500
cycles on dolphin distribution is another factor contributing to differences in habitat
use. Mixed groups were never observed, and is probable that these species avoid
physical interaction due to potential for aggression (Bearzi 2005). Observational
data suggest that when Guiana dolphins arrive in the core areas of franciscana dolphins, they temporarily leave the area.
The Babitonga Bay Guiana dolphin population is roughly four times larger than
the franciscana population. Both show a high degree of residency inside the bay,
however the distribution of Guiana dolphins extends to the adjacent coast for hunting while franciscana dolphins are more dependent on the estuary.
Physical and behavioral characteristics of the Guiana dolphin indicate that it is
probably dominant in the niche partitioning system in Babitonga Bay (Bonesi et al.
2004). In fact, resident populations of Guiana dolphins in southern and southeastern
Brazilian coast live in a broad sympatry with franciscana dolphins; it is only in
Babitonga Bay the two species live in direct sympatry. In the southern portion of the
Guiana dolphin range, another coastal species, the bottlenose dolphin (T. truncatus),
occupies bays and estuaries (Simões-Lopes and Fabian 1999). In this areas franciscana dolphins do not live in direct sympatry with bottlenose dolphins. In Argentina
and Uruguay it is uncommon to find T. truncatus near the coast, and in these areas
franciscana dolphin populations reside in bays and estuaries (Bordino et al. 1999;
Rodríguez and Bastida 2002; Failla et al. 2012; Negri et al. 2012).
There is no clear explanation for the resident and potentially isolated franciscana
dolphin population in Babitonga Bay, but one interesting characteristic of this bay
is the deep channel that connects the bay to the ocean. Unlike other estuaries in
which the entrance channels are narrow and sand banks are common, Babitonga
Bay has a wide, deep, natural channel (25 m depth, 1.8 km width) without buildings
and no necessity for dredging to maintain it.
The home range area of both dolphin populations has declined over the last 20
years, probably as a consequence of human disturbances like sound pollution. This
situation increases species overlap, potentially increasing competition for habitat
and other resources. When food is abundant, competition is reduced or absent
(Begon et al. 1996), but habitat degradation and overfishing can alter prey abundances in Babitonga Bay. Reduction of prey may then drive changes in the diets of
sympatric species (Bonesi et al. 2004).
Populations seems to be stable at this time, but the absence of abundance estimates in the beginning of the monitoring period makes analysis of long-term trends
difficult. Further, different methods were used among studies to estimate abundances over the history of monitoring in the area. Based on current data we estimate
that if disturbance pressure continues to increase, coexistence could be unsustainable in the future. Considering that the Guiana dolphin is the dominant species in
estuarine areas throughout its range in southern Brazil, is possible that franciscana
dolphins will not persist in Babitonga Bay.
Although both species vulnerable to the same threats, franciscana dolphins
seems to be more sensitive than Guiana dolphins to accidental capture in fishing
nets and effects of habitat loss. Franciscana dolphins do not occur near very impacted
areas like harbors and cities; this is in stark contrast to Guiana dolphins, which are
M.J. Cremer et al.
cycles on dolphin distribution is another factor contributing to differences in habitat
use. Mixed groups were never observed, and is probable that these species avoid
physical interaction due to potential for aggression (Bearzi 2005). Observational
data suggest that when Guiana dolphins arrive in the core areas of franciscana dolphins, they temporarily leave the area.
The Babitonga Bay Guiana dolphin population is roughly four times larger than
the franciscana population. Both show a high degree of residency inside the bay,
however the distribution of Guiana dolphins extends to the adjacent coast for hunting while franciscana dolphins are more dependent on the estuary.
Physical and behavioral characteristics of the Guiana dolphin indicate that it is
probably dominant in the niche partitioning system in Babitonga Bay (Bonesi et al.
2004). In fact, resident populations of Guiana dolphins in southern and southeastern
Brazilian coast live in a broad sympatry with franciscana dolphins; it is only in
Babitonga Bay the two species live in direct sympatry. In the southern portion of the
Guiana dolphin range, another coastal species, the bottlenose dolphin (T. truncatus),
occupies bays and estuaries (Simões-Lopes and Fabian 1999). In this areas franciscana dolphins do not live in direct sympatry with bottlenose dolphins. In Argentina
and Uruguay it is uncommon to find T. truncatus near the coast, and in these areas
franciscana dolphin populations reside in bays and estuaries (Bordino et al. 1999;
Rodríguez and Bastida 2002; Failla et al. 2012; Negri et al. 2012).
There is no clear explanation for the resident and potentially isolated franciscana
dolphin population in Babitonga Bay, but one interesting characteristic of this bay
is the deep channel that connects the bay to the ocean. Unlike other estuaries in
which the entrance channels are narrow and sand banks are common, Babitonga
Bay has a wide, deep, natural channel (25 m depth, 1.8 km width) without buildings
and no necessity for dredging to maintain it.
The home range area of both dolphin populations has declined over the last 20
years, probably as a consequence of human disturbances like sound pollution. This
situation increases species overlap, potentially increasing competition for habitat
and other resources. When food is abundant, competition is reduced or absent
(Begon et al. 1996), but habitat degradation and overfishing can alter prey abundances in Babitonga Bay. Reduction of prey may then drive changes in the diets of
sympatric species (Bonesi et al. 2004).
Populations seems to be stable at this time, but the absence of abundance estimates in the beginning of the monitoring period makes analysis of long-term trends
difficult. Further, different methods were used among studies to estimate abundances over the history of monitoring in the area. Based on current data we estimate
that if disturbance pressure continues to increase, coexistence could be unsustainable in the future. Considering that the Guiana dolphin is the dominant species in
estuarine areas throughout its range in southern Brazil, is possible that franciscana
dolphins will not persist in Babitonga Bay.
Although both species vulnerable to the same threats, franciscana dolphins
seems to be more sensitive than Guiana dolphins to accidental capture in fishing
nets and effects of habitat loss. Franciscana dolphins do not occur near very impacted
areas like harbors and cities; this is in stark contrast to Guiana dolphins, which are
M.J. Cremer et al.