the frequency of individuals, which remained constant throughout the day. These results demonstrate the influence of circadian factors on an equatorial population which is highly impacted by human activities and highlights a need for protective measures.

During a monitoring programme of stranded marine mammals conducted along the north-east coast of the Rio de Janeiro State, Brazil, we collected marine tucuxi dolphins (Sotalia guianensis) stranded along the beach. We examined the carcasses to identify characteristic evidences of fishery by-catch. From July 2001 to August 2007 43 S. guianensis were found in the study area, with an annual mean of 6.1 stranding events. Dolphins stranded with evidence of fishery interaction (‘by-catch’) occurred in almost every year (annual mean = 2). The majority of the by-catch events occurred during winter and spring. Most of the stranded dolphins were sexually mature. From 43 S. guianensis stranded from 2001 to 2007, 32.5% (N = 14) of these presented evidence of by-catch. Twenty-nine other dolphins did not show clear evidence of by-catch or it could not be evaluated. Most of the by-caught dolphins occurred in the northern zones of the study area where gill-net use is intense.

We present the result of a milk analysis collected from a dead female of Sotalia guianensis (191 cm) incidentally captured in gill-net fishery together with its calf (70 cm) in November 2002, in Barra de São João, east coast of Rio de Janeiro State, Brazil. The organochlorine pesticides with the highest concentration were endosulfan1 (5.86 ng g–1), followed by p-p′DDT (4.89 ng g–1), dieldrin (2.55 ng g–1) and HCB (2.41 ng g–1). These results show the high environmental persistency of these contaminants, and the transference capacity from the mother to the calf during lactation.

Guanabara Bay constitutes the most degraded habitat along the marine tucuxi dolphin (Sotalia guianensis) distribution. Considering that these mammals are found year-round in this area, where the dolphin population is estimated to be approximately 70 individuals, interactions between cetaceans and human activities are of concern. From 1995 to 2005, photo-identification techniques have been employed to study marine tucuxis in Guanabara Bay and data on the individual injuries has been collected. Seven out of 78 photo-identified and catalogued individuals presented noticeable injuries on their bodies, including cut-like wounds, skin ulceration and mutilation. Parts of fishing gear attached to the bodies of some dolphins were also observed. The concern elicited by the injuries reported here is amplified if we consider that this cetacean population is highly exposed to contaminants of known immunosuppressive action, such as organochlorine, organotin and perfluoroalkyl compounds.

Individual identification using Hi8 video images was tested on the population of estuarine dolphins, Sotalia guianensis that inhabit the Cananéia estuary, south-eastern Brazil. Between June 2001 and August 2002, 25 different dolphins were identified by nicks on the dorsal fin (80%) and by the association of cuts and scratches on the dorsal fin and the body's dorsal region (20%). The existent marks were used to create an identification key to ease the matching between dolphins. The animals were present on 100% of the sampled days, and some of them were repeatedly registered at the same sector in the area, suggesting that although they occupy an extensive area, they use it in an intended way. When compared to researches that used photographs to identify cetaceans, the video images showed some loss of information, nevertheless the analysis could be done without impairment of results, therefore this method proved to be efficient.

The coloration pattern of the estuarine dolphin, Sotalia guianensis, does not display variations in its distribution and the colour grey, as its common name suggests, is predominant. On the coast of the state of Rio Grande do Norte a dolphin was sighted with atypical coloration in relation to animals from that region. After observing its morphological characteristics, it was verified that the animal was a white specimen of S. guianensis. Although photographic recordings were made, the reddish coloration of the eye cannot be confirmed, a characteristic that would identify the animal as an albino dolphin. Despite not being very frequent, cases of coloration anomalies such as albinism have been previously recorded in 20 species of cetaceans. However, there have been no cases recorded for the estuarine dolphin.

During a photo-identification survey for a study of area use and social organization of marine tucuxi dolphins (Sotalia guianensis) in an estuarine area in south-eastern Brazil, a remora or discfish was recorded attached to one dolphin in a group of 25. The record place (water salinity 28 psu at the record time) is about 23 km from the closest connection between the estuary and the open sea. The discfish was identified as a sharksucker (Echeneis naucrates). This is the first record of a remora on a marine tucuxi dolphin. Besides adding a new host to the cetacean species which are known to carry remoras, our record strengthens the recent suggestion that sharksuckers use coastal dolphins as hosts.

This study tested the association between variation in precipitation and behavioural seasonality in Sotalia guianensis, at the Curral inlet (6º13′00″S 35º3′00″W) in north-eastern Brazilian coast. From October 1999 to September 2003, observations were carried out four days a month. Behaviours were registered instantaneously each five minute interval. The observation period consisted of six hours into the daytime (0600 to 1700 h). Two observers inspected the area simultaneously, from a fixed location at the top of the cliff. During June to November, socialization (MdJN = 1.524; MdDM = 0.667; U = –2.848; P = 0.004) and forage (MdJN = 3.268; MdDM = 1.762; U = –2.881; P = 0.004) were significantly higher than from December to May, although the number of individuals (MdJN = 6.000; MdDM = 5.000; U = –1.395; P = 0.163) did not change over the year. Such behavioural seasonality was not associated with dry/rainy periods (number of individuals: MdRS = 5.000; MdDS = 6.250; U = –1.672; P = 0.095; socialization: MdDS = 0.947; MdRS = 1.464; U = –0.858; P = 0.391; and forage: MdDS = 3.017; MdRS = 2.292; U = –1.506; P = 0.132). These results support behavioural seasonality in an equatorial odontocete population.