Biology Reference
In-Depth Information
Fishing effort values varied little across time periods (1996-1997 vs 1999-2001), seasonal
periods (winter, summer), or strata. (Table 8). Examples of mean vales were 15.35 (1999),
16.9 (2000) and 16.32 (2001).
T
HOUGHTS AND
C
ONCLUSION
The lack of information on the fishing fleet effort made by the artisan fisheries is one of
the greatest limiting factors in accurately assessing dolphin bycatch. There is some
information regarding the interaction of
S. guianensis
with artisan fishery along its
geographical distribution, although these data are incomplete because they do not include the
fishing effort units. Knowledge of this information is necessary to be able to compare among
fishing fleets, as has already been done for Central America (Vidal et al., 1994) and Brazil
(Siciliano, 1994; Di Beneditto et al., 1998, 2001).
Every month, dolphins get caught up in nets due to the large number of artisan fishers on
the West Atlantic coastal areas. The same event occurs over S.
guianensis's
distribution range
along the Brazilian sea shore (Barros & Teixeira, 1994; Pinedo, 1994
b
; Siciliano, 1994).
Crew number, days per trip, and trips per year, together with the annual catch and number
of vessels working in each area are the most reliable information to obtain on fishing fleets
that help conservation biologists to determine the fishing fleets' direct effects on dolphins.
Dolphin bycatch data presented in this chapter across years, underscored by the information
on fishing effort unit for artisan vessels using drift gillnets and the number of fishermen per
day of trip can be assessed in this study.
Another important fact, is that most information found in the literature on bycatch of S.
guianensis
is related to drift nets with mono-threaded nylon used to catch fish of the families
Scombridae, Scianidae and sharks (Schmiegelow, 1990; Barros 1991; Pinedo, 1994
a;
Siciliano, 1994; Vidal
et al
.,
1994; Di Beneditto et al.,
2001), whereas in the Amazonian
estuary bycatches very rarely occur with this sort of gear, but rather with multithreaded deep
fishing nets.
Our findings show that the sort of interaction between artisan fishery and dolphins is
indirectly predatory, where dolphins as well as fish species are being caught by fishermen,
and that both fishermen and dolphins are in pursuit of the same prey. Regarding this subject,
Beverton (1985) reported on the interaction dynamics of dolphins and fishermen and
concluded that their relationship can be mutualistic when there is abundant food. Feeding
dolphins can be readily observed by fishermen and help the fishing fleet to pursue targeted
fish, and abundant prey corralled by fishing vessels could theoretically provide a large supply
of food. On the other hand, with a higher number of dolphins, the less available fishery target
prey will become, lowering the resources for the fishing fleet. Such a shift in prey abundance
could facilitate interspecific competition.
Bycatch of
S. guianensis,
in the Amazon estuary, presents a remarkable seasonality, with
summer having the largest bycatch period and then having it decrease significantly over the
winter. Change in the estuary salinity could be one of the factors generating this phenomenon,
even though it doesn't directly affect the dolphins, but rather the distribution of the fishery
target prey. Therefore, with the onset of winter, the stratum one fleet moves to other areas.
Fishing nets are changed and the number of fishing days is lowered, decreasing the fishing
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