12. Killer Whales

A "spy hopping" Southern Resident killer whale in the San Juan Islands. Image courtesy of NOAA.

A "spy hopping" Southern Resident killer whale in the San Juan Islands. Image courtesy of NOAA.

Background

Three distinct groups of killer whales (Orcinus orca) occupy the coastal waters of the northeastern Pacific. These groups—northern and southern residents, transients, and offshores—are distinguished by diet, behavior, morphology, and other characteristics. Among these, Southern Resident and transient killer whales commonly are found in Puget Sound. Northern residents and offshore killer whales rarely enter Puget Sound (Wiles 2004, Kriete 2007), and therefore are not described in detail here.

While the taxonomic status of north Pacific killer whales remains unresolved (summarized in Krahn et al. 2004, NMFS 2008), the Southern Resident killer whale (SRKW) and transient killer whale populations are considered by NOAA to be separate stocks based on genetic, morphological, dietary and behavioral differences and are classified as endangered (SRKW) and threatened (transient) under the U.S. Endangered Species Act (2005). The SRKW population is found primarily in Washington and southern British Columbia and includes three groups or pods (J-, K- and L-pod) (Krahn et al. 2002, Krahn et al. 2004). Their home range during the spring, summer, and fall includes Puget Sound, the Strait of Juan de Fuca, and the Strait of Georgia (NMFS 2008) (Figure 1). During the late fall to winter, SRKWs travel as far south as central California and north to the Queen Charlotte Islands, British Columbia. The distribution of transient killer whales ranges from southern California to Icy Strait and Glacier Bay in Alaska (Ford et al. 2000). Transients are recorded along the Puget Sound and Vancouver Island shorelines during the summer and early fall (Wiles 2004).

Figure 1

Figure 1. Distribution of Southern Resident killer whale sightings from 1990-2005 (data from The Whale Museum 2005; figure reprinted from NMFS 2008, courtesy of NOAA Fisheries).

Resident killer whales are believed to principally consume marine fish, while transients prey solely on marine mammals (Ford et al. 1998, Ford et al. 2000). Diets of resident killer whales were found to include 22 species of fish and one species of squid (Ford et al. 1998). A detailed dietary study based on 529 observed predation events from 1997 – 2005 of both Northern and Southern Resident killer whales revealed that salmonids (particularly Chinook) comprised 96% of the killer whale diet. However, most of these observations (>85%) were based on Northern residents; less information is available on the Southern Residents that routinely inhabit Puget Sound (Ford and Ellis 2006). The diet of transient killer whales is less well known, but is thought to be comprised primarily of harbor seals and to include other marine mammals such as sea lions, harbor porpoise, Dall’s porpoise, minke whales and marine birds (Ford et al. 1998).

The movements and locations of SRKWs have been recorded by researchers, whale watchers and citizens since the early 1970s and a database of their distribution is maintained by The Whale Museum in Friday Harbor, Washington. Whales are most frequently observed in the San Juan Archipelago but are also found as far into Puget Sound as the southern portion of the South Sound (Figure 1)(Hauser et al. 2007, NMFS 2008). Southern Resident pods are present regularly in the Georgia Basin, and during warmer months all pods concentrate their activity from the south side of the San Juan Archipelago through Haro Strait northward to Boundary Pass (Hauser et al. 2007). Most transient sightings in the Puget Sound-Georgia Basin region are concentrated around southeastern Vancouver Island, the San Juan Archipelago, and the southern edge of the Gulf Islands. Transients appear to utilize a wider range of water depths and habitats than residents (NMFS).

Three main factors have been identified as potential threats to killer whales in Washington and British Columbia: reductions in prey availability, disturbance by underwater noise and vessel traffic, and exposure to environmental contaminants, particularly PCBs and PBDEs (NMFS 2008). Ford et al. (2010) suggests that declines in SRKW abundance in the mid 1990s were driven by a significant decline in range-wide abundance of Chinook salmon. NMFS has published a Final Recovery Plan that describes a recovery program designed to address each of the threats to the SRKW population. Due to their trophic position as apex predator, levels of contaminants such as polychlorinated biphenyl (PCBs) and dioxins in both resident and transient killer whales have been shown to be among the highest recorded (Ross et al. 2000, Krahn et al. 2007, Krahn et al. 2009).

Status

SRKW population: Photo-identification censuses of the SRKW population performed by the Center for Whale Research since the 1970s have shown several periods of growth and decline (Figure 2). Because the average life expectancy of killer whales is estimated to be 50 years and can extend to 80-90 years, the existing data on the SRKW populations have covered only a small portion of the lifespan. In response to a 20% population decline from 1996 to 2001, the SRKW stock was designated as depleted under the Marine Mammal Protection Act (MMPA) in 2003 and became listed as Endangered under the Endangered Species Act (ESA) in 2005. In 2006, NMFS designated approximately 2,500 square miles as critical habitat for Southern Residents. The designated area encompasses parts of Haro Strait, the waters around the San Juan Archipelago, the Strait of Juan de Fuca, and all of Puget Sound.

Transient population: Detailed estimates of population abundances for transient killer whale populations have not been made (NMFS 2008). It is hypothesized that historical transient killer whale populations experienced a large decline in abundance due to substantial prey losses in the early-to-mid 1900s (Springer et al. 2003). Because harbor seal populations in the region have increased over the last 30 years and currently are close to carrying capacity (Jeffries et al. 2003), it is believed that transients are no longer prey-limited (Ford et al. 2000). Approximately 225 transients have been identified in Washington, British Columbia, and southeastern Alaska (NMFS 2008) although current abundances are not known (NMFS 2008).

SRKW population: The historical population of Southern Residents in the mid- to late-1800s ws estimated to be approximately 200 whales (Krahn et al. 2002), although lack of data prior to the 1970s makes contributes to the uncertainty of this estimate. The capture of live killer whales for aquaria is thought to have removed approximately 50 Southern Resident and 5 transient killer whales between 1962 and 1977 (NMFS 2008). Since that time, the population has experienced fluctuations with periods of positive population growth followed by decline (Figure 2). Most notable was a substantial period of population growth between the mid 1980s and mid 1990s, during which total whale numbers expanded from 75 to nearly 98 animals. That period was followed by a brief period of decline, to 80 animals, followed by a moderate increase thereafter (Wiles 2004, Kriete 2007, NMFS 2008). The most recent estimate of 85 animals derives from a survey conducted in April 2009 (Center for Whale Research, (reported in PSP 2009)(Figure 2).

Figure 2

Figure 2. Abundance of Southern Resident killer whales from 1976-2009 (data from the Center for Whale Research)(reprinted from PSP 2009)

SRKW population predictions: Krahn et al. (2004) conducted a population viability analysis (PVA) (Morris and Doak 2002) to assess the future risk of extinction of the SRKW population, the predictions of which varied significantly according to the time period from which survival rates were estimated. Using the survival rates estimated from 1974-2003, they found that extinction probabilities for the SRKW whale populations ranged from <0.1-3% over the next 100 years and 2-42% over the next 300 years. However, extinction probabilities based on 1994-2003 survival rates ranged from 6-19% over the next 100 years and 68-94 % over the next 300 years (Krahn et al. 2004)

Transient population: Trends in abundance of the transient killer whale population cannot be estimated because accurate assessments of transient killer whale abundance have not been made.

Uncertainties

While the diets of Northern resident killer whales, which inhabit the coastal habitat of British Columbia and Alaska, have been well characterized (Ford and Ellis 2006), the extent to which diets of Northern resident killer whales are predictors of the diets of SRKW population (the primary users of Puget Sound habitats) remains under investigation. There is strong evidence for correlations between fluctuations in salmonids, especially Chinook salmon, and resident killer whales (Ford and Ellis 2006), but the drivers behind this relationship have not been elucidated. Furthermore, the unknown and potentially interactive effects of multiple stressors on killer whales introduces uncertainty in projections of future population abundances.

Summary

Killer whales are challenging to study because they spend much of their time below the water surface, are wide-ranging, and are highly migratory. Photo-identification and vigilant observations of predation events have allowed researchers to identify every individual in the SRKW population based on unique patterns and morphology, thereby facilitating accurate estimation of population abundance and diet of Resident killer whales. Human removal of SRKW appears to have driven population declines prior to the 1970s, yet 35 years after the removals for live capture ended, SRKW population numbers remain low. Data on transient killer whale populations are lacking.

Literature Cited

Ford, J. K. B., and G. M. Ellis. 2006. Selective foraging by fish-eating killer whales Orcinus orca in British Columbia. Marine Ecology-Progress Series 316:185-199.

Ford, J. K. B., G. M. Ellis, and K. C. Balcomb. 2000. Killer whales: The natural history and genealogy of Orcinus orca in British Columbia and Washington. UBC Press; University of Washington Press, Vancouver; Seattle.

Ford, J. K. B., G. M. Ellis, L. G. Barrett-Lennard, A. B. Morton, R. S. Palm, and K. C. Balcomb. 1998. Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters. Canadian Journal of Zoology-Revue Canadienne De Zoologie 76:1456-1471.

Ford, J. K. B., G. M. Ellis, P. F. Olesiuk, and K. C. Balcomb. 2010. Linking killer whale survival and prey abundance: food limitation in the oceans' apex predator? Biology Letters 6:139-142.

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Krahn, M. M., P. R. Wade, S. T. Kalinowski, M. E. Dahlheim, B. L. Taylor, M. B. Hanson, G. M. Ylitalo, R. P. Angliss, J. E. Stein, and R. S. Waples. 2002. Status review of Southern Resident killer whales (Orcinus orca) under the Endangered Species Act. Page 133 in N. T. Memorandum, editor.

Kriete, B. 2007. Orcas in Puget Sound. Puget Sound Nearshore Partnership, Seattle.

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PSP. 2009. Ecosystem Status & Trends. Puget Sound Partnership, Seattle, WA.

Ross, P. S., G. M. Ellis, M. G. Ikonomou, L. G. Barrett-Lennard, and R. F. Addison. 2000. High PCB Concentrations in Free-Ranging Pacific Killer Whales, Orcinus orca: Effects of Age, sex and dietary preference. Marine Pollution Bulletin 40:504-515.

Springer, A. M., J. A. Estes, G. B. van Vliet, T. M. Williams, D. F. Doak, E. M. Danner, K. A. Forney, and B. Pfister. 2003. Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling? Proceedings of the National Academy of Sciences of the United States of America 100:12223-12228.

Wiles, G. J. 2004. Washington State status report for the killer whale. Washington Department of Fish and Wildllife, Olympia, WA.