Sotalia Fluviatilis Research Articles

Riverine and marine ecotypes of Sotalia dolphins are different species

The current taxonomic status of Sotalia species is uncertain. The genus once comprised five species, but in the twentieth century they were grouped into two (riverine Sotalia fluviatilis and marine Sotalia guianensis) that later were further lumped into a single species (S. fluviatilis), with marine and riverine ecotypes. This uncertainty hampers the assessment of potential impacts on populations and the design of effective conservation measures. We used mitochondrial DNA control region and cytochrome b sequence data to investigate the spe- cific status of S. fluviatilis ecotypes and their population structure along the Brazilian coast. Nested-clade (NCA), phylogenetic analyses and analysis of molecular variance of control region sequences showed that marine and riverine ecotypes form very divergent monophyletic groups (2.5% sequence divergence; 75% of total molec- ular variance found between them), which have been evolving independently since an old allopatric fragmen- tation event. This result is also corroborated by cyto- chrome b sequence data, for which marine and riverine specimens are fixed for haplotypes that differ by 28 (out of 1,140) nucleotides. According to various species defi- nition methods, we conclude that marine and riverine Sotalia are different species. Based on priority criteria, we recommend the revalidation of Sotalia guianensis (Van Beneden 1864) for the marine animals, while riverine dolphins should retain the species name Sotalia fluviatilis (Gervais 1853), thus becoming the first exclusively riv- erine delphinid. The populations of S. guianensis show a strong subdivision (FST=0.628) along the Brazilian coast, with at least three evolutionarily significant units: north, northeastern and south/southeastern.

Social interactions between tucuxis and bottlenose dolphins in Gandoca-Manzanillo, Costa Rica

Studies measuring the extent of interspecific interactions between dolphin species are rare. We observed free-ranging tucuxis (Sotalia fluviatilis) and bottlenose dolphins (Tursiops truncatus) to document the frequency of occurrence of interspecific interactions relative to group size and behavioral state. We conducted opportunistic surveys in Gandoca-Manzanillo (9°36'N, 82°35'W), Costa Rica. Of the 71 groups analyzed, 46.5% were comprised only of tucuxis, 21.1% of bottlenose dolphins, and 32.4% of the two species. Social behavior was more frequent in mixed-species groups and in groups larger than four dolphins; foraging was more frequent in single-species groups and in groups smaller than five dolphins. Photographic documentation and sightings of putative hybrids suggest the occurrence of hybridization between both dolphin species. Results indicate that tucuxis and bottlenose dolphins interacted frequently and that these interactions were predominantly social in nature. Future studies will discern whether these interactions result in the development of hybrids.

Whistles of tucuxi dolphins (Sotalia fluviatilis) in Brazil: Comparisons among populations

A quantitative and comparative analysis of the acoustic whistles parameters of tucuxi dolphins (Sotalia fluviatilis) from eight areas in Brazilian waters were conducted. Tucuxi produced mainly upsweep whistles, and despite their similar characteristics to other delphinids, whistles were shorter and less complex in shape. Some signals had fundamental frequency components above 24 kHz, and a broader band system may be useful in further studies concerning tucuxi whistles. The frequency parameters of the whistles had the lowest variation coefficients, whereas the duration and number of inflections had the highest variation coefficients. About 65% of all whistles had common characteristics between study localities, but SF, MinF, MeF, F1/4, F1/2, and F3/4 increased in frequency from southern to northern populations. The magnitude of the whistle characteristics variation was comparatively smaller between adjacent sites areas than between nonadjacent ones. However, for some pairwise comparisons the whistles were more similar between nonadjacent areas than between adjacent ones, indicating that other factors besides genetic and mixing affected tucuxi whistle characteristics. Only a few numbers of whistle variables were significantly different between adjacent areas and the parameter ranges of whistles overlapped, which made it difficult to discriminate whistles between adjacent tucuxi populations.

Group characteristics of marine tucuxis (Sotalia fluviatilis) (Cetacea: Delphinidae) in Guanabara Bay, south-eastern Brazil

Thirty-six boat surveys were conducted in Guanabara Bay, south-eastern Brazil, from October 2002 to June 2004 in order to investigate marine tucuxi group characteristics. The average and the range of tucuxis group size in the Guanabara Bay are similar with areas where small group sizes have been recorded. Group size ranged between one and 40 individuals (13·0±9·5; median=10·0). Groups of two to ten dolphins were the most common (53·9% of observations). Group size and the maximum number of dolphins sighted in each survey day did not vary between seasons (Kruskal–Wallis test, H3,36=3·249; P=0·355). Spatial geometry varied with group size and mixed groups were the largest (Kruskal–Wallis test, H3,218=57·149; P<0·001). The presence of calves had a great effect on group size, and nursery groups (mean =14·3±9·1; median=13·0) were twice as large than non-calf groups (mean=7·1±5·2; median=6·0). It is suggested that larger groups may aid in the calves development and learning. Group size changed frequently, resulting in 82·2% of observations with no constant size. Group size of marine tucuxi was not affected by the dolphins' behaviour (Kruskal–Wallis test, H2,215=5·626, P=0·06) neither by water depth (R2=0·012; F1,219=2·82; P=0·094).

Sensitivity of a tucuxi (Sotalia fluviatilis guianensis) to airborne sound

Auditory systems of cetaceans are considered highly specialized for underwater sound processing, whereas the extent of their hearing capacity in air is still a point of issue. In this study, the sensitivity to airborne sound in a male tucuxi (Sotalia fluviatilis guianensis) was tested by means of a go/no go response paradigm. Auditory thresholds were obtained from 2 to 31.5 kHz. Compared to the hearing thresholds of other dolphins as well as of amphibian mammals, the sensitivity to airborne sound of the test subject is low from 2 to 8 kHz, with the highest threshold at 4 kHz. Thresholds at 16 and 31.5 kHz reveal a sharp increase in hearing sensitivity. Thus, although not obtained in this study, the upper aerial hearing limit is in the ultrasonic range. A comparison of the present data with the underwater audiogram of the same test subject referred to sound intensity indicates that the sensitivity of Sotalia to underwater sound is generally better than to airborne sound.

Biology of the marine tucuxi dolphin (Sotalia fluviatilis) in south-eastern Brazil

Age, growth and reproductive parameters related to the marine tucuxi are presented, as well as feeding habits and parasitism. The specimens' age ranged from zero (newborn) to 21 years for males and 0·5 to 30 years for females. In relation to the body dimension, length distributions were bell-shaped for both sexes with male marine tucuxi ranging from 86·0 to 200·0 cm in length and females from 117·5 to 198·0 cm. The body length of new-born and calves varied between 86·0 to 117·5 cm and the postnatal growth curve an asymptotic reached length of 191·0 cm. According to the relationship between age, body length and reproductive characteristics, male and female specimens were considered sexually mature when [ges ]6 years and body length [ges ]180·0 cm and [ges ]6 years and body length [ges ]160·0 cm, respectively. Males and females up to six years old represented around 80% of the captures, indicating a bias towards juveniles and individuals that have yet to reach sexual maturity. The youngest specimen with solid contents in the stomach was 119·0 cm in length and seven months old. The marine tucuxi feeds on neritic prey, preferentially on the teleost fishes Trichiurus lepturus and Porichthys porossisimus, and on the cephalopods Loligo sanpaulensis and L. plei. Back calculation of prey lengths indicated that fish ranged from 1·2 to 106·9 cm and cephalopods from 3·4 to 22·2 cm. The barnacle Xenobalanus globicipitis was recorded attaching to the caudal fin and the helminths Braunina cordiformis, Anisakis typica, Halocercus brasiliensis and Nasitrema sp. were found in the internal organs.

Home ranges and movement patterns of the marine tucuxi dolphin, <i>Sotalia fluviatilis</i>, in Baía Norte, southern Brazil

Marine tucuxi dolphins (Sotalia fluviatilis) studied in Baía Norte, southern Brazil, exhibited very small overall home ranges with daylight movements in consistent water depths of around 3m. Mean overall home range areas calculated through two methodologies were similar, measuring 13.38km² ± 1.92 using the Minimum Convex Polygon method and 15.22km² ± 0.66 through the kernel estimator. Mean core areas calculated by kernel were extremely small [mean = 1.49km² at 50% UD (utilization distribution) and 0.87km² at 25% UD levels]. The geographical distributions of home ranges and core areas overlapped extensively at both 50% UD and 25% UD. Daylight movements ranged from 0.16 to 28.97 km/day (mean = 5.65 ± 0.56km/day, SD = 5.36) with mean minimum rate of movement of 2.6 ± 0.2km/h (SD = 2.02). Daylight movement patterns varied seasonally with higher values of distance moved and rate of movement in winter and winter and autumn, respectively. The marine tucuxi home ranges and daylight movement patterns presented here are smaller than much of what is known for other coastal small cetaceans. The percentage of overall home range within the limits of a protected area designated for the tucuxis could be considered moderate to high depending on the estimator used (54.06% or 5.9km² by MCP and 74.71% or 11.35km² using the kernel). Nevertheless, core areas are completely within the protected area. Conservation implications of these findings include the need to: (i) create a buffer zone to the south of the existing protected area which will encompass the overall home range of the tucuxis; (ii) establish constant, adequate enforcement of fishery and boat traffic regulations in the core areas; and (iii) regulate boat traffic and aquaculture farming in important areas which are not yet subject to specific regulations.

Notes on the distribution of tucuxi, <i>Sotalia fluviatilis</i> (Cetacea: Delphinidae), in Ecuadorian Amazonia

Sotalia fluviatilis, commonly known as tucuxi, is widely distributed in the main tributaries of the Amazon Basin in northern Brazil, Peru, Ecuador, and Colombia (Borobia et al., 1991; Jefferson et al., 1993; da Silva and Best, 1996; Nowak, 1999). In Ecuador, tucuxi is an elusive species with apparently low population densities (Denkinger, 2001) and evidence of its geographical distribution is patchy (Utreras et al., 2001). Little is known about its habitat requirements, movement patterns or population dynamics (Utreras, 1996; Utreras et al., 2001). This paper reports occasional sightings of tucuxis made by different observers, including the authors, during the last 15 years in the Ecuadorian Amazon, in an attempt to add new information on the occurrence of the species in Ecuadorian waters. In Ecuador, S. fluviatilis has been recorded to date as far north as the Putumayo River along the Colombian border. The southernmost records include the Morona and Mangosiza Rivers, near the southeastern Peruvian border. The majority of records come from the Putumayo, Aguarico, Napo and Yasuni Rivers (Table 1). This, however, can reflect higher observation effort in those areas. S. fluviatilis records for

Analysis of whistles produced by the Tucuxi Dolphin Sotalia fluviatilis from Sepetiba Bay, Brazil.

From July 2001 to June 2002, we recorded a total of 2 h 55 min of Tucuxi Dolphin Sotalia fluviatilis vocalizations from Sepetiba Bay, Brazil (22 degrees 35'S-44 degrees 03'W). A total of 3350 whistles were analyzed quantitative and qualitatively and were divided into 124 types, by visual inspection of sonograms. The following parameters were measured: Initial Frequency, Final Frequency, Minimum Frequency, Maximum Frequency, Duration, Number of Inflections, Frequency at the Inflection Points, Frequency at 1/2, 1/4, and 3/4 of whistle duration, Presence of Frequency Modulation and Harmonics. Ascending type whistles (N = 2719) were most common, representing 82% of the total. Dolphin behavior and average group size observed during recording influenced the whistle's quantitative and qualitative parameters. The results demonstrate the great diversity of whistles emitted and indicate a functional role of these vocalizations during the observed behaviors.

RIVERINE HABITAT PREFERENCES OF BOTOS (INIA GEOFFRENSIS) AND TUCUXIS (SOTALIA FLUVIATILIS) IN THE CENTRAL AMAZON

AbstractThe distribution and density of the Amazon's two contrasting endemic dolphins–boto, or Amazon river dolphin, Inia geoffrensis, and tucuxi, Sotalia fluviatilis–were examined on two adjoining large rivers in western Brazil. Using a 17‐m river boat as a sightings platform, strip transects were used to cover areas within 150 m of the river margin and line transects were used in all other areas. Highest densities of both dolphins occurred near the margin, and lowest in the center of rivers. Seven different habitats were identified along river margins. The boto and the tucuxi differed in some elements of habitat choice, but they shared a preference for areas with diminished current and where two channels joined. Neither species favored the two most common edge types in this region of the Amazon‐mud banks and flooded forest margins. Overall, the most preferred habitat type was the least common, and known as “meeting of the waters.” In these areas a channel of sediment‐rich white water meets one carrying acidic black water; the resultant mixing produces particularly productive, and obviously attractive, conditions for dolphins. These results demonstrate that Amazonian dolphins selectively occur in areas known to be favored for gill net deployment by local fishermen, and may explain why entanglement is apparently a common cause of mortality.

Interactions between gillnet fisheries and small cetaceans in northern Rio de Janeiro, Brazil: 2001-2002

In northern Rio de Janeiro (21°18'S-22°25'S), Brazil, gillnets are responsible for the by-catch of a number of small cetaceans. Franciscanas ( Pontoporia blainvillei ) and marine tucuxis ( Sotalia fluviatilis ) are the most impacted species. From November 2001 to October 2002 data on fisheries and mortality of small cetaceans were obtained through interviews with fishermen of 20% of the gillnet fleet. Seasons were grouped into spring-summer and autumn-winter and the region was divided into two fishing areas: Area I (Barra do Itabapoana to Sao Tome Cape) and Area II (Sao Tome Cape to Macae). A total of 374 gillnet operations were recorded between 0.02-42.1n.miles from shore and in waters 5-67m deep. Gillnets were 3.1m in length and of 120mm mesh size. The fishing effort for the entire fleet (n=50 boats) was estimated at 7,161.8km of net and the total number of small cetaceans caught was 225. The CPUE was 0.031 animal × (km of net × day) -1 . Fisheries and by-catches did not show seasonal differences. Fishing trips were more prevalent in Area I, whereas the by-catch of franciscanas and marine tucuxis was higher in Area II. Fisheries were distributed uniformly in areas close to shore (first 10n.miles) and beyond. In Area I they occurred predominantly within the 30-m isobath. The by-catch of franciscanas and marine tucuxis also occurred within these limits, suggesting that 30-m isobath may constitute the offshore limit of their distribution along the coast. The variability in the width of the continental shelf in Areas I and II may explain the differences in the magnitude of the by-catch of small cetaceans in these areas. Gillnet fisheries may adversely impact the coastal populations of small cetaceans. In northern Rio de Janeiro, particularly between 22°00'S and 22°25'S where the 30-m isobath is found close to shore, fisheries operating within the first 10n.miles from shore can pose substantial threat to dolphins. Due to the distribution of their preferred prey, franciscanas may be at greater risk of by-catch in areas near the Paraiba do Sul river mouth. It is thus proposed that these areas be closed to gillnet fisheries year round, so that the impact on coastal dolphins is minimised.

Chlorinated pesticides and polychlorinated biphenyls in marine tucuxi dolphins ( Sotalia fluviatilis) from the Cananéia estuary, southeastern Brazil

The Cananéia estuary is an important biological area on the southeast coast of Brazil. In the past, it was impacted by both chlorinated pesticides and polychlorinated biphenyls (PCBs) due to its natural location. The marine tucuxi dolphin ( Sotalia fluviatilis) is a top predator in this ecosystem and can be found year round in Cananéia estuarine waters that represent an important nursing area for the species. This work investigated chlorinated compounds in the blubber of nine individuals from the Cananéia estuary. Residue levels of DDTs (0.541–125 μg g −1 lipid wt.) were the highest, followed by PCBs (0.2–9.22 μg g −1 lipid wt.), mirex (0.014–0.312 μg g −1 lipid wt.), chlordanes (0.001–0.047 μg g −1 lipid wt.), HCHs (<0.003–0.044 μg g −1 lipid wt.), and HCB (n.d.−0.024 μg g −1 lipid wt.). The mean p, p′-DDE/Σ DDT ratio was approximately 0.8 and is indicative of the former DDT application in the study area. PCB contamination is suggested to be associated with atmospheric transport and relative proximity to the Cubatão industrial complex—the most important along the Brazilian coast. Low levels of HCHs and HCB can be attributed to their high volatility in tropical environments. Concentrations of organochlorines in the blubber of marine tucuxis from the Cananéia estuary were lower than levels found in small cetacean species from developed countries, where the input of these compounds was considerably greater than in Brazil. At extremes, male dolphins can present DDT burden several orders of magnitude higher than females. Despite the high levels of total DDT found in males, the major detected compound was p, p′-DDE which is considered to be of low toxicity.

Concentrations of heavy metals in Sotalia fluviatilis (Cetacea: Delphinidae) off the coast of Ceará, northeast Brazil

Lead (Pb), cadmium (Cd) and mercury (Hg) concentrations on liver and kidney of Sotalia fluviatilis (Cetacea: Delphinidae) stranded in the coast of Ceará, Brazil, were studied from 1996 to 1999. Pb levels were usually lower than the detection limit (0.1 μg/g). Concentrations of Cd were significantly higher in kidney than liver, averaging 0.8 μg/g. Mercury accumulation took place mainly in liver with an average concentration of 4.6 μg/g. Both metals were significantly higher in larger mature individuals, but differences between sexes were not significant. The detection of Cd, Hg and Pb in tissue samples of S. fluviatilis off the coast of Ceará indicated that heavy metals are locally available in the water, and bioaccumulation may be occurring through the food web. Contamination levels were not considered critical, but could be related to Ceará's growing industrial development. The associated risks of pollution outfalls may pose a threat to marine organisms in a near future, especially for top predators such as S. fluviatilis.

Age estimation of marine tucuxi dolphins (Sotalia fluviatilis) in south-eastern Brazil

This study reports the age estimation of marine tucuxi dolphins (Sotalia fluviatilis) found dead along the south-east coast of Brazil (24°40′S–25°30′S). The investigated specimens were collected from August 1995 to December 1998. Ages were estimated through the number of Growth Layer Groups (GLGs) from decalcified and stained thin sections of teeth. A total of 36 individuals was investigated. Estimated ages of marine tucuxis varied from zero to 29 years. The growth curve was attained from a modification of the von Bertalanffy equation applied to body length and age data. It was possible to estimate the total length of 97·8 cm in newborns, physical maturity was attained at seven years of age, and an asymptotic length of 179·8 cm.

Vocalizations of Amazon River Dolphins, Inia geoffrensis: Insights into the Evolutionary Origins of Delphinid Whistles

AbstractOceanic dolphins (Odontoceti: Delphinidae) produce tonal whistles, the structure and function of which have been fairly well characterized. Less is known about the evolutionary origins of delphinid whistles, including basic information about vocal structure in sister taxa such as the Platanistidae river dolphins. Here we characterize vocalizations of the Amazon River dolphin (Inia geoffrensis), for which whistles have been reported but not well documented. We studied Inia at the Mamirauá Sustainable Development Reserve in central Brazilian Amazônia. During 480 5‐min blocks (over 5 weeks) we monitored and recorded vocalizations, noted group size and activity, and tallied frequencies of breathing and pre‐diving surfaces. Overall, Inia vocal output correlated positively with pre‐diving surfaces, suggesting that vocalizations are associated with feeding. Acoustic analyses revealed Inia vocalizations to be structurally distinct from typical delphinid whistles, including those of the delphinid Sotalia fluviatilis recorded at our field site. These data support the hypothesis that whistles are a recently derived vocalization unique to the Delphinidae.

Insights on site fidelity and calving intervals of the marine tucuxi dolphin (Sotalia fluviatilis) in south-eastern Brazil

Ecological aspects of the marine tucuxi dolphin, Sotalia fluviatilis (Cetacea: Delphinidae), remain poorly known. Important information can be gathered in long-term studies using photo-identification. Using this methodology, the authors present the first evidence of site fidelity for marine tucuxis in the Cananéia Estuary (25°03′S 48°01′ W), south-eastern Brazil. A total of 86 easily recognizable individuals was catalogued from June 1996 to August 2001, but most of them have been sighted from May 2000. From June 1997 to August 2001, three female tucuxis were sighted on nine (KN #19), 22 (KN #30) and 28 (KN #10) different days in a relatively small area (∼16 km2). Each of them was observed with a calf in 1997–1998 and again with a new offspring in 2000–2001. Thirteen other individuals were reported on eight to 19 different days in the same area between May 2000 and August 2001. Our observations lead to conclusions on the evidence of site fidelity and indicate that calving intervals may range between two and three years for this species in the surveyed area.

Sightings of Pontoporia blainvillei (Gervais &amp; D' Orbigny, 1844) and Sotalia fluviatilis (Gervais, 1853) (Cetacea) in South-eastern Brazil

Pontoporia blainvillei (Gervais &amp; D' Orbigny, 1844) and Sotalia fluviatilis (Gervais, 1853) have typically coastal habits and are sympatric in South-eastern Brazil. The purpose of this work was to record sightings and describe aspects concerning the behaviour of both species in this region, between 1993 and 1998. The sightings were accomplished by cruises and from a fixed point. Information provided by fishermen were also considered. In general, P. blainvillei was observed in all seasons of the year, up to five nautical miles away from the coast, up to 15 m deep and the groups comprised up to five specimens. The most part of the groups of S. fluviatilis, were observed in autumm and winter times and comprised up to 10 specimens. In about half of the sightings, the presence of calves and/or juveniles was noted. The cooperative (group) fishing was the foranging behaviour most frequently noted for S. fluviatilis.

Cephalopods in the diet of marine mammals stranded or incidentally caught along southeastern and southern Brazil (21–34°S)

Cephalopod remains found in 286 stomach contents of 13 species of odontocetes ( Pontoporia blainvillei, Lagenodelphis hosei, Sotalia fluviatilis, Stenella frontalis, Steno bredanensis, Tursiops truncatus, Delphinus sp., Globicephala melas, Orcinus orca, Pseudorca crassidens, Kogia breviceps, K. sima and Physeter macrocephalus) four species of pinnipeds ( Arctocephalus australis, A. gazella, A. tropicalis and Mirounga leonina) were identified and measured. The stomachs were collected from stranded or incidentally caught marine mammals from Rio de Janeiro to Paraná states (21–26°S) and Rio Grande do Sul (29–34°S), between 1985 and 1998. Twenty-five species of 16 families of cephalopods were identified. Cephalopod prey were small to medium sized. The diversity of cephalopods as prey was lower for coastal marine mammals and increased in offshore species that fed on diverse oegopsin squids including both the fast moving muscular squids and the less mobile neutrally buoyant cephalopods. In common dolphins, ommastrephid and loliginid squids and the sepiolid Semirossia tenera, were all important in the diet. Loliginid squids were the most frequent cephalopod in the diet of six other species of dolphins and three species of fur seals. Loligo sanpaulensis was recorded in specimens from throughout the study area, whilst Loligo plei and Lolliguncula brevis were frequent in the northern area. Benthic octopuses were found only in the diet of bottlenose dolphin and franciscana. Pelagic octopuses, particularly Argonauta nodosa, were only relatively frequent in the stomach contents of franciscana. In the stomach contents of larger odontocetes, as well as in Southern elephant seals and SubAntarctic fur seals, squids of the families Ancistrocheiridae, Chiroteuthidae, Cranchiidae, Enoploteuthidae, Histioteuthidae, Lycoteuthidae, Octopoteuthidae, Onychoteuthidae and especially Ommastrephidae were found.

Bone lesions in Sotalia fluviatilis (Cetacea) as a consequence of entanglement: Case report

The objective of the present study was to describe a set of lesions caused by entanglement of Sotalia fluviatilis accidentally captured by gillnet on July, 23, 1995, in the Northern Rio de Janeiro State (21o37'S-041o01'W), Southeastern of Brazil. The specimen presented lacerated lesions of the skin and subcutaneous tissue around the posterior extremity of the rostrum. Nylon twines were found associated to these lesions. Bone and teeth lesions were also observed in the underlying area and affected the maxillaries, pre-maxillaries and mandibles. The lesions were likely caused by non-lethal entanglement in fishing gears. Abnormalities in the growth layer pattern of the damaged teeth between the third and sixth growth layers suggest that this encounter had taken place when the specimen was around three years old. The data indicate that non-lethal encounters with fishing gears may cause serious health problems to small cetaceans and reduce their lifetime. This is the first description of fisheries-related chronic lesions in S. fluviatilis.

Low-frequency sounds emitted by Sotalia fluviatilis guianensis (Cetacea: Delphinidae) in an estuarine region in southeastern Brazil

Recordings of the vocalizations of the dolphin Sotalia fluviatilis guianensis were made in an estuarine complex at Cananéia in southeastern Brazil over a period of 10 years (1989–1998). This area is characterized by mangroves and the constant presence of dolphins. Recordings were obtained at depths of 2–4 m using digital and analog recorders at a speed of 19 cm/s. Four classes of sounds were identified. "Whistles," which are used in social activities, occurred with the greatest frequency (χ2 = 58.92, df = 3, P &lt; 0.001). "Calls," which were very variable in form, were the second most common class used by family groups (χ2 = 10.96, df = 2, 0.005 &gt; P &gt; 0.001). There were no differences in the rates at which schools emitted whistles and calls (χ2 = 2.12, df = 1, 0.25 &gt; P &gt; 0.10). "Gargles" were apparently emitted by calves and were similar in structure to a low-frequency call. The fourth class, "clicks," are used in echolocation. Clicks varied considerably in their frequency of occurrence and frequency of emission, and were not always detected. There were significant differences in emission rates among the four classes (χ2 = 18.73, df = 3, P &lt; 0.001). In addition, which class of sound was emitted depended on the type of activity exhibited by the dolphins (displacement, fishing, social) and on the social structure (family or school) adopted.