2021_Tixier_et_al_Crozet_killer_whale_photo_iD_report_CEBC_CNRS.pdf (148.91 MB)
Killer whales of the Crozet Archipelago and adjacent waters: photo-identification catalogue, population status and distribution in 2020.
preprint
posted on 2021-02-01, 14:20 authored by Paul TixierPaul Tixier, Nicolas Gasco, Jared R. Towers, Christophe GuinetThree forms of killer whales (Orcinus orca)
occur around the subantarctic islands of the southern Indian Ocean (42-53°S;
34-74°E). The form encountered in both inshore and offshore waters, described
as generalist in its feeding preferences (seals, whales, penguins and fish as
prey) and known to depredate toothfish from longliners has been
opportunistically photo-identified around the Crozet archipelago since the
1960s. Together with photo-identification data collected in the Prince
Edward/Marion EEZ, Kerguelen EEZ and international waters, this report provides
up to date information on the abundance and distribution of the Crozet killer
whales. In total, 124,313 photographs taken during 2,109 encounters since 1964
were analysed, allowing for 299 individuals to be identified. Most encounters
with available data were from the Crozet EEZ (1,432 from longliners, 602 from Île
de la Possession) and occurred after 2003 when photo-identification was implemented
in the fishery observer program. Among the 188 individuals recorded in the
Crozet EEZ since 2003, 22 (12%) were also photographed in the Kerguelen EEZ, 13
(7%) in the Prince Edward/Marion EEZ and 13 (7%) in adjacent international
waters. The frequently encountered subset of the Crozet killer whale population
was composed of 23 social units (maternal groups), 19 of which included
individuals alive in 2020. These social units ranged in size from 1 to 11
individuals with a mean (± SD) of 4 ± 3 per unit. As of June 2020 when
the latest photographs included in the study were taken, abundance of this
subset was 89-94 individuals. However, detailed analysis of data collected
between 2005 and 2020 shows that the number of confirmed deaths (n = 51) exceeds
the number of recorded births (n = 46), resulting in a 5% decrease of the
population size over this period. These deaths were distributed across the
population with the majority occurring in the most common sex and age classes -
adult females and juveniles. Factors contributing to mortalities are unclear. When paired with the
fact that the Crozet killer whales already underwent a severe mortality episode
in the 1990s, these findings raise strong concerns about the future of the
population and stress the necessity of conservation actions while maintaining
an intensive monitoring effort.