Biology Reference
In-Depth Information
effort. It is possible for dolphins to migrate in pursuit of their prey which lessens the chance
of them interacting with the fishing fleet. With zero salinity, fish and dolphins move to other
areas and the fishery - dolphin frequency interaction decreases. Again, there is seasonality for
bycatch of
S. guianensis
in the Amazonian estuary, and summer is the season with the largest
catch.
Type of net and the gauge of its thread did not have significant effects on dolphin
mortality, which suggests that differences in bycatch among vessels in the Amazonian estuary
may be due to additional, untested factors. The interaction between dolphins and fishing
industry may be related to factors not pertaining to the fishing activity, such as ecological
aspects and species behavior itself. The size of nets, depth fished, thread type and total area
fished by the fleet in the Amazonian estuary, may also help to explain how bycatches in the
Amazonian estuary are different from other fished areas.
Usually, artisan fishing fleets are subjected to the region's environmental and economical
fluctuations. A remarkable seasonality of fish catch within the Amazonian estuary resulting
from environmental factors in turn influences local economies. For example, those species
with high commercial value occur during the summer and the ones caught over the winter are
mainly exploited by the industrial fishing fleet. This strategy, that depends on target species,
results in economic instability. Bycatches of
S. guianensis
by the fleet using this strategy
should not be dangerous to this species (
S. guianensis).
A comparison of data from the two time- periods (1996-1997 vs 1999-2001) indicated
that the percentage of the fishing trips with bycatch almost doubled from 33.41% to 61.7%.
while the mean number of dead animals per fishing trip with bycatch decreased significantly
(7.2 to 5.4).
Similarly, the percent of fishing trips with bycatch also showed an increase in stratum
one (42.33% to 47.6%) and in stratum two (57.66% to 86.3%). In contrast, the means number
of dolphins caught per fishing trip (in bycatch) decreased significantly (5.3 to 2.6 in stratum
one and 9.7 to 8.1 in stratum two).
S. guianensis
bycatch seems quite large at 4,146 individual dolphins, for a 30 month
sampling period until one considers that the size of the fleet's fishing area is nearly 90,000
km
2
. Consider that this bycatch is equivalent to 4.5% of
S. guianensis's total
population size
in the Amazon estuary. This is the maximum bycatch percentage estimated for
Stenella
attenuata
in tuna fishing nets (Bjrge et al
.,
1994). Therefore,
S. guianensis's
estimated
population size in the estuary should be approximately 92,133 dolphins, that translates into an
estimated density of 1.02 ind/km
2
. Densities of
S. fluviatilis
on the upper Amazon, varies
from 0.9 ind/km
2
in river channels, 2-2.8 ind/km
2
in the Amazon River and 8.6 ind/km
2
in
lakes (Vidal
et al.
1997) and are the highest densities reported for small cetaceans. The
closest estimated densities of other cetaceans in the Amazonian estuary are the
Phocoena
phocoena
(0.56 ind/km
2
) and
Delphinus delphis
(0.3 ind/km
2
) (Barlow, 1988, 1995).
There are no data on the species life history and abundance, however, as a k strategist, it
shows a great muscle mass, it takes a long time to reach sexual maturity (6-7 years), it has a
slow growth rate (0,79), a long life span and has birth intervals of over one year apart (2
years) (Rosas, 2000). Their life history characteristics are similar to those of
Phocoena
phocoena
whose annual population increase was estimated to be between 8.3 and 9.4%
depending on the age (Barlow & Hanan, 1995). That is, estimated bycatch for
Sotalia
guianensis
in the Amazon estuary doesn't show an impending threat for the population, since
it has the potential to be able to regain 6% of its number within a year.
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