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to apply mark-recapture studies to this species. Secchi (2006) used two approaches for
estimating survival rates of franciscana from Rio Grande do Sul and Uruguay: by fitting the
Siler model (Siler, 1979) to age-at-death data of bycatch and beach cast animals and by using
life tables from similar model species. Despite their intrinsic limitations, the two methods,
after careful treatment to reduce bias effects, resulted in identical mean survival rates for calf
(0.67 to 0.74), juvenile (0.88 to 0.90) and adult franciscana (0.86 to 0.87). Life-tables from
other species may be the only option to model the survivorship of many cetacean species due
to a lack of data, although caution is needed when selecting the model species. This approach
relies on the assumption that the chosen species have similar life histories.
The use of distributions of age-at-death data to estimate mortality relies on the
assumption that the population has a stable age distribution. For age distribution to be stable,
age-specific differences in both death rates and birth rates across age classes must be
constant, and need to have been long enough for the age structure to equilibrate. Removal of
franciscana from the population through bycatch can lead to deviations from the stable age
distribution if the age structure of the bycatch fluctuates through time. This fluctuation leads
to unstable age distribution biasing survival estimates. If biases are small, it can be concluded
that survival rates of franciscana are lower than other small cetaceans (
e.g.
bottlenose and
Hector's dolphins - Wells & Scott, 1990; Cameron et al., 1999; Du Fresne, 2004) and much
lower than medium size and large cetaceans (
e.g.
killer whales - Olesiuk et al., 1990;
bowhead and humpback whales - Zeh et al., 1995; Givens et al., 1995; Barlow & Clapham,
1997). This seems biologically reasonable as there is strong evidence that body mass is
positively correlated with survival in mammals (Millar & Zammuto, 1983). Hector's dolphins
however, have similar body mass to franciscanas. The explanation for the higher survival rate
in Hector's dolphin is related, to some extent, to its much lower reproductive potential. A
female Hector's dolphin first reproduces at approximately 8 years of age and produces one
offspring every two to three years (Slooten, 1991; Slooten & Dawson, 1994). A female
franciscana, on the other hand, has its first calf at an age of approximately 4 years and
reproduces every one or two years.
P
OPULATION
G
ROWTH
R
ATE
Based on matrix population models using reproduction and survival rates data as input
parameters, growth rates for different franciscana populations were estimated at 0.8% to 3.8%
(Secchi, 2006). The estimates of population growth rates have to be interpreted with caution
due to the fact that survival rates used as input parameters were estimated based on limited
data and on the model life tables of similar species. These survival rates were also assumed to
be the same for all franciscana populations. Growth rate estimates can be sensitive to
estimates of survival rates. If the assumption that all franciscana populations have the same
survival rate is valid, differences in population growth rate will be determined by
reproduction. The higher estimated growth rate for the franciscanas from Rio Grande do Sul
State/Uruguay and Rio de Janeiro State is due to a higher reproductive potential of the
females from those areas. Females are approximately one year older when they attain sexual
maturity in the two populations adjacent to Rio Grande do Sul/Uruguay (
i.e.
from Santa
Catarina/Paraná/São Paulo states and Argentina, to the north and to the south, respectively).
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