Biology Reference
In-Depth Information
Group Size
Based on observations from fishermen, five group sizes of dolphins were defined:
Type
I,
solitaire animals;
Type II,
2 to 3 animals;
Type III
, 4 to 10 animals;
Type IV,
11 to 30
animals and
Type V
, more than 30 animals.
The presence of calves among groups varied and was considered rare (48.3%), common
(35.5%) or seasonal (19.4%). Their occurrence based on habitat type indicated that the calves
were more common in OW (46.7%) and less common in IB (33.8%). They were more
seasonal in BC (24.8%) and rare in IB (53.2%), while in BC there were no calves present in
96.8% of the cases. Within the OW habitat, the highest frequency of calf observation was in
Type II groups, where they were considered common (66.7%). Calves were commonly
observed in type V groups (50%) in this habitat (Table 2). Type II groups in the IB habitat
were considered common and rare because both frequencies were 42.1%. But for BC, they
were rarely in Type II groups (51%) and common in Type V groups (66.7%).
Calf observations inside groups of more than 10 animals were considered difficult in the
OW and BC habitats on the part of the observers (the interviewees). However, calves were
never observed alone in any habitat type.
Table 2. Percent occurrence of
Sotalia guianensis
calves in the Amazonian Estuary
between 1999 and 2001 by size group (I: one animal, II: 2-3 animals, III: 4-10 animals,
IV: 11-30 animals, V: >30 animals) in each habitat.
Habitat
Occurrence
I
II
III
IV
V
Common
-
66.7
52
28.6
50
Open waters
Rare
100
33.3
24
38.1
27.8
Seasonal
-
-
24
33,3
22,2
Common
-
42.1
42.9
75
66.7
Igarapés and/or Bays
Rare
100
42.1
14.2
-
33.3
Seasonal
-
15.8
42.9
25
-
Common
-
6.2
-
-
-
Ports and Communities
Rare
100
93.8
-
-
-
Seasonal
-
-
-
-
-
Common
-
32.1
38.4
26.7
66.6
Beach and/or Coast
Rare
100
50.9
30.8
33.3
16.7
Seasonal
-
17
30.8
40
16.7
Behavior
Fishermen's narratives identified dolphin feeding behavior (FB) such as pursuit of fish,
continuous immersions in the same spot with fish activity, and special formations for
capturing fish in fishing spots
and close to corrals.
They also identified dolphin movement
behavior (MB), like swimming in one direction without evidence of immediate return to the
observation spot or following vessels.
Fishermen from the Br and V areas, who work with corralled fish mentioned fish being
persecuted by groups from two to four dolphins during the ebb tide towards the curral (habitat
IB). In this case, dolphins used the corral as a barrier to make their fish capture easier.
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