Orcas who lived in petting pools show higher levels of aggression than others.
The violent death of SeaWorld trainer Dawn Brancheau in the jaws of an orca named Tilikum in early 2010 triggered a series of journalistic articles and books (see also) on the history and conditions of orca captivity resulting in severe injuries and deaths to both orcas and trainers. This culminated in the documentary "Blackfish" that has strongly impacted the public and has led to an open debate on the ethical propriety of keeping such large, intelligent, and socially complex creatures in captivity. A significant part of the discussion has been the question of why certain orcas turned violent. Alongside the public discussion, the scientific community has been asking many of the same questions.
One apparent reason for the aggression was the physically and psychologically abusive treatment that Tilikum suffered at SeaLand prior to arriving at SeaWorld. However, in the case of the aggressive SeaWorld orcas that never experienced SeaLand, the focus has been on such factors as the insufficient size of the pools, disruption of social structure, including premature separation of calves from their mothers, the constant pressure to perform, premature breeding of females, and a host of other possible reasons.
There was, however, a bit of early common history in the background of several of SeaWorld's most aggressive orcas and their elevated aggression continues in a number of their descendants. These orcas and descendants account for a majority of the documented, published aggressive encounters at SeaWorld. They constitute a majority of the aggressions shown in "Blackfish" other than those directly related to Tilikum. The orca profiles that SeaWorld released make no mention of this early history yet this history has been hidden in plain sight on the Internet for many years in old photo sets and discussions if one simply knew the correct search terms. However, after the public airing of "Blackfish," some of the human participants in this history created a website called "Exploring the Cetacean-Human Relationship" documenting these early events and have sought to raise public awareness.
The prehistory of orca aggression
SeaWorld's Icelandic captives from 1977 (Kandu 5, Canuck 2) and 1978 (Katina, Kasatka, Kotar, Shawn) were sent to the San Diego facility for performer training. While in training, they were rotated through the dolphin petting pool. Pool monitoring seemed to be an incidental function of people with other main jobs, so it was sparse to non-existent. The orcas were in their first and second years of captivity and ranged in ages from about two to five years.