SEASONAL ECOLOGY OF INIA IN THREE RIVER BASINS OF SOUTH AMERICA (ORINOCO,AMAZON, AND UPPER MADEIRA) - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 30

I NTRODUCTION
River dolphins live in an environment that is less-stable and more spatially complex than
the ocean, where most odontocetes (toothed whales) occur. The Amazon River dolphin Inia,
occurs in the Neotropics, where water temperature and photoperiod remain almost constant
throughout the year; seasonal differences are primarily due to changes in precipitation (wet
and dry seasons) which in turn corresponds to river water levels (low, rising, high, and
falling).Yearly seasonal fluctuations in river levels may be as great as 20 meters and these
differences result in seasonal extremes in quality and quantity of aquatic habitat available to
river dolphins. Dolphins restricted to deep waters of river channels, confluences, and
seasonally isolated oxbow lakes during the dry season are free to swim through submerged
primary rainforest and across the llanos (grassland plains) during the height of the rainy
season.
Changes in water levels affect not only the amount and type of aquatic habitat available
to Inia , but also to their prey. Inia are piscivores (fish-eaters), and therefore prey biomass and
availability are largely determined by seasonal water levels, via seasonal patterns of fish
reproduction and fish migrations. Fish reproduction and migrations are highly seasonal,
although the timing of these events varies according to species (Goulding, 1980). Ease of prey
capture may also be affected by water depth and aquatic habitat. Fish may be easier to catch
in shallow confined waters, than in areas of deep open water, or when they are dispersed and
hidden in the structure provided by flooded vegetation. Such seasonal extremes in habitat and
prey availability would be expected to be reflected in seasonal patterns of other aspects of
river dolphin ecology, such as distribution, habitat association, movement and residency
patterns, group size, reproduction, and mortality.
The Amazon River Dolphin, Inia geoffrensis, occurs in freshwaters of the Amazon,
Orinoco, and upper Madeira River basins of South America, in the countries of Bolivia,
Brazil, Colombia, Ecuador, Peru, and Venezuela (da Silva, 1994). Inia is listed by the
International Whaling Commission and the International Union for the Conservation of
Nature as a single species with three subspecies: Inia geoffrensis humboldtiana in the Orinoco
River Basin, Inia g. geoffrensis in the main Amazon River Basin, and Inia g. boliviensis in
the Bolivian sub-basin of the Amazon. The taxonomic status of Inia is in question however,
and many researchers have proposed classifying Inia boliviensis as a separate species, based
on genetic and morphologic differences (D'Orbigny, 1834; Pillieri & Gihr 1977; da Silva
1994; Hamilton et al, . 2001; Banguera-Hinestroza et al., 2002; Ruiz-García et al . , 2008).
In this chapter, we present results from our investigations of the seasonal ecology of Inia
from three river basins of South America: Inia geoffrensis humboldtiana in Venezuela's
Orinoco River Basin; Inia geoffrensis geoffrensis in Peru's Amazon Basin; and Inia
boliviensis in Bolivia's Mamore Basin (upper Madeira Basin). We also discuss seasonal
patterns in natural and anthropogenic mortality, including strandings, intra-specific
aggression, entanglement in fishing nets, vessel strikes, and deliberate killing by fishermen.
Although this chapter focuses on Inia , the discussion of seasonal ecology can be applied to
river dolphin species presented in other chapters of this topic.