Killer whales have been exploited at low levels in several regions world-wide (Jefferson et al. 1993). Norwegian whalers in the eastern North Atlantic took an average of 56 whales per year from 1938 to 1981. The Japanese took an average of 43 whales per year along their coastal waters from 1946 to 1981. The Soviets, whaling primarily in the Antarctic, took an average of 26 animals annually from 1935 to 1979 and then took 916 animals in the 1979/80 Antarctic season (Dahlheim and Heyning 1999; Reyes 1991). Killer whales are still taken in small numbers in coastal fisheries in Japan, Greenland, Indonesia, and the Caribbean islands (Reeves et al. 2003).

Fishermen in many areas see killer whales as competitors, and intentional shooting of whales is known to occur. This problem is especially serious in Alaska, where depredation of longline fisheries is extensive (Jefferson et al. 1993; Yano and Dahlheim 1995; Donohue et al. 2003). Depredation of long-line catches appears to be a recent and increasing phenomenon, and now occurs in many regions (e.g., Aleutian Islands Alaska, South Georgia, Crozet Island, and several other southern ocean island areas, Australia, and other locations in the south Pacific).

During the period 1976-1988, 59 whales were captured alive off Iceland, of which 8 were released, 3 died and 48 (an average 3.7 per year) were exported to aquaria (Reyes 1991). Live-captures of several killer whales have also taken place in Japanese waters (Reyes 1991). Bycatch in trawl and driftnet fishing operations occur, but are considered rare (Dahlheim and Heyning 1999).

Persistent bio-accumulating contaminants have recently been found to present a serious potential risk to some killer whale subpopulations. Ross et al. (2000) report that total PCB concentrations were very high in three killer whales subpopulations (2 resident and 1 transient) frequenting the coastal waters of British Columbia, Canada. Transient killer whales were particularly contaminated due to their high trophic position in the marine ecosystem. PCB levels in most killer whales sampled were greater than levels established at which adverse effects occur in harbor seals, suggesting that the majority of free-ranging killer whales in this region are at risk of toxic effects. The southern resident and transient killer whales of British Columbia and Washington can be considered among the most contaminated cetaceans in the world (Ross et al. 2000).

Habitat disturbance may be a matter for concern in areas inhabited by killer whales and supporting whale-watching industries (Reyes 1991). Moving boats can disrupt activities such as foraging and resting, and underwater boat noise could affect social and echolocation signals of the whales or otherwise interfere with foraging (Erbe 2002; Williams et al. 2002). For example, close approaches by whale-watching vessels have been shown to result in avoidance responses by resident killer whales in British Columbia, which may result in energetic costs for whales frequently subjected to whale watching activity (Williams et al. 2002, 2006). Fast-moving boats in the proximity of killer whales also present a risk of collision or injury from propellers. Visser (1999) reports on propeller scars observed on killer whales in New Zealand and their possible causes of mortality.

Large-scale catastrophic oil spills have the potential to cause significant mortality of killer whales. The Exxon Valdez oil spill in Alaska was strongly correlated with the subsequent loss of several whales from a pod that had been seen swimming through light oil slicks early in the spill (Dahlheim and Matkin 1994). Oil spills may also have an indirect effect by reducing prey abundance.

There have been large-scale reductions in predatory fish populations (Myers and Worm 2003; Baum et al. 2003) and over-fishing and collapse of several important “prey” fish stocks world-wide (Jackson et al. 2001). There have also been dramatic declines in marine mammal populations throughout the world. The effects of such reductions in prey populations (both fish and marine mammal) and subsequent ecosystem changes on world-wide populations of killer whales are unknown but could result in population declines.

Due to their dietary specialization, some populations of killer whales could be especially vulnerable to a reduction of their food supply. In British Columbia and Washington State, many salmon stocks have significantly declined as a result of over-fishing, habitat degradation and reduced ocean survival. This is likely to affect fish-eating resident killer whale populations in that region (Ford et al. 2005). Mammal-hunting killer whales in British Columbia likely experienced periods of reduced prey availability due to depletion of pinniped populations prior to 1970 (Ford and Ellis 1999). The depletion of the Mediterranean bluefin tuna stock is considered a source of concern for the survival of the Gibraltar killer whales (Cañadas and de Stephanis 2006).

Predicted impacts of global climate change on the marine environment may negatively affect certain killer whale subpopulations more than others through changes in prey availability (see e.g. Learmonth et al. 2006).