Biology Reference
In-Depth Information
Another fishing type described was in BC, at depths between 5 and 10 m, when groups from
4 to 10 dolphins dove many times, without any specific organization, to capture yellow mouth
salmon (
Macrodon ancylodon
) or White mullet (
Mugil curema
).
A more elaborate type of feeding strategy was mentioned for the open water (OW)
habitat. Groups in excess of 10 animals were positioned in a line, a circle and/or a "V", to trap
cutlass fish (
Trichiurus lepturus
) that swim in the stocking water during the day. The stomach
contents of
S. guianensis
within the study area (N = 50) contained
Trichiurus lepturus
(Beltran-Pedreros & Araújo, 2007).
Fishermen's observations also indicated a tendency for dolphins to have similar
frequencies for feeding (51.9%) and moving in BC (48.1%) and in OW (54.5% and 45.5%).
In IB the feeding frequency in relation to the movement was higher (68.8% and 31.2%).
There were no accounts of dolphins feeding in the PC habitat, but they did display movement
behavior. When the dolphin groups were in ―common occurrence‖ they were more often
observed feeding (63.4%) than moving (36.6%), while for groups of dolphins of rare
occurrence, they were often observed moving (80%).
Calf presence during the behavior observation was indicated in most cases as rare
(46.8%). But the results varied when each behavior was analyzed (Table 3). The results for
calf observation in different habitats for each behavior are in Table 4. Common or seasonally
observed calves were best seen feeding than moving, while when they were rare they were
best seen moving. Calf observation frequencies in each behavior varied according to the
habitat and group size. In OW habitat they were commonly observed feeding (59.5%)
especially in Type III groups (40%), while they were rarely observed moving (19.5%).
Table 3. Percent of
Sotalia guianensis
groups and occurrence of calves in the Amazon
estuary between 1999 and 2001 for each behavior. I = one animal, II = from 2 to 3
animals, III = from 4 to 10 animals, IV = from 11 to 30 animals, V = > 30 animals.
Group Size
Calves
Behavior
I
II
III
IV
V
Common
Rare
Seasonal
Feeding
29.3
59.7
59.2
50
30
75
32.7
55.9
Movement
70.7
40.3
40.8
50
70
25
67.3
44.1
In IB habitat, calves were commonly observed feeding in Type II groups (61.5%) and
rarely alone and were observed moving 54.5% of the time. They were never observed feeding
in port and community (PC) habitats and even their movements were considered rare when
solitary (Type 1) (100%) or in small groups of two to three (Type II) (93.7 %). Calves in BC
habitats were only observed commonly feeding in Type III groups (50%). For Type II groups,
the occurrences of feeding behavior during common and rare categories were similar (13 and
12 %).
The most observed group sizes were Type II (37.9%) and Type III (26.7%) in BC habitat
(44.5% and 61.9%). Solitary animals were more often observed in IB (53.7%) and in PC
(36.6%), and only a few times in OW and BC (2.4% and 7.3%). Type V groups were
frequently observed in OW (60%) while Types IV and V groups were not observed in the PC
habitat. Type V groups were observed moving in 70% of the cases, as well as Type I (70.7%);
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