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likely that several different geographic genetic lineages of pink river dolphins are living
together within these large populations. However, no such large populations were observed in
the Napo-Curaray River transect analyzed. Furthermore, it's unclear if the genetic structure
found in this study could be applied to these large pink river dolphin populations. It is logical
to think, as was determined by Martin et al. (2004) in rivers of central Brazilian Amazon, that
the highest densities of the dolphins occur close to the margin, the location where characins, a
significant component of
Inia
's diet, migrate (da Silva, 1983, migrate). Here, sediment-rich
white waters meet acidic black waters, creating a high productive area and an abundant
number and variety of fish. This water-meeting area has a reduced current, the lowest in the
river's center and is a much preferred area by the dolphins. A relatively high density of pink
river dolphins was only found in the river center due to annual minimum water levels,
probably, because fish were forced to migrate by them (Martin et al., 2004). It's likely that
different genetic lineages of pink river dolphins converge on these high productive areas
where these two river channels meet (Magnusson et al., 1980; Leatherwood, 1996; McGuire
& Winemiller, 1998). But their genetic integrities are maintained throughout this space no
matter if the animals are dispersed as solitaries or as different lineages converging on points
of high fish density when, for example, fish movements are produced by the Amazon's
seasonal changes in water level and dissolved oxygen. (Martin & da Silva, 2004b). Martin &
da Silva (2004b) showed that an exodus of pink river dolphins is produced from the
floodplain to the main rivers at Mamirauá during the low water season which results in the
highest reported density for any cetacean worldwide (18 dolphins/km
2
or 4.2 dolphins/linear
km in the várzea). However, the climatic, physical and ecological conditions of the Napo-
Curaray rivers, where the samples of dolphins for this study were obtained were different to
those found by these authors in Mamirauá. The animals were caught during the low-falling
water period of these rivers in October-November and almost 69 % of the animals present in
the diverse lagoons and in the beach were caught with our nets (33 out 48 animals).
Therefore, in the eight sampling points, a total of 15 animals escaped from our nets. In the
main rivers (Napo, Curaray and their affluents), 169 animals were counted (with the help of
M. F. Gómez and P. Escobar-Armel). Thus, a total number of 217 pink river dolphins were
documented in seven lagoons, one beach 280 km away and in the main rivers between these
lagoons. This yielded a minimal density of 0.775 dolphins/linear km. This density is within
the range of other densities found for
Inia
(for instance, 0.22 dolphins/linear km in 490 km
from Manaus to Jutica in the Amazon river, Magnusson et al., 1980; 0.56 dolphins/linear km
in the Apuré River and 1.15 dolphins/linear km in the Apurito River within Venezuela,
Schnapp & Howroyd, 1992; 1.12 dolphins/linear km in the Tijamuchí river, an affluent of the
Mamoré river in Bolivia, Aliaga-Rossel, 2002; a mean of 0.66 dolphins/linear km along
margins of Japurá (Caquetá) and Amazon River near to Tefé, Martin et al., 2004; 0.47-0.68
dolphins/linear km and 0.73-1.47 dolphins/linear km were determined for the Samiria and the
Tapiche rivers, affluents of the Ucayali River in the Peruvian Amazon, Henningsen, 1998;
0.38 dolphins/linear km in low water and 0.44 dolphins/linear km in high water in the lower
part of the Lagartococha river in the Ecuadorian Amazon, Utreras, 1996; or even lower
densities between 0.06 to 0.21 dolphins/linear km during three years in the Cuyabeno River
and 0.21-0.27 dolphins/linear km during three years in the Lagartococha River, in Ecuador,
Denkinger et al., 2000) and was substantially lower than what was determined for low water
within the Mamirauá Reserve by Martin & da Silva (2004b). Although the captures were
made in the falling-low water season, a significant fraction of the animals were living in the
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