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Recovery Strategy for the Northern and Southern Resident Killer Whale

1.3 Population Size and Trends

1.3.1 Global

Little is known of the historic abundance of killer whales, except that they were “not numerous” (Scammon 1874).  Since the early 1970s, photo-identification studies have provided reasonable population estimates for killer whales in the near-shore waters of the northeastern Pacific (Washington, British Columbia, Alaska, and California), and similar work is now underway in several other coastal regions (e.g. the Gulf of California, the Russian Far East, New Zealand, Patagonia, Iceland and Norway).  In other areas line transect surveys have been used to provide population estimates.  These include the Antarctic (25,000 whales, Branch and Butterworth 2001) and the Eastern Tropical Pacific (8,500 whales, Wade and Gerodette 1993).  As such, the worldwide abundance of killer whales is probably between 40,000 and 60,000 whales (Forney and Wade in press).  Trend data for killer whales are generally not available, with the exception of resident populations of whales in British Columbia  (discussed below) and southern Alaska (population increasing, Craig Matkin, North Gulf Oceanic Society personal communication, November 2005) and for a small population of transients in Prince William Sound (AT1s, currently in decline, not likely to recover, Saulitis et al. 2002).

1.3.2 British Columbia

There are no population estimates for killer whales in British Columbia prior to 1960. Population censuses for killer whales are now conducted annually using photo-identification of individuals.  Population trends vary by community and clan. For the purposes of the recovery strategy, data held by the Centre for Whale Research (CWR), Friday Harbor, Washington, were used to describe the population status and trends of southern resident killer whales.  Data held by the Cetacean Research Program, DFO Nanaimo, BC (CRP-DFO), were used to describe the northern resident killer whale population.  Whales are censused slightly differently by each research group.[3]

The southern resident count includes all whales that are seen during a calendar year, and mortalities are included in the count depending on when they take place. For example, a whale that is not seen from March onwards is assumed to be dead.  There is less certainty that a whale that is not seen in November or December is dead, and it may be included in the count.  In recent years, observer effort has been high and members of the southern resident community are photographed annually, so the count is reasonably precise.

The northern resident count includes all whales that are known to be alive on July 1 of each year. However, not all members of the resident community are seen each year, so the count data are generally less precise than for the southern residents. 

In 2003, there were a total of 290 northern and southern resident killer whales (unpublished data, CWR, and CRP-DFO).  By comparison there are approximately 220 transient and 200 offshore killer whales, although these numbers are less precise than the resident counts, because not all individuals are encountered each year (Ford et al. 2000).  

Southern Residents

The size of the southern resident community has been known since the first complete photo-identification census in 1976, and was estimated for the years prior to that (Olesiuk et al. 1990, unpublished data CWR).  Figure 2 shows the size of each pod as well as the fluctuation in the total population of the southern resident community from 1974-2003. 

Figure 2 : Population size and trends for southern resident killer whales from 1974-2003. Source: Unpublished data from the Centre for Whale Research 

Figure 2: Population size and trends for southern resident killer whales from 1974-2003. Source: Unpublished data from the Centre for Whale Research

Although the southern resident community was likely increasing in size in the early 1960s, the number of whales in the community dropped dramatically in the late 1960s and early 1970s due to live capture for aquariums (Bigg and Wolman 1975). A total of 47 individuals that are known or likely to have been southern residents were captured and removed from the population (Bigg et al. 1990).  The population increased 19% (3.1% per year) from a low of 70 after the live-captures ended in 1973 to 83 whales in 1980, although the growth rate varied by pod (Figure 2).  From 1981-1984 the population declined 11% (-2.7% per year) to 74 whales as a result of lower birth rates, higher mortality for adult females and juveniles (Taylor and Plater 2001), and lower numbers of mature animals, especially males, which was caused by selective cropping in previous years (Olesiuk et al. 1990).  From 1985 to 1995, the number of southern residents increased by 34% (2.9% per year) to 99 animals.  A surge in the number of mature individuals, an increase in births, and a decrease in deaths contributed to the population growth.  The latest decline began in 1996, with an extended period of poor survival (Taylor and Plater 2001, Krahn et al. 2002) and low fecundity (Krahn et al. 2004) resulting in a decline of 17% (-2.9% per year) to 81 whales in 2001. Since 2001, the number of southern residents has increased slightly to 85 in 2003[4] (unpublished data CWR).  The growth has been in J and K pods, whereas L pod has continued to decline. 

Population viability analyses (PVA) have been used to estimate the extinction risk of southern resident killer whales (Taylor and Plater 2001, and Krahn et al. 2002, 2004).  As would be expected, extinction risk increases when the frequency and magnitude of catastrophes such as oil spills and disease epidemics is elevated. The models predict that if the mortality and reproductive rates of the 1990s persist, there is a 6-100 % probability that the population will be extinct within 100 years, and a 68-100% risk that the population will be extinct within 300 years.  Extinction of the southern resident population can be regarded as inevitable in these scenarios under the assumptions of the analyses, and catastrophic events simply hasten its demise.  However, when the mortality and reproductive rates of the entire 1974-2000 period are used, the risk of the population going extinct declines to 0-55% over 100 years and 2-100% over 300 years.

In addition to analyses focused solely on the southern residents, Krahn et al. (2002) ran simulations assuming that the southern resident population was part of a larger breeding population including northern and southern Alaskan resident killer whales, which greatly decreased its extinction risk. However this scenario does not reflect present evidence that suggests that southern residents are genetically isolated from other populations (Barrett-Lennard 2000; Barrett-Lennard and Ellis 2001).

Northern Residents

The northern resident community was likely increasing in size during the early 1960s, but was cropped by the live capture fishery of 1964-1973, during which at least 14 individuals were removed.  Twelve of those are known to have been from one pod (A5, Bigg et al. 1990).  When first censused in 1974, the northern resident community was estimated to contain approximately 120 whales.  Although abundance estimates for northern residents are less precise than those for southern residents, because not all matrilines are seen each year, it appears that the northern community grew steadily during the period 1974 to 1991 (approximately 3.4% per year, Figure 3).  The census method used for northern residents is to estimate the population size based on the number of animals that are known to be alive on July 1 of each year.  The population increased to a peak of 220 animals in 1997 (growth of 3.0% per year, unpublished data CRP-DFO).  Several reasons have been postulated for the northern residents’ success relative to southern residents during this period:  the population’s larger size may have buffered changes in birth and death rates, fewer animals were captured during the live-capture fishery (Olesiuk et al. 1990), and in general they are exposed to less disturbance and environmental contamination.  Between 1997 and 2003, the northern resident community declined 7% to 205 whales in 2003 (unpublished data CRP-DFO, Figure 3).  As with southern resident killer whales, the cause(s) of the decline are not known.  No population viability analysis has yet been conducted for the northern resident killer whales exclusively.

Figure 3: Population size and trends for northern resident killer whales from 1974 to 2003.  Values reflect the minimum, maximum and estimated number of animals alive as of July 1 in each year.  Source: Unpublished data, CRP-DFO, Nanaimo.

Figure 3: Population size and trends for northern resident killer whales from 1974 to 2003.  Values reflect the minimum, maximum and estimated number of animals alive as of July 1 in each year.  Source: Unpublished data, CRP-DFO, Nanaimo.



[3]  Note that there are small discrepancies in the southern resident counts in the literature due to different methods of recording when whales are considered to enter or leave the population. For example Krahn et al. (2004) report 83 southern residents in 2003

[4] This estimate includes L98 or Luna, discussed in section 3.2.2 Social Organization