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observed groups from 6 to 15 individuals in coastal areas (Defran & Weller, 1999) and
typically larger groups in the open sea (Shane, 1990).
A similar situation happens with
S. fluviatilis
. Their group size ranges between one and
six animals, with groups of two being the most commonly observed (Silva & Best, 1996).
The group formation of more than six animals are rare, but this group type becomes more
common in lakes and wide rivers (ex: Solimões) during the flooding period.
The significant dependence among the variables: OD, H and B let us infer that there is a
relationship between habitat use and an ecological variable that allows this use. The species
invests most of its time in feeding and movement (Edwards & Schnell, 2001), but these
activities are linked to food availability and depth. Depth directly affects the occurrence of
dolphins because shallow depths do not allow movement without danger and because depth
will also limit the distribution of dolphin food resources. Therefore, depth is the ecological
variable that determines the habitat use of the species in the Amazonian estuary, which varies
according to time of day and type of tide.
During the high and ebb tides the igarapés, bays and exposed coast beaches habitats, are
explored by the species, for feeding as well as movement. Movement pattern and habitat use
have already been described by Borobia et al. (1991), Silva & Best (1994), Edwads & Schnell
(2001). Even though in the Amazonian estuary the food availability is not a limiting factor,
the
S. guianensis
feeding habit in the studied area marks a preference for continuous
occurrence of fish. The distribution of those prey species is linked to the salinity and depth in
the system (Barthem, 1985) and, consequently, the dolphin groups would be in constant
movement in the search of prey. This provides an explanation as to why movement behavior
was many times associated with feeding.
A
CKNOWLEDGMENTS
The present work was developed with the financial support of CNPq (Scholarship
143542/1998-2). Our gratitude to the National Institute of Researches of the Amazonia
(INPA), to the members of the Aquatic Mammal Laboratory, Dr. Vera Silva and Dr.
Fernando Rosas, and to the fishermen of the Amazonian estuary.
R
EFERENCES
[1]
Barthem, R. (1985). Ocorrência, distribuição e biologia dos peixes da Baía de Marajó,
Estuário Amazônico. Boletim do Museu Paraense Emilio Goeldi,
Série Zoologia
, 2(1),
49-69.
[2]
Barthem, R. & Schwassmann, H. (1994). Amazon river influence on the seasonal
displacement of the salt wedge in the Tocantins river estuary, Brazil, 1983-1985.
Boletim do Museu Paraense Emilio Goeldi,
Série Zoologia,
10(1), 119-130.
[3]
Begossi, A., Hanazaki, N. & Silvano,
R. (2002). Ecologia humana, etnoecologia e
conservação. In M. Amorozo, L. Ming, S. & Silva. (Eds.),
Métodos de coleta e análise
de dados em etnobiologia, etnoecologia e disciplinas correlatas
(pp. 17-32). Rio Claro,
São Paulo. Editora CNPq, UNESP, SBEE.
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