SEASONAL MOVEMENTS AND FORAGING BEHAVIOUR OF RESIDENT KILLER WHALES fOrcinus orca) IN RELATION TO THE INSHORE DISTRIBUTION OF SALMON (Oncorhynchus spp.) IN BRITISH COLUMBIA
By
Linda M. Nichol
B.Sc. (Zool.)/ University of British Columbia, 1985
A Thesis submitted in partial fulfilment of the requirements for the degree of Master of Science
In
The Faculty of Graduate Studies (Department of Animal Science)
We accept this thesis as conforming
to the required standard
University of British Columbia
July 1990
(§) Linda M. Nichol 1990 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.
Department of Animal Science
The University of British Columbia Vancouver, Canada
Date September 14, 1990
DE-6 (2/88) ABSTRACT
Sightings and acoustic recordings from 1984 to 1989 of
northern resident killer whales fOrcinus orca) from Johnstone
Strait off north eastern Vancouver Island and from King Island on
the central British Columbia coast were analysed to examine the
hypothesis that northern resident whales move seasonally in their
range to areas where salmon are available. Killer whales were most abundant in Johnstone Strait between July and October and
infrequent during the remainder of the year. The increase in
whale abundance during summer coincided with the migration of
salmon (Oncorhynchus spp.) from offshore into Johnstone Strait.
The occurrence near King Island in spring 1989 of the same
resident whales that are seen in Johnstone Strait during summer,
coincided with runs of sockeye and chinook salmon.
During July, August and September of 1984 through 1988,
killer whale sightings were recorded virtually daily in the
Johnstone Strait. Observations of killer whales in Johnstone
Strait during the summer of 1988 showed that whales foraged along
shore and in areas of strong current where salmon occur in high
densities. Of the 16 killer whale pods in the northern resident
community, however, less than half were present more than 15% of
summer days (1984 to 1988). Regression results between numbers of
whale days per week from each pod and numbers of salmon per week
showed that the occurrence pods that were present on more than
15% of summer days in Johnstone Strait was positively and iii significantly associated with the abundance of sockeye and pink salmon (six pods). In addition to these, the occurrence of one pod that spent less than 15% of summer days in the Strait was positively and significantly associated with chum salmon.
Together these results support the hypothesis that northern resident killer whales select their habitat seasonally to feed on available salmon. The results also lead to the hypothesis that within the northern resident community each pod has a seasonal home ranges. iv
TABLE OP CONTENTS
ABSTRACT ii
LIST OF TABLES V
LIST OF FIGURES. vi
ACKNOWLEDGEMENTS vii
INTRODUCTION 1
METHODS 7 1) Study Animals 7 2) Study Areas 7 3) Sighting Records 9 4) Acoustic Records 14 5) Salmon Abundance Estimates 16 6) Estimating the Abundance of Killer Whales in Johnstone Strait throughout the Year 17 7) Regression Analyses 19 8) Behavioural Sampling 20 RESULTS 2 3 1) Occurrence of Killer Whales in Johnstone Strait Throughout the Year.. 23 2) Salmon Abundance and Killer Whale Occurrence in Johnstone Strait 25 3) Salmon Abundance and Killer Whale Occurrence Near King Island 29 4) Pod Usage of Johnstone Strait During Summer 29 5) Regression Results Between the Occurrence of Killer Whales and the Abundance of Salmon in Johnstone Strait 33 6) Behaviour of Killer Whales 38
DISCUSSION 40 1) Occurrence of Killer Whales and Salmon In Johnstone Strait and Near King Island 40 2) The Relationship Between the Ocurrence of Killer Whales and Salmon in Johnstone Strait During Summer 40 3) Foraging Behaviour of Resident Killer Whales..42 4) Pod Home Ranges 46
CONCLUSION 52
LITERATURE CITED 53 V
LIST OP TABLES
Table I. Weeks of consecutive daily observer effort in Johnstone Strait used in analysis 13
Table II. Months from which acoustic recordings were analysed and from which previously analysed data were used 15
Table III. Pods present in Johnstone Strait during different seasons of the year 1985 to 1989 27
Table IV. Numbers of salmon (in thousands) in Johnstone Strait, July to October, 1984 to 1988 30
Table V. Multiple regression results, between weekly numbers of whale days, from each northern resident pod, and weekly numbers of salmon in Johnstone Strait during July, August and September, 1984 to 1988 36 vi
LIST OF FIGURES
Figure 1. Map of British Columbia coastline showing ranges of the northern and southern resident killer whale communities, major salmon migratory routes and the two study areas 8
Figure 2. Map of Johnstone Strait study area 10
Figure 3. Map of King Island study area 11
Figure 4. A comparison of the number of whales present in Johnstone Strait on 27 days during July, August and September of 1980, 1981, 1982 and 1983, estimated by visual observation and by acoustic monitoring 24
Figure 5. Mean number of whale days per month estimated from sightings and acoustic data i 2 6
Figure 6. Mean number of salmon (in thousands) and whale days per week in Johnstone Strait during July, August, September and October (1984 to 1988) 28
Figure 7. Frequency of occurrence of all or part of each northern resident pod seen in the King Island study area during May and June 1989 31
Figure 8. Number of whale days each week in the King Island study area during May and June 1989 32
Figure 9. Frequency of occurrence of all or part of each northern resident pod in Johnstone Strait during July, August and September, 1984 to 1988 34
Figure 10.Mean number of whale days per week during July, August and September, 1984 to 1988 35
Figure 11.Correlation between the percent time spent in Johnstone Strait by pods during July, August and September and the strength of the relationship with salmon 37 vii
ACKNOWLEDGEMENTS
I would like to thank Dr. David Shackleton, my supervisor for his support and encouragement. I would also like to thank Dr.
Kim Cheng for acting as my supervisor while Dr. Shackleton was on sabbatical. The members of my committee, Drs. A. Harestad, J. Ford and L. Gass also provided assistance through valuable criticism and discussion. To Dr. M. Bigg, a special thanks for taking an interest in my study and helping me at various points along the way.
Dr. D. Bain, Dr. M. Bigg, J. Jacobsen, P. Spong and H.
Symonds each gave me the use of their killer whale sighting records. Dr. J. Ford, P. Spong and H. Symonds also allowed me to use their acoustic recordings and C. Guinet allowed me to use his acoustic data. L. Hop-Wo, S. Hutchings, W. Luedke and P. Starr of the Canadian Department of Fisheries and Oceans provided data on salmon abundance.
Q. Muelder ten Kate and J. Ripley assisted me in the field in 1988. In 1989, Dr. D. Bain, B. Kriete and myself combined forces to carry out a killer whale survey near King Island. I thank them both for including me in this venture. I would also like to thank Jim and Ann Borrowman and Bill and Donna MacKay of Telegraph
Cove for giving support in so many ways during both field seasons.
Financial support was provided through a G.R.E.A.T. award from the British Columbia Science Council in cooperation with the
Vancouver Public Aquarium and by the Ann Vallee Ecological Fund.
West Coast Whale Research and Education Foundation, generously gave me use of a Boston Whaler and outboard engine in 1988. 1
INTRODUCTION
Food influences patterns of movement, home range sizes, social structure and territoriality in many animals. Seasonal movements are common among mammals because food availability
fluctuates seasonally in virtually all habitats (Sinclair 1983).
The size of areas over which individuals or groups range depends
on food availability and distribution (Macdonald 1983). Group
sizes of predators and the evolution of cooperative hunting are related to prey type and hunting tactics (Kruuk 1975; Packer and
Ruttan 1988). The occurrence of territoriality depends on the
abundance and distribution of a resource (in this case food) and whether it can be economically defended (Brown 1966).
This thesis describes how salmon (Oncorhynchus spp.)
abundance influences the movements and distribution of resident killer whales (Orcinus orca) in British Columbia. Killer whales
occur throughout most oceans in both pelagic and coastal habitats
(Dahlheim 1981; Leatherwood and Reeves 1983). The most detailed
studies, however, have been conducted in coastal British Columbia
and northern Washington state (Balcomb et al. 1982; Bigg 1982;
Ford and Fisher 1982). These studies show that killer whales in the Pacific Northwest are highly social and live in cohesive groups called "pods" (Bigg et socially isolated communities with different ranges (Bigg et al. 1987). The "northern resident community" ranges from mid Vancouver Island north to the Alaskan border. The "southern resident community" ranges from mid Vancouver Island south into Puget Sound and the Strait of Juan de Fuca. The range of the "transient community" overlaps both resident ranges. Transients and residents do not mix but actively avoid each other (Bigg 1982). World-wide, killer whales eat a variety of fish, cephalopods, pinnipeds and other cetaceans (Hoyt 1984; Lowry et al. 1987; Rice 1968). Within populations, however, killer whales may feed preferentially on specific prey types and move seasonally to areas where these prey are abundant (Heimlich-Boran 1986) . Early observers of killer whales in Puget Sound and off northern Vancouver Island suggested that killer whales fed on salmon (Rice 1968; Spong et al. 1970). Later studies (Bigg et al. 1987) suggested that killer whales in the northern and southern resident communities, feed on salmon and other fish because observations showed killer whales occurred predictably each summer in specific areas within their ranges where salmon are abundant during summer. These specific areas are Johnstone Strait, off north eastern Vancouver Island visited by northern resident whales, and Haro Strait, south of Georgia Strait, visited by southern resident whales. Both areas are along the salmon migration routes around Vancouver Island used by salmon returning to rivers in southern British Columbia (Groot and Quinn 1987; Gould et al. 1988; Gould and Stefansson 1985). Transient killer whales, on the other hand, feed primarily on marine mammals, and Bigg et al. (1987) observed that the seasonal movements of these whales were much less predictable 3 than those observed in resident whales. Observations from other parts of the world suggest similar seasonal shifts in killer whale distributions in relation to prey. In the Indian Ocean, killer whales appear at southern fur seal (Arctocephalus spp.) and southern elephant seal (Mirounga leonina) rookeries during the breeding season (Condy et al. 1978; Voisin 1972). Off the coast of Norway and around Iceland, killer whales are sighted most commonly inshore during the herring season (Jonsgard and Lyshoel 1970; Sigurjonsson et al. 1988). Similarly in British and Irish waters, sightings of killer whales inshore coincide with the inshore migration of herring (Clupea spp.) and salmon (Salmo spp.) and with the breeding concentrations of grey seals (Halichoerus grypus) (Evans 1988). Off the east coast of Canada, killer whales appear to move north in spring following the migration of rorquals (Balaenopteridae spp.) (Sergeant and Fisher 1957). Little has been done to quantitatively relate the movements of killer whales to those of their prey. This has been largely due to lack of data. Recent historical information, however, is available on the movements of resident killer whales and of salmon in the coastal waters of British Columbia and Washington State. These data provide a unique opportunity to quantitatively relate movement patterns to prey abundance in a killer whale population. Sighting records spanning many years provide data on the seasonal occurrence and distribution of each resident killer whale pod. These data are available from many 4 researchers and are maintained in a central database at the Pacific Biological Station in Nanaimo, British Columbia (M. Bigg pers. comm.). Information on the abundance and migration characteristics of salmon are available from the Canadian Department of Fisheries and Oceans (L. Hop-Wo pers. comm.; W. Luedke pers. comm.) and from the literature (Aro and Shepard 1967; Cooke and Groot, in press; Gould et al. 1988). Heimlich-Boran (1986) has made a quantitative analysis of the seasonal occurrence of southern resident killer whales over a three year period in Haro Strait and adjacent waterways. His results show that the occurrence of southern resident whales correlates positively with the abundance of salmon in the area. Guinet (1990a) has shown a positive correlation between the occurrence of northern resident killer whales and the abundance of salmon in Johnstone Strait during one summer season. A quantitative in depth study similar to Heimlich- Boran's (1986) has not been made of the northern resident killer whales. In my study I examined the seasonal movements of each of the northern resident killer whale pods in two areas of their range, Johnstone Strait and King Island over a six year period. The purpose was to examine the hypothesis that killer whales feed on salmon and move seasonally throughout their home ranges to areas where salmon are abundant. I considered three predictions of this hypothesis. 1) If killer whales follow migrating salmon, then 5 they will be seen more frequently in Johnstone Strait during summer months when salmon are abundant than during any other time of the year. Further, their occurrence in the Strait will be positively related to numbers of salmon. To examine this prediction, I used sightings and other available data to estimate the abundance of killer whales on a monthly basis throughout the year. I then regressed the number of whales present against the number of salmon, on a weekly basis during the salmon season. 2) The occurrence of killer whales will be positively related to the occurrence of salmon in other parts of the northern community range, particularly when salmon are not abundant in Johnstone Strait. To examine this prediction, I compared the occurrence of northern resident whales with estimates of salmon numbers during spring near King Island. 3) Foraging activity of killer whales in Johnstone Strait will indicate that they are feeding primarily on salmon. To examine this prediction, I observed killer whale behaviour and recorded where foraging occurred in Johnstone Strait and compared this to the areas fished by commercial salmon fishermen. 6 Drawing on the results of these predictions I then develop the hypothesis that within the northern resident community, pods have their own seasonal home ranges. 7 METHODS 1) Study Animals In the northern resident community there are approximately 172 killer whales in 16 "pods" (Bigg et al. 1987). Pod: a group of individuals which travel together the majority of the time (Bigg et al. 1990a). Pod membership is stable over time. Pods range in size from three to 23 individuals. When pods do split temporarily, they do so into groups called "subpods". Subpod: sub units of pods which also have stable membership. Members of subpods almost always travel together (> 95% of the time) (Bigg et al. 1990a). There are one to three subpods per pod. The basic social unit within subpods, and hence within pods, is the "matrilineal group". Matrilineal Group: A mother and her offspring including adult males but not adult daughters which have their own offspring. Daughters with offspring form their own matrilineal groups within the pod. Individuals always travel with members of their matrilineal group (Bigg et al. 1990a). 2) Study Areas Northern resident killer whales were studied in two coastal areas, 1) Johnstone Strait, off north eastern Vancouver Island and 2) King Island, on the mainland approximately 180 km north of Johnstone Strait (Fig. 1). a) Johnstone Strait: The study area is approximately 50 km from 8 130 125 0 130 125 Figure 1. Map of British Columbia coastline showing ranges of the northern (/////) and southern resident killer whale communities (after Bigg et al. 1987), major salmon migration routes (^^**^^ ) and the two study areas (Boxes). 9 east to west and the Strait is 3.5 to 4.5 km wide (Fig. 2). b) King Island: The study area encompasses Burke, Dean and Labouchere Channels all of which encircle King Island and includes North Bentick Arm to the mouth of the Bella Cbola River. This is a circuit of approximately 200 km and each channel is approximately 3.5 to 4.5 km wide (Fig. 3). 3) Sighting Records A sighting record is a report of a sighting of a subpod together with the date, location and the identity of the subpod. Bigg (1982) showed that killer whales can be individually identified by the unique shape of their dorsal fins and saddle patches, and by scars or nicks in these two areas. All researchers use this method to identify and report the individuals or subpods they see. I compiled sightings of killer whales from records collected over an 18-year period (1972 to 1989) mostly in Johnstone Strait but also from other parts of the northern community range. The majority of these sightings are stored in a database at the Pacific Biological Station in Nanaimo. Additional sightings were collected by myself and other observers. Combined there are over 4000 sighting records. Further references to this dataset will be made as "killer whale sighting database" (KWSD). a) Johnstone Strait From the KWSD I selected daily records from five years, 1984 to 1988, to compare with salmon abundance. I chose 10 Figure 2. Map of Johnstone Strait study area ( 11 12 these years because there was daily observer effort over consecutive weeks during the summer months of July, August and September when salmon are abundant. During each summer, one to two research boats, two whale-watching boats and observers at a cliff station overlooking the Strait (in radio contact with the boats) all monitored the study area. Not only were sightings of subpods reported, but days without whales were also reported. In total, I selected 61 weeks of consistent daily records (Table I). In addition to daily summer sighting records from Johnstone Strait, I selected records from the KWSD for the non- summer months between January 1985 and February 1989. I combined these records with the summer sightings and with acoustic data (see below) to describe the annual pattern of killer whale occurrence in Johnstone Strait. Non-summer sighting records were collected incidentally and they provided no indication of observer effort because days without observers and days when whales were not seen were not recorded, b) King Island Sightings of killer whales near King Island were collected by myself and two other researchers (D. Bain and B. Kriete) between April 28 and June 12 1989. In addition to recording the identity of subpods seen, we recorded days without whales and days without observer effort. Over the 46 days of this survey, 10 days were lost to bad weather, engine difficulties and supply runs. 13 Table I: Weeks of consecutive daily observer effort in Johnstone Strait used in analysis. Year First week Last Week Total Weeks 1984 July Sept. 1st week 2nd week 11 1985 June Sept. 4th week 4th week 14 1986 June Sept. 4th week 4th week 14 1987 July Aug. 3rd week 4th week 7 1988 June Oct. 4th week 1st week 15 Total 61 14 4) Acoustic Records Sighting records provide one source of data to monitor killer whale presence and movements. Another source are acoustic recordings of the vocalizations produced by the whales. Ford and Fisher (1982) determined that each resident killer whale pod produces a repertoire of eight to 15 discrete calls. Some pods share calls but each has a unique dialect. Pods can be identified by spectral analysis of taped recordings of their underwater sounds and comparing these with identified pod-specific calls. I analysed acoustic recordings made during non-summer months at Orca Lab (a permanent acoustic monitoring station on Hanson Island) and at Telegraph Cove (on Vancouver Island). As with incidental sightings, monitoring effort was not always known. Orca Lab monitors three to four hydrophone installations located in Blackney Pass and Johnstone Strait (Spong and Symonds 1990). At Telegraph Cove, the hydrophone is located at the entrance to the cove and monitors Johnstone Strait and Weynton Pass (J. Ford pers. comm.). The recordings I analysed represented 52 days from non- summer months from 1986 to 1989. In addition I used data collected and analysed by Guinet (1986) and J. Ford (unpublished) from 1985 and 1986 respectively (Table II). Recordings were analysed using a Kay DSP spectrum analyser, model 5500. Pod-specific calls were identified by comparing the screen display of a call structure with the call parameters documented by Ford (1987) and by aural comparison with 15 Table II: Months from which acoustic recordings were analysed and from which previously analysed data were used. Year Months Monitorincr location 1985* January, April T.C. 1985** October November O.L. December 1986** January, February, O.L. March, April, May June 1986 March, June October O.L., T.C. 1987 September, October, O.L. November, December 1988 October, November, O.L. December 1989 January, February O.L. * acoustic data previously analysed (J. Ford unpubl. data). Used with permission, (Ford pers. comm.) ** acoustic data previously analysed (Guinet 1986). Used with permission (Guinet pers. comm.) T.C. = Telegraph Cove on Vancouver Island O.L. = Orca Lab on Hanson Island 16 previously recorded and identified examples. 5) Salmon Abundance Estimates a) Johnstone Strait The Canadian Department of Fisheries and Oceans in Nanaimo (L. Hop-Wo pers. comm.; W. Luedke pers. comm.) provided estimates of the number of sockeye (0. nerka), pink (0. crorbuscha) and chum (0. keta) salmon passing through Johnstone Strait on a weekly basis during July, August, September and October from 1984 to 1988. Each estimate for each salmon species in each year is calculated from three pieces of information: i) The total size of the run that passed through Johnstone Strait. This is calculated as the catch plus the escapement. The escapement is salmon which escape the fishery and arrive at the mouth of their spawning river. ii) The migration pattern, or the percent of the total run that passed through the Strait during each week of the migration. This is estimated from average historical escapement curves and the run reconstruction method (See Hilborn and Starr 1988) . iii) The date of the peak of the migration through Johnstone Strait. This is estimated from test fishing results. The date of the peak catch estimates the date of the peak of the migration. Chinook (0. tschawytscha) and coho (0. kisutch) salmon also occur in Johnstone Strait during summer months but they are much less abundant than sockeye, pink, and chum and are not 17 considered commercially important in the area. Consequently the Canadian Department of Fisheries and Oceans has no estimate of their numbers or migration timing (P. Starr pers. comm.), and so I was unable to include these salmon in my analyses, b) King Island The Canadian Department of Fisheries and Oceans in Bella Coola (S. Hutchings per. comm.) provided catch and escapement data on sockeye, pink, chinook and chum, as well as estimates of the periods of peak abundance for each species in the area. From the catch and escapement data I estimated the total number of each salmon species in the study area during spring. Unlike Johnstone Strait, there are no estimates of weekly abundance. 6) Estimating the Abundance of Killer Whales in Johnstone Strait Throughout The Year The daily sighting records from summer months and the acoustic data and incidental sightings covering the rest of the year represented 3 6 months between January 1985 and February 1989. These data were used to calculate the average number of whales present during each month of the annual cycle, a) Acoustic Versus Visual Estimates of Whale Abundance Estimates of whale abundance during non-summer months were derived mainly from acoustic recordings whereas summer estimates were derived from sightings. It was important, therefore, to compare estimates of whale abundance derived by the 18 two methods when collected simultaneously, to determine if they could be used interchangeably to estimate whale abundance throughout the year. I compared the pods identified acoustically with those identified visually using acoustic data already analysed (J. Ford unpubl. data) and the corresponding sightings (KWSD) from a total of 27 days in July and August of 1980 to 1983. The acoustic recordings were made from a boat and were collected simultaneously with the sighting records. A comparison of the pods detected acoustically with those detected visually, tests the likelihood that killer whales vocalize when present. The number of whales estimated by each method, assuming that all members of a pod were present, was then compared in a simple regression using analysis of variance to test the significance of the relationship (Wilkinson 1988: 475; Zar 1984: 268). I used the student t-test to test whether the slope of the regression differed from 1.0 (Zar 1984: 271). b) Estimating Numbers of Whales Visual data show that pods sometimes split into subpods which may travel separately for up to a month (Bigg et al. 1990a). Killer whale vocalizations allow identification of pods but not which of their subpods are actually present. Furthermore, Gl and G12 pods are not yet distinguishable acoustically. To account for this limitation of acoustic data, I assumed for the purposes of this analysis that when a pod was identified acoustically all members of the pod were present. In 19 the case of Gl and G12 pods, when "G" calls were identified I assumed both pods were present. For comparative purposes in this analysis only, I converted summer sighting of subpods to pods when at least one subpod of a pod was present. Multiple sightings on one day, of subpods belonging to one pod were counted as only one sighting. The pod present was then converted to the total number of whales in that pod because pod sizes vary greatly in the northern resident community. I then considered each whale from the pod present on one day as one 11 whale day" and summed the number of whale days for each month. Each month of the year was represented by at least 2 years of data (with the exception of May for which there was only 1 years data). I calculated the mean number of whale days each month in the annual cycle by averaging replicate months between January 1985 and February 1989. 7) Regression Analyses a) Johnstone Strait I used only the 61 weeks of daily killer whale sighting records from the summers of 1984 to 1988 (Table 1). I computed a regression statistic between numbers of whales present and numbers of salmon present for each of the 16 northern resident pods. I converted each sighting of a subpod directly to the number of whales in that subpod. This can be done with the assumption that all members of the subpod are present because members of subpods travel together greater than 95% of the time 20 (Bigg et al. 1990a). Each whale present on one day represented one "whale day". I then summed the number of whale days each week for each of the 16 northern resident pods. Then treating each pod separately, I regressed the number of whale days per week against weekly numbers of each salmon species (sockeye, pink and chum) in a multiple regression analysis. Analysis of variance was used to test the significance of each multiple regression (Wilkinson 1988: 480; Zar 1984: 335). I present both the coefficient of multiple determination (R2) and the corresponding multiple correlation coefficient (R) for comparative purposes with other studies, b) King Island The co-occurrence of killer whales and salmon in this area was examined qualitatively because weekly estimates of salmon numbers were not available. 8) Behavioural Sampling a) Observations Between July 7 and September 1, 1988, I observed killer whales in Johnstone Strait and recorded where they foraged and the amount of time they spent in different activities. A subpod was located and observed at a minimum distance of 100 m. I increased observation distances to at least 300 m when the whales entered the Robson Bight Ecological Reserve because they are very sensitive to disturbance when rubbing (Briggs 1987). Observation sessions ranged from 3 to 8 h. 21 Sessions were terminated after a maximum of 8 h to avoid observer fatigue, while shorter sessions were terminated by weather and sea conditions, time of day or movement of the whales out of the study area. The behaviour of each individual in the group was observed and recorded at 15 min scan intervals (Altmann 1974). The direction of travel of the whales and their position relative to landmarks and distance from shore were also recorded on a detailed map of the study area. I classified the behaviour of the whales into one of five main behavioural categories, foraging, travelling, socializing, resting and rubbing. These behaviours have been defined by recognizable surface behaviours, surfacing intervals and the degree of group synchrony in respiration (Ford 1988; Jacobsen 1986; Osborne 1986) . Foraging: Whales in the group are spread out often as much as several hundred metres along the shore or within a few hundred metres of shore. Individuals swim in the same general direction parallel to shore but surface asynchronously. Brief bouts of erratic swimming indicate pursuit of prey. Travelling: Whales in the group swim abreast of each other and surface synchronously. Resting: As in travelling, whales swim abreast and individuals surface synchronously. Their horizontal progress is very slow, however, and surfacings are frequently followed by brief periods of surface floating during which members of the group float with their blowholes exposed. Socializing: Individuals make physical contact while chasing, breaching and rolling over each other. Other surface behaviours observed include fluke slaps, pectoral slaps, breaches, penis displays and spyhops. Rubbing: Individuals in the group rub their bodies on the smooth pebble substrate at specific beaches in the Robson Bight Ecological Reserve, b) Activity Budget During each 15 min interval the behaviour of the group was taken as the behaviour exhibited by the majority (> 50%) of the individuals in the group (Ford 1988). In all sample sessions this criteria was met, which reflects the fact that whales within a subpod typically engage in the same behaviour at the same time. To determine the percent of time whales spent engaged in each behaviour I calculated the total time spent in each behaviour over all sessions and divided this by the total minutes of observation over all sessions. RESULTS I begin with a comparison of acoustic and visual estimates of whale abundance. I then present the pattern of killer whale occurrence in Johnstone Strait throughout the year and in more detail the temporal pattern during summer months of both whales and salmon. I then present the temporal pattern of killer whale and salmon numbers in the King Island study area. From considering all whale sightings combined, I next consider whale sightings from each pod. I examine how much time each pod in the northern resident community spends in Johnstone Strait during summer. I then present the regression results between the occurrence of whales from each pod and numbers of salmon in the Strait. Finally, I describe the behaviour of whales in both the Johnstone Strait and King Island study areas. 1) Occurrence of Killer Whales in Johnstone Strait Throughout the Year A comparison of acoustic and visual estimates of whale abundance (Fig. 4) shows that, with similar levels of effort and the assumption that all members of a pod are present, the two methods provide similar estimates of whale numbers (r2 =0.78 P<0.001, n=27). The acoustic method, however, under-estimated slightly the number of whales, because not all pods identified visually vocalized during some recording sessions or they were out of range of the recording equipment. This is reflected in the slope of the regression equation which is significantly less than 24 100 -i 80 - "5 60 - CO CD O -*—» CO oZ3 40 - o < 20 0 0 20 40 60 80 100 Visual estimate Figure 4. A comparison of the number of whales in Johnstone Strait on 27 days during July, August and September of 1980, 81, 82 and 83 estimated by visual observation and by acoustic monitoring. r2=0.78, P<0.001, n=27 days, slope 1.0 (slope = 0.78, t-test: P < 0.05, Y = 3.95 + 0.78X). Nonetheless, I have combined sightings and acoustic data from January 1985 to February 1989. These data show that killer whales are most abundant in Johnstone Strait during summer (July to October) and are least abundant between December and June (Fig. 5). Furthermore, killer whales are not only less frequent in Johnstone Strait outside of summer, but only a few pods seem to visit the area (Table III). 2) Salmon Abundance and Killer Whale Occurrence in Johnstone Strait The timing of salmon migrations, especially of pink and sockeye, through Johnstone Strait during July, August and September coincides well with the occurrence of killer whales (Fig. 6). During these months sockeye and pink salmon peak in abundance between late July and mid August, chum salmon peak in abundance in early October while killer whales sightings peak between late July and early September. Total numbers each summer of these three salmon species have varied from four to 15 million (1984 to 1988) (salmon estimates provided by the Canadian Department of Fisheries and Oceans, L. Hop-Wo pers. comm.; W. Luedke pers. comm). Interannual variation in salmon numbers results because different stocks of each species return in different years and stock sizes vary. In a similar manner, the number of salmon during the peak of the migration varied. Numbers of sockeye during weeks of peak abundance ranged from 150,000 to 26 n= 3 2221 344444 3 months 1500 -i GO > "a J32 1000 - ctf CD E c c 500 co CD 0 I T T T M A MY JU JL AU S 0 N D Months Figure 5. Mean number of whale days per month estimated from sightings and acoustic data. Bars represent means. Vertical lines represent standard error of the mean, n = number of replicate months (1985 -1989). 27 Table III: Pods present in Johnstone Strait during different seasons of the year (1985 - 1989). Pods identified from both visual and acoustic records. July - October November - June Al Al A4 A4 A5 A5 BI CI CI G1/G12 Dl 111 G1/G12* 131 HI 11 12 111 118 131 Rl Wl * G1/G12 = Gl and/or G12 because these pods are not yet distinguishable acoustically. 28 Figure 6. Mean number of salmon and whale days per week in Johnstone Strait during July, August, September and October. • numbers of sockeye in thousands. numbers of pink in thousands, x numbers chum in thousands.© numbers of killer whales, n = 5 summers (1984 - 1988). 29 1.4 million per week between 1984 and 1988. During weeks of peak abundance, numbers of pink ranged from 250,000 to 2 million per week and numbers of chum salmon ranged from 280,000 to 700,000 per week (Table IV). 3) Salmon Abundance and Killer Whale Occurrence Near King Island Between late-April and mid-June, neither killer whales nor salmon are abundant in Johnstone Strait. Near King Island, however, killer whales from nine northern resident pods, Al, A4, A5, CI, Dl, G12, II, 12 and Rl, which all visit Johnstone Strait during the summer, were observed for 3 6 days between April 28 and June 12 1989 (Fig. 7). At the same time, there were salmon in the King Island area and the predominant runs were of sockeye and chinook (Fig. 8). Chinook salmon were present in these channels during early April with the majority returning to the Bella Coola River system. The number of returning chinook is estimated to have been about 3 0,000, with peak numbers in the channels by mid- June (S. Hutchings pers. comm.). Sockeye salmon, migrating to the Bella Coola River at the head of North Bentick Arm and to the Kimsquit River at the head of Dean Channel appeared in Burke and Dean Channels in mid-May, and peaked in abundance by early July (S. Hutchings pers. comm). The total number of returning sockeye was about 38,000 salmon. 4) Pod Usage of Johnstone Strait During Summer Although in general, killer whales are most abundant in Table IV: Numbers of salmon (in thousands) in Johnstone Strait July to October, 1984 to 1988 from the Canadian Department of Fisheries and Oceans estimates (L. Hop-Wo pers.comm; W. Luedke pers.comm.). Total numbers of each species and the peak weekly number of each species. TOTAL JULY to OCTOBER PEAK WEEK Year Sockeye Pink Chum Sockeye Pink Chum 84 1796 820 1838 636 241 285 85 4398 7739 3667 1387 2045 570 86 3492 1756 738 1119 566 696 87 2379 2804 1924 659 779 298 88 546 2624 3195 155 818 895 31 100 -i 80 - 00 cd "a c o =cSd 60 > CD" 00 o 40 c CD iO_ CD Q_ 20 - 0 1 T T r? A5 C1 D1 A4 G1 IT 12 R1 A1 Pods Figure 7. Frequency of occurrence of all or part of each northern resident pod seen in the King Island study area during May and June 1989. n = 36 days of observations. Pods are arranged from most frequent to least frequent. 32 n • 6 6 4 6 6 days 150 CO & 100 TJ CD E 50 - 0 May June Weeks chinook sockeye v/////;/////;/~x Figure 8. Number of whale days each week in the King Island study area during May and June 1989. n = number of observation days each week. Horizontal bars = weeks when chinook and sockeye were present. Arrow = week of peak chinook abundance. Johnstone Strait during July, August and September (Fig. 5), only whales from Al and A5 pods were present in Johnstone Strait on more than 50% of the days during these months (1984 to 1988). Most other pods spent considerably less time in the Strait (Fig. 9). Consequently, peak weekly numbers of whale days during summer represents only about 25% of the northern resident population. If the entire northern resident community were present there should have been about 1204 whale days per week (172 whales * 7 days), whereas the peak number of whale days was 413 or an average of 277 whale days per week (Fig. 10). 5) Regression Results Between the Occurrence of Killer Whales and the Abundance of Salmon in Johnstone Strait When considered individually, sightings of seven of the 16 pods in the northern resident community were positively and significantly associated with estimates of one or more! species of salmon. The occurrence of three of these pods, Al, A5,j and CI, associated positively with sockeye and pink salmon, while the occurrence of A4, Dl and HI associated positively with only sockeye and Gl pod associated positively with only chum salmon (Table V). The more time pods spent in Johnstone Strait during July, August and September (data from Fig. 9), the stronger was the regression (Fig. 11, r =0.83, p<0.001, n=16) with salmon numbers (data from Table V). Of the six pods whose occurrence were positively related to sockeye and pink salmon, all were present on more than 15 % of summer days in Johnstone Strait. The 34 100 -i 80 - c CD 00 CD 60 00 CO 40 - c CD iO_ CD Q_ 20 - nnn JZL 0 i T i i i i \ i i i i—i—i—i—i—r A1 A5 C1 A4 D1 H1 131 B1 R1 W1 G1 2 111 118 G12 11 Pods Figure 9. Frequency of occurrence of all or part of each northern resident pod in Johnstone Strait during July, August and September, 1984 to 1988. Bars represents percent of all observation days from 1984 to 1988 (n = 403 days). r 35 n = 4555555 555444 weeks 300 n 0 i i i i i i i i i i July Aug Sept Weeks Figure 10. Mean number of whale days per week during July, August and September. Dots represent means. Vertical lines represent standard error of the mean, n = number of replicate weeks 1984 - 1988. 36 Table V. Multiple regression results between weekly numbers of whale days from each northern resident pod and weekly numbers of three species of salmon, in Johnstone Strait during July, August and September, 1984 to 1988 (n=61 weeks). R2 = the coefficient of multiple determination for each pod, R = the multiple correlation coefficient, n = the number of weeks of whale sightings and salmon abundance data used in each regression. Salmon = those species which contributed significantly in each multiple regression. POD R2 R n Salmon Al 0.28** 0.53** 61 S, P A4 0. 10* 0.32* 61 S A5 0.37** 0.61** 61 S, P Bl 0. 04 0.19 61 CI 0.35** 0. 59** 61 S, P Dl 0. 10* 0.31* 61 S Gl 0. 10* 0.32* 61 c G12 0. 05 0.22 61 HI 0.17* 0.41* 61 S 11 0.01 0 . 12 61 12 0. 03 0. 18 61 111 0. 06 0.24 61 118 0. 07 0. 27 61 131 0. 04 0.20 61 RI 0.06 0.25 61 Wl 0. 04 0.20 61 ** P < 0.001 * P < 0.01 S = Sockeye, P = Pink, C = Chum 37 0.4 n , A5 C1, 'C o "cca 0.3 - "E A1 "5 •o a? a. 0.2 - H1. c Q 'o A4 ^ 0.1 - G1i # CD D1# O o 0.0 -f 0 20 40 60 80 100 Percent presence Figure 11. Correlation between the percent time spent in Johnstone Strait by pods during July, August and September and the strength of the regression with salmon. r=0.83, p<0.001, n=16. Points representing pods with significant, positive relationships with salmon abundance are labelled with the pod's name (see Table V). 38 whales in these pods represent 3 5% of the northern resident community (approximately 59 whales). For whales which spent less than 15% of summer days in the Strait, the occurrence of only Gl pod was positively related to salmon abundance and this was with chum salmon. Gl pod represents an additional 13% of the community (approximately 2 3 whales). Combined, numbers of whale days from these seven pods constituted the majority of weekly whale days between 1984 and 1988 (X=87.6%, S.E.M.=1.52, n=59). The occurrence of the remaining nine pods, BI, G12, II, 12, 111, 118, Rl and Wl (approximately 9 3 whales or 52% of the community) did not relate significantly to salmon abundance and all spent less than 15% of summer days in the Strait. 6) Behaviour of Killer Whales a) Johnstone Strait A total of 13 3 h of observations were made of two subpods, A12 and A2, of the Al pod during 22 daily sessions between July 7 and September 1, 1988. Of all the northern resident pods, Al pod whales use Johnstone Strait the most during summer (see Fig. 9) and tend to stay within the study area for extended periods making it possible to follow them for 4 to 8 h sessions. Killer whales foraged along the Vancouver Island shore from Robson Bight to Blinkhorn Point, a distance of about 13 km. The whales also forage along West Cracroft Island, Hanson Island and in the strong current at the junction of Blackney Pass and 39 Johnstone Strait. Pursuit of prey by killer whales was evident by their brief bouts of erratic swimming. Other researchers and I made 5 direct observations of salmon being pursued or captured by killer whales in these shoreline areas: on two occasions I observed Al pod whales capturing salmon; J. Jacobsen (pers. comm.) observed A5 whales on two occasions and a CI whale on one occasion, corner salmon in rock crevices along the shore. The proportion of time spent in each behaviour category was: foraging 38%, travelling 32%, resting 15%, socializing 12% and rubbing 3%. b) King Island Although detailed behavioural observations were not made in this area, killer whales swam mainly along shorelines and frequently moved long distances. For example, on eight occasions when killer whales were tracked for six to 10 h, they travelled distances of 40 to 85 km. Furthermore, resighting of the same whales occurred in all parts of the approximately 200 km long study area around King Island, providing further evidence that the whales were travelling long distances on a daily basis. This is in striking contrast to their movements in Johnstone Strait during summer, which are very localized. 40 DISCUSSION 1. Occurrence of Killer Whales and Salmon in Johnstone Strait and Near King Island Despite the varying quality of data used to describe the occurrence of killer whales in Johnstone Strait throughout the year, the seasonal distribution of killer whales is consistent with the impressions of researchers and tour operators who have worked in Johnstone Strait over the past 18 years (M. Bigg pers. comm.; B. MacKay pers. comm.). The annual increase in whale occurrence in the Strait during the salmon season (July to October) supports the hypothesis that northern resident killer whales select their habitat seasonally in response to the temporal availability salmon. The occurrence in the King Island study area of the same northern resident pods that visit Johnstone Strait during summer coincided with local spring runs of sockeye and chinook salmon, providing additional support for this hypothesis. Southern resident killer whales appear to behave similarly, using their habitat seasonally in response to salmon (Heimlich-Boran 1986). 2. The Relationship Between the Occurrence of Killer Whales and Salmon in Johnstone Strait During Summer The positive and significant relationships between the occurrence of killer whales and of salmon illustrates statistically that the seasonal occurrence of killer whales in 41 Johnstone Strait is explained in part by the seasonal abundance of salmon. Although statistically significant, none of the coefficients of multiple determination, nor the corresponding correlation coefficients are very high. However, comparing the correlation coefficients with those reported by Heimlich-Boran (1986) for the southern resident community, and by Guinet (1990a) for the northern resident community, my results are similar to there's. In considering the variation unaccounted for in these relationships it is important to recognize that killer whales are probably highly efficient predators which can use sight and sonar to locate prey and then move with speed and agility to capture them. Consequently small changes in overall salmon numbers may be unimportant to the whales. Salmon estimates may be another source of variation which is unaccounted for in the regressions because the estimated number of salmon per week is likely inaccurate to some degree. This error is related to the migration curves used by the Department of Fisheries and Oceans to make weekly estimates of salmon abundance. The migration curve for each salmon species is an average derived from the migration curves of all stocks of a species that passes through Johnstone Strait. For example, both chum salmon and even-year pink salmon passing through the Strait are comprised largely of a mixture of stocks destined for different rivers along the south coast of British Columbia (Aro and Shepard 1967; Gould et al. 1988). In different years, the number of salmon from each stock of a species can vary and thus 42 the composite migration curve for a species will be inaccurate in shape and therefore in its value of weekly salmon numbers (W. Luedke pers. comm.). Unforunately, the amount of error from this source is unknown (W. Luedke pers. comm.). The sport fishing catch data used by Heimlich-Boran (1986) to correlate the occurrence of southern resident whales with salmon abundance are also likely to have provided inaccurate estimates of weekly salmon numbers, because he did not used any measure of fishing effort. Similarly, Guinet (1990a) used commercial catch data without any measure of fishing effort in his single season correlation of northern resident whales. Nonetheless, despite inaccuracies in each of our salmon data sets, my results are consistent with theirs, and combined our results illustrate that a seasonal relationship does exist between resident killer whales and salmon in coastal British Columbia and northern Washington. 3) Foraging Behaviour of Resident Killer Whales The observations of killer whales pursuing and eating salmon while in the Johnstone Strait, which I reported, provides evidence, in addition to the regression results, that the occurrence of whales and salmon in Johnstone Strait reflects a predator-prey relationship. Similar observations of foraging whales have been made in the southern resident community (Heimlich-Boran 1986). In addition to observations of foraging behaviour, there is evidence from fish scales collected on the water near foraging resident killer whales to indicate that 95% 43 of detected kills are of salmon (Bigg et al. 1990b). My observations of killer whales in Al pod showed that they foraged primarily along shorelines and in areas of strong current. Around King Island, killer whales were also observed to swim mainly along the shore. There are at least two possible reasons for foraging in shoreline areas: 1) Areas where salmon travel close to shore may be important to killer whales because they are able to use the shore as a barrier against which to trap salmon (Jacobsen 1986; Ford 1989). Johnstone Strait and Burke, Dean and Labouchere Channels, all have steep sides which may make them even better areas for trapping salmon. Underwater observations of killer whales cornering salmon in rock crevices in Johnstone Strait (J. Jacobsen pers. comm.) support this potential advantage of steep shorelines. Southern resident killer whales forage significantly more in areas with high relief subsurface topography and shallow reefs along salmon migratory routes (Heimlich-Boran 1988). These are areas where salmon could be easily herded into higher densities (Heimlich-Boran 1988). Killer whales off Argentina and off the Crozet Islands use shorelines as barriers when hunting southern sea lions (Otaria flavescen) and southern elephant seals (Mirouncra leonina). These whales intentionally beach themselves and capture their mammalian prey at the water's edge. Hunting success is higher with this technique than in open water where pinnipeds are faster and more agile than on land (Lopez and Lopez 1985; Guinet 1990b). The use of barriers to herd and capture prey has also been described in studies of bottlenose dolphins (Tursiops truncatus) (Wursig 1986; Irvine et al. 1981). 2) Commercial fishing activity, catch data and ultra-sonic tagging of salmon all suggest that the types of areas where killer whales forage most in Johnstone Strait are areas of high salmon densities (Cooke and Groot, in press; Gould and Hop-Wo 1986; Quinn and teHart 1987). In Johnstone Strait, "hotspots" reported by fishermen and test fishing locations used by the Department of Fisheries and Oceans are along the Vancouver Island shore, along the Hanson Island shore, off Cracroft Point and in Blackney Pass (Cooke and Groot, in press; Gould and Hop-Wo 1986). Ultra-sonic tagging of salmon reveals (Quinn and teHart 1987) that migrating salmon encountering headlands, entrances to bays or intersections of channels, become temporarily disoriented, possibly because of confusing current flows resulting from tides and wind deflection in these areas. Consequently, salmon tend to aggregate in these places for a time until they reorient themselves and move on. Around King Island, killer whales travelled over much greater distances than when in Johnstone Strait during summer. This difference in daily movement between the two areas may be in response to densities and dispersal of salmon. Near King Island, whales may have needed to cover more area to locate salmon than when in Johnstone Strait where salmon can number in the millions (see Table IV). Similar seasonal changes in travel distances have been reported for other mammals and have been linked to food abundance and distribution (African wild dogs, Frame et al. 1979; baboons, Devore and Hall 1965). From late fall to spring, salmon are fewer inshore than during summer, mainly small numbers of coho (Aro and Shepard 1967) and chinook (Aro and Shepard 1967; Healey 1983) salmon and they are more widely dispersed. I suggest that outside of summer months, resident killer whales regularly cover more area and spend less time in any one area in response to this change in salmon densities and dipseral. So far, I have illustrated that 1) the occurrence of northern resident whales in both Johnstone Strait and King Island is seasonal, 2) weekly numbers of whales and salmon in Johnstone Strait during summer are positively related, and 3) observations of resident killer whales indicate that they do prey on salmon in Johnstone Strait. Together, these results support the hypothesis that killer whales use their habitat seasonally to take advantage of varying availability of salmon. My study, however, also revealed that there are differences in the frequency of occurrence of each northern resident pod in Johnstone Strait, leading to the hypothesis that within the northern resident community, each pod has its own unique home range and pattern of home range use. I will refer to pods as the social unit maintaining home ranges, because my analyses were at the pod level rather than the subpod level, however, the hypothesis I will now develop could apply equally well to subpods. 46 4) Pod Home Ranges My study has shown that only part of the northern resident community visits Johnstone Strait frequently during summer and that the occurrence in Johnstone Strait of less than 50% of the whales in the community is positively associated with salmon abundance there. This raises the question as to, why do so many northern resident killer whales spend so little time in Johnstone Strait during summer when numerically there would seem to be enough salmon between July and October to feed the entire community? I propose that the reason for the relatively low numbers of whales in Johnstone Strait during summer may in part be that their presence affects the actual availability of their prey. Though salmon are abundant in Johnstone Strait, they may be wary of foraging killer whales and become harder to catch once killer whales have been present for a time. Certainly, fishermen state that salmon, particularly chinook salmon, respond strongly to the presence of killer whales and can be difficult to catch for some hours after whales have passed through (J. Ford unpubl.). This effect of predators on their prey, known as resource depression, can be as important or more important than the abundance of prey in determining the movements and foraging decisions of a predator (Charnov et al. 1976). Consequently, killer whale pods may need to distribute themselves in such a way as to avoid excessive overlap in any one area if they are to forage efficiently. The various corollaries (bold-type) of my home range hypothesis follow. 47 A pod's seasonal home range consists of a several feeding locations where salmon are abundant during the same period and a pod travels among these areas. This is supported by my observation that even among pods whose occurrence was positively associated with salmon abundance in Johnstone Strait, most, still spent the majority of summer elsewhere (Fig. 9). In studies of primates (Goodall 1977; Oates 1987), it has been argued that systematic utilization of the home range leads to more efficient resource exploitation because animals know the abundance and distribution of food within their range. This may also be true for killer whales. Heimlich-Boran1s (1986) study of southern resident whales provides additional support for this corollary. He showed that southern resident whales travel among areas, and that their occurrence in many areas correlates with salmon abundance. Pod home ranges overlap and many feeding locations are used simultaneously by more than one pod. This is clearly supported by the observation that different pods visited Johnstone Strait at the same time and that a number of pods were observed together near King Island. Although home ranges overlap spatially there may be temporal separation in some cases. For example, Gl pod spent very little time in Johnstone Strait during summer and yet their occurrence was positively associated with chum salmon which peak in fall. This suggests temporal partitioning (Schoener 1974) of the salmon resource in some areas. Sightings and acoustic data from the fall suggest that Gl 48 and perhaps G12 pods are more common during the fall than during the summer. This pattern has been noted by other researchers for a number of years (D. Bain pers. comm.; J. Ford pers. comm.; P. Spong pers. comm). It is also possible that Gl pod whales prefer chum salmon. Certainly, dietary differences among groups of mammals which have similar food resources in their respective home ranges have been reported (chimpanzees, Goodall 1986). However, because sockeye, pink and chum salmon each have different migration timings through Johnstone Strait, it is difficult to separate differences in timing of use from prey preference. Seasonal feeding locations are visited annually by the same pods which have developed preferences for feeding in these specific areas. This is supported by the observation that each summer it is the same pods that are frequent visitors to Johnstone Strait. Traditional use of certain areas could develop because of the advantage of feeding in areas where individuals of the pod have acquired knowledge over many years about local salmon distribution. Salmon distribution is affected by current, subsurface topography, temperature and salinity (Cooke and Groot, in press; ; Madison et al. 1972; Quinn and teHart 1987; Stasko et al. 1973). Furthermore, different species (Argue 1964) swim at different depths and different stocks of the same species use different migration routes (Cooke and Groot in press). Killer whales are long-lived animals and the stable kin associations within pods mean that experience could be easily maintained in 49 the pod and transferred between generations. The advantage of familiarity from past experience of seasonal food availability are evident in home range use by groups of mountain gorilla (Gorilla gorilla berinqei). In one study (Goodall 1977) , gorillas anticipated the seasonal growth of bamboo and travelled to specific areas to dig for the young shoots before they had emerged from the soil. In another study (Fossey and Harcourt 1977), observations suggested younger gorillas learned from the experience of older individuals in the group where to find food and in particular, where to find the best food. The summer home ranges of over half of the northern resident community include feeding areas distant from Johnstone Strait. This is supported 1) by the observation that pods whose occurrence in Johnstone Strait was not significantly associated with salmon abundance were infrequent visitors to the Strait and 2) by a small number of incidental summer sightings of northern resident killer whales (KWSD) away from Johnstone Strait, most of which, have been of pods which spend less than 15% of summer days in Johnstone Strait. These sightings are mainly from coastal areas north of Vancouver Island and from the west coast of Vancouver Island. 3) salmon are abundant in many areas besides Johnstone Strait between July and October. In particular, the salmon which migrate through Johnstone Strait must first pass through Queen Charlotte Strait. This area, like Johnstone Strait, is an important commercial fishing area (Cooke and Groot, in 50 press). Rivers and Smith Inlets, on the mainland north of Vancouver Island, are also important sockeye fishing areas where peak weekly numbers of sockeye during mid July can range from 150,000 to 1,000,000 (1984 to 1988) (Aro and Shepard 1967; Goruk and Thomson 1988). Other important fishing areas for sockeye, pink and chum salmon exist throughout many coastal areas northward from Rivers and Smith Inlets to include the Skeena and Nass Rivers near the British Columbia - Alaska border. Commercial catches in these coastal areas are made between late June and the end of August (Aro and Shepard 1967). On the west coast of Vancouver Island, small runs of salmon are present from June to October (Aro and Shepard 1967; Pacific Region Salmon Resource Management Plan 1988 Vol. H). I predict that resident killer whales will be found to exploit salmon in many of these areas. Resident killer whales pods not only co-occur in certain areas and therefore have overlapping home ranges, but also show neither aggression nor avoidance when encountering other resident pods. In a survey of primate societies, Cheney (1987) concluded that, in general, the more overlap that existed among home ranges, the less likely were aggressive encounters, and if they did occur, it was over a specific food item. In another survey, Mitani and Rodman (1979) reported that ranges too large to be patrolled daily were undefended. In resident killer whales, the lack of either aggression towards other pods or avoidance of pods suggests 1) that home ranges are very large and 2) prey are not in short supply, at least not during the spring 51 and summer when I observed them. A third possibility, however, is that their reproductive behaviour may influence their movements. My observations of killer whales in Johnstone Strait were that when different pods encounter each other, they tended to travel together intermingling and engaging in social activity without any signs of aggression. Social contact may be important among pods for breeding opportunities because individuals do not disperse from their pod, and breeding is believed to occur between rather than within pods (M. Bigg pers. comm.). For these reasons, it would seem that pods need to seek out other pods for mating, if not for other social requirements. This may account for some of the overlap among home ranges of resident killer whale pods, and particularly for the occurrence of infrequent pods in Johnstone Strait during summer whose presence was not significantly related to salmon abundance. Although virtually nothing is known about the mating system of killer whales, it is known that calves are born throughout the year, with birth peaks in fall and winter (Bigg 1982). 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