Population density of Sotalia guianensis (Cetacea: Delphinidae) in the Cananéia region, Southeastern Brazil
Liisa Havukainen1,2, Emygdio Leite de Araujo Monteiro Filho1,3 & Gislaine de Fatima Filla1
1. Instituto de Pesquisas Cananéia, IPeC, R. Tristão Lobo 199, CEP 11990-000, Cananéia, São Paulo, Brazil; www.ipecpesquisas.org.br, gica_filla@yahoo.com.br 2. Universidade de Mogi das Cruzes, UMC, Av. Dr. Cândido Xavier de Almeida Souza 200, CEP 08780-210, Mogi das Cruzes, São Paulo, Brazil; liisabio@yahoo.com.br 3. Departamento de Zoologia, Universidade Federal do Paraná, UFPR, Setor de Ciências Biológicas, Departamento de Zoologia, Caixa Postal 19020, CEP 81531-980, Curitiba, Paraná, Brazil; elamf@ufpr.br Dirección para correspondencia
Abstract
Population density in cetaceans can be estimated through photo-identification, mark-recapture, land-based observations and visual estimative. We the aim to contribute with conservation strategies, we used line transects (distance method) to estimate the population density of the river dolphin, S. guianensis, in the estuarine region of Cananéia, Southeastern Brazil. The ]]>
Key words: ]]>
Resumen
El delfín estuarino S. ]]>
, habita en aguas tropicales costeras y estuarinas. A pesar de su amplia distribución no se conoce suficiente, por lo tanto, recientemente se han intensificado sus estudios poblacionales. Transectos de línea (Método Distancia) fueron utilizados para estimar la densidad de población de S. guianensis en la Bahía Trapandé, región de estuario de Cananéia, Sudeste de Brasil. El muestreo aleatorio se realizó en tres sectores de la bahía desde mayo 2003 ]]>
Palabras clave: densidad de población, transectos en línea, delfines, estuario Cananéia, conservación.
The estuarine dolphin, S. guianensis, inhabits tropical coastal and estuarine waters (Carvalho 1963). In the Western Atlantic Ocean the species can be found as far South as off the coast of Florianópolis in Southern Brazil to as far north as Honduras. Despite its large distribution, in 2001 the genus Sotalia, which also includes the species Sotalia fluviatilis, was ]]>
et al. 2008).
Several methods have been used to estimate cetacean population densities. These methods include mark-recapture, land-based observations and quadrat and transect sampling (Odum 1988, Bonin et al. ]]>
et al. 1979, Gaskin 1982, Bonin et al. 2008). When using a transect method, the sampling is made randomly, and it also considers weather and ocean (fluvial) conditions, as well as the level of experience from the observer and the species behavior (Gaskin 1982, Bonin et al. 2008).
Previous studies on cetacean population densities have given important preliminary information from Brazilian areas: these included the species of Tursiops truncatus, commonly known as the bottlenose dolphin (Leatherwood 1979, Barham et al. 1980, Barco et al. ]]>
S. fluviatilis and the pink river dolphin, Inia geoffrensis (Magnusson et al. 1980, Vidal et al. 1997).
Along with distance methods, aerial surveys have also been carried out to estimate the population density ]]>
Stenella attenuata, Stenella longirostris and Delphinus delphis) during feeding activities off the Mexican and Central American coasts (Smith 1981). Aerial surveys have also been used in previous studies to analyze the movement of cetaceans off California coast (Forney & Barlow 1998) and population density of humpback whales, Megaptera novaeangliae, off the Brazilian coast between 2002 and 2005 (Andriolo et al. ]]>
For the estuarine dolphin, S. guianensis, four population density estimations are currently known. Edwards & Schnell (2001) studied the species in the Northern limit of its distribution in the Cayos ]]>
et al. (2008a) found a population density of 2.79ind/km² in Sepetiba Bay, state of Rio de Janeiro. Geise et al. (1991) found a population density of 5.1ind/km² in the Guanabara Bay, also in the state of Rio de Janeiro. Geise et al. (1999) also conducted a study at the ]]>
This study aimed to estimate the current population density of the top predator S. guianensis in the Brazilian Cananéia region, to support and strengthen conservation efforts, since any disturbance may reflect in ]]>
Material and Methods
Studied Area: The study area in ]]>
Fig. 1) is located in the South São Paulo state (24º59’-25º04’ S and 47º54’-47º56’ W). The region has three islands. Cardoso Island is located within Trapandé Bay plus Cananéia and Comprida Islands which are separated by a River Channel (Schaeffer-Novelli et al. 1990).
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The vegetation of all these islands consists of a large mangrove swamp with three species of mangrove: the white mangrove, Laguncularia racemosa, the black mangrove, Avicennia schaueriana and the red mangrove, Rhizophora
mangle. According to Schaeffer-Novelli et al. (1990) this lagoon estuary system is one of the best preserved ecosystems in the Brazilian coast, being legally protected as a State and Federal Environmental Protected Area. Lagoon watercourses that make up the coastal Cananéia system tend to be silted, creating shallows or sandbars (Cunha-Lignon 2005).
Sampling methods: Sampling efforts were carried out once a month for ten months from May 2003 to April 2004. The area was divided into three sectors according to their proximity to open sea: Sector I has the greatest influence from the ocean and makes up the area of the Cananéia Estuary mouth, between Cardoso and Cananéia Islands; Sector II is an intermediate region between the three islands and a large flow of fresh water, and a salient ]]>
Fig. 1).
In each sector, transect lines of approximately the same length were used in order to cover the entire ]]>
Fig. 1) and all thirteen transects were covered each day during all ten days of sampling, meaning that each transect was covered ten times. The order in which transects were covered was randomly chosen (by lottery methods). Due to the short sighting time and the small body ]]>
During sampling, dolphins were detected with the naked-eye. The Distance method was used and all ]]>
bow were registered with their distance and angle in relation to the transect (Eberhardt et al. 1979, Buckland et al. 1993, 2001, Bonin et al. 2008). Radial-distances were visually estimated exclusively by the same researcher, a methods previously carried out by Smith (1981), Gaskin
et al. (1985) for other species of small cetaceans and Geise et al. (1999) for the estuarine dolphin.
The boat was always driven by the same pilot, and maintained a low, steady speed (five knots) in order to confer a greater reliability to the data (Leatherwood 1979, Gaskin 1982, Bonin et al. 2008). The ]]>
During field work, the four premises of the Distance method were followed: (1) all animals on the ]]>
et al. 1993, 2001, 2004, Cullen et al. 2004, Bonin et al. 2008).
The overall data was ]]>
et al. 1998). At first, three potential functions of detection were considered: uniform, half-normal and hazard-rate, together with various adjustments. Other variables such as season, methods (with or without binoculars), Beaufort sea scale, observer and models were added to the analysis and compared with the likelihood-ratio tests and Akaike Information Criteria (AIC), and those that adjusted better to each group of data were used (Thomas et al. ]]>
In order to eliminate outliers we truncated the distributions of perpendicular ranges at 600m (Buckland et al. 2001). This measure ]]>
Abundance was calculated with the Horvitz-Thompson estimator, as described in Marques & Buckland (2003).
Being the present study done with ]]>
transect line (g(0)) may not be 100% certain. In order to minimize this potential underestimation it was assumed that (g(0) ) was equal to one, because S. guianensis emerges at short intervals to breathe, which exposes them during sampling periods. Moreover, the area is an estuary, small and protected, which makes it easier to detect the animals. ]]>
Seasonality: The present study estimates the dolphin’s population density for the entire year and also the relative density for rainy and dry seasons. The dry season is considered to be from April to September with a pluvial index of 125mm/month in average. The rainy seasons is considered to be from October to March, when the pluvial index rises to 212mm/month (CIIA-GRO 2008). ]]>
Periods of the day: The authors also calculated the density related to different periods of the day: the morning (between 06:00h and 11:59h) and afternoon (from 12:00h to 18:30h). All the analyses were done separately for each sector of the bay.
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Groups: The present study used the same classification for “groups” as the one adopted by Monteiro-Filho (2000). According to this author, one or two adults in the company of a calf is considered to be a family (whether related or not); this type of group is the most common in the region. Groups of more than four individuals who meet for a certain purpose (displacement, cooperative feeding, among other) are considered to be schools. Isolated ]]>
Results
Between May 2003 and April 2004, ]]>
Population density: The total density of individuals found in the Cananéia region was ]]>
2 (CV=25.53%). Density of groups in the studied area was 5.77groups/km2 (CV=25.31%) while the abundance for the entire study area were 195 individuals (CV=25.53%). The number of individuals per square kilometer varied between sectors: Sec-tor I D=33.1ind/km2 (CV=13.34%); Sector II D=7.8ind/km2 (CV=21.97%) and Sector III D=3.04ind/km2 (CV=34.04%). ]]>
The Half Normal model with Cosine adjustments was used as an estimator. According to the premise of the method, the probability of detection becomes lower as perpendicular distance from the track line increases and makes it possible to graphically represent the possibility of detection for the whole sampled region as showed in Fig. 2.
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Seasonality: During the dry season 253 groups were registered with a total of 474 individuals. Density of individual was 5.77ind/km2 (CV=27.87%) while the density of groups was 2.94groups/km2 (CV=27.46%). Total abundance found for the dry season was 91 individuals (CV=27.87%). During the rainy season 208 groups were registered with a total of 434 individuals. Density of individuals ]]>
2 (CV=31.95%) as well as the density of groups, which rose to 8.84groups/km2 (CV=31.57%). Total abundance for the rainy season was also greater, totalizing 319 individuals (CV=31.95%). Densities for each season and sectors are shown in Table 1. Again the estimator Half Normal/Cosine was used.
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Periods of the day: Density varied between different periods of the day. During the morning 156 groups were registered with a total of 297 individuals. Density of individuals was equal to 7.29ind/km2 (CV=29.63%) while the density of groups was 3.29groups/km2 (CV=28.88%). Total abundance during the morning was 115 individuals (CV=29.63%). During the afternoon 305 groups were ]]>
2 (CV=30.19%), density of groups 6.57 groups/km2 (CV=29.92%) and the total abundance was 217 individuals (CV=30.19%).
As shown in Table 2, the dolphins occupation of the studied area was uneven at different times of the ]]>
Groups: The average size of the groups was 2.2ind/group. The animals were most likely to be seen alone, ]]>
individuals were also recorded. The largest group registered was formed by 12 individuals. The average number of individuals per group differed between each sector (Sector I had an average of 2.1ind/group; Sector II 2.4ind/group; and Sector III 2.1ind/group) and between the two seasons (the dry season had 1.9ind/group while the rainy season had 2.4ind/group). The number of ]]>
Discussion
Most of the estuarine dolphins sampled in the Cananéia region of Southeastern Brazil were found ]]>
et al. (1999) found that in the same area groups were commonly of two and five individuals, specifically. Pairs made up 30% of the total groups. Monteiro-Filho (2000) states that families are the most common group type found. The Cananéia region represents an area of high density of the estuarine dolphin, being one of the places in Brazil where the dolphins are most likely to be seen. In Guanabara Bay (state ]]>
et al. (2005) reported group sizes varying from one to 40 individuals and commonly found in group sizes ranging from two to 20 individuals. Average group sizes reported in ParatyBay and Sepetiba Bay (state of Rio de Janeiro) were 32.4 individuals (Lodi 2003) and 30.2 individuals (Flach et al. 2008b) respectively. According to Araújo & Azevedo (2001), those bays share the same fish assemblages that form large schools and make them distinct from other ]]>
In the South of Brazil, Filla & Monteiro-Filho (2009) found an average group size of 2.13ind/km² and 2.9ind/km² in the Guaratuba and Paranaguá Bays (state of Paraná), respectively. Also in the Paranaguá Bay, Santos et al. (2010) registered group sizes ranging from two to 90 ]]>
et al. 2005).
In Nicaragua, Edwards & Schnell (2001) registered group sizes ranging from two to 15 individuals. According to those authors, the group sizes varied with activity patterns. The average size group documented was 3ind/km².
All these studies were done in protected regions with shallow bays, little current and wave action and a low predation rate.
Lodi & Hetzel (1998) documented exceptional group sizes in the Ilha Grande Bay (state of Rio de ]]>
The results found in Cananéia regarding population density can be considered high when compared to other studies carried out with the same species (Geise et al. 1999, Edwards & Schnell 2001, Flach ]]>
et al. 2008a). This could be a result of the premise adopted in the study that the closer the animals are to transects, the greater the estimated density. On the other hand, this could also prove that the extrapolation of data for an area may lead to overestimations. This second possibility was evident when analyses were done separately for each sampled sector because the results indicated that the animals used the studied area heterogeneously, with a much larger use of certain localities in detriment of other. This uneven use can be explained by heterogeneity found in the environment itself and that the dimension of ]]>
et al. 1999, Oliveira et al. 2008).
Sector I was the one most used by the dolphins. In this sector, salinity is greater than in the others sectors and is considered to be constant throughout the year. This ]]>
et al. 2008), which would explain the permanence of the dolphins in this area, including during the night (Atem & Monteiro-Filho 2006). Moreover, this region has salient and steep sloping beaches, which are used by the dolphins as part of their fishing strategy. The dolphins corral the fish against the steep walls, reducing their escape route and thus disorganizing the school (Monteiro-Filho 2008). In the study carried out in Nicaragua, estuarine dolphin densities were also more common in ]]>
et al. (2008a) also observed more sightings and a higher encounter rate with estuarine dolphin at the entrance of Sepetiba Bay, in comparison to its interior. Additionally, Sector I has areas of steep sloping beaches: a topography that facilitates the dolphins feeding strategies as it reduces prey escape routes (Monteiro-Filho 2008), as aforementioned. Heterogeneity found on thebottom of distribution areas has also been considered to be a key determinant in ]]>
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The population density of the same region found in the present study was greater during the rainy season, as published by Geise et al. (1999). Similar results were also found in Paraty Bay (Lodi & Hetzel 1998, Lodi 2003). Santos et al. (2010) report that the largest groups of S. guianensis in the protected areas of Paranaguá Estuarine Complex (state of Paraná) occurred ]]>
Differences regarding the period of the day were also noted in this study. In general, the dolphins density ]]>
et al. (1999). Sectors also proved density differences. More individuals were found in Sector I and II during the morning, as opposed to what was found in Sector III, which had a greater density during the afternoon. A logical explanation is the dolphins movement between sectors.
Finally, the present study provides ]]>
Acknowledgments ]]>
We specially thank the boatman, Clovis Ribeiro Xavier Júnior. Also the Instituto de Pesquisas Cananéia (IPeC) and the Universidade de Mogi das Cruzes (UMC), Roberto Fusco Costa, Luciana Leite; and financial support from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). References
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Correspondencia a: Liisa Havukainen. Instituto de Pesquisas Cananéia, IPeC, R. Tristão Lobo 199, CEP 11990-000, Cananéia, São Paulo, Brazil; www.ipecpesquisas.org.br / Universidade de Mogi das Cruzes, UMC, Av. Dr. Cândido Xavier de Almeida Souza 200, CEP 08780-210, Mogi das Cruzes, São Paulo, Brazil; liisabio@yahoo.com.br Emygdio Leite de Araujo Monteiro Filho. Instituto de Pesquisas Cananéia, IPeC, R. Tristão Lobo 199, CEP 11990-000, Cananéia, São Paulo, Brazil; www.ipecpesquisas.org.br / Departamento de Zoologia, Universidade Federal do Paraná, UFPR, Setor de Ciências Biológicas, Departamento de Zoologia, Caixa Postal 19020, CEP 81531-980, Curitiba, Paraná, Brazil; elamf@ufpr.br Gislaine de Fatima Filla. Instituto de Pesquisas Cananéia, IPeC, R. Tristão Lobo 199, CEP 11990-000, Cananéia, São Paulo, Brazil; www.ipecpesquisas.org.br, gica_filla@yahoo.com.br
Received 12-X-2010. Corrected 18-I-2011. Accepted 17-II-2011.
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