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Recovery Strategy for the Transient Killer Whale [Proposed]

1.5 Threats

1.5.1 Characterizing Threats Using a Weight-of-Evidence Approach

West Coast transient killer whales are long-lived organisms that sit at the top of the trophic ladder. However, these killer whales appear to have low reproductive rate, which significantly reduces their ability to recover from catastrophic events or population declines.  Their small population size (currently estimated at ~250 animals, CRP-DFO unpublished data) places them at additional risk for anthropogenic threats. Since scientific, ethical, logistical and legal challenges preclude direct or causal experimentation with killer whales, a weight-of-evidence approach provides a framework for characterizing and prioritizing the threats that they face.  Such an approach is common in the human pharmaceutical sector, where toxicity, safety and efficacy data from controlled laboratory animal experiments are used to extrapolate to humans. In the case of transient killer whales, a weight-of-evidence approach draws upon the collective scientific results from controlled laboratory, captive, and field studies on other marine mammals (such as harbour seals), as well as opportunistic observations and naturally occurring experiments on killer whales and other cetaceans in the wild.  This approach is used wherever possible to describe the threats to which killer whales may be vulnerable. 

There are numerous anthropogenic threats to the viability of transient killer whales. These include chemical contaminants (both legacy and emerging), biological pollutants, trace metals, physical disturbance, acoustical disturbance (both chronic and acute), toxic spills, disease, collision with vessels, and the effects of culls on their prey (currently prohibited).  Of these, the most pressing anthropogenic threats to transient killer whales are: 1) environmental contaminants, and 2) noise and disturbance.  As a result of their small population size and their extremely limited reproductive potential, the population is particularly vulnerable to any sources of mortality that may be considered as above ‘background’.  However, the extent to which current threats may act synergistically to impact killer whales is unknown, but in other species multiple stressors have been shown to have strong negative and often lethal effects, particularly when animals carry elevated levels of environmental contaminants (Sih et al. 2004).

1.5.2 Threat classification

See Appendix B for clarification of the termsused in categorizing the threats and note that Persistent Organic Pollutants are divided into legacy and emerging contaminants because different strategies are required to address them.)

Table 1  Anthropogenic Threat Classification Table
1Stress: Persistent Bioaccumulating Toxins (PBTs): Legacy Contaminants Stressor Information
Stressor CategoryPollution and Changes in natural processes (Food supply)ExtentWidespread and locally concentrated
General StressorPersistent Bioaccumulating Toxins (PBTs) OccurrenceCurrentCurrent
Specific StressDirect toxic effects and transfer (and bioaccumulation) of contaminants to killer whales through preyCausal CertaintyExpectedExpected
EffectReproductive impairment, endocrine disruption, skeletal abnormalities, cancer, etc.Level of ConcernHigh
2Stress: Persistent Bioaccumulating Toxins (PBTs): Emerging Contaminants Stressor Information
Stressor CategoryPollution and Changes in natural processes (Food supply)ExtentWidespread and locally concentrated
General StressorPersistent Bioaccumulating Toxins (PBTs) OccurrenceCurrentCurrent
Specific StressDirect toxic effects and transfer (and bioaccumulation) of contaminants to killer whales through preyCausal CertaintyExpectedExpected
EffectReproductive impairment, endocrine disruption, skeletal abnormalities, cancer, etc.Level of ConcernHigh
3Stress: Chronic NoiseStressor Information
Stressor CategoryHabitat DegradationExtentWidespread
General StressorVessel NoiseOccurrenceCurrentCurrent
FrequencyContinuous, with some seasonal variabilityContinuous, with some seasonal variability
Specific StressMasking of  communication signals, inability to forage successfullyCausal CertaintyPlausible but requires further studyPlausible but requires further study
EffectPhysiological and physical harmLevel of ConcernModerate
4Stress: Acute NoiseStressor Information
Stressor CategoryDisturbanceExtentLocal point sources throughout range
General StressorIntense impulsive soundOccurrenceCurrentCurrent
Specific Stress

Seismic surveys

Military sonar

Underwater explosions

Causal CertaintyExpectedExpected
SeverityLow at current frequencyLow at current frequency

Physiological impairment and possible physical harm (from military sonar & underwater explosions only)

Behavioural effects

Level of ConcernHigh because of potential to expand
5Stress: Physical DisturbanceStressor Information
Stressor CategoryDisturbanceExtentLocalized but widespread
General Stressor

Recreational activities

Whale-watching operations

FrequencyContinuous, with some seasonal variability 
Specific StressInterruption of foraging and social behavioursCausal CertaintyExpected but requires further study 
EffectPossible displacement

Level of Concern


6Stress: Biological PollutantsStressor Information
Stressor CategoryPollution and Changes in natural processes (Food supply)ExtentLocalized
General StressorPrey reduction and toxic effectsOccurrenceAnticipated 
Specific Stress

Prey species are

vulnerable to pollutants that can spread quickly throughout the marine environment. May also impact killer whales directly.

Causal CertaintyPlausible 
EffectPhysiological changes, disease, reduced prey availabilityLevel of ConcernUnknown
7Stress: Toxic Spills Stressor Information
Stressor CategoryHabitat Degradation and PollutionExtentLocalized
General StressorToxic spills, including hydrocarbonsOccurrenceAnticipated 
Specific StressIngestion/ exposure to noxious materialsCausal CertaintyDemonstrated 
EffectPhysiological impacts/ deathLevel of ConcernHigh
8Stress: Collision with VesselsStressor Information
Stressor CategoryAccidental MortalityExtentLocalized
General StressorHigh speed vessel trafficOccurrenceCurrent 
Specific StressBlunt force trauma and/ or lacerationsCausal CertaintyDemonstrated 
EffectDirect or indirect mortality (via infection)Level of ConcernLow
9Stress: Decline in Prey Availability and/or QualityStressor Information
Stressor CategoryConsumptive use or cullingExtentWidespread
General StressorCulling OccurrenceCurrentHistorical
FrequencyUnknownContinuous until early 1970s
Specific StressPrey reduction Causal CertaintyPlausiblePlausible
Effect Lack of foodLevel of ConcernLow (based on current seal management and cetacean protections)

1.5.3 Description of threats


Transient killer whales are the most polychlorinated biphenyl (PCB)-contaminated marine mammals in the world described to date (Ross et al. 2000), underscoring concerns that they may be at elevated risk for adverse health effects. Within the generic class of contaminants known as Persistent Bioaccumulating Toxins (PBTs), or alternatively Persistent Organic Pollutants (POPs), PCBs, are the greatest toxicological concern in high trophic level organisms in the northern hemisphere.  PBTs are persistent, toxic, and bioaccumulate, all features that render transient killer whales vulnerable to heavy contamination and to health risks. PBTs are not typically acutely toxic, but rather are considered as ‘hormone mimics’, or ‘endocrine disruptors’ because of their chronic, slow-acting and insidious effects on normal growth and development of organ systems. As such, affected populations have been shown to suffer from diminished reproductive health, decreased immune function (and increased incidence of disease), skeletal abnormalities, and neurological impairment.

Transient killer whales are at particular risk to PBT contamination because they are long-lived animals that feed high in the food web, with their diet comprising other animals that are already contaminated with PBTs (Ross et al. 2004, Mos et al. 2006).  Adult females of both the resident and transient killer whales are less PBT-contaminated than their male counterparts, due to the reproductive transfer of PBTs to their offspring during gestation and lactation (Ross et al. 2000, 2002, Rayne et al. 2004, Ross 2006).  Harbour seals, one of the principal prey species of transients, are known to be relatively contaminated with PBT chemicals, particularly near urban areas (Ross et al. 2004). Levels of PCBs in Puget Sound harbour seals have been associated with immunosuppression and endocrine disruption (Mos et al.  2006,  Tabuchi et al 2006.)

Legacy Contaminants

PBTs include ‘legacy’ contaminants, such as PCBs and dichloro-diphenyl trichloroethane (DDT), which are no longer widely used in industrialized countries but continue to persist in the environment.  Dioxins and furans have declined in the environment and are found at relatively low levels in killer whales, reflecting the metabolic removal of the compounds at increasing trophic levels in the food web (Ross et al. 2000). Transient killer whales contain PCB levels that are two to four times higher than those of the threatened St. Lawrence beluga whales (Delphinapterus leucas) (Martineau et al. 1987, Béland et al. 1993, Ross et al. 2000).  While unequivocal evidence is a near-impossibility in the real world of complex contaminant mixtures, these belugas are suspected of having contaminant-associated reproductive impairment and immunosuppression, which may explain the failure of the population to recover since they were afforded protection from hunting in 1979 (De Guise et al. 1995). These levels are considerably higher than those known to cause PCB-associated reproductive impairment, skeletal abnormalities, endocrine disruption and immunotoxicity in pinnipeds (Ross 2000, Ross et al. 2004).  Although PCB levels are declining in the environment, recent models suggest that it will take decades before the PCB levels in killer whales decline below the thresholds for adverse effects (Hickie et al  2007).  Because transient killer whales feed on contaminated prey, their contaminant levels will not decline as quickly as they will for resident killer whales, even if the contaminant is no longer used.

Emerging Contaminants

While legacy PBTs have been largely regulated in the industrialized world, a number of contaminants with similar properties remain on the market, or represent by-products of current practices. These include the polybrominated diphenyl ethers (PBDEs), which are used as flame retardants in applications ranging from textiles to televisions and computers. Two of the three commercial formulations (penta and octa) have been banned in Europe or withdrawn from the marketplace in North America, but decaBDE remains in use. Since decaBDE breaks down into penta- and octa-like forms in the environment, the exposure of killer whales to increasing levels of endocrine-disrupting PBDEs remains a significant concern.  PBDE levels in humans and in pinnipeds have been doubling approximately every four to five years (Hites 2004, Ross 2006).  While many questions remain unanswered about the nature of its toxicity, growing evidence of endocrine disruption and immunotoxicity (Darnerud 2003, Hall et al. 2003) highlight the emerging concern associated with this currently-used flame retardant. Analyses suggest that transient killer whales carry even higher levels of PBDEs than members of the Endangered southern resident killer whale population (Ross 2006).

A number of other PBTs may also affect transient killer whales, including persistent aromatic hydrocarbons, di- and tri-butyltin, perfluoro-octane, alkylphenol ethoxylates, and polychlorinated naphthalenes, paraffins and terphenyls. Appendix C lists PBTs and their potential risk to transient killer whales and their prey, as well as a brief summary of their sources. 

There is a high level of concern about the potential impacts of PBTs on transient killer whales.  A weight-of-evidence approach needs to be incorporated into research, conservation planning and regulatory decision-making, in order to better protect killer whales and their prey from these highly toxic compounds.

Biological Pollutants

Transient killer whales may be at heightened risk to the impacts of exotic diseases or ‘biological pollution’ as a result of their preference for marine mammals as prey. Viruses, bacteria and macroparasites typically cross species barriers more readily when the two species are more closely related.  Transients may be exposed to pathogens that are endemic to their mammalian prey or from spill-over from terrestrial sources, such as domestic pets or livestock. Evidence of sewage- or runoff-related infectious diseases in Puget Sound harbour seals (Lambourn et al. 2001)and in California sea otters (Miller et al. 2002) highlight this route as one of concern for transient killer whales.

A number of high profile mass mortalities in several species have drawn attention to the potential threat that biological pollution poses to marine mammals, and identifies these pollutants as emerging conservation concerns (DeSwart et al. 1995, Miller et al. 2002, Ross 2002, Mos et al. 2003, Mos et al. 2006).  Biological pollutants may act via two routes, either by infecting and impacting the prey of transient killer whales, or by infecting transient killer whales themselves. In addition, the immunotoxic nature of the PBTs found at very high levels in transient killer whales may predispose the whales to increased risk or severity of infection by biological pollutants (Jepson et al. 1999, Ross et al. 1996, Mos et al. 2006).  

Pathogens are capable of spreading quickly in marine mammal populations. For example, Morbillivirus epidemics in seals and dolphins spread at a rate of 3000-6000 km per year (McCallum et al. 2003).  Certain pathogens, such as Morbillivirus spp., occur naturally in the marine environment.  Some of the more well-known species of Morbillivirus that have been identified include canine distemper virus, phocine (seal) distemper virus, and two forms of cetacean morbillivirus (dolphin and porpoise).  Infection can result in pneumonia, reduced lymphocyte production and encephalitis.  Cetacean morbilliviruses were responsible for the deaths of more than 50% of the bottlenose dolphin population along the east coast of the US in 1987-1988 (Di Guardo et al. 2005).  Cetacean morbilliviruses have been detected in stranded dolphins off California, but there have been no epidemics in the Pacific (Reidarson et al. 1998).

Other pathogens, such as Brucella spp. and likely Toxoplasma gondii, spill over from terrestrial sources through sewage and agricultural runoff (Lambourn et al. 2001, Miller et al. 2002, Mos et al. 2006).  Blood testing of 12 stranded killer whales revealed that nine tested positive for Brucella (S. Raverty, BCMAFF, Abbotsford, personal communication Jan. 17, 2007).  In cetaceans, Brucella is associated with lesions in the reproductive tract as well as encephalitis (González et al. 2002, Steven Raverty, BCMAFF, personal communication Jan. 17, 2007).  Harbour seals exposed to runoff from urban and agricultural areas carry a number of bacterial and protozoan pathogens, which they are more vulnerable to due to their increased chemical contaminant burdens (Mos et al. 2006). 

There are approximately 100,000 harbour seals in British Columbia (P. Olesiuk, PBS, DFO, unpublished data).  If a large pathogenic outbreak caused mass mortalities of harbour seals in British Columbia, such as occurred in northwestern Europe in 1988 (18,000 dead) and again in 2002 (21,000 dead, Di Guardo et al. 2005), there could be potential consequences for transient killer whales due to the loss of one of their principal prey species.  As transient killer whales are also heavily chemically contaminated and likely immuno-compromized, they may also be more vulnerable to direct infection with the same pathogens. 

Climate change may play a significant, although indirect, role in the development of infectious disease epidemics.  For example, changes in the El-Niño Southern Oscillation have resulted in measurable effects on the development of pathogens, survival rates, and disease transmission in the marine environment (Harvell et al. 2002).  Exactly how climate change and global warming may affect the vulnerability of killer whales, and in particular, their prey, to pathogens is unknown, but it may become a larger threat in the future as ocean temperatures continue to increase.

Trace Metals

Little information is available on the levels and effects of trace metals on marine mammals.  Trace metals occur naturally in the marine environment, and killer whales have evolved the ability to detoxify some of these substances, such as mercury (Martoja and Berry 1980).  However, elevated levels can be found in urban and industrial areas, and may be of concern to both killer whale populations and their prey (Grant and Ross 2002). 

Acoustic Disturbance

At the time of writing of the COSEWIC status report on killer whales (Baird 2001), there was relatively little known about the potential impacts of noise on marine mammals.  Since then, there has been a growing awareness that noise likely represents a significant threat to marine life that degrades their habitat. It also may affect their ability to detect prey and predators, to communicate and to acquire information about their environment.  It can do so by disrupting natural behaviours such as foraging, displacing prey, potentially impairing hearing, either temporarily or permanently, and causing physiological damage (Barrett-Lennard et al. 1996, Erbe 2002, NRC 2003). 

It is challenging to describe and measure the effects of disturbance, as responses may be subtle and/or difficult to interpret.  As well, animals may show no obvious behavioural response to disturbance, yet still be negatively affected.  Todd et al. (1996) found that humpback whales remained in close proximity to underwater explosions and showed no obvious behavioural responses to them.  However, there were significantly higher entanglement rates during this time, and subsequent necropsies of two whales that drowned in nets revealed acoustic trauma (Ketten et al. 1993).  Although the study of how anthropogenic sources of sound affect marine mammals is relatively new, killer whales rely heavily on the use of sound, and the costs of hearing loss could be severe.

Acoustic disturbance can be of two types: chronic and acute. Potential impacts of these two types of disturbance may differ and require separate mitigation strategies.  For this reason, chronic and acute acoustic disturbance are considered separately in this discussion and in Table 1.

Chronic Noise

Chronic noise is associated with vessel traffic, particularly shipping, and in some areas of the coast, whale watching.  Studies that have measured changes in ambient underwater noise levels over the past 100 years attribute much of the increase in underwater noise to the dramatic increase in commercial shipping.  Vessel noise covers a broad band of frequencies, and is now the dominant source of ambient noise in the 0-200 Hz range (NRC 2003).  Exactly how this increase in underwater sound may affect killer whales is not well understood.  Chronic noise can result in masking, such that animals may find it difficult to communicate.  Masking could lead to disruption of social contact or interference with acoustically-coordinated behaviours. This is of particular concern with transient killer whales, as they vocalize much less frequently than resident killer whales (Deecke et al. 2005).  Transients also rely heavily on being able to acoustically detect their prey (Barrett-Lennard et al. 1996), so increased underwater noise may reduce their foraging efficiency. 

Acute Noise

Sources of acute noise in the marine environment include military sonars, seismic surveys, commercial sonars and underwater explosions usually associated with construction.  Many of these intense impulsive sounds have the potential to travel large distances underwater (>10-100+ km).  Recent evidence suggests that such sounds may have significant impacts on cetaceans, although further research is needed to provide insight into the mechanisms by which these effects occur.  In other species of marine mammals, acute noise has been associated with hearing threshold shifts, the production of stress hormones, and tissue damage, which is likely due to the formation of air bubbles or as a result of resonance (Ketten et al. 1993, Crum and Mao 1986, Evans and England 2001, Finneran 2003, Jepson et al. 2003, Fernandez et al. 2004).  Marine mammals may be particularly vulnerable to resonance because of the air-filled cavities in their sinuses, middle ear, and lungs, and small gas bubbles in their bowels. 

Low-mid frequency sonar has been associated with increased strandings of humpback and beaked whales (IWC 2004), and with unusual behaviours of resident killer whales (K.C. Balcomb, personal communication, in Wiles 2004).  Systematic surveys of cetaceans during seismic surveys have been undertaken in UK waters and have shown that killer whales and other cetaceans were generally seen further away during periods when the survey was active (Stone 2003).  Although they did not see killer whales at the time, during seismic surveys in southern British Columbia and northern Washington, Bain and Williams (2006) found that harbour porpoises and Steller sea lions showed significant avoidance responses to intense sounds even at relatively low levels, and at distances of up to 70 km or more.

While there is no direct evidence of the effects of high intensity sound on transient killer whales in particular, by inference from other cetacean species, high intensity sound would likely have a detrimental effect.  Transients are particularly vulnerable to exposure to these high intensity sounds and because transients are difficult to detect, both visually and acoustically, it is extremely difficult to develop adequate mitigate measures to address exposure to acute sound. They typically travel in small groups, and the likelihood of visually detecting them falls off markedly at distances greater than 1 km (Wade et al. 2003).

Physical Disturbance

Cetaceans are being subjected to increasing amounts of physical disturbance from both vessels and aircraft (IWC 2004).   How this may affect transient killer whales is not well understood, but there is concern that it could reduce their foraging success, close vessel approaches may disrupt hunting behaviour.  Killer whale attacks on marine mammals are often prolonged and may take place over several kilometres, so the more boat traffic in an area, the greater the possibility that the attack may be interrupted.

Commercial whale watching has increased dramatically in British Columbia in recent years (Baird 2002, Osborne et al. 2003). While the majority of these encounters are with resident killer whales, occasionally whale-watchers encounter transient killer whales.  Resident killer whales are likely much more habituated to the ‘behaviour’ of whale watching boats than transients, yet residents show responses to boats following them at a distance of 100 m (Williams et al. 2002).  These responses included reduced foraging time, which has the potential to significantly reduce their energy intake due to lost feeding opportunities (Williams et al. 2006).   Recognizing that the specialized hunting techniques of transients likely makes them more vulnerable than residents to disturbance, the Whale Watch Operators Association Northwest (WWOANW 2006) suggests that all boaters maintain a distance of 200 m from transients that are actively engaged in a kill. However, simply the close proximity of vessels, and their associated noise, may serve to disrupt an attack.

Collision with Vessels

Until recently there have been relatively few reports of killer whales being struck by boats, but within the last three years there have been four such reports in British Columbia, two of which were fatal for the whales (CRP-DFO, unpublished data).  These mortalities suggest that killer whales are at an increasing risk of collision, either as a result of blunt force trauma, and/or through blood loss associated with lacerations received from the boat’s propeller.  Both commercial shipping and cruise ship traffic have increased dramatically over the last two decades, and are likely to continue to increase, further increasing the risk of collision with killer whales. It is not known whether the often erratic and unpredictable diving behaviour of transient killer whales (Morton 1990) puts them more at risk of collision than resident killer whales.

Toxic Spills

Killer whales do not appear to avoid toxic spills, as indicated by the behaviour of a group of transients in the vicinity of the Exxon Valdez oil spill in 1989 in Prince William Sound, Alaska (and described in Section  This spill was associated with unprecedented mortality of both transient and resident killer whales, which likely died from the inhalation of petroleum vapours (Matkin et al. 1999).   Spills on a smaller scale have occurred in British Columbia, such as the Nestucca oil spill (875 tonnes in December 1988) in Gray’s Harbor, Washington, which drifted into Canadian waters, and the more recent spill of 50 tonnes of bunker fuel into Howe Sound from a ruptured tanker in August 2006.  There is currently a considerable amount of tanker traffic in and out of Puget Sound and the Strait of Georgia, which poses a risk for killer whales (Baird 2001, Grant and Ross 2002).  If the proposed 30-inch 400,000 barrel/day Gateway Pipeline is built near Kitimat, the risk of an oil spill associated with tanker traffic running from inshore waters to California and Asia will increase significantly. 

Spills other than hydrocarbons also pose a risk to killer whales, and a recent spill highlights the fact that these are not merely hypothetical events. 

Changes in Prey Availability and/or Quality

In western Alaska, there have been dramatic declines in populations of harbour seals, sea lions and fur seals.  These declines are hypothesized to have caused a shift in transient killer whale prey to less desirable species such as sea otters (Estes et al. 1998).  There is virtually no information on the abundance or trends of small cetacean and minke whale populations in British Columbia available to determine potential changes in importance or availability of cetaceanscetacean prey for transients.

Much more is known about the historic trends and present abundances of pinnipeds, particularly harbour seals and Steller sea lions.  Until the early 1970s, there was an active program to cull both species in British Columbia.  By the time these programs were concluded, harbour seal populations were 1/10th of what is assumed to be their historic population size, and their numbers have since rebounded to their pre-cull abundance (Olesiuk 1999). Steller sea lion numbers have also doubled since the culling program ended (DFO 2003).  These recovering prey populations have likely had a significant positive effect on the population of transient killer whales (Ford and Ellis 1999), since both seals and sea lions are important in their diet. 

In recent years, there have been calls for culls of harbour seals and sea lions because they interfere with commercial and recreational fisheries, by feeding on the targeted species as well as by depredating fishing gear.  As seals and sea lions are important prey of transients, any pinniped cull program has the potential to reduce the supply of food available to transients, and potentially negatively affect the total population.

Contaminant loading in small cetacean and pinniped populations can also reduce quality and/or quantity transient killer whale prey.  For example, a mass mortality of harbour seals, associated with an infectious disease, could have a significant impact on the available food supply of transient killer whales.  Increasing PBT contaminant levels and changing ocean climate associated with global warming may increase the frequency of these epidemics (Walther et al. 2002).