Biology Reference
In-Depth Information
In consideration of all the environmental characteristics that determine prey distribution,
depth would be the most limiting factor. Prey search and hunt depend on this factor. Authors
have reported the species likes to fish
at shallow depths
using the margin or beach or the
fishing corral as a barrier to capture fish. This behavior is also common in the Amazonian
estuary and it was also described by Monteiro-Filho (1995) in Paraná State coast. In the BC
habitat, type II and III groups exhibited higher frequencies during feeding when they fished at
random and/or formed attack lines on shoals and/or fish that escape from the fishing corrals.
In stream, bay and mangrove lagoon habitats (igarapés), where depth is critical during
low-tide, dolphins were seen in the high tide and at the beginning of the ebb tide, when depth
was sufficient for prey capture. There is an added benefit for the dolphins during the ebb tide
because the current disperses the fish shoals and thus facilitating their capture.
Dolphin group size and feeding strategy were dependent on habitat characteristics as
indicated by the positive interactions among group size, behavior, and habitat (GS*B*H). In
igarapé, and channel habitats (IB) small groups of dolphins were observed as solitary animals
(Type I), or in small groups of two to three (Type I, II), performing random fishing, without
presenting special formations. In OW habitat, dolphins were most frequently found in group
types III, IV and V. They demonstrated organized hunting strategies, with even line
formations, in "V" and circle patterns, strategies that have already been documented for this
species (Silva & Best, 1996).
Calf observations were dependent on dolphin behavior, habitat and group size,
demonstrated by the significant interactions among these variables. But, Type II groups call
for special attention, where group structure or group composition was linked to differences in
the frequency of calf observations. In IB and BC habitats calves were more commonly
observed feeding than moving. But, this frequency varied if the group was comprised of a calf
and one or two adults (when they were considered common) or groups composed of a calf in
the presence of another calf and/or young animal (when they were considered rare). Seasonal
observations of calves for those habitats supported that feeding as well as moving were
related to the periods that the tides allow for the exploration of these habitats, times when
there is a reduced risk of injury.
Calf feeding was common for Type III groups in OW (40%) and BC (28.8%) habitats.
But there were little evidence of groups of calves moving possibly because it is difficult to
distinguish calves during successive dives and by the tendency of adults to protect them.
The most observed group sizes were of two (Type II) to ten (Type III) animals, but the
observation frequency varied according to the habitat type and the dolphin activity. Therefore,
habitat and behavior are the main factors that influence group size, as has been previously
recognized by Edwards & Schnell (2001) and which demonstrates the significant dependence
among the interaction variables.
In other studies with
S. guianensis
, group size varied from 1 to 30 animals, with groups
of 2 to 10 animals being the most commonly observed (Edwards & Schnell, 2001). Those
group sizes are common in internal areas of bays, while group sizes tend to be larger and
more open, oceanic waters. Within the Amazonian estuary, Type I and II groups were more
frequent in IB habitats. Type II and III groups were common in BC habitats and Type IV and
V frequently occurred in OW habitats. There was a tendency of group size to increase as the
habitat became wider and deeper (oceanic). Herds of hundreds of animals were observed in
the largest habitats. That pattern is similar to that described for
Tursiops truncatus
, who
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