Biology Reference
In-Depth Information
Although data on abundance exist only for the southern range of the species (Secchi et
al., 2001; Crespo et al., 2009), empirical evidence suggests that the southern population is
larger than the northern one. Relative abundance of franciscanas is suspected, based on bycatch
per unit of effort (CPUE) data, to be much higher to the south (
e.g.
Crespo et al., 1986; Corcuera,
1994; Praderi, 1997; Secchi et al., 1997; Ott, 1998; Secchi & Ott, 2000), than to the north of
Santa Catarina (
e.g.
Di Beneditto et al., 1998; Di Beneditto & Ramos, 2001; Bertozzi & Zerbini,
2002). Furthermore, as previously hypothesized by Secchi et al., (2003a) the franciscana
abundance might be limited in the north by the presence of abundant sympatric species such as
the Guiana dolphin (
Sotalia guianensis
), which has its southern limit at Santa Catarina (Borobia
et al., 1991) and other less abundant or occasionally sympatric species such as the Atlantic
spotted (
Stenella frontalis
), rough-toothed (
Steno bredanensis
) and bottlenose (
Tursiops
truncatus
) dolphin (Moreno et al., 2005; Bastida et al., 2007). Because these sympatric species
may compete for the same resources (habitat or food), it may be reasonable to expect that the
northern population is less abundant and a more opportunistic feeder than the southern one. In
northern Rio de Janeiro, franciscana feed upon about 25 species and a low degree of competition
for the same resources with Guina dolphin has been reported (Di Beneditto & Ramos, 2001). To
the south, franciscana is probably widely distributed from northern Argentina to Santa Catarina
(as suggested by the aerial surveys in southern Brazil and northern Argentina). The only
cetaceans that are sympatric with the southern population year round are the highly coastal or
estuary-dependent small populations of bottlenose dolphins, mainly in southern Brazil and
Uruguay (Bastida et al., 2007), and Burmeister's porpoises,
Phocoena spinipinnis
, in northern
Argentina (Brownell and Praderi, 1984; Corcuera et al., 1994; Molina et al., 2005). Moreover,
genetic diversity was greater within samples collected from animals from the southern
populations than within the samples of franciscanas from the northern populations (Secchi et al.,
1998; Ott, 2002; Lázaro et al., 2004; Mendez et al., 2007).
P
REY AND
P
REDATORS
Likewise the true river dolphins, franciscana has an elongated, narrow rostrum filled with
many small and sharp teeth (up to 65 in each jaw) well adapted to its trophic niche,
characterized by small and soft prey. Franciscanas feed upon several small-sized shallow-
water fish, cephalopods, and crustaceans (Brownell, 1989; Di Beneditto & Ramos, 2001;
Rodriguez et al., 2002; Danilewicz et al., 2002).
Prey are typically smaller than 80 mm in
length and 5 g (fish) or 10 g (squids) in weight, regardless of the geographic location (
e.g.
Bassoi, 1997; Di Beneditto & Ramos, 2001; Rodriguez et al., 2002). The diet of adults
consists of at least 76 food items (see Danilewicz et al., 2002 for a revision) that includes
fishes (83%) with the teleost Sciaenidae family as the predominant prey
(
mainly
Cynoscion
guatucupa),
crustaceans (9%), and molluscs (8%) (specially, the small squid species such as
Loligo
sanpaulensis)
,
although geographic variation exists
(
e.g.
Fitch & Brownell, 1971; Ott,
1994, Bassoi, 1997, 2005; Di Beneditto & Ramos, 2001; Rodríguez et al., 2002). Shrimps are
very important in the diet of these young dolphins (Rodríguez et al., 2002; Bassoi, 2005).
Three diet categories were defined during the first year of life of franciscanas: lactating,
mixed diet, and solid diet (Rodríguez et al., 2002). The first solid food is composed of very
small fish and shrimps. This ontogenetic variation might be related to a learning process with
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