Killer
Whales of Crozet islands: a declining population
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Between 1964 and 2000, the
Killer Whales
(Orcinus
orca) occurring in the coastal waters around Possession island,
Crozet archipelago, were photo-identified.
We only considered the years with at least
15 available photos (1977, 1980, 1982, 1985-1990, 1998-2000) to run Mark-Recapture
models.
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We estimated that the Killer Whale population
visiting Possession Crozet was 93 individuals in 1988-1989 and only
43 individuals in 1998-2000 (figure 2).
The survival model (Cormack-Jolly-Seber)
indicates that survival (including all age groups, excepted animals less
than one year old, i.e. new-born) decreased from 16,4 % over 23 years
(figure 1).
This estimate cannot separate true mortality
from emigration. |
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Figure
1
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The fecundity rate is very small: 2,82
% as the ratio number of new-borns surviving at least one year / number
of year of monitoring mature females, confirms that this population lives
in a period of strong demographic stress today.
During 1991-2000, we identified only one
juvenile of Killer whale on 9 during 1981-1990.
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Figure
2
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The decline of the population size
of Killer Whale at Crozet may be explained by several factors:
1) their small
fecundity rate,
2) the
decline of their main prey type:
the large cetaceans, due to the past whales tapping, and the Elephant
Seals (Mirounga leonina),
3) the possible
mortality induced during
interactions with long-lined fishing (Disostichus eligenoides),
and
4) the possible
dispersal of individuals
or groups from the inshore of the Possession island.
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