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Recovery Strategy for the Northern and Southern Resident Killer Whale

1.4 Natural Factors Affecting Population Viability and Recovery

It is important to appreciate that northern and southern resident killer whales have been studied primarily in protected waters during the months of May to October (Ford et al. 1998, 2000).  Their behaviour and ecology in other areas and seasons is poorly known.

1.4.1 Biological Limiting Factors

The following description of the biology of killer whales is based on data from both the northern and southern resident populations.  Essentially, resident killer whales feed on fish and do not switch to marine mammals when their principal prey species are not abundant.  They are long-lived animals with no natural predators.  On average, females produce a single calf every five to six years during a 25-year reproductive period, and as a result the population has an inherently slow rate of growth.  Resident killer whales have strong cultural traditions that influence their association and mating behaviours, which also limits the capacity for the population to grow.  More detailed information on the factors that may limit the ability of resident killer whale populations to grow is provided below.

Diet

Although killer whales feed on a wide range of prey species globally, northern and southern resident killer whales are dietary specialists, feeding primarily on fish (Ford et al. 1998).  Unlike transient killer whales, resident killer whales do not feed on marine mammals and the breadth of their diet appears to be quite limited.  Extensive surface observations and collection of prey fragments from sites of kills by resident whales have shown that these whales forage selectively for certain salmonids regardless of their abundance (Ford and Ellis 2005).  Chinook salmon (Oncorhynchus tshawytscha) is the predominant prey species taken by both northern and southern resident communities during May-August, but chum salmon (O. keta) is more prevalent in September-October.  Coho salmon (O. kisutch) are taken in low numbers in June-October, but sockeye (O. nerka) and pink (O. gorbuscha) salmon are not significant prey species despite their high seasonal abundance.  Non-salmonid fishes do not appear to represent an important component of resident whale diet during May-October. 

Resident whales likely forage selectively for chinook salmon over other available salmonids because of the large size, high fat content, and year-round availability of this species in coastal waters (Ford et al. 1998, Ford and Ellis 2005).  Killer whales feeding at Langara Island in Haida Gwaii (Queen Charlotte Islands) are known to feed on chinook from stocks returning to rivers as far north as the Skeena River near Prince Rupert and as far south as the Columbia River in Oregon (unpublished data CRP-DFO). 

Despite over 30 years of study in British Columbia, only 14 stomachs from resident killer whales have been recovered and examined (Ford et al. 1998, unpublished data CRP-DFO).  The extent to which stranded individuals provide accurate insight into the dietary preferences of healthy, free-ranging killer whales is not certain.  However, salmon was identified in all seven stomachs that contained prey, including four in which chinook was positively identified.  Two contained squid and one also contained bottom fish.  It is possible that bottom fish (including ling cod, kelp greenling and sablefish), as well as squid, comprises a significant component of killer whale diet in some areas or during certain times of the year, but more research is needed to determine the year-round diet of killer whales.

It is not known whether resident killer whales depend on specific salmon runs, but their occurrence has been correlated with the abundance of various salmonid species in several past studies (Heimlich-Boran 1986, Nichol and Shackleton 1996, Osborne 1999).  The role of these geographical correlations with regard to prey selection is uncertain, since some of these species (sockeye and pink salmon) are not taken in significant numbers compared to chinook salmon (Ford et al. 1998, Ford and Ellis 2005). It is likely that whale occurrence in such areas is driven primarily by the availability of migrating chinook salmon, especially in summer months, and correlations with pink and sockeye salmon are an incidental result of their great abundance during the same period.  In fall, the presence of chum salmon appears to influence the movements of resident whales.  In Johnstone Strait, chum salmon is the primary prey species taken by northern residents from late September through October (Ford and Ellis 2005).  Fall movements of southern resident pods into Puget Sound were roughly correlated with runs of chum salmon, as well as chinook (Osborne 1999).  Recent winter sightings of southern resident killer whales in central California were coincident with high local densities of chinook salmon (N. Black, Monterey Bay Whale Watch, unpublished. data). 

Social Organization

The social structure of killer whales in British Columbia appears to be complex and differs among the three ecotypes (Ford and Ellis 1999, Ford et al. 2000).  The social structure of resident killer whales is the best understood, and one of its unique features is that there is no permanent dispersal of either sex from the natal group.  The basic social unit of resident killer whales is the matriline, composed of an older female (or matriarch) her male and female offspring, and the offspring of her daughters (Ford et al. 2000).  Because matriarchs have long life spans, some matrilines may contain up to four generations.  In over three decades of study, immigration and emigration have rarely been observed (Bigg et al. 1990, Ford et al. 2000).  Two recent cases of juvenile whales leaving their matrilines and traveling alone are considered to be exceptional, isolated incidents.  One, a female calf referred to as A73, or Springer, was separated from her pod shortly after her mother died and was observed alone after a brief period of association with a pod from another clan.  She was subsequently reunited with her pod and joined another matriline. The second incident involved a male calf L98, or Luna, who became isolated from his pod and all other killer whales for unknown reasons in 2001.  Although individuals do not disperse from their natal group, sisters often begin to spend more and more time apart after their mother dies, and their own matrilines may eventually become socially independent (Bigg et al. 1990, Ford et al. 2000, Ford and Ellis 2002). 

Reproductive Parameters

Females reach sexual maturity, defined as the age of first successful pregnancy, at 14.9 years on average (range 12-18 years,Olesiuk et al. 1990).  Males reach sexual maturity, defined as when the dorsal fin shape changes sufficiently to distinguish males from females, at 15 years on average (range, 10 -17.4 years).  Males reach physical maturity (when the dorsal fin reaches its full height) at about 20 years.  Genetic paternity testing indicates that males rarely reproduce before 25 years of age (Barrett-Lennard 2000). The gestation period of killer whales is typically 16 to 17 months, one of the longest of all whales (Walker et al. 1988, Duffield et al. 1995).  Only single calves are normally born.  Only one possible case of twins has been reported (Olesiuk et al. 1990.

Approximately equal number of males and females are born (Dahlheim and Heyning 1999) and newborn calves are between 218 and 257 cm long (Olesiuk et al. 1990).  Haenel (1986) estimated that calves are weaned at 1.0-1.5 to two years of age.  The interval between calving is usually about 5.2 years for northern residents and 6.2 years for southern residents (unpublished data CRP-DFO).  However the interval is highly variable, and ranges from two to 12 years, and increases with age until menopause (Olesiuk et al. 1990).  Overall, females have an average of 5.25 viable calves in a 25.2 year reproductive lifespan (Olesiuk et al. 1990).  Calving occurs year-round in the northern resident community, but appears to peak between fall and spring.   Southern residents do not appear to calve in the summer (unpublished data CWR).

Mating Behaviour

Mating behaviour between male and female killer whales has rarely been observed in the wild.  However, genetic evidence has revealed that resident killer whales have a propensity to mate outside their matriline (and clan, in the case of northern residents) but inside their community (Barrett-Lennard 2000, Barrett-Lennard and Ellis 2001).  This minimizes the possibility of inbreeding very effectively, but restricts the options for mating if the population becomes very small.  For example, in the southern resident community there may be an extreme shortage of sexually mature males, particularly for L pod females, assuming females select mates outside their pod.

Survival and Longevity

Survival of resident killer whales varies with age.  Neonate mortality (from birth to six months of age) is high, reported at approximately 43% for all residents (Olesiuk et al. 1990), and 42% for northern residents (Bain 1990).  Accordingly, average life expectancy is reported for an animal that survives the first six months, and is estimated to be 50.2 years for females and 29.2 years for males (Olesiuk et al. 1990).  Maximum longevity for females is an estimated 80-90 years and for males is 50-60 years (Olesiuk et al. 1990).  Although a typical trait in most mammals, the shorter lifespan of males could be related to sexual selection (Baird 2000) or to higher levels of persistent chemicals, such as PCBs (Ross et al. 2000).  The bioaccumulation of toxins is discussed in greater detail in Section 2.2.1. Atypical, however, is the prolonged post reproductive period of females, discussed in the following section.   Recent evidence suggests that declines in both the northern and southern resident populations (all age and sex classes) can be attributed to an increase in mortality rates (Ford et al. 2005) as well as a decrease in fecundity for southern residents (Krahn et al. 2004).  The potential causes of the population declines are discussed in Section 2.

Reproductive Senescence

The average life span of female resident killer whales is approximately 50 years, but on average they produce their last calf at 39, and a significant number live to 70 years or more (Olesiuk et al. 1990).  The ‘grandmother hypothesis’ suggests that the presence of older females in a group can increase the survival of offspring, and this may indeed be true for killer whales (see discussion under Culture below).  In any case, when evaluating the status of killer whale populations, it is important to consider the age structure of the population and to note that post-reproductive adult females are no longer able to contribute directly to population growth.  In an endangered population of transients in southern Alaska (AT1s), no calves have been born since 1984.  Since the remaining females are near or beyond their reproductive years, the population is on the verge of extinction (Saulitis et al. 2002), with virtually no prospect for recovery, even though it may persist for many more years.

Culture

Culture refers to a body of information and behavioural traits that are transmitted within and between generations by social learning.  Until recently, culture was generally considered a distinguishing feature of human societies.  Of late, the concept of culture has been broadened to include non-human mammals and birds (reviewed in Rendell and Whitehead 2001) and there is strong evidence for it in both northern and southern resident killer whales, and southern Alaskan resident killer whales (Ford 1991, Ford et al. 1998, Barrett-Lennard et al. 2001, Yurk et al. 2002).  There is also evidence for culture in other cetaceans, such as sperm whales (Whitehead and Rendell 2004), although not to the same extent as for resident killer whales (Rendell and Whitehead 2001). 

Dialects are the best studied form of culture in killer whales. A calf learns its dialect from its mother and other closely related adults, retains it for life, and passes it on to the next generation with few modifications (Ford 1991, Deecke et al. 2000, Miller and Bain 2000).  These culturally-transmitted dialects may play an important role in inbreeding avoidance, since females apparently prefer males from dialect groups other than their own (Barrett-Lennard 2000, Yurk et al. 2002).  Culture also appears to play an important role in feeding, with dietary preferences and probably foraging techniques and areas passed on culturally (Ford et al. 1998).  Culture may also select for longevity in killer whales, as it provides a mechanism for older individuals to increase the fitness of their offspring and relatives by transferring knowledge to them (Barrett-Lennard et al. 2001).  In African elephants, older matriarchs are better able to discriminate between threatening and non-threatening disturbances than younger animals, and pass this knowledge on to other members of their group (McComb et al. 2001).

Culture may help animals to learn to adapt to changing environments by allowing them to learn from each other in addition to learning from experience.  For example, based on differences in foraging success by sympatric clans of sperm whales under different climatic regimes, Whitehead et al. (2004) suggest that cultural diversity may be even more significant than genetic diversity in helping sperm whales to deal with a changing ocean climate.  While we do not know if this is true for resident killer whales, we do know that they respond culturally to anthropogenic changes in their environment.  In Alaska, resident killer whales responded to longline fishing in areas of Alaska by learning to raid the gear and take fish, and this behaviour spread rapidly throughout the population (Matkin and Saulitis 1994). 

Depensation

Resident killer whale populations are at risk simply by virtue of their low population size. In general, small populations generally 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.  Pacific resident killer whale populations are considered small, at 85 southern residents in 2003[1] (unpublished data, CWR), and 205 northern residents in 2003 (unpublished data, CRP-DFO).  If either resident population continues to decline, they may be faced with a shortage of suitable mates.  Among the southern residents, L pod females may be particularly vulnerable to this scenario because of the small number of reproductive males in J and K pod. Even under ideal conditions, the population will recover slowly because killer whales calve relatively infrequently. 

Inbreeding appears to be less of a risk for resident killer whales than might be expected based on the small size of their populations.  They may avoid inbreeding and its inherent risks through non-random mate selection.  Resident killer whales select mates from outside their natal pod, which may make small populations of killer whales more genetically viable than would be expected from population size alone (Barrett-Lennard and Ellis 2001).  

Natural Mortality

Killer whales have no recorded predators, other than humans.  There are several potential sources of natural mortality that may impact killer whales:  entrapment in coastal lagoons or constricted bays, accidental beaching, disease, parasitism, biotoxins, and starvation (Baird 2001).  However, it cannot be ruled out that anthropogenic factors may make killer whales more vulnerable to natural sources of mortality. For example, disturbance from intense noise may cause animals to strand (Perrin and Geraci 2002).  The proximate cause of death, stranding, is a natural source of mortality, but the death is ultimately human-caused. 

1.4.2 Other Natural Limiting Factors

Entrapment and/or Accidental Beaching

Accidental beaching and entrapment are sometimes a source of mortality for killer whales.  At least four mass strandings involving more than 36 individuals occurred in BC in the 1940s (Carl 1946, Pike and MacAskie 1969, Mitchell and Reeves 1988, Cameron 1941).   Although the causes of mass strandings in toothed whales are uncertain, disease, parasitism, and disturbance from intense underwater noise have been suggested as possible causes (Perrin and Geraci 2002).  Two possible cases of temporary entrapment have been reported for southern resident killer whales (Shore 1995, 1998).  In 1991, J-pod spent 11 days in Sechelt Inlet, apparently reluctant to exit through a constricted entrance with tidal rapids.  In 1997, nineteen killer whales spent 30 days in Dyes Inlet, Puget Sound, possibly because they were reluctant to pass under a noisy bridge (Shore 1998). 

Disease and Parasitism

Diseases in captive killer whales have been well studied, but little is known of diseases in wild killer whales (Gaydos et al. 2004).  Causes of mortality for captive killer whales include pneumonia, systemic mycosis, other bacterial infections, and mediastinal abscesses (Greenwood and Taylor 1985).  Of 16 pathogens identified in killer whales, three have been detected in wild individuals: marine Brucella, Edwardsiella tarda, and cetacean poxvirus (Gaydos et al. 2004).  A severe infection of E. tarda resulted in the death of a southern resident male in 2000 (Ford et al. 2000).  Marine Brucella may cause abortions and reduced fecundity in killer whales (Gaydos et al. 2004).  Cetacean poxvirus can cause mortality in calves and causes skin lesions (Van Bressem et al. 1999).  Twenty-seven additional pathogens have been identified in sympatric odontocetes that may be transmittable to killer whales (Gaydos et al. 2004). 

External parasites of killer whales have been reported in Mexico (Black et al. 1997), but none have been observed on killer whales in BC (Baird 2001).  Internal parasites of killer whales include various trematodes, cestodes, and nematodes (Heyning and Dahlheim 1988, Raverty and Gaydos 2004).  These endoparasites are usually acquired through infected food, but the amount of infestation and their contribution to killer whale mortality are not known at this time.

Algal Blooms

Harmful algal blooms (HABs) are blooms of algae that produce biotoxins such as paralytic shellfish poison, domoic acid, saxitoxin and brevitoxin.  Such toxins can accumulate in the tissues of species that ingest them and are magnified up the food chain.  Mortality of humpback whales (Megaptera novaeangliae) off Massachusetts in 1987 and California sea lions (Zalophus californianus) in California in 1998 have been linked to biotoxin exposure (Geraci et al. 1989, Scholin et al. 2000).  Several species of marine mammals have been shown to have a potential susceptibility to the neurotoxic effects of biotoxins (Trainer and Baden 1999).  Given the apparent increase in HAB event frequency, and the potential for toxic effects in killer whales, there may be some risk to resident killer whales exposed to biotoxins through HABs, although the risk is thought to be low (Krahn et al. 2002). 

Regime Shifts

In the North Pacific, there are widespread changes that occur in the circulation and physical properties of the ocean. These changes take place on decadal time scales and are referred to as ‘regime shifts’ (see reviews in Francis et al. 1998, Benson and Trites 2002).  Such shifts may happen quite quickly, and result in dramatic changes in the distribution and/ or abundance of many species, ranging from zooplankton to fish and possibly marine mammals and seabirds.  If the distribution or abundance of resident killer whale prey changed significantly following a regime shift, it is possible that killer whales could be impacted.



[1] including L98 or Luna, discussed in section 3.2.2 Social Organization.