Depredation of Patagonian toothfish (Dissostichus eleginoides) by two sympatrically occurring killer whale (Orcinus orca) ecotypes: Insights on the behavior of the rarely observed type D killer whales
Résumé
Sympatric forms of ecologically distinctive killer whales (Orcinus orca) have been
documented worldwide. This study focused on a new case of such sympatric occurrence
of the “Crozet” type and the recently described “type D” killer whales off the
Crozet Islands. The two ecotypes are morphologically and genetically distinct, but
they both depredate the same local longline fishery. We used observational,
photo-identification, and fishing data, collected between 2003 and 2015, to examine
differences in their patterns of depredation. Of the 828 sets where ecotype could be
confirmed, type D killer whales interacted with 82 (11%) of the sets, including 9
(1%) sets that were simultaneously depredated by both ecotypes. Associations
between the two types were never observed. Type D killer whales typically occurred
in larger groups and both ecotypes preferentially depredated Patagonian toothfish
(Dissostichus eleginoides). GLMM modeling revealed that the probability of type D
depredation significantly increased throughout the study period, especially in deep
waters, and photo-identification data suggested that a subset of all individuals were
habituating to depredation. This study documents the partitioning of resources
between two distinct ecotypes of killer whales and provides preliminary insight into
the feeding ecology of the rare type D killer whale.