Foraging behavior and ecology of transient killer whales (Orcinus orca)

Staff >> Robin Baird >> Foraging behavior and ecology of transient killer whales (Orcinus orca)

Three of the four chapters of my Ph.D. dissertation have been published and are thus available. I've included here most of the unpublished sections. 

Baird, R.W. 1994. Foraging behaviour and ecology of transient killer whales (Orcinus orca). Ph.D. Thesis, Simon Fraser University, Burnaaby, B.C. 159 pp. 


The foraging behaviour and ecology of transient killer whales (Orcinus orca) around southern Vancouver Island was studied from 1986 through 1993. Predation on marine mammals (mostly harbour seals) was observed on 136 occasions, and no predation on fish was observed. Transient killer whale occurrence and behaviour varied seasonally and between pods; some pods foraged almost entirely in open water and were seen throughout the year, while others spent much of their time foraging around pinniped haul-outs and other near-shore areas, and used the area primarily during the harbour seal weaning and post-weaning period. Overall use of the area was highest during that period, and energy intake at that time was significantly greater than during the rest of the year. Energy intake varied with group size, with groups of three having the highest energy intake rate per individual, and the lowest risk of an energy-shortfall. The typical size of groups comprised of adult and sub-adult whales, engaged primarily in foraging and feeding, was 3.29, implying that these individuals are found in groups consistent with the maximization of energy intake hypothesis. However, larger groups were also regularly seen. 

Near the end of this study, a time-depth recorder/VHF radio tag was deployed on six residents and one transient, to look for differences in diving behaviour between the two forms. While detailed information was only obtained for 23 hours, the data suggest that foraging-related differences in diving behaviour may exist. The proportion of time spent at depth differed between the two forms, with the residents spending the majority of their time at shallower depths than the single transient individual. 

Utilizing information collected during this study and from previous research, a model of indirect interactions between transient and resident killer whales was formulated. The model suggests that the evolution of foraging specializations in these populations may have occurred through frequency-dependent indirect interactions acting in concert with density-dependence within populations and disruptive selection on prey-type specific foraging characteristics. I suggest the two forms of killer whales may be in the process of speciating, i.e, they may be incipient species. 



I would like to thank Michael Corry for starting the ball rolling in 1983 when he introduced me to the study of ecology and of killer whales. In 1985 Thomas Learholm gave me the opportunity to begin the study of transient killer whales around southern Vancouver Island, with encouragement, guidance and logistical support from Michael Bigg then and for many years afterwards. Dave Duffus gave me my first full-time paying job working with killer whales, in Johnstone Strait in 1986, and has helped with encouragement, friendship and loans of equipment ever since. My friend and colleague Pam Stacey was instrumental in the inception and development of this research in 1985, and worked with me on every aspect of this research through 1990. Working as a field assistant and colleague from 1990 through 1993, Tamara Guenther provided incredible assistance with all aspects of the research. Alex Fraser provided assistance with logistical aspects of the research throughout. I would especially like to thank Larry Dill, for initially taking me on as a student in 1988, for guiding the development of this research through to its current state, and for his support - professionally, financially and personally. 

Personal financial support was obtained through scholarships from the Natural Sciences and Engineering Research Council of Canada, Simon Fraser University and the Anne Vallée Ecological Fund. Funding for the research was primarily through NSERC Canada (grant A6869 to LMD) and the Science Subvention Program of the Department of Fisheries and Oceans. Small grants or provision of services were also supplied by the Friends of Ecological Reserves, Cetacean Society International, The Whale Museum, BC Cellular, and BC Telephone Co. Logistical support was provided by the Canadian Pilotage Authority, the Center for Whale Research, the Victoria office of the Department of Fisheries and Oceans, Lester B. Pearson College, the Pacific Biological Station, and Seacoast Expeditions, Victoria. Alex Rhodes and Ron Ydenberg provided access to vessels during the early years of this study. Michael Bigg, David Ellifrit and Graeme Ellis provided identifications and details on whale ages, sighting histories, and gender. Marilyn Dahlheim also provided information on sighting histories. Many individuals helped collect behavioural data and transcribe tapes, but Tamara Guenther and Pam Stacey provided especially significant assistance. Similarly, numerous individuals assisted by reporting and locating whales, but I would particularly like to thank Vinz Eberl, Gerry Toner and Eric Walters. My research benefitted from numerous discussions with Marilyn Dahlheim, Dave Duffus, Tamara Guenther, Alex Morton, Peter Olesiuk, Rich Osborne, Pam Stacey, Peter Watts, and Michael Bigg. Research within several Provincial Ecological Reserves was conducted under permit from the Ministry of Parks Ecological Reserves Unit. Sherry Smrstik and the National Marine Mammal Laboratory library provided access to numerous references. The work in Chapter II would not have been possible without the assistance of Jeff Goodyear, who built the tags and modified them several times, loaned a crossbow to the project, and helped with three early trackings. Encouragement and loans of equipment by Dave Duffus, Peter Olesiuk, and particularly Marilyn Dahlheim and the National Marine Mammal Laboratory, Seattle, were also crucial to the work presented in Chapter II. Numerous individuals helped in deploying (or attempting to deploy) tags and tracking whales, particularly Tamara Guenther, Louise Hahn, Glen Hvenegaard, Bryan Nichols, Nicole Phillips, Pam Willis, Colin Wilson, and Steve Wischniowski. Don Horn and the University of Victoria provided access to the MV John Strickland for calibration of the depth sensors. Malcolm Ramsay and Susan Chivers provided access to unpublished (or in press) manuscripts. Graeme Ellis, John Ford, Luc-Alain Giraldeau, Craig Packer, Eva Saulitis, and an anonymous reviewer reviewed Chapter III. Peter Watts provided assistance with the development of an early version of the model in Chapter IV, and Peter Abrams formulated the final version presented in that chapter. For the analyses presented in Appendix II, John Ford and Ted Miller provided access to sonographs, Dave Duffus loaned me a Sony Professional cassette recorder, and Tamara Guenther, Pam Stacey and Kim Parsons assisted with analysis of recordings. The entire thesis benefitted from the suggestions of Larry Dill, Dave Duffus, Tamara Guenther, Alton Harestad, Hal Whitehead and Ron Ydenberg. 


This thesis is about the behavioural ecology of foraging killer whales (Orcinus orca), and is unique in several respects. First, cetaceans have played little role in the development or testing of behavioural ecological theory for the function and evolution of animal behaviour. Second, behavioural studies of predation by mammalian carnivores have largely been limited to a few species that hunt in open areas, a situation conducive to the observation and recording of predation events, including information on prey species, and the size and composition of the hunting group. While research on the behaviour of cetaceans has increased dramatically in recent years with respect to the diversity of species studied, the geographic scope of research efforts, and the range of research foci, relatively little research has been done on the foraging ecology or foraging behaviour of cetaceans. Of that which has been done, most has been descriptive and inferential, utilizing information on stomach contents, estimated energetic expenditures, surfacing patterns, and a largely incomplete knowledge of the populations and behaviour of potential prey species. 

The most intensive studies on the killer whale have focused on populations that feed primarily on fish well beneath the water's surface, thus limiting the researcher's ability to study the interactions among the predators or between them and their prey. In several areas of the world, however, the predictable occurrence of killer whales hunting near-shore marine mammals has allowed more detailed investigation of these types of interactions. These sites include the Crozet Archipelago in the Indian Ocean (Guinet 1992), and the Punta Norte region of Argentina (Hoelzel 1991). Shore-based observations of killer whales hunting elephant seals or sea lions in the surf zones of these areas have provided extensive information on the dynamics of group hunting, hunting tactics, and various factors affecting prey capture. Unfortunately, such studies have been limited by the relative inaccessibility of these sites, as well as by the lack of opportunity to study the whales when they were not in nearshore areas. One geographic area has been identified where killer whales regularly feed on marine mammals, where weather and logistical considerations allow for year-round vessel-based observations, and where it is possible to make frequent observations of prey capture, including information on prey species, size, duration of handling time, and the sex, size and identity (and often presumed relatedness) of individuals in the hunting group. That location is southern Vancouver Island, British Columbia, the site of my dissertation research - a study of the foraging behaviour and ecology of the so-called transient killer whales. 

Some background on the history and development of killer whale research is relevant. Prior to 1970, research on this species world-wide was largely based on the examination of beach-cast animals or those taken in whaling operations, as well as a few studies with captive animals. Field studies in British Columbia were first initiated by Spong et al. (1970), and have been continued by a variety of investigators. Notable has been the work of Bigg and his colleagues (Bigg et al. 1976, 1987, 1990; Bigg 1982), using photo-identification of individual animals based on distinctive acquired and congenital characteristics of the dorsal fin and saddle patch. They first described the occurrence of the two forms of killer whales recognized today. These two forms were originally termed residential or transient to particular areas based on movement patterns; throughout this dissertation they are referred to as resident and transient, as the names have subsequently been shown not to be descriptive. Occurrence of transient killer whales is much less predictable than that of residents, both temporally and geographically. Combined, the smaller group sizes and erratic surfacing patterns have made transients more difficult to find and follow, and the vast majority of research to date has focused on the resident populations around northern and southern Vancouver Island. An extensive network of spotters around the southern tip of Vancouver Island and the discovery of the somewhat predictable occurrence of transients in that area led to the initiation of my study of transients in 1986. 

My dissertation is divided into four chapters, each representing a stand-alone investigation of a specific aspect of transient foraging behaviour or ecology. Chapter I sets the context, describing the occurrence and behaviour of transientkiller whales in the study area, and how both vary seasonally and between transient pods (maternal groups); this variation is also shown to be related to differences in foraging tactics between pods. Chapter II provides a preliminary examination of killer whale diving behaviour; this is a previously uninvestigated topic that intrigues me both for its ability to provide insight into what the animals do the 95% of the time they are invisible below the water's surface, as well as for the opportunity to investigate differences in diving behaviour between transient killer whales and the sympatric fish-eating residents. Because of the dichotomy in prey choice between these two forms of killer whale (residents eat fish, while transients eat marine mammals), interpretation of diving behaviour can be undertaken in the comparative context of prey searching strategies. Chapter III investigates the meat and potatoes of transient hunting - the energetic benefits of foraging in different sized groups and how that relates to transient killer whale dispersal patterns and social structure. In Chapter IV a simple model is presented outlining potential indirect ecological interactions between transient and resident killer whales through the food web. The model itself is not unusual, simply applying Lotka-Volterra equations to a killer whale food web, but it provides a basis for the development of a verbal model which might explain the evolution of the foraging specializations seen in killer whales today. Together these papers represent an investigation into the foraging behaviour and ecology of a large social carnivore, providing a new understanding of the complexity of killer whale foraging tactics, differences between the sympatric residents and transients which may be relevant to processes of speciation between the two forms, and how factors such as the relationship between food intake and group size may have influenced transient killer whale social structure and dispersal patterns. 


Bigg, M.A. 1982. An assessment of killer whale (Orcinus orca) stocks off Vancouver Island, British Columbia. Rep. Int. Whal. Commn. 32:655-666. 

Bigg, M.A., I.B. MacAskie, and G. Ellis. 1976. Abundance and movements of killer whales off eastern and southern Vancouver Island with comments on management. Unpublished report, Arctic Biological Station, Ste. Anne de Bellevue, Quebec. 

Bigg, M.A., G.M. Ellis, J.K.B. Ford, and K.C. Balcomb. 1987. Killer whales - a study of their identification, genealogy and natural history in British Columbia and Washington State. Phantom Press, Nanaimo, B.C. 

Bigg, M.A., P.F. Olesiuk, G.M. Ford, J.K.B. Ford, and K.C. Balcomb. 1990. Social organization and genealogy of resident killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Rep. Int. Whal. Commn. Spec. Iss. 12:383-405. 

Guinet, C. 1992. Comportement de chasse des orques (Orcinus orca) autour des iles Crozet. Can. J. Zool. 70:1656-1667. 

Hoelzel, A.R. 1991. Killer whale predation on marine mammals at Punta Norte, Argentina; food sharing, provisioning and foraging strategy. Behav. Ecol. Sociobiol. 29:197-204. 

Spong, P., J. Bradford, and D. White. 1970. Field studies of the behaviour of the killer whale (Orcinus orca). Pages 169-174 in Proc. 7th Ann. Conf. on Biol. Sonar and Diving Mammals. 




The diving behaviour of killer whales (Orcinus orca) around southern Vancouver Island was investigated using a recoverable, suction-cup attached time-depth recorder (TDR)/VHF radio-tag. TDR tags were deployed on six residentsand one transient, resulting in a total of 23 hours of diving data, with depth recorded once per second. The shape of dive profiles was extremely variable, with parabolic, flat-bottomed u-shaped, irregular u-shaped (frequent changes in bottom depths), and v-shaped dives recorded. Dive depth was strongly correlated with dive duration for both the transient and all residents, although residents were more variable, with some long dives near the surface, some to mid-water, and others to the bottom (to 173 m). Long dives for the transient were less variable, with the majority to between 20 and 60 m depth, even when bottom depth was greater. The proportion of time spent at different depths also differed between the residents and the transient. While residents typically dove much deeper than the transient, the majority of their time (>66%) was spent at depths less than 20 m; the transient spent the majority of its time deeper than 20 m. I suspect these differences in use of the water column result from differences in prey species, since residents feed primarily on fish and transients feed primarily on harbour seals (Phoca vitulina). In open water, transients may spend the majority of their time at depth and detect prey visually, using silhouettes of prey against the surface. 



Case studies of incipient speciation provide an important window into understanding the general causes and consequences of reproductive isolation between populations (Otte and Endler 1989). My research into the foraging behaviour and ecology of transient killer whales provides an increased understanding of the differences between transients and residents, as well as of their possible causes and consequences. This study provides heretofore unavailable detail on many aspects of transient killer whale biology, that can be combined with information collected in the few other studies undertaken on transients. This information can be compared with the extensive base of knowledge of the biology of resident killer whales gained through the far more numerous studies on these animals. The purpose of this epilogue is to briefly review the current state of knowledge regarding differences between these two forms and the taxonomic implications of these differences. 

An early idea regarding the two forms was that transients were likely individuals who were rejected from resident pods (M.A. Bigg, pers. comm.), accompanied with the stigma of low productivity and relegation to a less desirable lifestyle (Bigg 1979). By 1987, Bigg et al. had termed these forms "races", and this term has been adopted, I suggest uncritically, by many investigators. The term "race" is usually defined in a geographic sense, implying geographically isolated populations which are typically given subspecific designation (Mayr and Ashlock 1991). In Chapter IV, I suggested that transients and residents should be considered incipient species, that is, in the process of speciation. The two forms might even be considered separate species, according to the biological species concept (Mayr 1969; Stuessy 1990). 

An updated list of differences, and potential differences, between transients and residents is shown in Table E.1. Genetic differences reported by Stevens et al. (1989) and Hoelzel (1989) were based primarily on mitochondrial DNA. While these differences suggest reproductive isolation between the two forms, the maternal inheritance of mitochondrial DNA precludes absolute determination of such isolation. Although several morphological differences between the two forms have been noted (Table E.1), also implying reproductive isolation, no information is available to determine how complete such isolation might be. I argued in Chapter IV that disruptive selection on prey-type specific foraging specializations may have favoured reproductive isolation of these populations. Such a scenario has been postulated with other species-pairs in sympatric situations (Benkman 1993; Grant and Grant 1989; McPhail 1992; Schluter 1993; Schluter and McPhail 1992, 1993; Tauber and Tauber 1989). 

If residents and transients were allopatric, no conclusions regarding their status as biological species could be reached. However, as noted by Mayr (1969), sympatry can be viewed as a test for the validity of biological species; if reproductive isolation is maintained in sympatry, divergent forms should be considered good biological species. I believe the available information, much of which is summarized in this thesis, is conclusive enough to suggest thatresidents and transients currently behave as different biological species. However, the tradition of applying a morphological species concept (rather than a biological one) to cetacean taxonomy makes such a suggestion unlikely to be accepted by the majority of cetacean taxonomists. Regardless, I do not mean to imply that the capacity to exchange genetic information does not exist between the two forms, especially considering the frequency with which interspecific and even intergeneric hybrids have been recorded in cetaceans (both in the wild and captivity; e.g., Nishiwaki and Tobayama 1982; Herzing 1990; Arnason and Gullberg 1993), only that such exchange does not appear to be occurring today, consistent with the biological species concept. 

Several pieces of evidence are needed about the resident/transient system to flesh-out the causes and consequences of reproductive isolation. One of these is the determination of a behavioural isolating mechanism. The clear differences in underwater sounds produced by these two forms (Ford and Hubbard-Morton 1990) is the obvious candidate; monitoring the reactions of resident whales to playbacks of transient sounds, and vice versa, as well as of residents andtransients to their own sounds, would demonstrate experimentally whether the differences in sound are used as a behavioural isolating mechanism, and would supplement the few observations of reactions of transients when nearresidents in the wild. More information on ecological separation of the two forms, through expanded studies of diving behaviour using TDR tags like those used in Chapter II, as well as studies of the behaviour and ecology of both forms at night, is also necessary. Lastly, the consequences of reproductive isolation, in terms of skeletal and other morphological differences between the two forms, needs to be investigated, particularly looking for the kinds of foraging-related differences suggested in Chapter IV. As noted in Chapter III, due to the large geographic range of individuals, the low frequency of resightings of known animals and the long calving intervals, continuing long-term studies will be necessary to provide detailed information on dispersal, as well as to provide the sort of life-history information available for residents (Olesiuk et al. 1990). The information in Chapter I on pod-specific differences in behaviour, habitat use and seasonal occurrence also suggests that studies must be expanded geographically and seasonally to take into account the intra-form variability evident for killer whales. When studies of transients expand andresident research matures, information from this system may become of more general interest to investigators working on the mechanisms, causes and consequences of reproductive isolation between populations, that is, the processes of speciation. 

Another consequence of this work, often overlooked in biological studies, is its implications for the conservation and management of killer whales. Information on transient diet presented throughout the thesis and the food web connections described in Chapter IV imply that human perturbations of any of the components of the transient/resident food web may affect residentstransients or both. Clearly, an ecosystem approach to management must be taken if these populations are to be maintained in spite of increasing human presence and disturbance of their environment. Information on the importance of group hunting for killer whales (Chapter III), the presence of pod-specific foraging tactics (Chapter I), and the probable role that learning and familiar hunting associates may play in prey capture (Chapter IV), also imply that live-capture programs for this species world-wide must be reconsidered in light of the potential for disruption of the social groupings of the animals which remain in the wild. Increased understanding of the biology of killer whales, and in particular their habitat use, is needed to properly manage these populations. 


Table E.1. Evidence to suggest reproductive isolation between residents and transients

Differences in mitochondrial DNA - Hoelzel 1989; Stevens et al. 1989 

Differences in the shape of the dorsal fin - Bain 1989 

Differences in saddle patch pigmentation - Baird and Stacey 1988 

Possible differences in eye patch pigmentation - D. Ellifrit, pers. comm. 

Differences in behaviour and ecology 

- diet - Chapter III; Bigg et al. 1990 

- travel patterns - Morton 1990 

- respiration patterns - Morton 1990 

- vocalizations - Morton 1990; Ford and Hubbard-Morton 1990 

- echolocation - Barrett-Lennard 1992 

- amplitude of exhalations - Chapter IV; Appendix II 

- diving patterns - Chapter II 

- group size - Chapter III; Morton 1990 

- dispersal from maternal group - Chapter III; Bigg et al. 1987 

- seasonal occurrence - Chapter I; Morton 1990 

- geographic range - Bigg et al. 1987 

Avoidance of residents by transients - Chapter I; Morton 1990; 

Possible aggression by residents towards transients - G. Ellis, pers. comm. 


Arnason, U., and A. Gullberg. 1993. Comparison between the complete mtDNA sequences of the blue and the fin whale, two species that can hybridize in nature. J. Mol. Evol. 37:312-322. 

Bain, D.E. 1989. An evaluation of evolutionary processes: studies of natural selection, dispersal, and cultural evolution in killer whales (Orcinus orca). Ph.D. Thesis, University of California, Santa Cruz. 

Baird, R.W., and P.J. Stacey. 1988. Variation in saddle patch pigmentation patterns in populations of killer whales (Orcinus orca) from British Columbia, Alaska, and Washington State. Can. J. Zool. 66:2582-2585. 

Barrett-Lennard, L. 1992. Echolocation in wild killer whales (Orcinus orca). M.Sc. Thesis, University of British Columbia, Vancouver. 

Benkman, C.W. 1993. Adaptation to single resources and the evolution of crossbill (Loxia) diversity. Ecol. Monogr. 63:305-325. 

Bigg, M.A. 1979. Interaction between pods of killer whale off British Columbia and Washington. Page 3 in Abstracts of the Third Biennial Conference on the Biology of Marine Mammals, October 7-11, 1979, Seattle, Washington. 

Bigg, M.A., G.M. Ellis, J.K.B. Ford, and K.C. Balcomb. 1987. Killer whales - a study of their identification, genealogy and natural history in British Columbia and Washington State. Phantom Press, Nanaimo, B.C. 

Bigg, M.A., G.M. Ellis, J.K.B. Ford, and K.C. Balcomb. 1990. Feeding habits of the resident and transient forms of killer whale in British Columbia and Washington State. Page 3 in Abstracts of the Third International Orca Symposium, March 1990, Victoria, B.C. 

Ford, J.K.B., and A.B. Hubbard-Morton. 1990. Vocal behavior and dialects of transient killer whales in coastal waters of British Columbia, California and southeast Alaska. Page 6 in Abstracts of the Third International Orca Symposium, March 1990, Victoria, B.C. 

Grant, P.R., and B.R. Grant. 1989. Sympatric speciation and Darwin's finches. Pages 433-457 in D. Otte and J.A. Endler, Editors. Speciation and its consequences. Sinauer Associates, Inc. Sunderland, Massachusetts. 

Herzing, D. 1990. Underwater and close up with spotted dolphins. Whalewatcher 24(3):16-19. 

Hoelzel, A.R. 1989. Behavioural ecology and population genetics of the killer whale. Ph.D. Dissertation, Cambridge University. 

Mayr, E. 1969. Principles of systematic zoology. McGraw-Hill, Inc. New York. 

Mayr, E., and P.D. Ashlock. 1991. Principles of systematic zoology. Second Edition. McGraw-Hill, Inc. New York. 

McPhail, J.D. 1992. Ecology and evolution of sympatric sticklebacks (Gasterosteus): evidence for a species-pair in Paxton Lake, Texada Island, British Columbia. Can. J. Zool. 70:361-369. 

Morton, A.B. 1990. A quantitative comparison of behavior in resident and transient killer whales of the central British Columbia coast. Rep. Int. Whal. Commn. Spec. Iss. 12:245-248. 

Nishiwaki, M., and T. Tobayama. 1982. Morphological study on the hybrid between Tursiops and Pseudorca. Sci. Rep. Whales Res. Inst. 34:109-121. 

Olesiuk, P.F., M.A. Bigg and G.M. Ellis. 1990. Life history and population dynamics of resident killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Rep. Int. Whal. Commn. Spec. Iss. 12:209-243. 

Otte, D., and J.A. Endler (Editors) 1989. Speciation and its consequences. Sinauer Associates, Inc. Sunderland, Massachusetts. 

Schluter, D. 1993. Adaptive radiation in sticklebacks: size, shape and habitat use efficiency. Ecology 74:699-709. 

Schluter, D., and J.D. McPhail. 1992. Ecological character displacement and speciation in sticklebacks. Am. Nat. 140:85-108. 

Schluter, D., and J.D. McPhail. 1993. Character displacement and replicate adaptive radiation. Trends Ecol. Evol. 8:197-200. 

Stevens, T.A., D.A. Duffield, E.D. Asper, K.G. Hewlett, A. Bolz, L.J. Gage and G.D. Bossart. 1989. Preliminary findings of restriction fragment differences in mitochondrial DNA among killer whales (Orcinus orca). Can. J. Zool. 67:2592-2595. 

Stuessy, T.F. 1990. Plant taxonomy. Columbia University Press. 

Tauber, C.A., and M.J. Tauber. 1989. Sympatric speciation in insects: perception and perspective. Pages 307-344 in D. Otte and J.A. Endler. Speciation and its consequences. Sinauer Associates, Inc. Sunderland, Massachusetts. 





Observation Group Composition

Periods Per Group


1 (Q7); (X10); (Y1); (Y3); (O2, O3); (M1, M2, M9); (P26, P28, P32); (Q9, Q11, Q13); (Y2, Y3, E10); (AO1, AO2, AL4, AL14); (O20, O21, O22, E12); (Q3, Q12, Q4, Q8); (Q4, Q8, E10, E12); (T125, T126, T127, T128); (Y1, Y2, Q3, Q12); (M1, F15, F16, F17, F18); (O4, O5, Q3, Q7, Q12); (O20, O21, O22, O4, O5); (Q4, Q8, Q9, Q11, TO12C); (T3, T6, T11, E10, E13); (F19, F20, F21, F22, F23, AM33); (O20, O21, O22, Q3, Q7, Q12); (O20, O21, O22, Q9, Q11, Q13); (Y1, Y2, Y3, T3, T11, E10); (AO1, AO2, AO4, AL4, AL14, M2, M9, M4); (O4, O5, T3, T6, T11, M20, M21, E12); (O20, O21, O22, E12, F15, T75A, F17, F18); (O4, O5, Q1, Q2, Q10, Q9, Q11, Q13, P1); (O20, O21, O22, Q9, Q1, TO12C, M2, M9, M4) 

2 (F1); (M1, M2, M4); (M2, M4, M9); (O4, O5, M2, M9); (Y1, Y2, Y3, E10); (M1, M2, M9, O4, O5); (T3, T6, T11, Q9, Q11, Q13); (M1, M2, M4, M9, F15, F16, F17, F18); (M1, M2, M4, M9, F15, F16, F17, F18, T2) 

3 (AO1, AO2, AO4, M4); (M1, M2, M4, M9); (Q3, Q7, Q12, P1); (Q3, Q7, Q12, T3, T11); (O20, O21, O22, Q9, Q11, T012C); (Y1, Y2, Y3, Q3, Q7, Q12) 

4 (V1); (Q3, Q7, Q12); (Y1, Y2, Q3, Q7, Q12) 

5 (Y1, Y2) 

6 (O4, O5); (O20, O21, O22) 

7 (Y1, Y2, Y3) 

8 (T3, T6, T11) 

Each observation period represents a single datum (a continuous period of time

during which group size and composition remained constant). Only those periods lasting

longer than 59 minutes, for group sizes with three or more observation periods,

are shown. 



In Chapter IV, I suggested that transients might modify the amplitude of their exhalations to minimize detection by potential prey. During field research in 1989 I noted that transient exhalations (blows) sounded quieter than residentblows. Such differences were apparent over a range of killer whale behavioural states, whether foraging, feeding or travelling. In an attempt to quantify this difference, I made recordings of killer whale blows in the field in 1990, 1992 and 1993. Recordings were made using a Sony Professional WM-D6C cassette recorder, and a Audio-Technica AT815a Condenser ("shot-gun") microphone. Due to masking by other noises, recordings could only be made on days when wind speed was less than 5 kph, sea state was Beaufort 0, the whales were travelling slowly, and no other power vessels were in the area. Due to the difficulty of measuring and maintaining a precise distance and orientation from the whales, the sound pressure level (SPL) of an exhalation could not be measured directly. Consequently, the duration of the exhalation was chosen as a potential indicator of SPL, since SPL should be greater for exhalations of shorter duration (assuming a constant exhalation volume). Exhalation duration was measured from sonograms using a Kay DSP Sona-Graph model 5500. 

Two hundred and sixty three exhalations were recorded from residents, and 391 exhalations were recorded from transients. Because the whales were usually in groups, information on individual identity was not available for all recorded blows; thus classification of individuals as to age and sex was not always possible. In many cases exhalations of two or more individuals overlapped, which prevented the measurement of blow duration. Distance from the subjects and the presence of other vessels in the area also affected the quality of recordings; measurement of duration for many blows recorded at large distances or with vessels nearby was not possible. Considering only those individuals with five or more blows of sufficient quality to measure exhalation duration, and utilizing only adult individuals (since relatively few measurements were available for sub-adult animals), resulted in a total of 56 usable blows for residents and 145 for transients. These included exhalations recorded from 13 individuals: 3 transient males, 2 resident males, 5 transient females, and 3 resident females. Number of exhalations analyzed for each individual ranged between 7 and 49 (mean = 15.5, SD = 11.0). 

The mean exhalation duration for each of these whale types is shown in Table A.II.1. Exhalations of transient males are significantly longer than those of transient females (Mann-Whitney U-test, p < 0.001), but no difference between the blows of male and female residents was found (Mann-Whitney U-test, p = 0.814). A comparison of transients versus residents for each sex separately indicates that transient male exhalations are of longer average duration than those of residents (Mann-Whitney U-test, p = 0.005), while no difference exists between the exhalation durations for females of the two types (Mann-Whitney U-test, p = 0.478). 

The increased duration of the exhalations of transient males should result in a decrease in loudness, and such a decrease could function to decrease detection by harbour seals. Alternatively, such differences in the characteristics of the exhalation may simply reflect differences in the lung capacity of the two forms, since transients typically take much longer dives than residents (Morton 1990). Why no differences were found between females of the two forms is unclear. Differences in amplitude noted in the field appear consistent for both male and female transients, suggesting that some mechanism other than an increase in the duration of the exhalation must be partly responsible for the apparent decrease in amplitude. Measurements of amplitude from audio recordings made using a directional microphone and a cassette recording system, in combination with determination of distance to the whales using a video system might allow for more direct quantification of differences between transient and resident blows. 


Table A.II.1


Descriptive statistics for killer whale exhalation durations (sec).


Killer whale type Mean SD N 


Individuals Blows


Resident males 0.636 0.121 2 22 

" females 0.614 0.144 3 34 

Transient males 0.723 0.116 3 83 

" females 0.613 0.121 5 62 




Morton, A.B. 1990. A quantitative comparison of behavior in resident and transient killer whales of the central British Columbia coast. Rep. Int. Whal. Commn. Spec. Iss. 12:245-248.