Biology Reference
In-Depth Information
A
BUNDANCE
There is no current abundance estimate for the species as a whole. The only estimates
available are for the animals inhabiting the Rio Grande do Sul State, southern Brazil, and
Argentina. In 1996, thirty-four franciscanas (in 29 groups) were recorded during aerial
surveys in the area, giving a mean density of 0.657 individuals/km
2
(95%CI: 0.516 to 0.836)
for the 435 km
2
study area after correcting for the probability of missing submerged dolphins.
Extrapolating this density to the entire stock's range,
i.e.
Rio Grande do Sul State and
Uruguay (Secchi et al., 2003a) would result in an estimate of 42,078 franciscanas (95% CI:
33,047-53,542). This extrapolated result, however, should be interpreted cautiously as it is
based on a density estimate for a small fraction of the coastline, representing only 0.7% of the
possible range of the stock (
ca
64,045 km
2
). If only the Rio Grande do Sul coast, from
shoreline up to the 30 m isobath (ca. 24,315 km
2
) is taken into account, the extrapolated
abundance would be around 15,975 animals. More recently, in 2004, another aerial survey
was conducted covering a much larger area off the coast of Rio Grande do Sul State
(Danilewicz et al., 2007). Thirty-one animals were seen in 25 groups, from which the authors
estimated a density of 0.51 ind/km
2
. If extrapolated to the same area (i.e. 24,315 km
2
), this
density would result in an abundance of 12,400 individuals. This difference should not be
viewed as a population decline because the area covered in both studies differed greatly in
magnitude, mainly because of the constraints imposed by the flight autonomy of the single-
engine aircraft utilized in 1996. The first study was covered an area about 30 times smaller
than the second study and was concentrated in shallower waters close the Patos Lagoon
estuary, a potentially more productive area for both fish and franciscanas. The aircrafts and
the observers also differed among surveys. The main difference was that the aircraft had only
flat windows in the first survey while in the second survey bubble windows were present in
the rear allowing the observers to see right below the plane.
Crespo et al., (2009), carried out aerial surveys in 2003 and 2004 for estimating
franciscana abundance in Argentina. The area was divided into two sections, northern sector,
from Lavalle to Mar del Plata and from Mar del Plata to Claromecó, Buenos Aires Province;
and a southern sector, from Bahía Blanca to the mouth of Río Negro River and along the
northern coast of Golfo San Matías, Rio Negro Province. One hundred and one franciscanas
were observed in 71 sightings. For the northern sector, the density was estimated to be 0.106
ind/km
2
. Density declined with depth (0.05 ind/km
2
between the
30 m and 50 m isobaths) and
was lower in the southern sector (0.056 ind/km
2
). After correcting for submerged dolphins
and extrapolation for unsurveyed areas, the abundance for the Argentine coast was estimated
to be between 15,062 to 16,335 individuals.
Although these numbers cannot be considered absolute abundance estimates because the
surveyed area does not cover the entire population (or stock, Secchi et al., 2003a) range, they
can be viewed as reasonable approximations for assessing the potential effects of non-natural
removals (
e.g.
Secchi, 1999, 2006; Kinas, 2002; Secchi et al., 2003b; Crespo et al., 2009).
Densities, on the other hand, provide good insights about habitat use in both latitudinal and
depth gradients. The values presented above suggest that although franciscana might occur up
to the 50 m isobath or even deeper, their density is higher along the coast up to a depth of 30
m. Furthermore, the density decreases from southern Brazil towards the austral distribution
limit of the species.
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