Collapsed or collapsing dorsal fins are rare in most wild populations and usually result from a serious injury to the fin, such as from being shot or colliding with a vessel. After exposure to the 1989 "Exxon Valdez" oil spill, two male resident killer whales experienced dorsal fin collapse, and the animals subsequently died. In 2002, the dorsal fin of a stranded killer whale showed signs of collapse after three days but regained its natural upright appearance as soon as the orca resumed strong normal swimming upon release. A study in 1998 recorded that 7 out of 30 (23%) adult male killer whales off the coast of New Zealand had abnormal dorsal fins. Fins were considered abnormal if they displayed collapse, partial collapse, or bends. This is a higher prevalence of these deformities than in other areas of the world, as studies have reported rates of abnormal fins in wild adult males at 4.7% in British Columbia and 0.57% in Norway. Researchers in 1994 found that of the ~300 killer whales photographed off the coast of British Columbia, fewer than 1% were observed to have "droopy" dorsal fins. ABC News reported that captive killer whales have attacked nearly two dozen people since the 1970s. Studies of killer whales in the wild have identified at least two categories, based on their territorial range. Those living in a limited area, such as Puget Sound or the Strait of Juan de Fuca, are termed "resident" whales, while "transient" whales roam the oceans at will. These "transient" types have to be more aggressive, in order to assert themselves in a wide range of territories and to prey on a variety of different species. This increased aggressiveness does not disappear in captivity. Furthermore, captivity itself has been asserted to aggravate aggressive behavior, resulting in a "cetacean equivalent of anxiety disorder." Captive killer whale attacks on humans seem to fall mostly into the categories of: biting during feeding, ramming in the water, and holding under water. Killer whales biting trainers during feeding or shows is generally the mildest form of attack seen, but can escalate to an animal dragging the trainer underwater and holding them there until they lose consciousness or drown. Trainers who have had killer whales ram into them in the water tend to suffer from a range of injuries including internal bleeding, broken bones, ruptured organs, and heart attack. Tilikum, a large bull male killer whale who died in early 2017, had been involved in the death of three individuals since his capture near Iceland in November 1983.