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in rivers and lakes generally did not differ among seasons once differences in seasonal
sampling effort and the high variation within seasons were taken into account. Other studies
have reported seasonal changes in river dolphin distribution and abundance, although the
patterns of seasonal changes have varied. For example,
Inia
were observed most often in the
Amazon River in Brazil during low water (Martin et al., 2004), but during high water in the
Marañón River in Peru (Leatherwood, 1996). Maximum
Inia
encounter rates were observed
during high and rising water in the Ecuadorian Amazon (Utreras, 1996), in the Marañón River
of Peru (Leatherwood, 1996), and Rio Caquetá in Colombia (Galindo, 1998).
An increase in aquatic habitat from rising water should by itself result in lower
Inia
encounter rates, as density per unit area declines. Furthermore, the total surface area, water
depth and quantity of submerged vegetation during falling water are very similar to that
during rising water (i.e., water levels during these two seasons are mirror images of each
other). If temporal constancy in the number of dolphins within the study area and changes in
vulnerability to detection were the only factors affecting sighting rates, one would predict
sighting rates to be highest during low water, when total aquatic habitat and submerged
vegetation are reduced, lowest during high water, and intermediate and approximately equal
during rising and falling water, but this was only the case in Bolivia, not in Venezuela or
Peru. An alternative hypothesis is that some dolphins left the study areas during rising waters,
and that dolphins re-entered the study areas as waters fell. This may have been the case in
Bolivia, where the number of dolphins in the study area decreased during the high water
season, perhaps due to the dispersion of individuals in the inundated forest, or migrations of
the dolphins to the Mamoré River and its numerous lagoons and seasonally inundated
channels.
It is possible that seasonal differences did exist in the study areas, but were not detected if
the four broad categories of seasons that were used did not accurately represent the complex
seasonality of the study area. In general, within-season variability was higher than between-
season variability; for example during one 5-day period in Peru, the water level of a lake
dropped by 2 m, and the number of
Inia
decreased by 32. Dolphins may well respond to
changes in water depths and fish abundance, but perhaps not on the scale likely to be detected
with only four categories of season. Movements and spawning of many fishes are triggered
by small changes in water level, rather than by season (Lowe-McConnell, 1975). Short-term
rises in water level are associated with changes in fish migrations (Goulding, 1980). Local
fishermen respond to these fluctuations with changes in fishing location and effort, and it is
likely that dolphins would do the same.
In general, fish abundance declines during the dry season as young of the year and other
fishes are either eaten or strand in shallow, oxygen-poor stagnant waters, while fish biomass
achieves its maximum during the late rainy and falling water periods (Lowe-McConnell,
1975, 1979). The dispersal of fishes across the flooded forest and floodplain decreases fish
densities per unit area during the rainy season. Fish biomass approaches its maximum at the
end of the rainy season and beginning of falling water. As waters recede, young-of-the-year
fish move out of lakes and into the rivers. Thus, we had predicted maximum dolphin
encounter rates in rivers during falling and low water, and lowest encounter rates in rivers
during rising and high water as fish travel up tributaries and disperse onto the floodplain.
Why this pattern was observed in some rivers and not all is unclear. Dolphins likely respond
to not only the relative abundance of fishes in an area but also the relative ease of capturing
them. During the rising and high water period, flooded vegetation permits fish dispersal and
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