Biology Reference
In-Depth Information
other seasons, and when present, were in lower densities than in other seasons. Confluences,
with their deep water and high density of prey, appear to provide refuge to dolphins during
low water periods, when many lakes and small tributaries are dry or shallow, and fish have
left the shallow areas for deep water. Conversely, during high water, dolphins probably leave
the confluence areas and follow the rising waters into lakes, tributaries and inundated forests,
where fish density is high. Confluence areas in Venezuela however appeared to be favored
Inia habitat year-round; dolphin densities in confluences were higher than in lakes or rivers,
and did not change seasonally. Confluences throughout the Neotropics are known to provide
year-round availability of deep water and high densities of fish, as fishes pass through
confluences while leaving oxbow lakes and tributaries during falling water, and pass again
during rising water when they return (Lowe-McConnell, 1975).
Rates of Travel
Maximum rates of re-sightings of identified dolphins might have been expected during
the low water season, when Inia were predicted to be concentrated in deep water areas and
relatively easy to locate and photograph. In all three study areas however, identified dolphins
were more likely to be resighted during transition seasons (i.e., falling and rising water).
Although sample sizes are too small to allow for anything more than speculation at this point,
the high rates of travel observed for a few identified dolphins during the high and falling
water seasons in Peru are consistent with the idea that some dolphins may extend their ranges
during this period as they search for dispersed prey.
Group Size
Mean group size remained less than two animals for most habitats and seasons in Peru
and Venezuela. Although seasonal differences were statistically significant, in Venezuela,
they were numerically small and may have been biologically irrelevant. Mean group size in
Bolivia did increase slightly during the low water season, but was never more than three
dolphins. A significant seasonal change in group size might indicate seasonal changes in prey
distribution and abundance. A distinct seasonal change in group size might also be indicative
of the onset of a distinct mating season and/or calving season.
Few studies have examined seasonal changes in Inia group size, and fewer still accounted
for seasonal differences in sampling effort. It is interesting to note that among the few studies
that examined seasonality of Inia group size, seasonality existed in the Orinoco River Basin
and in the Mamoré River Basin, but rarely in the main Amazon River Basin. The Amazon
River is just a few degrees south of the equator, and its seasonal flux is generally not as
extreme as those rivers located at higher latitudes (Lowe-McConnell, 1979; Lewis et al.,
1995). Compared to the rest of South America, seasonality is minimal in the northwestern
Amazon Basin (Lewis et al . , 1995), in which the Pacaya-Samiria Reserve is located. The
Orinoco River has a more pronounced dry season than the Amazon (Meade & Koehnken,
1991), and it is likely that a similar situation exists for the Mamoré River system, located at
about15 o S latitude. Perhaps seasonal differences in dolphin group size are more pronounced
where seasonal differences in aquatic habitat are most predictable and extreme.
Search WWH ::




Custom Search