Biology Reference
In-Depth Information
48 % of the local rain being recycled (Salati & Vose, 1984). Additionally, a major fraction of
the deforestation is related to the destruction of the river banks by farming at the edges of the
rivers and in the floodplains (Moran, 1990). The destruction of floodplain has an
incontrovertible effect on the fish production (Wellcome, 1979) and the fish production is a
vital process in the life of the river dolphins and their movements throughout the year (Martin
et al., 2004). Mining sand and gravel from river banks alters the hydraulics and substrate
composition of fish spawning and also altering the morphology of river channels (Smith &
Smith, 1998). The remove of woody debris from the river channels reduces the complexity
and biotic diversity of lotic systems destroying essential components for some fishes which
are an important part of the dolphin's diet. 2- The use of mercury for the gold amalgamation
process, where up to 10 % of the mercury is lost, has a tremendous impact in the local food
chains, especially in piscivorous animals, such as river dolphins. A study in the sediments and
floating plants in the Tucurui Reservoir in the Tocantins River showed the risk of mercury
accumulation in the bed of non-flowing waters (Aula et al., 1995). Petts (1989) claimed that
stable gravel-bed rivers might act as sinks for the progressive accumulation of heavy metals.
The use of pesticides and fertilizers are other threats because they are responsible for
eutrophication of the rivers. Additionally, oil spills along with gas and fuel oil explorations
have an undoubtedly impacted the food chains. For instance, Denkinger et al. (2000) revealed
a population density declination of
Inia
in the Cuyabeno and Lagartococha Rivers of the
northeastern Ecuadorian Amazon from 1996 to 1998. This declination seemed to be due to
contamination by six oil spills and waste waters of the oil fields. In fact, the other Amazon
dolphin (
Sotalia fluviatilis
) disappeared after 1990 from the Lagunas Grandes in the upper
Cuyabeno River, an affluent of the Aguarico River presumably because of oil spills. 3-
Another extreme threat for river dolphins is the construction of hydroelectric dams. They
wholly transform the hydrologic river cycles altering the flood regime and affecting the rain
regimes. Da Silva (1995) claimed that dams stop fish migrations and reduce the trophic
resources affecting the biomass and diversity of the fishes, primary aliments of the river
dolphins. Dams also affect sediment load and water quality of rivers and suppress the
seasonal flow peaks preventing the formation of adjacent floodplains which are extremely
vital for the subsistence of river dolphins in certain years and seasons. But dams, also
constitute absolute barriers to the potential movements of the river dolphins. Hence, they can
fragment the river dolphin populations. The fragmented populations could be more
susceptible to the action of endogamy and genetic drift and then more vulnerable to
catastrophic environmental and demographic events, inbreeding depression or external
pathogenic microorganisms. For instance, two dams in Brazil have unknown consequences on
pink river dolphin populations. One of these, the Tucurui dam, is on the Tocantins River,
isolating an upstream pink river dolphin subpopulation, while the second is the Balbina Dam
on the Uatuma River. Projected dams could also have catastrophic consequences on pink river
dolphins, such as the plan to build dams on the Atures rapids (Venezuela), in the Orinoco
between Caicara and Ciudad Bolivar as well as on the Madeira River in Brazil very near to
the frontier of Bolivia in the separation geographical point of
Inia geoffrensis
and
Inia
boliviensis
. 4- The over-fishing could be another important problem for the river dolphins.
Nylon gill nets catch more fish with the fishermen expending less effort. Small nylon-mesh
gill nets are extremely damaging because they are indiscriminate in catching fishes of all size
and age classes and therefore could have major and negative impacts on river dolphin prey.
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