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provides greater refuge from predators, likely making prey more difficult to locate and
capture. Although fish abundance declines during the low water period, fishes are presumably
easier for predators to catch because the reduced water volume spatially concentrates them. In
Venezuela, maximum fish CPUE for gill nets and seines occurred during low water. This
index reflects not only the relative abundance of fishes caught, but also the ease of capture.
The association between dolphin abundance and absolute water depth was examined
separately from season because there was substantial variation in depth within seasons, and
seasons were classified according to a combination of relative depth and month of the year.
The flexible body of
Inia
allows them to maneuver in shallow water, and their long beaks
allow them to extract fish from submerged vegetation (da Silva & Best, 1996), thus equipping
them to exploit extremes in water depth. In general, dolphin abundance in rivers was not
related to absolute water depth. The relationship between
Inia
abundance and depth was not
significant in any of the rivers surveyed in Peru. Other factors such as depths of nearby rivers
and lakes, river currents, human activity, and distribution and abundance of prey probably
affect dolphin abundance as well.
The relationship between seasonal lake depth and dolphin sighting rates appeared to vary
according to study area and lake. Maximum dolphin densities in lakes would be expected
during transition seasons, when dolphins enter lakes from rivers with the rising water and
migratory fish, or leave the floodplain and forest with the receding waters and enter lakes;
however, these seasonal patterns of dolphin distribution were not observed in the three study
areas. In Venezuela,
Inia
densities in lakes were greatest during falling water, and lowest in
rising water (high water season not sampled; McGuire & Winemiller, 1998). In the Bolivian
Amazon, Aliaga-Rossel (2002) recorded the highest density of
Inia
in lakes during rising and
high water, and lowest densities during the dry season. In one lake in Peru,
Inia
encounter
rates were constant year-round, regardless of season or water depth. The findings from this
lake suggest that large lakes may be year-round dolphin habitat due to their buffering
capacity, as they do not experience the same reduction in surface areas with depth changes as
do shallower lakes, and therefore may provide refuge to fish and dolphins during low water
periods when shallower lakes and rivers become dry. If dolphins track not only water levels,
but also fish movements, as some have suggested (da Silva, 1994; McGuire & Winemiller,
1998; da Silva and Martin, 2000a), dolphin densities in lakes should be minimal during low
water as fish migrate to river habitats (Lowe-McConnell, 1975; Goulding 1980), and also
during high water, as fish disperse onto the floodplain and into the flooded forest. In the
central Brazilian Amazon, da Silva & Martin (2000) observed that
Inia
densities in lakes were
greatest during rising water, and that
Inia
moved from lakes to rivers during the dry season,
and from rivers to the flooded forest during high water. Trujillo (1990) reported that dolphins
in Colombia were found in lakes during the rainy season, and then moved to the Amazon
River during falling water, although some
Inia
remained in the lakes during this season. In
summary, seasonal dolphin abundance in lakes can be explained only partially by water
depth, and other factors such as distance to nearby rivers, lake volume, human activity,
seasonal variability, and distribution and abundance of prey probably affect dolphin
abundance as well.
As with other river dolphins, confluences appear to be important habitat for
Inia
. There
were significant seasonal differences in occurrence of dolphins in confluences in Peru. During
low water,
Inia
persisted longer in the confluences and occurred at higher densities than in
any other season. Dolphins were least likely to be in confluences during high water relative to
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