Killer Whales have been exploited (i.e., deliberately hunted) in several regions. Norwegian whalers in the eastern North Atlantic took an average of 56 whales per year from 1938 to 1981. Whalers in Japan took an average of 43 per year in coastal waters from 1946 to 1981. Soviet commercial whalers took an average of 26 Killer Whales annually from 1935 to 1979, primarily in the Antarctic, and then took 916 animals in the 1979/80 Antarctic season (Dahlheim and Heyning 1999,). Killer whales are also taken in small numbers for food (or as a population control measure) 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 is known to occur. This problem was especially serious in Alaska, where depredation of longline fisheries is extensive (Jefferson et al. 1993, Yano and Dahlheim 1995, Donohue et al. 2003), and retaliation for depredation was credited with contributing to a temporary population decline at Crozet Island, in the Southern Ocean (Poncelet et al. 2010). Depredation of longline catches appears to be a recent and increasing phenomenon, and now occurs in many regions (e.g., Aleutian Islands and Southeast Alaska, South Georgia, Crozet Islands, and several other Southern Ocean Subantarctic island areas, Australia, and other locations in the South Pacific). Increased depredation of Toothfish hooked in fisheries in the Crozet Archipelago appears to have had a positive effect on calving rates of female Killer Whales (Tixier et al. 2015).
Incidental mortality in trawl, longline and driftnet fishing operations occurs, but is considered rare (Dahlheim and Heyning 1999, Muto et al. 2016).
Persistent bio-accumulating contaminants present a serious potential risk to some Killer Whale populations. Ross et al. (2000) reported that total polychlorinated biphenyl (PCB) concentrations were very high in three Killer Whale populations (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 are considered among the most PCB-contaminated cetaceans in the world (Ross et al. 2000). More recently, Jepson et al. (2016) found very high mean blubber PCB concentrations in Killer Whales in Europe. Small or declining populations of Killer Whales in the northeastern Atlantic (e.g., the very small population associated with the Strait of Gibraltar) have been associated with low recruitment, which is consistent with PCB-induced reproductive toxicity. Also, increasing levels of polybrominated diphenyl ethers which, like PCBs, can cause immunotoxicity, neurotoxicity, and reproductive impairment, have become a concern in the southern resident population in British Columbia and Washington (Mongillo et al. 2012).
Disturbance is another concern in areas where Killer Whales are subject to whale-watching tourism. Moving boats can disrupt foraging and resting, and underwater boat noise can affect social and echolocation signals of the whales or otherwise interfere with foraging (Erbe 2002, Williams et al. 2002, Noren et al. 2009). For example, close approaches by whale-watching vessels have been shown to result in avoidance responses by resident Killer Whales in British Columbia, which presumably has energetic costs to whales that are frequently subjected to this activity (Williams et al. 2002, 2006). Fast-moving boats in the proximity of Killer Whales also present a risk of vessel strikes (e.g., Visser 1999).
Potential disturbance and acoustic masking effects of increasing ambient noise levels associated with shipping and other vessel traffic is a growing concern in some regions (Williams et al. 2014). Disturbance responses and masking of echolocation signals in areas with heavy ship activity have the potential to disrupt foraging behavior and reduce prey acquisition with possible population-level consequences (Williams et al. 2014, Veirs et al. 2016).
The Exxon Valdez oil spill in Alaska in March 1989 was strongly correlated with the subsequent loss of Killer Whales from transient and resident pods that had been seen swimming near or through oil slicks early in the spill (Dahlheim and Matkin 1994). The AT1 pod had at least 22 individuals when first censused in 1984, before the spill. Eleven individuals have been missing from this pod since 1990 and two more since 1992. Four more whales from this pod died in the early 2000s and there have been no recorded births within the pod since 1984. As of 2012, only 7 of the original 22 AT1 pod members remained. A resident Killer Whale pod affected by the oil had failed to recover 16 years after the spill (Matkin et al. 2008, 2012). Oil spills may also have indirect effects on Killer Whales by reducing prey abundance.
There have been large-scale reductions in predatory fish populations (Myers and Worm 2003; Baum et al. 2003) along with overfishing and collapse of several important “prey” fish stocks worldwide (Jackson et al. 2001). There have also been large declines in some marine mammal populations but many of those populations have recovered or are recovering under protection, and this may mean that some mammal-eating populations of Killer Whales are recovering as well. The same is not likely true of many fish-eating populations, however.
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 Pacific Salmon stocks have significantly declined as a result of overfishing, habitat degradation, and reduced survival at sea. Declines in survival and reproductive rates of resident Killer Whales were found to correlate with a multi-year reduction in Chinook Salmon abundance in coastal British Columbia and Washington State (Ward et al. 2009, Ford et al. 2010). Mammal-eating Killer Whales along the west coast of North America likely experienced periods of reduced prey availability due to depletion of pinniped populations prior to 1970 (Ford and Ellis 1999). Depletion of Bluefin Tuna stocks in the Mediterranean Sea has almost certainly had a serious impact on the survival of Killer Whales near the Strait of Gibraltar (Cañadas and de Stephanis 2006, Esteban et al. 2016).
How global climate change and its impacts on the marine environment will affect the different populations and ecotypes of Killer Whales is uncertain (e.g., Learmonth et al. 2006). However, in the Arctic, declining extent and duration of sea ice has given Killer Whales more access to marine mammal prey (Belugas, Narwhals, Bowhead Whales) that depend on ice for refuge from predation (Ferguson et al. 2010, Higdon et al. 2012, George et al. 2015). Similarly, in the Antarctic, the abundance and availability of Killer Whale prey is mediated through the presence of ice (e.g., Pitman and Durban 2012), which means that many species, including both Killer Whales and their prey, face an uncertain future.
