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young franciscana preying upon slower prey.
Franciscana's feeding strategy seems to be
opportunistic, eating the most abundant prey in the area (Danilewicz et al., 2002). Seasonal
fluctuations in the franciscana's diet match with patterns of variation in the abundance of the
prey species throughout the year (Di Beneditto & Ramos, 2001; Rodriguez et al., 2002;
Bassoi, 2005). Furthermore, decadal changes in franciscana's diet seems also to match with
decline in fish stock abundance (
e.g.
Secchi et al., 2003b; Secchi, unpubl. data), though
further studies are necessary for certainty. To date there is no information about daily food
consumption in both juvenile and adult specimens, although feeding studies carried out in
captivity indicate that a medium sized franciscana might eat approximately 10% of its body
weight daily with a diet composed by a variety of fishes and invertebrates of low, medium
and high caloric values (Loureiro et al., 2000). Determining franciscana's nutritional
requirements and preferred prey is crucial for assessing the potential competition with coastal
fisheries and, most importantly, to understand its role in the ecosystem functioning.
Predation is a natural cause of mortality for
P. blainvillei.
The top most predators in the
marine ecosystem, the killer whales (
Orcinus
orca
)
and several shark species are known to
prey upon franciscana throughout most of its distribution range (Di Beneditto, 2004; Monzón
et al., 1994; Ott & Danilewicz, 1996; Praderi, 1985; Santos & Netto, 2005). In Uruguayan
waters, the broadnose seven-gill (
Notorhynchus
cepedianus
), hammerhead (
Sphyrna
spp), the
sand tiger (
Eugomphodus
taurus
), tiger (
Galeocerdo
cuvieri
), and requiem (
Carcharhinus
sp.)
sharks are known predators of franciscana
(Brownell, 1975; Pilleri, 1971; Praderi, 1985).
Other shark species are also potential predators. Fresh wounds and scars caused by shark bites
found on individuals incidentally caught in fishing nets suggest that predation might also be
an important source of natural mortality of franciscana in Argentina (Monzón et al., 1994) as
well as in Brazil. A male killer whale was observed preying on a free-swimming franciscana
in coastal waters off Parana State, southern Brazil (Santos & Neto 2005). Ott and Danilewicz
(1996) found a female killer whale washed ashore in Rio Grande do Sul State, southern
Brazil, with remains of three franciscana in its stomach.
P
ARASITES
A few species of ectoparasite or epizoit crustaceans have been reported for
P. blainvillei
.
The barnacle,
Xenobalanus globicipitis
, was found attached to the trailing edge of the flukes
and fin of franciscana (Brownell, 1975; Di Beneditto & Ramos, 2000). Specimens of an
unidentified necked-barnacle were also observed in the teeth row of the lower jaw of a large
and skinny, possibly old and sick franciscana. These epizoits are known to settle during their
larval stage on slow moving animals or drifting objects. The isopods
Cirolana
sp. and
Nerocila
sp. which normally infect the gill of some fishes and sharks were found on several
occasions, respectively, in the blowholes or stomachs and on the skin of franciscanas from
Uruguay and Rio Grande do Sul State, Brazil (Brownell, 1975). Likewise, the isopod
Riggia
sp. was found in the vagina of one franciscana in Santos, São Paulo State, Brazil (Ferreira et
al., 1998). The association of isopods and franciscana is not yet well understood. Films of
diatoms
Navicula
sp. and mainly
Cocconeis ceticola
have been documented to partially
covering the skin of franciscana (Nemoto et al., 1977; Santos et al., 2009).
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