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Recovery Strategy for the Transient Killer Whale [Proposed]

1.4 Habitat and Biological Requirements, Ecological Role and Limiting Factors for Transient Killer Whales

1.4.1 Habitat and Biological Requirements

The habitat requirements of transient killer whales are not well understood.  Their specialized hunting techniques for capturing acoustically-sensitive marine mammals suggest that their habitat must be sufficiently quiet enough for them to acoustically detect their prey.  These conditions would also help them to maintain other vital functions such as communication.

Transient killer whales rely completely on abundant marine mammal populations in order to survive.  In the coastal waters of British Columbia, their principal prey are pinnipeds and small cetaceans (Ford et al. 1998, Ford et al. 2005).  Populations of two of their known prey, harbour porpoises (Phocoena phocoena), and Steller sea lions (Eumetopias jubatus), are designated as Special Concern under COSEWIC (COSEWIC 2003) and if these species decline, there may be an impact on the available food supply of transients.  This could have consequences on the ability of the transient population to grow.   More detailed information on the prey preferences of transients is provided in Section 1.4.3 Biological Limiting Factors, Diet.

1.4.2 Ecological Role

Transient killer whales feed on warm-blooded animals, and are considered to be apex level predators.  The specifics of their dietary preferences are discussed in Section 1.4.3.  In British Columbia, the role that killer whale predation plays in the population dynamics of their prey populations is not well understood.  However, in western Alaska, several prey populations are in serious decline, and there has been considerable debate in the literature as to whether killer whale predation is the cause.  These include the western populations of Steller sea lions, sea otters (Enhydra lutris) and harbour seals (Barrett-Lennard et al. 1995, Estes et al. 1998, Springer et al. 2003, Williams et al. 2004, Trites et al. 2007, Wade et al. 2007).  While the debate is not resolved, it seems plausible that once prey populations are reduced, for whatever reasons, killer whales are capable of maintaining their prey in a ‘predator pit’ where ongoing predation prevents prey populations from recovering.

1.4.3 Biological Limiting Factors

Biological limiting factors that can affect the population growth of transient killer whales include:  diet, social organization, survival, dispersal, reproductive success, mating behaviours, reproductive senescence, culture, small population size (depensation) and various souces of natural mortality that are described below.  Predation is not a factor limiting the population growth of transient killer whales since they have no natural predators.

Diet

Field observations (Baird and Dill 1996, Barrett-Lennard et al. 1996, Ford et al. 1998, Ford and Ellis 1999) and fatty acid analyses (Herman et al. 2005) clearly show that transients prey on marine mammals and occasionally seabirds, but they do not eat fish.  The capture of three transient killer whales in 1970 reinforces the strength of this dietary preference.  After 75 days of refusing fish, one whale died, and after 79 days the remaining two whales began to eat fish.  When they were later returned to the wild, the two surviving whales resumed their diet of marine mammals (Ford and Ellis 1999).

In coastal waters of British Columbia, harbour seals are the most frequently documented prey species of transients, followed by harbour porpoises, Dall’s porpoises (Phocoenoides dalli) and Steller sea lions.  They also consume California sea lions (Zalophus californianus), Pacific white-sided dolphins (Lagenorhyncus obliquidens), grey whales (Eschrichtius robustus), minke whales (Balaenoptera acutorostrata), and, less commonly, river otters (Lutra canadensis) and elephant seals (Mirounga angustirostris) (Baird and Dill 1996, Ford et al. 1998, Ford et al. 2005).

The prey of transients is generally available year-round, although there are seasonal peaks associated with calving and pupping.  This may be why transients are seen more reliably throughout the year than resident killer whales (Ford and Ellis, 1999), which are found in inshore waters most frequently in the summer and fall when their principal prey, chinook salmon (Oncorhynchus tshawytscha), are most readily available as they return to freshwater to spawn.

Hunting warm-blooded, acoustically sensitive prey has shaped the social and acoustic behaviours of transient killer whales. In coastal waters of British Columbia, they typically travel in small, acoustically quiet groups and they hunt with stealth.  Their dive times are long relative to residents and they often swim erratically (Morton 1990).  They typically do not vocalize when foraging until after they have made a successful kill (Deecke et al. 2005).  While harbour seal kills may occur very quickly, attacks on other species, such as sea lions, Dall’s porpoises and minke whales, may be prolonged and involve high speed chases over several kilometres (Ford et al. 1998, Heise et al. 2003, Ford et al. 2005).

Social Organization

Transient killer whales are most often seen in groups of two to six, although they occasionally may be found alone or in much larger groups (Ford and Ellis 1999).  In their study of transients around southern Vancouver Island, Baird and Dill (1996) found that they were most commonly encountered in groups of three to four, and that mothers frequently traveled with their adult sons.  However, unlike resident killer whales, the social groupings of transient killer whales are much more fluid and difficult to interpret.  Transients do not necessarily remain in their natal matrilines for life and there can be some dispersal.  The dispersers may mix widely within the population (Ford and Ellis 1999) although there can be strong long-term associations (Baird and Whitehead 2000).

The entire transient population is linked through association; all individual members of the transient population have been seen travelling with at least one other known member of the transient population at some time.  As well, West Coast transients are all acoustically linked through the sharing of calls (Ford and Ellis 1999, Deecke 2003).  These calls differ from those used by AT1 transients and those used by Gulf of Alaska transients (Saulitis et al. 2005).  Transient killer whales that are seen more frequently in California share some calls with those that are most often found in British Columbia waters, but produce several unique calls as well (Ford 1984, Deecke et al. 2005, CRP-DFO unpublished data). 

Survival and Longevity

It is difficult to estimate mortality rates for transients since they may disappear for extended periods of time (years) and then reappear. Indeed, transients have been known to reappear after not having been seen for as long as 15 years (CRP-DFO, unpublished data).  As a result, there are currently insufficient survival data specific to transients that can be used to estimate survival and longevity rates, and the discussion below is based on data for resident killer whales.

Killer whale survival rates vary with age, and mortality is highest for neonates (from birth to six months) at 37-50% (Olesiuk et al. 1990).  The average life expectancy is described for animals that survive the first six months of life, based on data collected between 1973 and 1996 for northern resident killer whales and is estimated to be 46 yrs for females and 31 yrs for males.  The maximum longevity is 80 yrs for females and 40-50 yrs for males (Olesiuk et al. 2005).

ReproductiveParameters

As with survival and longevity data, there are few detailed records of reproductive data specific to transient killer whales and much of what is known about killer whale reproduction comes from resident killer whales.  Since transients are not reliably sighted yearly, it is possible that females may have given birth to a calf that may or may not have survived.  As a result, the discussion below is based on data from resident killer whales. 

On average, male and female killer whales reach sexual maturity at 14.2 and 12.8 yrs respectively (Olesiuk et al. 2005).  The gestation period is 16-17 months, one of the longest of all whales (Walker et al. 1998, Duffield et al. 1995).  Females give birth to their first calf between 12 and 17 yrs of age (ave. = 14.1 yrs), and produce a single calf every five years over a 24 yr reproductive period, although the calving interval is highly variable and can range from two to 11 yrs (Olesiuk et al. 2005).  Calving occurs year-round, but peaks in fall through spring.  Calves are approximately 2.2-2.5 m in length at birth (Olesiuk et al. 1990).

Mating Behaviour

Mating behaviour between male and female killer whales has rarely been observed in the wild and there is no information about mating behaviour or mate choice in transient killer whales.  Genetic and photo-identification studies suggest that the transient, resident and offshore ecotypes are closed to immigration, and that successful mating between these, if it ever occurs, is extremely rare (Barrett-Lennard 2000).  Resident killer whales in British Columbia give birth mostly in fall and winter, which implies a peak in mating during spring and summer (16-17 month gestation period; Olesiuk et al. 1990, 2005).  There are insufficient data to determine whether transients have a similar seasonality in mating and calving.

Reproductive Senescence

Little is known about reproductive senescence in transient killer whales, but female resident killer whales generally produce their last calf at approximately 40 years, and may live to age 70 or older (Olesiuk et al. 2005).  This post-reproductive period is relatively rare in animals, and only occurs in species with maternal care where offspring remain dependent for an extended period.

Culture

Strong cultural traditions that are passed along from generation to generation through learning are evident within the West Coast transient killer whale population.  In particular, their preference for mammalian prey is an important feature that helps to define this population.  The strength of this tradition is highlighted by the transient killer whale held in captivity that ultimately died because of its refusal to eat fish (discussed in Section 1.4.3.).   West Coast transients also have their own unique repertoire of acoustic calls, which is different from those of resident killer whales, as well as with other populations of transients (Ford 1984, Deecke 2003, Deecke et al. 2005, Saulitis et al. 2005).

Transients do not compete with resident killer whales for food, yet they appear to actively avoid residents (Baird 2000), and have even been aggressively attacked by them in one well-documented account (Ford and Ellis 1999).  There is much to be learned about the role of culture in transient society, but it likely enhances foraging efficiency as whales pass along hunting techniques and information on important feeding areas to each other. 

Depensation

In British Columbia, the West Coast transient killer whale population numbers approximately 250 whales (CRP-DFO unpublished data), and is considered to be at risk because of its low population size.  In general, small populations have an increased likelihood of inbreeding and lower reproductive rates, which can lead to low genetic variability, reduced resilience against disease and pollution, reduced population fitness and elevated extinction risks due to catastrophic events.   There is evidence of at least one genetic disorder within the West Coast transient population, although it is relatively rare. 

In Alaska, the story of the AT1 transient population demonstrates the risks associated with low population size.  This population was first encountered in 1984 in Prince William Sound, and numbered 22 whales.  These whales were regularly encountered in the area between 1984 and 1989.  In the spring of 1989 the Exxon Valdez oil spill occurred in Prince William Sound and in 1990 nine individuals were missing from the group, three of which were seen swimming through oil in the vicinity of the ship shortly after the spill.  The following year an additional two whales went missing and all 11 whales are now presumed dead.  Since 1990 an additional three whales have disappeared and the population now numbers eight animals (Angliss and Outlaw 2005).  As there are no longer any reproductive age females in the group, this will lead to the extirpation of the AT1 population (Saulitis et al. 2002).  It is worth noting that the Gulf of Alaska transient population is sympatric with the AT1 population, but there is no interbreeding between these two groups of killer whales (Barrett-Lennard 2000).

Natural Mortality

As discussed above, many transient groups are not encountered on a regular basis, and carcasses are rarely recovered, so there is little known about natural sources of mortality.  It can be assumed that they are vulnerable to many of the same risks as resident killer whales, including: entrapment in coastal lagoons or constricted bays, accidental beaching, disease, parasitism, biotoxins and starvation (Baird 2001). Anthropogenic factors may make transients more vulnerable to natural sources of mortality as well.  For example, intense high-energy sound may cause animals to strand (Perrin and Gerraci 2002).  Death is ultimately due to stranding, but the proximate cause is anthropogenic. 

Little is known of the diseases that may affect the survival of killer whales in the wild.  Sixteen pathogens have been identified in captive killer whales, and four pathogens have been detected in wild killer whales (Gaydos et al. 2004, Stephen Raverty, BC Ministry of Agriculture, Food and Fisheries, Abbotsford, personal communication Jan 17, 2007).  These include marine Brucella spp., Edwardsiella tarda, Toxoplasma gondii, and cetacean poxvirus. As well, twenty-seven additional pathogens have been identified in sympatric species of odontocetes that may be transferable to killer whales.  Because transients feed on marine mammals, it is possible that they may be exposed to these pathogens at a higher rate than resident killer whales.  These diseases may cause abortions, reduced fecundity and/or increased mortality (Gaydos et al. 2004).

Little is also known of the role of parasites in transient killer whale mortality.  There have been no reports of external parasites on West Coast transients (Baird 2000) but they can be infected with internal parasites. These include various species of trematodes, cestodes and nematodes (Dahlheim and Heyning 1999, Raverty and Gaydos 2004), which are likely acquired through infected prey. 

Transients may on occasion be vulnerable to accidental beaching or entrapment, although there is only one record of a known transient killer whale accidentally beaching itself.  This was a lone male that stranded and subsequently died on a sandbar while foraging near Tofino in 1976 (Ford and Ellis 1999).

Any factor that dramatically alters the abundance of their prey could be a significant source of mortality for transient killer whales.  In other areas of the world, there have been massive outbreaks of diseases and biotoxins in pinnipeds and small cetaceans.  Morbillivirus outbreaks have caused mass mortalities of dolphins and seals (Aguilar and Borrell 1994, Kennedy et al. 2000) and in ocean-living river otters in British Columbia (Mos et al. 2003).  There are three ways in which one of the members of the Genus Morbillivirus may present a risk to transient killer whales: 1) an outbreak in their community, perhaps most likely due to Dolphin/Cetacean Morbillivirus (DMV), 2) an outbreak of phocine distemper virus (PDV) or canine distemper virus (CDV) among their principal prey – harbour seals – which could dramatically reduce prey abundance for transients, and/or 3) a transfer of PDV or CDV from pinnipeds, river otters or other species to transients, the likelihood of which is unclear.

Harmful algal blooms (HABs) have the potential to put transient killer whales at risk, either through bioaccumulation or through loss of prey.  HABs result in the production of biotoxins, such as paralytic shellfish poison, domoic acid, saxitoxin and brevitoxin.  Several species of marine mammals have been shown to have a potential susceptibility to their neurotoxic effects (Trainer and Baden 1999).  As HABs seem to be increasing in frequency, and have been linked to the deaths of California sea lions (Scholin et al. 2000), transients may be at risk in future.

Changing climatic conditions may also result in a reduction in the food supply of transient killer whales, and ultimately reduce their survival.  El Niño events have been linked to large-scale die-offs of pinnipeds in California (Angliss and Lodge 2004).  Widespread changes in the circulation and physical properties of the ocean, known as ‘regime shifts’ also have the potential to affect the distribution and abundance of the prey of killer whales (Benson and Trites 2002).