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Recovery Strategy for the Transient Killer Whale [Proposed]

1. Background

1.1 Species Assessment Information from COSEWIC

Date of Assessment: November 2001

Common Name (population): Transient Killer Whale, Orca, West Coast Transient

Scientific Name: Orcinus orca

COSEWIC Status: Threatened

Reason for designation: A small population that eats marine mammals. Individuals have high levels of toxic pollutants.

Canadian Occurrence: Pacific Ocean

COSEWIC Status History: Designated Special Concern in April 1999. Status re-examined and designated Threatened in November 2001. Last assessment based on an existing status report with an addendum. Met criterion for Endangered, D1, but not the definition of Endangered (i.e. not in imminent danger of extinction), therefore designated Threatened.

1.2 Description

Killer whales are the largest members of the dolphin family (Family Delphinidae, Sub-Order Odontoceti, Order Cetacea).   Their size, distinctive black and white colouring and tall dorsal fin make them easy to distinguish from other cetaceans.  Killer whales are sexually dimorphic.  Males are larger and heavier than females, and the dorsal fin of adult males is taller, (averaging 1.8 m in height) than that of females and juveniles of either sex (usually less than 1 m) (Dahlheim and Heyning 1999).  Killer whales are relatively easy to recognize individually due to differences in the shape, size and position of the white eye patch and the saddle patch (behind the dorsal fin), as well as variations in the size, shape, and angle of the dorsal fin, and (in many cases) naturally-acquired nicks and scars.

Only a single species is recognized at present, Orcinus orca, but variation in the diet, size, colouration, vocalizations and genetic characteristics of different populations of killer whales may lead to a revision of the taxonomy in future years (Dahlheim and Heyning 1999, Ford et al. 2000, Barrett-Lennard and Ellis 2001, Hoelzel et al. 2002, Pitman and Ensor 2003, Reeves et al. 2004).   Along the continental shelf and in inshore waters from California to western Alaska, three forms, or ecotypes, are recognized: residents, transients and offshores.  These forms rarely, if ever, associate, and differ in their diet and foraging behaviour, vocal behaviour, social structure, genetics and dorsal fin shape (Ford et al. 1998, 2000, Barrett-Lennard and Ellis 2001).

Resident killer whales feed exclusively on fish and cephalopods and travel in acoustically active groups of 10 to 25 or more whales (Ford et al. 2000).  Unlike transient killer whales, resident killer whales have an usually stable social structure, with no dispersal of either males or females from their natal matriline (Bigg et al. 1990; Ford et al. 2000). 

Transient killer whales feed on marine mammals, particularly harbour seals (Phoca vitulina), porpoises and sea lions (Ford et al. 1998).  They travel in small, acoustically quiet groups, and generally rely on stealth to find their prey (Morton 1990, Barrett-Lennard et al. 1996, Ford and Ellis 1999). Their dive times are also significantly longer than those for residents (Morton 1990).  They are known to attack and kill baleen whales, and although this is observed infrequently in the coastal waters of British Columbia (Ford et al. 2005), groups of transient killer whales may coalesce when attacking and feeding on baleen whales at sea (Barrett-Lennard and Heise 2006). In coastal waters, kills of minke whales by small groups of transients typically involve a strategy of confining the whale in a bay or inlet (Ford et al. 2005).

Offshore killer whales are the least known of the three ecotypes.  They were first identified in the late 1980s, and are most often found on the outer part of the continental shelf (Ford et al. 2000) with occasional sightings in inshore waters.  They are most often seen in large acoustically active groups of 20 or more animals, and are thought to prey on fish and elasmobranches (Heise et al. 2003, Jones 2006), although they may also take marine mammals (Herman et al. 2005).

1.3 Populations and Distribution

1.3.1  Global

Killer whales are found in all of the world’s major ocean basins, and are estimated to number at least 40,000-60,000 animals (Forney and Wade 2006).  The description of resident, transient, and offshore ecotypes can only reliably be applied to killer whale populations in the northeastern Pacific. In other parts of the world, killer whales are not as well studied, and in some areas it is possible that they may prey on both marine mammals and fish.  

1.3.2 Canadian Pacific

The three distinct ecotypes of killer whales in the northeastern Pacific are further subdivided into at least seven socially, genetically, culturally and acoustically distinct populations.  The majority of these populations use both Canadian and US waters, and are recognized by the governments of both countries.

Three putative populations of transient killer whales have been described to date in the northeastern Pacific. These include the so-called West Coast transients, distributed from Washington State to southeastern Alaska, the AT1transients, centred in Prince William Sound and Kenai Fjords, Alaska, and the Gulf of Alaska transients, usually sighted in waters of the central and western portion of the Gulf of Alaska (Angliss and Outlaw 2005).   The AT1 population has declined precipitously in recent years and is believed to comprise only eight individuals, none of which are reproductive females (Saulitis et al. 2005).  The Gulf of Alaska transient population numbers at least 314 individuals, and are most reliably seen between southeastern and western Alaska (Angliss and Outlaw 2005).  Although the ranges of the AT1 and the Gulf of Alaska populations overlap, they have never been observed interacting.

The West Coast transient population is the only one known to frequent Canadian waters, and is the focus of this recovery strategy.  Approximately 250 individuals are known to travel throughout the waters of British Columbia, although they range from Washington to southeastern Alaska (Cetacean Research Program (CRP)-DFO unpublished data).  Defining members of the West Coast transient population is not as straightforward as it is for members of resident killer whale populations, largely because transients are not seen as reliably as residents.  As well, unlike resident killer whales, transients disperse from their natal group.  As a result, a number of criteria are combined in a weight-of-evidence approach to define the West Coast transient population.  These criteria include: 1) association (members frequently associate with other members, and rarely if ever associate with the members of other populations), 2) shared acoustic repertoire of distinct vocalizations, 3) genetic relatedness, 4) shared range, and 5) shared diet and suite of foraging behaviours.  In future, similar fatty acid and/or contaminant profiles may also help to define membership within this population (see Herman et al. 2005, Krahn et al. 2007).

An assemblage of approximately 100 transient-type killer whales has been documented off the California coast (Ford and Ellis 1999). This group is poorly-studied and has in the past been considered an extension of the West Coast transient population.  A group of killer whale experts at a technical workshop convened in Vancouver 16-17 January 2007, for the purpose of advising Fisheries and Oceans Canada on technical issues relevant to this recovery strategy, determined that the available evidence suggests that the California assemblage belongs to one or more distinct, currently undefined populations.  Acoustically, the repertoire of calls from these whales is similar, but not identical to that of transients found in British Columbia (Deecke 2003).  Approximately 10 of these individuals have been seen in British Columbia and Alaska, and at times they have been observed interacting with members of the West Coast transient population.  These interactions, although rare, suggest that there may be limited gene flow between the two groups.  Very little is known about the status of the California assemblage, as these animals are encountered relatively infrequently, even in Californian waters.  

Most sightings of transient killer whales in British Columbia tend to take place during the summer and fall, when more people are on the water, but transients are observed in all months of the year.  However, they are not evenly distributed throughout the area, and are most frequently found where their prey is particularly abundant.  Some transient groups travel throughout the range of the population, including one group that traveled 2,660 km from Glacier Bay Alaska to Monterey California, (Goley and Straley 1994).  Other whales have only been seen in particular regions, such as the Queen Charlotte Islands.  It is possible that they may have ‘home ranges’ or preferred areas where local knowledge gives them a hunting advantage (Ford and Ellis 1999).  Unlike resident killer whales that may remain in an area for several weeks or more, particularly during peak salmon runs, transient killer whales usually pass through an area relatively quickly, likely because their mammalian prey leave the water or become highly evasive once alerted to their presence.