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Echolocation in wild killer whales (Orcinus orca) Barrett-Lennard, Lance 1992

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ECHOLOCATION IN WILD KILLER WHALES (ORCINUS ORCA). by LANCE BARRETT-LENNARD B .Sc., University of Guelph, 1980  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology)  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA December 1992 © Lance Barrett-Lennard, 1992    In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study . I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives . It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  (Signature)  Department of  Zoology  The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  Dec.  ABSTRACT  Echolocation by odontocete whales has been demonstrated in captive settings many times, yet little is known about its use and function in the wild . In this thesis, I describe echolocation sounds in pods of killer whales (Orcinus orca) off the coasts of British Columbia and Alaska. I examine the relationships between echolocation and water clarity, ambient noise, and behavioural activity. I compare echolocation by two populations with different feeding habits and foraging patterns . The study provides new insight into the functional role of odontocete echolocation, and into the relative contributions of vision and passive listening to spatial perception and prey detection.  Underwater recordings were made during 111 encounters with killer whales . Known members of the resident (fish-eating) population were photographically identified 85 times, and transients (mammal eaters) 23 times . Click sounds characteristic of echolocation were identified in the recordings aurally, and by spectrographic and waveform analysis . Most clicks occurred in series (trains), but isolated clicks were occasionally detected.  Echolocation behaviour differed strikingly between residents and transients. An echolocation index (EI) was defined as the average percentage of time that an individual produced click trains . The mean EI for residents was 4 .24%, 27 times greater than for transients . The duration of resident click trains averaged 6 .83 s, compared to 0 .86 s for transients. Resident click trains were comprised of evenly spaced clicks, whereas transient trains had uneven click spacing . Transient individuals used isolated clicks once every five minutes,  111  four times as often as residents . For resident killer whales, EI values were significantly higher during foraging and travelling than during other behavioural activities . In residents, EI declined with group size . This was also true of transients, for the recordings in which click trains were detected. Transient EI levels were significantly higher when close to shore than when offshore.  No relationship between EI and ambient noise level was found for either residents or transients. Residents increased the amplitude of their clicks in response to increasing ambient noise levels . No relationship was found between water clarity and EI for either type of killer whale .  I suggest that the differences in echolocation behaviour between residents and transients are accounted for by their different prey . Fish have little or no aural sensitivity in the frequency range of killer whale clicks . Marine mammals are able to detect clicks, and may use them to evade killer whale attacks . The use of isolated clicks and short, irregular, quiet click trains makes transient echolocation less detectable by marine mammals than the echolocation used by residents . Passive listening is probably the principal technique that transients use to locate prey, whereas residents use echolocation in combination with passive listening when foraging. Vision is not a major factor in locating prey, but may be used by either whale type during pursuits. Finally, I suggest that both residents and transients obtain much of their positional and orientational information using passive listening alone .    iv  TABLE OF CONTENTS Page ABSTRACT  ii  TABLE OF CONTENTS  iv  LIST OF TABLES  vi  LIST OF FIGURES  vii  ACKNOWLEDGEMENTS  ix  CHAPTER ONE : General Introduction  1  CHAPTER TWO : Transient killer whales seldom click: intraspecific divergence in patterns of echolocation use  5  Introduction  5  Study Species  7  Methods  8  Study Locations  8  Sampling Methodology  8  Behaviour Categories  10  Acoustic Analysis  11  Photographic analysis  15  Results  16 Killer Whale Encounters  16  Detectability of Echolocation Sounds  16  Characteristics of Echolocation Pulses  17  Click Train Characteristics  18  v Echolocation Index Values  19  Contexts of Echolocation Use  19  Behavioural Activity  19  Group Size  22  Water Clarity  22  Ambient Noise  23  Discussion  24  Resident and Transient Echolocation Levels  24  Patterns of Echolocation Clicks  25  Behavioural Contexts of Echolocation Use  27  Group Size and Echolocation Use  28  Echolocation Use and Water Clarity  29  Echolocation Use and Ambient Noise  31  Conclusions Echolocation and Prey Detection Strategies  33 33  CHAPTER THREE : General Conclusions  53  REFERENCES  60  APPENDIX  69  vi LIST OF TABLES Page  Table I.  Study locations, dates and areas surveyed .  35  Table II .  Characteristics of resident and transient click trains .  36  Table III.  Echolocation by resident and transient killer whales .  37    vii  LIST OF FIGURES Page Figure 1 . Map of the northwest coast of North America showing the study areas .  38  Figure 2 . Waveform of a killer whale echolocation click .  39  Figure 3 . Energy distribution of two echolocation clicks .  40  Figure 4. Waveform and spectrogram of four seconds of a resident killer whale click train .  41  Figure 5. Waveform and spectrogram of four seconds of a transient killer whale click train.  42  Figure 6. Number of analyzed five minute recordings of resident and transient killer whales, by behavioural activity .  43  Figure 7. Mean echolocation index values for resident and transient killer whales, by behaviour category.  44  Figure 8. Cumulative frequency distribution of echolocation index levels for resident and transient killer whales .  45  Figure 9 . Echolocation index level as a function of group size for resident killer whales .  46  Figure 10. Echolocation level as a function of group size for transient killer whales . 47 Figure 11 . Photocell resistance as a function of depth .  48  Figure 12. Echolocation index as a function of Secchi depth for resident killer whales .  49  vii' Figure 13. Echolocation index as a function of Secchi depth for transient killer whales .  50  Figure 14. Echolocation index plotted against environmental and vessel noise levels for resident killer whales .  51  Figure 15. Echolocation index plotted against environmental and vessel noise levels for transient killer whales .  52  Figure Al . Cumulative frequency histograms of echolocation level by resident killer whales engaged in four behavioural activities .  70  Figure A2 . Cumulative frequency histograms of echolocation level by transient killer whales engaged in four behavioural activities .  71  Figure A3 . Plots of resident and transient killer whale echolocation level against Secchi depth, by behaviour category .  72  Figure A4 . Plots of resident killer whale echolocation level against environmental and vessel noise levels, by behaviour category .  73  Figure A5 . Plots of transient killer whale echolocation level against environmental and vessel noise levels, by behaviour category .  74  ix ACKNOWLEDGEMENTS A remarkable number and variety of people contributed ideas, inspiration, equipment, and direct assistance to this project . I owe a debt of gratitude to each one of them. The late Michael Bigg, friend and mentor, helped plan this project, and blessed it with his infectious enthusiasm and perpetual good humour . I believe that he would have been tickled at the further unfolding of the resident/transient killer whale story, and I hope I have done his memory justice . I was privileged to have had two supervisors . One was John Ford, who got me started in killer whale acoustics, and who encouraged me to return to graduate school after a long hiatus in the free world . John suggested this project, and shared my excitement as the results emerged. Jamie Smith co-supervised . Jamie was a wellspring of enthusiasm, advice, and support . This thesis was improved immeasurably by the application of his legendary editing skill. I owe him many red pencils and many thanks. Graeme Ellis deserves special mention for doing the photo-identifications, and for knowledge and advice freely offered . Craig Matkin of the North Gulf Oceanic Society made it possible to carry out research in Alaska . Eva Saulitus, Molly Freeman, and Olga von Ziegesar, my co-researchers in Prince William Sound, were constant sources of support and ideas, and the best of friends. The stalwart crew of the "Lucky Star" supplied engine parts and fresh vegetables, and countered the angst of biologists with the practical wisdom of fishermen. In British Columbia I was assisted by the Haida Watchmen, and by Ron Hamilton and Doug Buries of the Canadian Parks service . Anna Reid, Julie Kimmel, Leah Saville, Jamie Smith, and my father Godfrey Barrett-Lennard endured heavy seas, bad food, cramped accommodation, and cranky leadership while serving as crew . Alex Morton and Robin Baird shared details of their observations of transient killer whale foraging behaviour. David Bain, Marilyn Dalheim, Paolo Domenici, Christophe Guinet, Gillian Muir, Oskar Painter, Dick Repasky, Tiu Simila, Andrew Trites, David Ward and David Westcott offered ideas and advice . Robin Liley served on my committee and supplied feedback and comments on earlier versions of this thesis . Don Ludwig offered many helpful suggestions throughout, and provided support for a summer assistant to develop acoustic analysis software . Pete Matthews gave many hours of his time helping me come to grips with the physics of underwater acoustics. I owe the most to Kathy Heise, who was involved in the project from beginning to end, who was indefatigable in the field, and whose immaculate field notes helped patterns emerge from chaos . Kathy reminded me on the most difficult days to open my eyes : after all, we were alive and well and working in paradise. I received financial support for research expenses from the Vancouver Aquarium, and from an NSERC operating grant awarded to Jamie Smith . I received personal support from a UBC teaching assistantship, and from a BC Science Council G .R.E.A.T. scholarship . Equipment was loaned by the North Gulf Oceanic Society, the Vancouver Aquarium, the Canadian Department of Fisheries and Oceans, and the University of British Columbia . I thank the Canadian Parks Service for permitting me to conduct research in Gwaii Haanas/South Moresby, and the US National Marine Fisheries Service for allowing me to combine my research with an ongoing population study of Prince William Sound killer whales .  1 CHAPTER ONE  General Introduction  Sound is well-suited for use as a carrier of information . It takes little energy to produce, it propagates efficiently through air and water, and its bandwidth properties allow it to carry many signals simultaneously (Norris and Evans 1988) . Numerous animal species have the ability to both detect and create sounds . Thus, it is not surprising that many species also have the ability to actively gather information using the technique of echolocation . Species for which echolocation has been reported include rodents, insectivores, birds, bats, humans and whales (Fenton 1980 ; Norris & Evans 1988) . Perhaps what is most surprising is that only the bats and toothed whales are known to have highly-evolved echolocation capabilities.  The advantage of echolocation seems evident: it is a technique that can be used to acquire spatial information when visual, olfactory and passive acoustic cues are not sufficient for a particular activity . The most obvious liability of echolocation is that the user risks revealing its location and identity to competitors, predators, and prey . Owls and pinnipeds suffer similar constraints to vision as bats and toothed whales, and have obvious hearing and vocal preadaptations for echolocation, yet they use echolocation rarely, if at all (Konishi 1973 ; Watkins & Warztok 1985). Thus, conditions that restrict vision do not necessarily result in net selection for echolocation.  Researchers of bat sonar have long been aware of both the costs and the benefits of echolocation (eg Fenton 1980) . Echolocation facilitates the pursuit of insects and the avoidance  2 of obstacles (Fenton 1980) . However, some species of prey take effective evasive measures upon exposure to bat echolocation sounds (Roeder 1975), competitors can eavesdrop on echolocators and share prey (Barclay 1982), and predators may track bats by homing in on their echolocation calls (Fenton 1980) . Many bats are known to hunt part or most of the time using passive listening rather than echolocation . In these species the costs of echolocation exceed the benefits (Andersen & Racey 1991 ; Faure & Barclay 1992) . In general, echolocation is a mixed blessing sensu Fenton (1980), and a cost-and-benefit approach is important to understanding and predicting its use.  The role of echolocation in the behavioural ecology of bats is much better understood than for toothed whales (odontocetes) . Echolocation by bottlenose dolphins (Tursiops truncatus) was discovered in the 1950's (McBride 1956 ; Kellogg 1958) . Since then, much research effort has been devoted to describing the sonar capabilities of various odontocete species in captivity (eg Diercks et al. 1971 ; Au 1980; Moore & Pawloski 1990 ; Au 1990; Kamminga & Beitsma 1990, Turl 1990 ; Turl et al . 1991) . By contrast, the published literature on echolocation in wild cetaceans is limited to a few papers describing the acoustic analysis of single encounters (Ford & Fisher 1978 ; Steiner et al . 1979; Goodson et al . 1988), or listing incidental observations (Watkins 1980).  Field studies have lagged behind research on captive animals for several reasons . First, field studies of echolocation lack the precision of behavioural observation possible in a captive setting. For example, it is seldom possible to link specific individuals in a group with specific sequences of echolocation pulses . Second, acoustic characterisations of sounds recorded in the  3 field are inexact. For example, the frequency structure of echolocation sounds varies depending on an animals's orientation with respect to the hydrophone (Au et al . 1978) . Since orientation cannot be controlled or determined in the field, only general descriptions can be made of the frequency characteristics of echolocation sounds (Watkins 1980) . Third, overlapping sequences of clicks from large groups may be difficult to differentiate, and sounds from nearby animals may mask quieter sounds from those of more distant individuals (Watkins 1980).  These problems, however, do not rule out studies of wild populations aimed at describing the behavioural, social and environmental contexts of echolocation use (Evans & Awbrey 1988). Identification of specific echolocators and precision in the frequency component of acoustic analysis are not essential to answering "when" and "why" questions about echolocation use . The problem of overlapping sequences may be . Mressed to some extent by focusing on smaller groups . Studying echolocation in a wild population of odontocetes requires a sacrifice in precision in order to see general patterns.  Populations of killer whales (Orcinus orca) along the north-west coast of North America are well-suited for echolocation research . Killer whales in this area are accessible, predictable in distribution, and familiar with the presence of small vessels (Bigg et al . 1987 ; Leatherwood et al. 1990) . They travel in small groups, and pass through areas where ambient noise levels are low . Both of these characteristics facilitate acoustic analysis . Their non-sonar vocal behaviour has been described (Ford 1984, 1989, 1991), the majority of individuals have been identified using natural markings (Bigg et al. 1987 ; Ellis 1987; Heise et al . 1992), and research has been carried out on their social structure (Bigg et al . 1990) . Diercks et al . (1971) confirmed  4 that the species uses echolocation, and described its echolocation sounds . Most significantly, two non-associating populations of killer whale inhabit this region, one of which eats fish and the other marine mammals (Ford 1984 ; Bigg et al. 1987 ; Morton 1990) . This makes it possible to relate echolocation use to foraging strategy, and to assess the flexibility of echolocation use.  In this thesis I examine echolocation in the two forms of coastal killer whale from British Columbia and Alaska . In Chapter Two, I compare echolocation use between the two forms in various behavioural and environmental contexts. I consider the question of whether echolocation substitutes for vision by examining the relationship between water turbidity and echolocation use . I also ask whether echolocation substitutes for passive listening by examining whether ambient sounds that potentially restrict passive listening are associated with increased echolocation use . Finally, I propose strategies of echolocation use for fish-eating and mammaleating killer whales . In Chapter Three I discuss the "mixed blessing" of echolocation in light of these findings .  5 CHAPTER TWO  Transient killer whales seldom click: intra-specific divergence in echolocation use.  Introduction  Since the discovery in the late 1950's that some toothed whales use echolocation, the sonar abilities of various species have been investigated extensively. This research has focused on the following topics : 1) the acoustic structure and directionality of echolocation pulses (Au & Hammer 1978 ; Hol & Kamminga 1979); 2) target resolution ability (Au et al . 1978; Kamminga & Beitsma 1990) ; 3) effects on sonar performance of noise and reverberation (Au 1980; Au & Turl 1984; Turl et al . 1991) ; and 4) the effects of target size and distance on sonar pulse structure, amplitude and repetition rate (Morozov et al . 1972 ; Turl & Penner 1989 ; Moore & Pawloski 1990) . These questions have all been addressed through experiments on captive dolphins and porpoises . In most cases individual animals have been trained to perform tasks such as detecting silent targets or discriminating between similar objects while blindfolded, prior to the running of trials.  This experimental paradigm has provided many answers to "what" and "how" questions of sonar capability . Unfortunately progress in this area has outstripped progress on "when" and "why" questions of sonar utility . For example, there is certainly overlap in the function of vision and sonar, yet we know very little about how and when individuals make decisions about which to use . Several authors have suggested that passive listening is a third means of orienting and  6 locating prey (Norris 1967 ; Wood & Evans 1980 ; Evans & Awbrey 1988) . This has not been investigated systematically in captive studies, apart from measuring hearing capabilities (Andersen 1970 ; Thompson & Herman 1975 ; Thomas et al . 1990 ; Bain 1992).  Four recent sets of observations indicate that wild odontocetes may use echolocation less often or for different functions than was previously suspected. 1) Equipping fishing nets with acoustically-reflective targets generally does not reduce accidental entanglements (Evans & Awbrey 1988 ; Lien et al . 1990 ; Dawson 1991). 2) Recently-captured or untrained dolphins are often unable to use echolocation effectively for simple tasks such as obstacle avoidance (Wood & Evans 1980) . 3) Recordings of wild killer whales (Orcinus orca) indicate that silent travelling is not uncommon, even under conditions of restricted visibility (J . Ford, pers . comm .). 4) Blindfolded captive bottlenose dolphins can pursue live fish without emitting echolocation sounds (Wood & Evans 1980), presumably using passive listening alone . Since the advantages of echolocation to individuals living in dark or turbid environments seem obvious, these observations imply significant costs to echolocation use.  Various costs of echolocation use have been reported for bats (Fenton 1980 ; Barclay 1982). These include attracting predators and competitors, and stimulating evasive behaviour in prey . Similar costs may apply to odontocetes . In addition, echolocation in social odontocetes is probably affected by information sharing, and by the use of echolocation sounds in communication . These questions have not been addressed to date, due to the lack of descriptive data on echolocation use by wild populations .  7 This study provides baseline data on echolocation in the wild for one species, the killer whale . I describe echolocation in free-ranging populations on the north west coast of North America, and examine the costs and benefits of echolocation use by comparing populations of killer whales with different feeding specializations . I investigate the behavioural correlates of echolocation use, to test whether echolocation functions principally in food-detection and orientation. I test the hypothesis that information acquired using echolocation is shared between group members. Finally, I assess tradeoffs between echolocation, vision, and passive listening by comparing echolocation use across a range of water clarities and ambient noise levels.  Study Species  The coastal killer whale populations of the northern Gulf of Alaska and British Columbia have been actively studied for 12 and 20 years, respectively. Almost all individuals have been identified and catalogued on the basis of natural markings and fin shape (Bigg et al . 1987 ; Heise et al. 1992), and their seasonal movements, diet, social structure, and life history parameters have been described. (Stevens et al. 1989 ; Bigg et al. 1990 ; Nichol 1990; Olesiuk et al . 1990). Two sympatric non-associating forms, known as residents and transients, inhabit these areas (Bigg et al. 1987) . Residents live in stable matrilineal groups or pods of 5 to 40 individuals and forage principally (perhaps entirely) on fish (Bigg et al . 1987) . Transients live in less stable groups of 2 to 10 animals and feed principally or entirely on marine mammals (J . Ford and G. Ellis, pers . comm .). Killer whales produce three distinct type of vocalisations: pure tones or whistles, pulsed tones, and clicks (Ford 1989) . In odontocetes, whistles and pulsed tones are believed to be primarily social signals, and clicks are used in echolocation (Popper 1980) . Ford  8 (1984, 1991) reported pod-specific repertoires of discrete pulsed calls for residents, whereas all transient groups are believed to share a single repertoire (Ford 1984).  Methods  Study Locations The study sites and dates are listed in Table I, and the locations are shown in Figure 1. All three sites were sheltered from heavy seas and had relatively low levels of vessel traffic, making it possible to record under low noise conditions . In Prince William Sound and the central coast of British Columbia, the water varied widely in clarity between different locations, due to glacial meltwater and surface run-off. The killer whale populations of British Columbia and Prince William Sound do not overlap, hence two discrete resident and two discrete transient populations were examined.  Sampling Methodology In Alaska I used an 8 m vessel for field accommodation and for all aspects of the research . The vessel was driven by a single-screw stern drive, and had a maximum cruising speed of 16 knots . In British Columbia a 13 m vessel was used for accommodation and research activities . It was driven by a single screw on a straight shaft, and had a maximum cruising speed of 8 knots . A 5 m inflatable boat powered by a 25 hp outboard engine was also used for searching and observing in British Columbia, when sea conditions permitted . I worked with one assistant in Alaska, and one or two assistants in British Columbia .  9 In order to locate killer whales we visually searched areas known to be commonly used by killer whales . Sighting information received from mariners or aviators by radio was used whenever possible to focus searches . During searches we travelled at approximately 8 knots, scanning with binoculars . If local conditions permitted, we periodically dropped an observer off on shore to scan from a high vantage point with 7X50 binoculars or a 20X spotting scope . At approximately 1 hour intervals a hydrophone was deployed to listen for vocalising pods out of visual range. If distant whales were detected, we attempted to find them using a directional hydrophone.  When killer whales were located we approached slowly to within about 25 m, and then paralleled their course while taking identification photographs with a 35 mm camera fitted with a 300 mm lens and shoulder brace . We photographed the left side of each individual, using the protocol described by Bigg et al . (1986) . When all animals present had been photographed, we moved 500 m ahead of the animals and shut off the vessel engine . A Bruel and Kjaer 8101 hydrophone was lowered on a 25 m cable, and recordings were made on Ampex 407 tape using a custom-built calibrated pre-amplifier and Nagra IV-SJ reel-to-reel recorder running at 37 cm/s (effective system frequency response 25 Hz - 35 kHz +/- 1 dB) . Recordings were made at hourly intervals . Five minute recording sessions were used, since the animals generally remained within range for detecting echolocation for that period . Simultaneous voice recordings of behavioural observations were made on a separate track of the same tape . Backup recordings were made concurrently using a Sparton 60CX123 hydrophone on a 20 m cable and a Sony WM-D3 cassette recorder.  10 Immediately after each recording session, water clarity was measured using a 20 cm Sechii disk divided into black and white quadrants . The disk was lowered on the lee side of the vessel until it disappeared from view . It was then raised until it re-appeared, and the Secchi depth was taken to be the average of the two . The surface of the water was shaded from direct sun during this measurement. To assess whether water clarity varied with depth, an ambient light profile was made using a Archer 276-116A photocell attached by a 30 m cable to a resistance meter . Readings were taken every meter as the cable was lowered . Clear layers were revealed when depth increments resulted in little or no change in resistance reading : rapid changes indicated the presence of turbid layers.  Behaviour Categories During the recording sessions the behavioural activities of the whales were noted . These were grouped into the six categories listed below . The first four are based on categories used by Ford (1989). Foraging : This included all occasions when the whales were known or suspected to be feeding or searching for prey. Direct evidence was available when whales were seen carrying prey, or if fish scales, tissue, blood, or blubber were seen floating at the surface . Kills were confirmed whenever possible by collecting remains with a fine mesh net for close inspection . Erratic highspeed swimming, sudden lunges, and swimming in tight circles were also considered to be evidence of foraging by residents . Swimming close to shore, entering into shallow bays, and circling kelp patches and small islands was evidence of foraging by transients (Ford 1984). Transients swimming in dispersed formation across open areas were also considered to be  11 foraging (Baird & Stacey 1989). Travelling: A group was considered to be travelling when its members formed a single large  group or several tightly-knit subgroups and moved on the same course at speeds > 6 km/hr. Resting : Groups were categorized as resting when members formed tightly-knit groups, moved  at speeds < 4 km/hr, and surfaced in unison in a co-ordinated manner, without travelling consistently in any direction. Socializing: Whales grouped into clusters and engaged in physical interactions such as chasing,  rolling, and thrashing were considered to be socializing . Breaches (when an animal threw most or all of its body out of the water), spyhops (vertical surfacing with the head lifted clear of the water), and the slapping of flippers, dorsal fins and tail flukes on the surface were all characteristic of social activities. Milling : Groups were considered to be milling when their members grouped loosely and made  little or no progress through the water . Unlike resting, breathing during milling was neither regular nor co-ordinated. Slow-travelling: Groups that moved at about 3-6 km/hr, showed no signs of foraging, engaged  intermittently in surface activities such as breaches and tail fluke slapping, and organized into moderately dispersed subgroups of 3-4 animals, were considered to be slow-travelling.  Acoustic Analysis The recordings were analyzed using a Kay Elemetrics DSP-5500 sound analyzer, capable of performing acoustic analysis in real time . The tapes were played back at reduced speed (1/8 to 1/2 original speed, depending on the complexity of the sounds being analyzed) . I displayed the analysis results on a divided video screen . Half of the screen displayed a colour-enhanced  12 512 point spectrographic display with a 64 kHz bandwidth (compensated for tape speed) in a 4 second time window . This display was useful for detecting fine-scale variations in frequency. Although the system response declined beyond 35 kHz, useful information extended to 50-55 kHz. Killer whale echolocation pulses or clicks have been reported to lie well within this range (Diercks et al . 1971) . The second half of the screen displayed the signal waveform, which was useful for detecting signal strength fluctuations over time . Input and dynamic range levels were adjusted periodically to compensate for varying signal-noise ratios . I also monitored the playback aurally during all phases of the analysis.  Sonar pulses usually occurred in regular series, hereafter referred to as click trains. During the analysis, the beginning and end times of each click train were recorded using a digital stopwatch . The individual pulses in any given click train were typically consistent in timing and frequency structure, and were readily-distinguishable from those in other trains. This made it possible to distinguish overlapping trains . If the whales were widely-dispersed or travelling rapidly during a recording session, it was sometimes difficult to determine whether certain trains had ended or faded into background noise . If this occurred, the analysis of that session was discarded . Five minute recordings of grouped pods moving at normal speeds could usually be analyzed successfully . Recordings of rapidly-travelling or very widely-dispersed pods could not always be analyzed, particularly under high noise conditions.  I used a simple echolocation use index (EI) to quantify sonar activity . This was the average percentage of time that an individual whale present during a recording session emitted click trains, and was calculated as follows :  13 EI=100 d sn  where d is the sum of the durations of all the click trains, s is the duration of the recorded session, and n is the number of whales present. Values of the echolocation index are also referred to as echolocation index levels throughout this paper.  Echolocation indices for the pooled resident and transient data were compared by t-test. Differences among EI means across all behaviour categories were square-root transformed to equalize variances and were tested using a one-way ANOVA . In the case of transients, a nonparametric analysis of variance was used (Kruskal-Wallis test, Zar 1984, p 176) since the variances could not be equalized by transformation . When means differed, Scheffe's multiple contrast test (Zar 1984, p 196) was used to test whether echolocation was more strongly associated with travelling and foraging than with other behavioural activities.  Click trains were selected using random number tables for measurements of average pulse repetition rate (R), calculated as  R= c-1 t  where c is the number of clicks in the train, and t the duration of the train . Repetition rates were compared between residents and transients using t-tests . The variances in repetition rate were compared using Bartlett's test (Zar 1984, p 181) . Every train detected was also rated for the evenness of intervals among its component clicks . Trains with irregular timing were flagged  14 and counted during analysis . A train was defined as irregular if the interval between any pair of adjacent clicks differed by more than 10% . Thus, trains that graded gradually from one repetition rate to another were considered to be regular. Most click trains could be readily classified by listening to a slowed playback of a recorded session and viewing its spectrographic form. When irregular trains were detected, I paused the playback and compared click intervals using time cursors on the spectrographic display . The percentages of irregular trains were calculated session-by-session. These data were normalized using a modified Freeman and Tukey arcsine transformation (Zar 1984, p 240) . Mean percentages were then compared using t-tests.  Isolated short-duration broadband sounds closely resembling sonar pulses but not occurring in trains were detected periodically . These are referred to as isolated clicks, and were quantified using the following index (IC):  IC=1000  C.  ns  where c ; is the number of isolated clicks, n the number of animals present and s the duration of the recorded session in seconds . Isolated click use was compared between transients and residents using t-tests on the arcsine-transformed data.  Finally, the average background levels of both environmental and vessel noise were rated on discrete scales of 0-5 . Each unit increment represented a change in average sound pressure level of 10 dB . This was determined by reference to the input levels of the recorder and the pre-amplifier at the time of recording, and the analyzer input level settings . The absolute sound  15 pressure level of the baseline was not determined, however it was consistent between all recording sessions . Resolution beyond 10 dB was not practical since noise levels fluctuated considerably within a 5 minute recording session. Environmental noise was caused by wind, rain, waves, surf, and the fizzing of melting glacial ice ; it was generally broadband and relatively constant. Vessel noise was produced by boat engines and propulsion systems, and was also broadband although lower in peak energy than environmental noise . Vessel noise varied from a constant roar to an intermittent throbbing . When both environmental and vessel noise were present, the relative contribution of each to total ambient noise was estimated . Associations between these noise types and echolocation index levels were sought using simple and multiple linear regression techniques on the raw and log-transformed data pooled across all behaviour categories, for each behaviour separately.  In many instances more than one recording session was analyzed from the same encounter. To ensure that recordings were statistically independent, I selected sessions for analysis which were separated by a distinct break in behaviour . Thus, if an encounter consisted of 3 h of foraging followed by 2 h of resting and then 3 h of foraging, I analyzed at most one session from each of the three activity periods . On occasion I analyzed concurrent transient foraging sessions that were separated by a kill and feeding session or by a transition between foraging modes (for example a move from an inshore area to an offshore area).  Photographic Analysis The whales present during each encounter were photographed to identify them as resident or transient. Ilford HP-5 black and white film was exposed at 1600 ASA using a shutter speed  16 of 1000/sec or less and developed as described by Miles (1990). The negatives were examined under low-power magnification with a dissecting microscope, and the animals identified using catalogues by Bigg et al . (1987), Ellis (1987), and Heise et al . (1992), and the photograph collections of the Pacific Biological Station, Nanaimo, British Columbia, and the National Marine Mammal Laboratory, National Marine Fisheries Service, Seattle, Washington . All animals photographed were positively identified and were known previously as residents or transients, except for three encounters with "new" groups . Recordings of these new animals were excluded from the data set .  Results  Killer Whale Encounters Killer whales were encountered 111 times during this study . Residents were present 85 times, transients 23 times, and uncategorized groups the remaining 3 times . Both residents and transients were observed in a range of behavioural activities and under varying ambient noise and water clarity levels . An average of 5 h was spent photographing, observing and recording the animals during each encounter. Successful acoustic analysis was performed on 162 recorded sessions of approximately 5 min duration each, comprising 74 sessions with residents and 88 with transients.  Detectability of Echolocation Sounds Echolocation clicks were often detected from approaching killer whales up to or beyond 3 km ; under most conditions complete trains could be distinguished when the animals were  17 within 1 km . When a pod passed the boat, clicks dropped in volume quickly, and detection became unreliable within 500 m . This directional effect was most evident in the high frequency part of the sound spectrum, as has been found in other odontocetes (Diercks et al . 1973 ; Au 1980). Transients produced quieter clicks than residents, but this was offset by their smaller group sizes, making it possible to be closer to all animals . Choppy sea conditions sometimes created sufficient slapping on the vessel hull to mask quiet clicks ; however, more than 75% of recorded sessions were analyzed successfully.  Characteristics of Echolocation Pulses Sonar pulses occurring within trains were broad-band, short duration signals characterized by rapid rise times and exponential decay (Fig. 2). Generally four to six peaks were resolvable before the signal became buried in noise, corresponding to an interval of approximately 0 .5 to 15 msec . Peak frequencies ranged between 4 and 18 kHz (Fig . 3), however some clicks had measurable energy extending to the upper limit of the recording system (c . 50 kHz). Similar frequency peaks and ranges were reported by Awbrey et al. (1982) for wild killer whales . As the hearing of killer whales extends to 105 kHz (Bain 1992), I paid careful attention to the upper frequency portion of the spectrographic display for evidence of the lowest components of very high frequency clicks. I found no indication of such clicks in the 162 sessions analyzed.  Isolated clicks were often very difficult to distinguish against background noise . Indeed, early in my analysis these sounds were missed entirely . I eventually realized that single or double pulses closely resembling those found in trains were recorded in the presence of killer whales, and were not detected in recordings made in the same areas when whales were absent  18 (12 sessions, 60 min total duration) . Isolated clicks bore similar characteristics to clicks occurring within trains, both in duration and frequency structure, and when paired had inter-click intervals of 100 msec or less.  Click Train Characteristics The characteristics of resident and transient click trains are summarized in Table II. Residents produced click trains that were more than eight times as long, on average, than those produced by transients . The percentage of irregular click trains was approximately 18 times higher for transients than residents . During spectrographic analysis I noted that resident click trains were more consistent in frequency structure and distinctly louder relative to background noise than those produced by transients . However, I did not systematically quantify these factors . Figures 4 and 5 are examples of resident and transient trains . Note the irregular timing and frequency structure of the latter.  Resident click repetition rates were about twice those of transients, but this difference was not statistically significant (Table II) . However, the variance in repetition rate was significantly greater for residents than transients (Bartlett's test, p< .001) . In the bottlenose dolphin the inter-click interval (the inverse of repetition rate) varies with target distance such that the interval is greater than the time required for a sound to travel to a target and back (Morozov et al . 1972; Au et al. 1974; Au et al . 1982; Roitblat et al . 1990) . The beluga (Delphinapterus leucas) differs in that clicks may be produced in consistently-structured "packets" of several clicks, with some inter-click intervals less than the two-way travel time to the target (Turl & Penner 1989) . Click packets were not detected in this study and have not  19 been reported by other authors . Based on the assumption that inter-click intervals exceed the two-way travel time to a target, the mean maximum target distances for residents and transients were 55 m (95% confidence interval : 34-133 m) and 109 m (95% confidence interval : 89-143 m) respectively.  Echolocation Index Values Residents and transients also differed strikingly in their echolocation index values, as summarized in Table III . On average, echolocation trains were produced 4 .24% of the time for every resident individual in a group, compared to 0 .16% for every transient, a 27-fold difference. In contrast, isolated clicks were used four times more frequently by transients than residents. The indices in Table III are equivalent to intervals between isolated clicks of 21 .4 minutes for residents and 5 .0 minutes for transients.  Contexts of Echolocation Use Behavioural Activity Figure 6 shows the number of sessions successfully analyzed within each behaviour category. This is only an approximate behaviour budget, since some categories of behaviour presented greater difficulties in acoustic analysis than others and were discarded more frequently. Transients spent a greater percentage of their time foraging than residents (as reported by Ford 1984, and Morton 1990), and were never observed socializing. Residents travelled infrequently (as reported by Ford 1989) and rested more often than transients.  Strong differences in echolocation index levels between transients and residents were  20 consistent across all behaviour categories (Fig . 7). For residents, analysis of variance indicated significant differences in EI between behaviours (p= .012). Foraging and travelling combined had significantly higher EI's than the remaining four behaviour categories combined (Scheffe's test, p=.004). Norris (1967) reported that different types of echolocation were used in foraging and orientation by bottlenose dolphins . To test whether this was true of resident killer whales, mean EI levels of foraging and travelling groups were compared . No significant difference was found (Scheffe's test, p= .574).  For transients, a nonparametric analysis of variance was used, since the variances differed markedly between categories despite the transformation . No differences in EI between behaviour categories were found (Kruskal-Wallis test, p= .924). However, the pattern of relative levels of echolocation between the various behaviour categories is similar to that seen in residents.  No recordings of socializing transients were obtained during the course of this study, reflecting the rarity of this behaviour (Morton 1990) . Four recordings of socializing transients were provided by E . Saulitus from four years of research on transient killer whales in Prince William Sound. These records are of interest in that they show EI values of 1 .11 to 7 .08, with a mean of 3.36, i .e . approximately 20 times higher than the mean values I observed for nonsocializing transients . These data are not included in the analyses in this paper.  Residents and transients differed in their EI frequency distributions, in addition to mean EI's (Fig. 8). The transient frequency distribution is heavily skewed towards zero, with no echolocation at all during the highest proportion of sessions . The resident frequency distribution  21 is much less skewed. This may indicate that for transients a cost to echolocation is experienced even at very low levels, whereas with residents constraints to echolocation increase more gradually. The general pattern of echolocation distribution is consistent across behaviour categories (Appendix, Figs . Al and A2).  Transients were observed foraging both inshore and offshore. Harbour seal kills were confirmed 15 times during inshore foraging and seven probable kills were seen but not confirmed. During offshore foraging we confirmed three kills of Dall's porpoise and saw three probable kills . Echolocation index levels were significantly higher when transients were near shore than when offshore (inshore: n=57, mean EI= .230, sd=.498 ; offshore : n=26, mean EI= .025, sd= .075; t-test: p<.041) . Transients were observed using two techniques for catching seals. The first was to travel long distances underwater to attack seals that were swimming near the shore. The second was used after seals were already alerted by a kill or attack . Seals near shore usually hauled out immediately in response to the presence of killer whales, but seals some distance from shore took refuge in kelp patches, on islets or shoals, or in underwater crevices or caves. In this case, the transients either moved away a short distance and returned, or milled in the area. Seals were caught when they left underwater refuges to surface for air or when a rising tide made them accessable to the whales . During these waiting periods, quiet echolocation clicks were relatively frequent.  During offshore foraging, transients swam more than 500 m underwater before attacking Dall's porpoises in three observed incidents . In the remaining three cases rapid surface chases began 25 to 50 m from the porpoises, which fled along the surface and frequently changed  22 direction. Although the porpoise groups contained one to three individuals, no more than one kill per attack was confirmed. I detected no echolocation sounds prior to or during a chase.  Group Size Echolocation index levels by resident killer whales declined significantly with increasing group size (Fig . 9 ; for EI vs log group size, r2= .142, p=.001). To test whether this result simply reflected sampling error due to large groups being spread out over longer ranges than small groups, I repeated the analysis using only recordings made when the animals were in close proximity to each other . This did not affect the result.  A similar but much weaker trend was observed for transients (Fig . 10; r2 =0.023, p=0.157) . However, when I removed all sessions with no echolocation at all, the negative association between EI and group size became stronger (Fig . 10; r2=0.143, p=0 .052). This suggests that group size does not strongly affect whether or not a transient group uses echolocation, but if it is used, echolocation per individual declines with increasing group size. Groups of three transient individuals were most common (49 of 88 sessions analysed) . Three transient groups of this size were recorded. One of these accounted for 28 sessions . Thus, it is possible that the observed trend is due to differences between pods rather than an effect of group size.  Water Clarity Echolocation index levels appear to be independent of water clarity across the range of clarities encountered . Secchi depth was taken to be a reliable indicator of water clarity, since  23 (Fig. 11) . No significant associations were found between EI level and Secchi depth for either residents or transients (residents : r2=0.000, Fig. 12; transients : r2=0.007, Fig. 13). Associations were also sought separately for each behaviour category, without success (Fig . A3). Lines were fitted on the latter plots using a Lowess smoothing technique (Wilkinson 1990) to help identify non-linear relationships : none were apparent.  Ambient Noise Echolocation activity levels for both residents and transients were independent of both environmental and vessel-produced noise levels. Simple linear regressions were attempted using the echolocation data pooled from all behaviour categories on environmental and vessel noise separately, and multiple linear regressions were attempted on both noise types . None of these models were statistically significant. Pooled EI's plotted against both noise types are presented in Figures 14 and 15 ; the surfaces, which are fitted using distance-weighted least squares methods (Wilkinson 1990), show the absence of clear trends.  I also searched for relationships between the two types of noise and EI for each behaviour category separately (Figs . A4 and A5) . Lowess smoothing was used to explore the possibility of non-linear trends, as described above ; again, no clear relationships emerged . It was noted consistently that resident clicks increased in intensity during periods of high ambient noise, whereas transient clicks did not do so in an obvious way . Thus it is possible that some transient echolocation, difficult to discern at the best of times, was not detected during noisy periods.  24  Discussion  Resident and Transient Echolocation Levels The most striking finding of this study is that resident and transient killer whales differed more than 25-fold in average echolocation index levels . I suggest that this is a consequence of the different responses of fish and marine mammals to echolocation sounds . Peak energies of killer whale echolocation clicks in this study ranged between 4 and 18 kHz ; Diercks et al. (1971) reported peak energies of 25 kHz for a captive killer whale . Audiograms have not been performed on Pacific salmon (Onchorhynchus spp.), the most common prey of resident killer whales (Bigg et al . 1987; Nichol 1990) . However, most bony fish that have been tested, including Atlantic salmon (Salmo salar), have very low auditory sensitivity above 3 kHz (Hawkins & Johnstone 1978 ; Hawkins 1986; papers in Tavolga 1976) . Thus these species may perceive little or no energy from killer whale clicks . Marten et al. (1988) tested the reactions of 13 species of bony fish to high intensity bottlenose dolphin sonar and detected no behavioural reactions . Pinnipeds and cetaceans, the prey of transients killer whales, have acute hearing to frequencies beyond 30 kHz (Mohl 1968 ; Andersen 1970 ; Mohl & Andersen 1973 ; Moore & Schusterman 1987 ; Terhune 1988).  Pinnipeds that detect approaching killer whales may evade capture by leaving the water, moving into shallow areas, or hiding in kelp patches or amongst rocks on the bottom . During my study harbour seals were seen using these techniques successfully . Frost et al . (1992) reported similar behaviours by walruses . Porpoises were seen swimming away from transients at high speed; other authors have reported dolphins and grey whales moving into shallow water  25 when killer whales were nearby (Baldridge 1972 ; Wursig 1989). Thus the risk of alerting prey may be a significant cost of echolocation for transients . The finding that transients echolocated more when nearshore than when offshore may have resulted from the fact that during many of the nearshore recording sessions, seals in the area were already alerted to the whales . The cost of echolocation was presumably reduced at this point, since the element of surprise was lost.  Are there alternative explanations for the difference in echolocation use between residents and transients? Residents and transients differ strikingly in behaviour : residents have large group sizes, do not follow shorelines closely, make short dives, and spend approximately 50% of the time foraging; transients live in small groups, often travel in shallow water following shorelines closely, have relatively long dive times and spend approximately 77% of the time foraging (Morton 1990 ; Heimlich-Boran 1988). Transients may require more precise orientation skills than residents, since near-shore travelling has the attendant risks of accidental stranding and collision with the bottom .  In addition, longer dive times provide reduced opportunities to  confirm position by visual reference to above-surface features . Finally, if positional information is shared among group members as proposed by Norris (1980), then transients should require more information per individual than residents simply by virtue of group size . All these differences in travel patterns, group sizes and dive-times suggest that frequent echolocation would be more beneficial to transients than residents, the opposite of the observed pattern.  Patterns of Echolocation Clicks I found in this study that transient killer whales produced short and irregular click trains compared to the trains of resident killer whales. I also noted incidentally that transient clicks  26 were more variable in frequency structure and quieter than resident clicks . These observations are consistent with the hypothesis that stealth is more important to the hunting success of transients than residents . Sequences of short duration sounds that are irregular in timing and frequency structure resemble random noise more closely than regular sequences . Similarly, the longer and louder a sequence, the greater the opportunity for recognition . Thus, potential prey of killer whales are more likely to detect resident echolocation trains against background noise than they are transient trains.  The use of isolated clicks may also yield echolocation information without alerting potential prey . My difficulty in recognizing isolated clicks while listening to slowed playbacks may be indicative of the problems faced by an acoustically-vigilant porpoise or pinniped . I suggest that isolated clicks are cryptic echolocation "snapshots" . Isolated clicks could only provide crude information compared to that possible with click trains (Norris 1967) . However, this disadvantage may be offset by the stronger echoes returned by pinnipeds and small cetaceans relative to fish.  The pulse repetition rate was significantly more variable in residents than in transients. This suggests that residents use echolocation to investigate wider distance ranges than do transients. For example, residents may use it for long-range navigation, detecting distant schools of fish, identifying and selecting specific fish to hunt, and tracking individual fish during pursuit. Transients may use it primarily to detect prey that are close enough to capture even if alerted. In the latter case, there should be a close match between click intensity and the range under examination, with the sound being sufficiently intense to return detectable echoes, but not so  27 intense as to warn prey at greater distances . This agrees with my qualitative observation that transient clicks were consistently quieter than those of residents.  Behavioural Contexts of Echolocation Use. The patterns of echolocation use by resident killer whales resembled the patterns of call use reported by Ford (1989), except that the high call rates of socializing groups were not matched by high echolocation rates . In addition, foraging groups in this study used echolocation consistently, while Ford (1989) reported periods with no calls ranging from 1 min to 1 hr during foraging . Evans and Awbrey (1988) warned that : "Assignment of sounds to mutually-exclusive communication or echolocation categories may or may not match their uses by dolphins". However, the more consistent use of echolocation than calls during foraging implies that echolocation plays a greater functional role in that activity . Ford (1989) suggested that calls were used as social signals between scattered pod members, since the first call in a series seemed to trigger a chorus of responses . Click trains did not appear to elicit responses, and there was no evidence that they had a similar function to calls . Clicks were louder and pulse repetition rates more variable during foraging than during other activities, also suggesting an active role of sonar in foraging . Average echolocation index levels were high during travelling, but silent travelling was also observed on occasion . This suggests that residents may experience some cost to echolocation . It may be advantageous to move to new foraging areas without revealing this information to other resident pods, as has been described in bats (Barclay 1982). Silent travelling by residents was twice preceded by the arrival of transient killer whales within 1 km . In both instances the resident pods abruptly terminated their previous activities, ceased all vocalizations and grouped up tightly before beginning to travel . This uncharacteristic  28 behaviour appeared to be an avoidance response, and is evidence that resident killer whale echolocation may be suppressed in certain social contexts.  Transient killer whale echolocation use differed less among behaviour categories than in residents, but a similar trend was seen . The infrequent use of echolocation in all behaviour categories, (except, possibly, socializing), might be explained in two ways . Firstly, transients may be alert for opportunistic encounters with prey even when not actively searching . Secondly, prey behaviour patterns may be affected by the detection of transients so that foraging success is reduced in the future . These ideas are both supported by my observations . Twice transient groups engaged in travelling and slow-travelling (not classified as foraging because the whales were swimming in close formation) suddenly pursued porpoises . Frequently killer whales passing close to harbour seals caused them to haul out or move rapidly into shallow water or kelp patches, although no pursuits or aggressive interactions took place . Increased vigilance by these seals might reduce their risk of being attacked for some time, thus, transients could pay a future cost for present detection.  Group Size and Echolocation Use The negative correlation between echolocation use and group size for resident killer whales suggests information sharing . There would seem to be little advantage to each individual separately acquiring information about prey or surroundings, and possible interference costs. Information sharing may occur in three ways . First, socially dominant individuals may use echolocation to make decisions and then lead other group members in movements and activities. Second, by monitoring the echolocation activity of distant group members, individuals may be  29 able to infer the location of foraging "hotspots" . Wilkinson (1992) reported eavesdropping of this type in bats . Third, killer whales may be able to interpret the echoes of pulses produced by other group members directly, or "share" clicks. Scronce and Johnson (1976) showed that a blindfolded captive bottlenose dolphin could detect a target using artificially-produced pulses from a sound projector . Click sharing may be more useful to a resident group that is swimming in formation and scanning ahead with echolocation than one engaged in foraging, since foraging animals are usually dispersed and often randomly oriented (Ford 1989) . In addition, resident killer whales foraging on schooling fish such as salmon are likely to be presented with a "clutter" of small acoustic targets, from which they select one or more to pursue . This activity requires greater intensity and frequency of clicks than coarser-scaled discriminations (Au & Turl 1983), and therefore may require greater individual control of click production.  Unlike residents, transients usually forage in a co-operative manner, (Heimlich-Boran 1988; Baird et al. 1992) . This almost certainly means that information exchange occurs. However, the intermittent and low-intensity nature of transient echolocation makes it unlikely that click-sharing is a principal mechanism for this exchange . In this study the effects of group size were weak if all recorded sessions were considered, but much stronger if the sessions with no echolocation were dropped . A possible explanation is that in transient groups only one individual echolocates, and that individual does not base its decisions on group size.  Echolocation Use and Water Clarity. I found no significant association between echolocation index and water clarity for either residents or transients . This is an important observation since it implies that caution must be  30 used in extrapolating the results of echolocation experiments in captivity . Echolocation does not always take over where vision leaves off . Wood and Evans (1980) described a study designed to examine echolocation use by a dolphin during the capture of live fish . Contrary to expectations, the dolphin repeatedly located and approached live fish without producing any detectable echolocation sounds . The study demonstrated that delphinid cetaceans can detect, track and catch fish using passive acoustic cues alone . This is highly relevant to understanding the foraging behaviour of free-ranging populations . Unfortunately, this type of experiment has not been repeated on other odontocete species . However, sound production by many species of fish is well documented (Myrberg 1981 ; Boyes 1982; Hawkins 1986), and it seems likely that many species of cetaceans make use of this source of information.  Evans and Awbrey (1988) described wild beluga whales feeding actively on salmon in silt-laden water without producing detectable sounds except for an occasional buzz . Either the animals were producing pulses with virtually no energy below 60 kHz, the limit of the equipment used (which is unlikely, see Kamminga & Wiersma 1981), or they were relying on passive acoustic cues . The same authors reported silent feeding under low-visibility conditions by bottlenose dolphins.  Norris et al . (1961) reported the opposite lack of correspondence between water clarity and echolocation . They found that a blindfolded bottlenose dolphin retrieved dead fish using echolocation, but also used echolocation when performing the same task in clear water without a blindfold . Evans and Awbrey (1988) also reported that wild bottlenose dolphins eating dead flying fish used intense echolocation despite clear water conditions . They speculated that the  31 echo characteristics of a fish may reveal more about its condition than does visual appearance. In my study foraging residents used echolocation under clear conditions . However, because inter-click intervals can be used to infer maximum but not actual target distances (Au et al. 1982), and because maximum distances exceeded the clarity, it is not known whether clicks were used to investigate fish within visual range.  In this study, transient killer whale attacks on Dali's porpoises were not associated with echolocation . Since in every case the prey were detected by the whales well beyond underwater visual range, and visual scanning above the surface was not seen, it appears that passive listening was the most likely detection mechanism . Dall's porpoise echolocation is centred on frequencies of 135-149 kHz (Au & Jones 1991) and thus may not be detectable by killer whales, which have little sensitivity above 105 kHz (Bain 1992) . However, I was able to detect the underwater sounds of porpoises breaking the surface and breathing within 25 m under quiet ambient conditions, and transients may make use of such sound at much greater distances . Jefferson et al. (1992) cite several reports of cetaceans becoming silent and motionless when approached by killer whales . This supports the idea that transients use swimming and surfacing sounds to find marine mammal prey.  Echolocation Use and Ambient Noise. Several authors have suggested that passive acoustic cues play important roles in the navigation, foraging and social behaviour of delphinid cetaceans (Norris 1967 ; Wood & Evans 1980; Evans & Awbrey 1988 ; Hoelzel 1991) . Passive listening cannot be easily measured in an observational study of free-ranging populations, however, the probability of its use may be  32 inferred from the relationships between ambient noise levels and echolocation behaviour. Echolocation sounds are generally louder than those providing information about prey, and potentially louder than those providing orientational cues, such as distant surf . Thus, as ambient noise levels increase, passive listening should become ineffective as a sensory mode sooner than echolocation . The masking effects of broadband noise on echolocation by beluga and bottlenose dolphins has been described by Turl et al . (1987) and Au (1980, 1990) . Both species responded to increased ambient noise by increasing the intensity and frequency structure of their clicks in target detection experiments . If this is also true of killer whales, then echolocation is probably preferred over passive listening under noisy conditions.  I found no correspondence between echolocation index and ambient noise level for residents . This agrees with the above conjecture that for residents echolocation is not costly and is normally used. Transients also showed no response in echolocation index to ambient noise of either type, which suggests that they do not trade off echolocation and passive listening . Any attempt by transients to overcome the masking effect of noise by increasing their echolocation index levels or the loudness of their clicks is likely to result in their detection by their acoustically-sophisticated prey . Anecdotal reports indicate that transients avoid vessels more actively than residents (E . Saulitus, pers . comm .), perhaps as a response to interference with passive listening. Twice during this study transient groups suddenly began pursuit of Dall's porpoises immediately after my vessel engine shut down, and similar observations have been made by G. Ellis and J. Ford (pers . comm .). These observations support the conjecture that vessel noise affects the ability of transients to find prey using passive listening .  33  Conclusion  Echolocation and Prey Detection Strategies Based on the foregoing discussion, I propose that resident and transient killer whales use echolocation strategies as outlined below . I do not imply that these strategies are proved, but rather that they fit well with my findings and those of others, and form a useful basis for future research.  Resident Killer Whales : Residents detect fish using passive listening, echolocation and vision. Echolocation is normally low in cost . Thus, it is used even when conditions for vision are good, both to probe beyond the visual range, and to obtain additional information such as the size, species and condition of prey . When vision is blocked by turbidity, click rates do not rise greatly, because the echolocation channel is already in use . Passive listening is also used to detect schools or aggregations of fish . Passive listening is an important mechanism in orientation and navigation, thus echolocation index levels are reduced when resident whales are not searching for prey . Echolocation use may be curtailed altogether if costs to echolocation arise . The risk of attracting competitors may be one such cost.  Transient Killer Whales : Echolocation is used sparingly in the form of isolated clicks or short, quiet, and irregular trains to reduce the risk of alerting potential prey (marine mammals) . Since the prey of transients are large and sparsely distributed, random underwater visual searching is seldom effective . Transients run a "trap-line" between areas of known local abundance such as harbour seal haulouts, and search other areas using passive listening . As such, their behaviour  34 is not affected by water clarity, except perhaps during the final phases of a pursuit . Since transients depend to a greater extent on passive listening than residents, particularly in open water, their hunting efficiency may be reduced by high ambient noise levels .  35  Table I . Study locations, dates and areas surveyed Location  Lat./Long.  Area Surveyed (km2)  Prince William Sound, Alaska  60°05'N., 147°30'W .  1600  Central Mainland Coast, British Columbia  52°0'N ., 127°50'W .  900  May 27-June 18/92 May 20-June 2/92  South-Eastern Queen Charlotte Islands  52°30'N., 131°0'W.  1200  June 3-July 9/92  Dates May 25-Aug. 30, 1990 July 8- Sept . 10/91 July 25-Sept . 6/92  36  Table II. Characteristics of resident and transient click trains Mean Click Train Duration +/- SE (s) [number sessions]  Mean Percentage of Irregular Trains +/- SE [number sessions]  Mean Repetition Rate +/- SE (/s) [number of trains]  Resident  6.83 +/- 0.54 [74]  3.46 +/-1 .08 [70]  13.75+/-3.98 [32]  Transient  0.86 +/- 0 .20 [88]  60.84 +/- 8 .16 [28]  6.83+/-0.77 [23]  t-test  p<.001  p<.001  p=.151  37  Table III . Echolocation by resident and transient killer whales, based on 74 recorded sessions for residents, 88 for transients Mean Echolocation Index (EI) +/- SE  Mean Index of Isolated Click Use (IC) +/- SE  Resident  4.24 +/- 0.51  0.78 +/- 0.23  Transient  0.16 +/- 0.04  3.29 +/- 0.99  t-test  p<.000  p=.024  38  away sarea  Central British Columbia Coast Study Area  e  Figure 1 . Map of the northwest coast of North America showing the study areas .    39  '' ,  i 11111 .4 I  0  Time (cosec)  7.4  Figure 2. Waveform of a killer whale echolocation click . The duration of the click is approximately 2.2 msec .    40  (a)  0  16 Frequency (kHz)    (b)  6  +  I  b  1  0  n  I'  i  0  1  32  ~  16 Frequency (kHz)  32  Figure 3. Energy distribution of a click from a click train (a), and of an isolated click (b) .     41  (a)  V a E  a  32 ("  00  (b)  re  0  '  ' 1 t  .r '•, _  ;  N,  ,, .  ISM  i  1, r  Iii1~ _  0  1  `•  ,t  4  00  ,vy  : .i„  •r i .~  • .  tiH 1„' x ''1 ,  h L P  ,  '  ~y  ~ .' '~~t . .  4 ,. itOyu' '  1  Y  .ri r,•L Y  i . a. ,  .  i"'fit, ,  1 . :~`  Y,~~ t ,  t~  ~•  •ryy~  " ~" ,''r1'  1  '  r.'',l ..  u  r. .~l~f,  2 Time (s)  4  Figure 4 . Waveform and spectrogram of four seconds of a resident killer whale click train. The dotted line on the waveform plot is a measure of relative amplitude . Clicks are dark vertical lines on the spectrogram plot ; the diffuse shaded areas from 20 kHz up are click echoes.    42  (a)  aE 0  Q  11  Ir  0  ~~.1 r. 2 Time (s)  4  Figure 5 . Waveform and spectrogram of four seconds of a transient killer whale echolocation click train . The dotted line on the waveform plot is a measure of relative amplitude .    43  50 q Transients Residents  40  30  20  10  f  ml  r  s  St  t  Behaviour category  Figure 6 . Number of analyzed five minute recordings of resident and transient killer whales, by behavioural activity . Abbreviations: f=foraging, ml=milling, r=resting, s=socializing, st=slow-travelling, t=travelling .    44  3.5 3 .0 2.5 2 .0 1 .5  f  1 .0 0 .5 0.0 f  ml  r  S  St  t  Behaviour category  Figure 7 . Mean echolocation index values for resident and transient killer whales, by behaviour category . Resident means are stars, transient means are circles . The echolocation index is squareroot transformed as follows : x'=(x+ .05).5. Standard error bars are shown .    45 (a)  1 .0  0.8  0.6  0.4  0.2  0  3  6  9  12  3  4  Echolocation index (b)  1 .0  0 .8  0.6  0.4  0.2  0  1  2 Echolocation index  Figure 8 . Cumulative frequency distribution of echolocation index levels for resident (a) and transient (b) killer whales . Note the differing axes .  46  Group size  Figure 9 . Echolocation index level as a function of group size for resident killer whales . The data points have been "jittered" (shifted by small random amounts) to reveal overlapping values .  47  Group size  Figure 10 . Echolocation level as a function of group size for transient killer whales . The continuous line is a logarithmic regression of echolocation on group size, for all the data shown. The broken line is a logarithmic regression of echolocation on the non-zero values shown . The data points have been jittered to reveal overlapping values .    48  20 (a)  00  0 X E  Sept 1 - - - - Aug 11  Aug 8 - - - July 28  July 25  15  m  5  10  15  20  25  20  25  Depth (m) 30  (b) June 15 June 10 - - - May31  May 30   20  10  10  15  Depth (m)  Figure 11 . Photocell resistance as a function of depth . Readings were taken in Prince William Sound, Alaska (a), and on the central mainland coast of British Columbia (b), in the summer of 1991 .    49  3 .5  ►  0  3 .0 ° 0  ° 0  0  2 .5 o  °  °  0 °  2 .0  ° °  •  °  0 0  °  °°  q  °  1 .5  °° 0°  °  °  ° °  0  1 .0  °0 °  0 0  0  0.5  00  0 0 °  °  0 o  0.0  0  0  °  q  ►  -0.5 0  2  4  6  8  10  12  Secchi depth (m)  Figure 12 . Echolocation index as a function of Secchi depth for resident killer whales . The data points have been jittered to reveal overlapping values .    50  2 .0  1 .5 0 0  1 .0  0  0 0  0  0.5  0  0  0  0 o °°  0 0 °  00 00  O •  0.0  q  0  0  0  0  o  W  ~O  -0.5 0  2  4  6  8  10  o  12  Secchi depth (m)  Figure 13 . Echolocation index as a function of Secchi depth for transient killer whales . The data points have been jittered to reveal overlapping values .  51  Figure 14 . Echolocation index plotted against environmental and vessel noise levels for resident killer whales . Noise is in 10 dB increments from an arbitrary baseline . The surface was fitted using least distance weighted squares techniques .  52  2  1  0  Figure 15. Echolocation index plotted against environmental and vessel noise levels for transient killer whales . Noise is in 10 dB increments from an arbitrary baseline . The surface was fitted using least distance weighted squares techniques .  53 CHAPTER THREE  General Conclusions  My work has shown that resident and transient populations of killer whales vary markedly in their use of echolocation . The extent of this difference suggests that generalisations about the echolocation behaviour of any species of odontocete may be risky at best . Populations may develop echolocation strategies that are dependent on available prey, predators, and other local ecological factors . Such adaptive strategies would maximize the functional aspects of echolocation, such as prey detection, and minimize the incidental costs, such as predator attraction. I assume in this thesis that the echolocation strategies of both resident and transient killer whales are indeed well-suited to their particular ecological niches . This is not an adaptationist approach sensu Gould and Lewontin (1979) : I have not assumed that because particular traits have evolved, they are necessarily adaptive in their present context . Based on evidence that echolocation skills are learned (Wood & Evans 1980 ; Evans & Awbrey 1988), I suggest instead that odontocetes adapt their innate echolocation capabilities to suit their specific ecological circumstances.  Transient killer whales used isolated clicks and short, quiet, irregular click trains, which were difficult to discern against background noise . I suggested in Chapter Two that this use of echolocation is part of a hunting strategy involving stealth . This does not necessarily mean that transients are proficient echolocators that derive large amounts of information from their cryptic clicks . On the contrary, there are two reasons to suspect that transient echolocation may be relatively undeveloped . First, transient foraging behaviour probably affords few opportunities  54 for learning sophisticated echolocation techniques, since echolocation is used relatively infrequently . Second, the task of hunting marine mammals with sonar may be relatively simple compared to the task of finding fish, since the lungs of marine mammals are good reflectors of underwater sound.  I reported in Chapter Two that resident killer whales used echolocation much more often than transients . They used longer, louder, and more regular click trains and used isolated clicks less frequently than transients . Resident echolocation was particularly strongly associated with foraging and travelling. During this study resident killer whales fed principally on salmon (Onchorhynchus spp.) that were aggregated into schools . A difficulty for predators feeding on schooling fish is that unless they are able to track and chase down specific individuals, they face an array of prey that never tires . I suggest that a function of echolocation for residents is that it enables them to "lock on" to specific fish and chase them through visually-confusing schools. This mechanism would depend on the fish being individually-recognizable by echolocation. Since tissue in water is relatively transparent to sound, the strongest echo from a fish comes from its air bladder . The air bladder forms a resonant chamber that rings on exposure to sound with a frequency that varies with its volume . Thus, it is likely that individual fish do indeed return unique echoes . There have been no tests of the ability of an odontocete to recognize individual fish . However, Thompson and Herman (1975) showed that bottlenose dolphins are capable of discriminating very slight differences in frequency, which is a requirement for this technique. A similar mechanism may enable residents to discriminate desirable from undesirable prey from a distance. On several occasions in Prince William Sound, I saw killer whales rapidly travel 25-50 m, bypassing schools of pink salmon, (O. gorbuscha) to catch larger fish, probably  55 chum or coho salmon (O. keta or O. kisutch).  I suggested in Chapter Two that passive listening is probably used in important ways by foraging killer whales, particularly transients . Transients use echolocation sparingly, and often travel silently . They do not appear to locate prey using underwater vision, since they initiate attacks from well beyond the range of visibility . Transients were seen raising their heads above the surface ("spyhopping") near seal haulouts during this study, but were not seen scanning above the surface in open water . As noted in the previous chapter, transients have been observed to locate and pursue prey immediately after vessel noise was reduced, which is evidence of passive listening . In addition, Guinet (1991,1992) in a study of predation by killer whales on elephant seals, was able to attract killer whales close to shore by striking the surface of the water to simulate seals entering the water . Hoelzel (1991) reported similar evidence of the use of passive listening by killer whales hunting sea lions near shore.  It is harder to draw inferences about the role of passive listening in foraging resident killer whales . In Chapter Two, I suggested that fish sounds probably enable killer whales to detect schools under quiet conditions using passive listening . However, residents usually echolocate during foraging, arguably for the reasons described above . Residents commonly produce little or no echolocation in other behavioural contexts . Passive listening may be the principal mechanism maintaining orientation at those times (Norris 1967) . Schevill & Watkins (1966) reported that a killer whale (subsequently identified as a resident, Ford 1984) held in a net pen collided with an obstacle that was quietly lowered into its path at night . After a single collision, the whale clicked briefly, then continued to swim silently, avoiding subsequent  56 collisions unless the object was moved . This experiment was repeated many times with the same result . Thus the whale was orienting reliably without echolocation or vision, presumably by using passive acoustic cues.  Echolocation in wild bats has been investigated much more thoroughly than in wild odontocetes . Insectivorous bats echolocate more than carnivorous species that forage for small terrestrial mammals (Fiedler 1979) . Bats hunting flying insects may experience the same kind of advantage of echolocation that I have proposed for resident killer whales : it may enable them to pick out and track individual prey and follow them through a swarm of other insects. Carnivorous bats, like transients, are probably silent to avoid alerting their prey (Fiedler 1979).  Perhaps the most obvious point of comparison between resident killer whales and insectivorous bats is that they both exploit food supplies that are seasonally superabundant. Resident killer whales run little risk of depleting incoming schools of salmon in the summer and fall (Nichol 1990) . Similarly, bat predation probably does not appreciably affect the density of swarming insects (Wilkinson, 1992) . Thus, neither suffers an appreciable cost by attracting competitors with echolocation . During seasons when prey are not abundant, the cost of echolocation in attracting competitors may be much higher . At the same time, a proposed advantage to echolocation use (increased efficiency in hunting aggregated prey) would decrease. Thus, I predict that both whales and bats decrease their use of echolocation when food is scarce or dispersed . The risk of attracting predators does not apply to killer whales, which have no known predators, and is probably also low for bats (Fenton 1980) .  57 Like many field studies of the behaviour of social mammals, this research has raised more questions than it has provided answers . Some of these questions are listed below, with suggestions of how they might be addressed in future projects.  1) Role of passive listening: effect of masking noise . I attempted to assess the role of passive listening by examining the relationship between ambient noise levels and echolocation behaviour. The only effect I detected was increasing click amplitude by residents in response to increasing noise levels . However, these levels were not manipulated directly . If noise levels were artificially elevated by projecting broadband random noise, changes in echolocation behaviour could be monitored more systematically, in order to test the model of echolocation strategies presented in Chapter Two . This model predicts that residents, but not transients, should increase their click amplitude in response to increasing broadband noise, and that transient foraging success should decline under conditions of elevated noise.  2) Role of passive listening : response to simulated prey sounds . Guinet (1991,1992) reported that killer whales responded to simulated sounds of elephant seals entering the water . It would be worthwhile to conduct similar trials simulating the sounds of offshore prey, such as Dall's porpoise . This would test whether passive listening is used in offshore foraging, and if so, whether swimming or vocal cues are most important in prey detection.  3) Seasonal effects on echolocation use . One limitation of this study is that it was carried out during a time of year when resident killer whales were feeding principally on a single abundant prey type, namely salmon (Nichol 1990) . Based on scattered observations and the stomach  58 contents of several beach-cast carcasses, it appears that residents in the winter may have a varied diet of non-schooling midwater and bottom fish (G . Ellis, pers . comm .). A study similar to this one but conducted throughout all seasons could evaluate whether residents echolocate less when their prey are dispersed. One difficulty with such a study is that the winter ranges of resident killer whales are poorly known.  4) Reactions of fish to killer whale echolocation . In order to explain differential costs to echolocation, I have argued that fish are unlikely to detect echolocation sounds . This has not been empirically tested. It is possible that killer whales voluntarily adjust the bandwidth of their clicks to control their audibility to fish . At times it may be desirable to produce audible sounds to herd fish (Simila & Ugarte 1991 ; Osborne 1986) . At other times it is probably advantageous for residents to remain undetected, particularly when chasing individual fish . Unpublished experiments that used recorded killer whale sounds have been attempted (J. Ford, pers.comm .). These were inconclusive, partly as a result of the difficulty involved in accurately reproducing high intensity, short duration sounds . Tests of the reaction of fish to real killer whale echolocation could be carried out by placing fish in visually-opaque but acoustically-transparent pens, in tanks with captive killer whales.  5) Role of learning in the development of echolocation strategies .  I have suggested that  odontocetes adapt echolocation strategies to fit local ecological conditions . Do individuals develop their own strategies by trial and error, or are they learned from other pod members? In the latter is true, how long does it take to add new techniques to a repertoire of echolocation skills? The answers to these questions may help to explain the separation of resident and  59 transient populations . The echolocation skills that residents use for hunting fish are poorlysuited for transients hunting marine mammals, and vice versa . Are the differences in echolocation simply consequences of the feeding specializations, or are they also factors maintaining those specializations? To determine whether a commitment to a given echolocation strategy reflects an inability to alter techniques, it is necessary to know how much innovation occurs naturally . The best approach to these questions might involve a combination of work on captive and wild killer whales . First, the ability of individuals to develop new echolocation strategies could be tested directly . This could be done by presenting novel echolocation tasks to captive individuals and rewarding innovative forms of echolocation . Categories of innovation might include click repetition rate, click interval regularity, click amplitude, click frequency structure, the use of isolated clicks, and/or the suppression of echolocation in favour of passive listening . A second experiment could assess the ability of individuals to learn by example . This could be carried out by testing whether captive killer whales learn echolocation skills for new tasks from individuals already possessing those skills.  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Wood, F .G. & Evans, W.E. 1980 . Adaptiveness and ecology of echolocation in toothed whales. In: Animal Sonar Systems (Ed. by R.-G. Busnel, & J .F. Fish), pp . 381-425 . New York: Plenum Press.  Wursig, B. 1989 . Cetaceans . Science, 244, 1550-1557. Zar, J .H. 1984 . Biostatistical Analysis . Second Edition . New Jersey : Prentice Hall Inc .  69 APPENDIX  This appendix contains supplementary figures .     70  - 15  (a)  25  (b)  1 .0  .0 20 -,0 0 0 z  -5  cc 5  0  4  8  12  2  Echolocation index'  4  6  8  Echolocation index'  - 15  4  (d)  (c) 10-  1 .o -  3  -10  5  4  8  12  Echolocation index'  16  3  5  7  9  11  Echolocation index"  Figure Al. Cumulative frequency histograms of echolocation level by resident killer whales engaged in four behavioural activities . The behaviour categories are as follows : (a) foraging, (b) resting, (c) socializing and (d) slow-travelling .     71  25  40 t .0  t .0 20 30  m  15 0 0 z  0 20  2z 10  10 5  0  t  2  3  4  0  Echolocation index  1  2  3  4  Echolocation index  (c)  (d)  15  30 ID  1 .0  10  20 0 0 c z  0  2z  5  0  t  2 Echolocation index  3  4  10  0.0  04  0.8  1 .2  1 .6  Echolocation index  Figure A2 . Cumulative frequency histograms of echolocation level by transient killer whales engaged in four behavioural activities . The behaviour categories are as follows : (a) foraging, (b) milling, (c) travelling and (d) slow-travelling .     72 2A  14  OA  -03  0  2  4  8  8  10  2  12  Secchi Depth (m)  2  4  8  8  10  2  12  Secchi Depth (m)  (e)  4  8  8  10  12  10  12  10  12  10  12  Secchi Depth (m)  4  8  8  Secchi Depth (m)  ,a  to  0o  2  4  8  8  10  12  0  2  Secchi Depth (m)  4  8  8  Secchi Depth (m)  (h)  ' 1A  0.8  oo  OA  0  2  4  8  8  Secchi Depth (m)  10  12  0  2  4  8  8  Secchi Depth (m)  Figure A3 . Plots of resident and transient killer whale echolocation level against Secchi depth, by behaviour category . The top four plots are residents engaged in : (a) foraging, (b) resting, (c) slow travelling and (d) socializing . The bottom four plots are transients engaged in : (e) foraging, (f) milling, (g) travelling and (h) slow-travelling . The lines are fitted using Lowess smoothing techniques (see methods) .    (a)  73  35  25  25  16  1a  06  06  -0a -08  06  16  25  36  45  06  15  25  36  46  Relative Vessel Noise  Relative Environmental Noise  3.5  25  16  V  _ 06  06  -06 -05  06  16  25  36  4b  -06 -05  Relative Environmental Noise  (C)  06  15  25  3.5  45  Relative Vessel Noise  3a  25  to  0a  -oa  -06  05  16  25  36  45  08  Relative Environmental Noise (d)  3a  35  25  25  1a  to  0a  as  -0b -0 .6  06  16  25  36  Relative Environmental Noise  15  25  36  45  36  46  Relative Vessel Noise  46  -06 -06  06  16  26  Relative Vessel Noise  Figure A4 . Plots of resident killer whale echolocation level against environmental and vessel noise levels, by behaviour category . Behavioural activities (by rows) : (a) foraging, (b) resting, (c) socializing and (d) slow-travelling. Lines fitted using Lowess techniques .    (a)  2A  74  2.0  .  15 -  -  16  t .0 -  -  IA  06 -  -  06  Q0  -05 -05  Ob  1 .5  25  36  46  06  Relative Environmental Noise  (b)  2A  2.0  tb  1 .5  to  to  O6  05  Q0  00  -0b -06  06  1 .6  26  36  16  26  3.6  45  Relative Vessel Noise  -05  46  -06  05  16  2b  36  4.5  Relative Vessel Noise  Relative Environmental Noise 2A  2A  ( C)  -  t5 -  x  c  15  -  -  IA-  -  0b -  -  c  to -  05  -  -  OA-  -0b -05  06  -  15  25  35  45  0 L U W ce  0 J  ao-06  -06  Relative Environmental Noise  0.6  15  2b  35  48  Relative Vessel Noise  2.0  16  15  IA  1 .0  05  O6  0.0  0.0  -05 -06  as 1 .b 2b 36 Relative Environmental Noise  46  -0b 1-  -0b  06  16  26  36  46  Relative Vessel Noise  Figure AS . Plots of transient killer whale echolocation level against environmental and vessel noise levels, by behaviour category . Behavioural activities (by rows) : (a) foraging, (b) milling, (c) travelling and (d) slow-travelling . Lines fitted using Lowess techniques.  

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