Biology Reference
In-Depth Information
marine mammals) as they grow (Caurant et al., 1994; Feroci et al., 2005). Conversely,
selenium is recognized as an essential element for metabolic activity of aquatic mammals,
acting as a protective agent against the toxicity of exogenous metals such as mercury (Feroci
et al., 2005). Due to the relatively low level that these exogenous elements were found in the
tissues of franciscanas, there probably is no reason for concern about their potential effect on
the populations' viability. However, the additive effect of other pollutants, habitat degradation
and bycatch should be taken into account.
Depletion of fish stocks and temporal changes in franciscana's diet
: Historical catch
records of commercial fishes have demonstrated a decline in yearly landings of the sciaenids
Micropogonias furnieri
and
Macrodon ancylodon
in southern Brazil (Haimovici et al., 1997;
Haimovici, 1998). This is consistent with a reduction in the occurrence of these two species in
franciscana's diet (Bassoi & Secchi, 2000; Secchi et al., 2003
b
).
M. furnieri
has been heavily
exploited by gillnet and trawl fisheries for more than three decades (Reis, 1992; Haimovici,
1998) and a drastic decrease in the density of juveniles in coastal waters has been observed
(Ruffino & Castello, 1992). During that same period,
M. ancylodon
and
M. furnieri
decreased
drastically from 41% to 7% and 27.5% to 4% in frequency of occurrence, respectively, in the
diet of franciscana (Bassoi & Secchi, 2000). Haimovici (1998) showed that stocks of these
sciaenid species have been extensively exploited and are currently at very low levels in the
region. On the other hand, frequency of occurrence of cutlassfish
Trichiurus lepturus
and
sciaenid
Umbrina canosai
in the diet of the franciscana increased from about 5% and 3% in
the late 1970s to about 39% and 20% in the mid 1990s, respectively.
T. lepturus
together with
Cynoscion guatucupa
represents 47% of the total estimated bony fish biomass in this region
(Haimovici et al., 1996). Both species have only experienced moderate fishing pressure
(Haimovici et al., 1997; Haimovici, 1998). While
C.
guatucupa
has always been the most
important prey for franciscana,
T. lepturus
has had only a little importance in the
franciscana's diet in the past. However, now it is the second most important prey for the
species in this region. These values suggest that changes in the franciscana diet parallels
reduced availability of certain prey species due to their over-exploitation. Although the
effects of this major dietary change on the franciscana are unknown, the energetic
implications might be of some concern.
Incidental Mortality in Fishing Nets
Mortality due to incidental entanglement in gillnets seems to be by far the greatest threat
to the franciscana (Figure 1). There is no indication of direct exploitation of the species.
Reports of bycatch in the shark gillnet fisheries of Punta del Diablo, Uruguay, date back to
the early 1940s (Van Erp, 1969). Although gillnetting in southern Brazil began around this
time (Haimovici et al., 1997), gillnet fisheries for bottom-dwelling fish were only
documented as a major threat to the franciscana in the 1980s. Bycatch has since been reported
from the main fishing villages along most of the species' distribution (
e.g.
Corcuera, 1994;
Moreno et al., 1997; Praderi, 1997; Secchi et al., 1997, 2003
b
; Di Beneditto et al., 1998;
Bertozzi & Zerbini, 2002; Rosas et al., 2002; Ott et al., 2002). In Uruguay, Praderi (1997)
estimated that between 1974 and 1994 at least 3,683 dolphins were killed. The highest and
lowest annual estimates were 418 and 66 dolphins caught in 1974 and 1994, respectively. The
bycatch was even higher prior to that period. In the late 1960s the annual bycatch was
Search WWH ::
Custom Search