MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 310: 235–246, 2006
Published April 3
Have North Pacific killer whales switched prey
species in response to depletion of the
great whale populations?
Sally A. Mizroch*, Dale W. Rice
National Marine Fisheries Service, Alaska Fisheries Science Center, National Marine Mammal Laboratory,
7600 Sand Point Way NE, Seattle, Washington 98115, USA
ABSTRACT: Springer et al. (2003; Proc Natl Acad Sci USA 100:12223–12228) hypothesized that populations of seals, sea lions and sea otters in the northern North Pacific Ocean and Bering Sea declined
because of increased predation by killer whales, in what they termed a ‘sequential megafaunal collapse’. They hypothesized that the killer whales had been dependent on large whales for food, and
that their increased predation on the smaller marine mammals was directly due to the depletion of
great whale populations as a result of post-World War II industrial whaling. The maps presented by
Springer et al. (2003) masked the development and precipitous decline of post-World War II industrial whaling. Our analysis shows that north of 50° N, whaling developed slowly from 1948 to 1951,
expanded steadily from 1952 to 1962, and increased very sharply from 1963 to 1967. By 1968, there
was near total drop-off in catches north of 50° N as the whaling fleets moved south. Because of the
extraordinary whale biomass removals in the mid-1960s, any whaling-related prey shifting should
have started by 1968, not the mid-1970s as they suggested. We also present data that refute their
assumption that North Pacific killer whales depended on large whales as prey either prior to or concurrent with the whaling era. During the years of the development and pulse of whaling (i.e. prior to
1968), less than 3% of the mammal-eating killer whales were observed to have large whale remains
in their stomachs. Killer whales attack healthy, adult large whales only rarely, and such attacks are
usually unsuccessful. Neither minke nor gray whales were depleted by post-World War II industrial
whaling, and they have always been available as prey for North Pacific killer whales.
KEY WORDS: Whaling · Killer whale · Sequential decline · Sequential megafaunal collapse
Resale or republication not permitted without written consent of the publisher
Springer et al. (2003) hypothesized that post-World
War II industrial whaling caused a ‘sequential megafaunal collapse’— the sequential collapse of populations of seals, sea lions and sea otters in the northern
North Pacific Ocean and Bering Sea because of
increased predation by killer whales Orcinus orca.
They asserted that mammal-eating killer whales
switched to pinnipeds as prey after their preferred
prey, the great whales, had been depleted by whaling.
This whaling targeted 4 species of large balaenopterid
whales: the blue whale Balaenoptera musculus, fin
whale B. physalus, sei whale B. borealis, and humpback whale Megaptera novaeangliae and 1 large
toothed whale, the sperm whale Physeter macrocephalus. We assess whether Springer et al. (2003)
mis-characterized the depth and timing of the depletion of the great whales in describing post-war industrial whaling and whether their description of the
development and decline of whaling in the North
Pacific is accurate either temporally or geographically.
Also, their assumptions about the food habits of killer
whales were based mainly on conventional stereotypes and untenable extrapolations from anecdotal
accounts. The scientific literature and anecdotal re-
*Email: sally.mizroch@noaa.gov
© Inter-Research 2006 · www.int-res.com
INTRODUCTION
236
Mar Ecol Prog Ser 310: 235–246, 2006
ports fail to provide any evidence that North Pacific
killer whales were ever dependent on great whales as
prey.
In this paper we present a clearer, more accurate picture of the start, development and decline of postWorld War II industrial whaling based on the dataset
used by Springer et al. (2003). We also explore the historical literature and provide both quantitative and
qualitative information on killer whale prey choices in
the North Pacific before and during the era of postWorld War II industrial whaling.
MATERIALS AND METHODS
Catch data. We used the same dataset as Springer et
al. (2003), which was collected by the Bureau of International Whaling Statistics (BIWS) and is now curated
by the International Whaling Commission (IWC). Data
used in our analysis are from the dataset distributed by
the IWC in February 2003.
Plots of densities of whale catches (catch in numbers of whales) as carried out by Springer et al. (2003)
cannot be used as a proxy for biomass removals as
the whalers shifted from larger whales to smaller
whales (e.g. fin whales to sei whales) after the stocks
of larger whales were depleted. Because the species
mix shifted, the biomass removals cannot be compared year to year. Although Springer et al. (2003)
mentioned whale biomass, they did not actually show
whale biomass at all in their Fig. 1 and possibly not
in their Fig. 2.
Our maps present catch biomass rather than the
number of whales caught and use shaded density grids
that show the zones of high densities, using the ‘natural breaks’ algorithm (Jenks 1963) within the program ArcMap 8.2 to determine optimal class size. Dot
patterns of grouped whale catches (as presented in
Springer et al. 2003) cannot pictorially show the actual
densities of the number of whales caught because the
dots overlap.
Biomass estimates. Whale biomass was calculated
using actual whale length data in the BIWS/IWC catch
database. Weights were estimated for each whale
caught based on length/weight curves developed for
each species; see Mizroch (1983) for the method of
calculation. We used the weight/length parameters
calculated by Lockyer (1976) for all species except
Berardius bairdii; for the latter species we used data
from Balcomb (1989). Appendix 1 (available as Supplementary Material at www.int-res.com/articles/suppl/
m310p235_app.pdf) shows the length/weight parameters used for each species, as well as detailed information for each species for the entire range of the dataset,
and also for those catches from 50° N and above.
Biomass estimates on the maps are the sums of
weights for all whales caught in each 1 × 1° cell. Biomass estimates on the figures are the sums for all
species combined.
Killer whale prey choice based on analysis of stomach contents. Groups of killer whales show a consistent
preference for a particular kind of prey. In the northeastern Pacific, most groups of killer whales fall into
one or the other of 2 categories which, for lack of a better term, may be called ecotypes (Bigg et al. 1987, Ford
et al. 1998, Heise et al. 2003). These ecotypes are
(1) fish-eating whales that prey on salmon, herring,
and other fishes; these whales return each summer to a
traditional home range in coastal waters, and (2) mammal-eating whales that prey on seals, sea-lions, porpoises, and dolphins; these animals are somewhat
nomadic, with occasional incursions into the coastal
habitat of the fish-eating whales. There is virtually no
overlap in the diets of these 2 ecotypes.
We surveyed the literature for studies on stomach
contents analyses of killer whales sampled in the North
Pacific prior to 1968. Our analyses were stratified by
whether the killer whale consumed fish or mammals.
In the stomach contents, baleen whales are usually
represented by sections of baleen and large chunks of
blubber, whereas small cetaceans are represented by
fragments of blubber and flesh including recognizable
parts such as flukes, and pinnipeds are represented
by bones, teeth, skin, hairs, vibrissae, and claws. We
do not know what possible biases may be introduced
by differential times of passage of these different body
parts.
RESULTS
Catch data
In our analysis, we recreated Springer et al. (2003)’s
Fig. 1A–D, but we selected different sets of years that
better characterize the development and abrupt
decline of whaling in the northern North Pacific and
Bering Sea (latitude 50° N and above). Our Fig. 1
shows catch levels throughout the North Pacific, presenting total catch in the North Pacific, catch taken
between the latitudes 30° and 50° N and catch taken
from 50° N and above. Catches north of 50° N began in
1948 and were nearly non-existent after 1975.
Post-World War II industrial whaling in the northern
North Pacific had a development period (1946 to 1951,
Fig. 2, similar to Springer et al. (2003)’s Fig. 1A), a
period of increasing catches (1952 to 1962, Fig. 3), and
then a sharp increase in 1963 with a very short era of
extremely high catches (1963 to 1967, Fig. 4), a maximal catch in 1964, high catches from 1965 to 1967, then
Mizroch & Rice: Whaling and killer whale prey switching
237
north of 50° N until 1948. During the period 1952 to 1962, catches expanded to
the Aleutians, Bering Sea, and to a small
degree, the Gulf of Alaska (Fig. 3). Note
20000
that catches near the Japanese mainland
Total catch in the North Pacific
Total catch North of 50°N
remained high during this period as fleet
Total catch between 30° and 50°N
capacity expanded.
15000
By far the largest catches in the
northern North Pacific and Bering Sea
10000
(50° N and above) occurred during the
period 1963 to 1967 (Fig. 4). Catches
were high throughout the northern
5000
North Pacific, off Kamchatka, the Aleutians, the Bering Sea and the Gulf of
0
Alaska, as well as near the Japanese
mainland. Land stations catches at Vancouver Island and off California were
Year
also at or near maximum during this
Fig. 1. Post-World War II industrial whaling catch levels throughout the North
period.
Pacific, including total catch in the North Pacific, catches between 30 and 50° N
From 1968 to 1972, the whaling fleets
and catches from 50° N and above. Note that by 1968 most of the catch was
taken between 30 and 50° N
moved further south and took large
catches of sperm and sei whales in the
rich grounds along the Subarctic
Boundary (ca. 42° N) (Fig. 5). By 1973,
50°N
there was almost no whale catch north
of 50° N (Fig. 6).
Fig. 7 shows whale catch for each
species by year. Note the peak in sperm
whale Physeter macrocephalus catches
in 1969, and the increase in sei whale
catches after 1966. Figs. 8 & 9 show the
distribution of sperm whale catches
30°N
from 1946 to 1967 and from 1968 to
1987. These show the high density of
sperm whale catches in the traditional
whaling areas through 1967, and the
Removals in biomass, metric tons
shift of sperm whale catches much farWhale catches, 1946–1951
1–1616
ther south, starting in 1968. There were
1617–5855
10°N
5856–12505
no sperm whale catches in most of the
12506–32039
areas north of 50° N after 1967. It is
32040–55336
55337–92501
likely that sperm whale catches south
of the area of interest caused the sec140°E
160°E
180°E
160°W
140°W
BIWS/IWC Catch Data: February 2003 files
ond big spike in Springer et al.’s (2003)
Fig. 2. Catches from 1946 to 1951, represented as biomass. These catches were
Fig. 2, which would give an erroneous
taken in the first phase of whaling after World War II
timeline of whale removals in the
northern North Pacific.
a near total drop-off north of 50° N (Fig. 5). The IWC
Fig. 10 shows all years of sei whale catches; note that
prohibited the killing of humpback whales in 1963 and
the primary concentration zone of sei whales has
fin whales in 1967. By 1968, most of the catch in the
always been much farther south, in the Subarctic
North Pacific was taken between 30 and 50° N (Fig. 1),
Boundary area (ca. 42° N). So, the spike in catch levels
south of the area where the pinniped declines were
of sei whales in 1967, as shown in Fig. 7, was prelater discovered.
dominantly caused by catches from the Subarctic
Catches taken between 1946 and 1951 were in waters
Boundary.
near the Japanese mainland and to a small degree, off the
Biomass removal totals for each species were comKamchatka peninsula (Fig. 2). Whaling did not move
puted using a length/weight relationship that should
North Pacific catch data (BIWS/IWC): all species combined
192
4
192
6
192
8
193
0
193
6
193
8
194
6
194
8
195
0
195
2
195
4
195
6
195
8
196
0
196
2
196
4
196
6
196
8
197
0
197
2
197
4
197
6
197
8
198
0
198
2
198
4
198
6
198
8
200
1
Catch in numbers of whales
25000
238
50°N
Mar Ecol Prog Ser 310: 235–246, 2006
(Fig. 11). This figure confirms that the
last big year of whale biomass removals
north of 50° N was 1967, and subsequent removals were negligible.
Killer whale prey choice based on
analysis of stomach contents
30°N
Data on stomach contents are
available for 442 killer whales taken in
the North Pacific prior to 1968; 400 of
the 442 contained food items, and 110 of
Removals in biomass, metric tons
these contained marine mammals, indiWhale catches, 1952–1962
cating that they were of the mammal3–4567
4568–15207
10°N
eating ecotype (Table 1).
15208–31686
31687–51494
The first biologist to examine any
51495–109492
109493–440100
stomachs of North Pacific killer whales
was Zenkovich (1938) who examined 2
140°E
160°E
180°E
160°W
140°W
BIWS/IWC Catch Data: February 2003 files
animals taken by whalers between
Fig. 3. Catches from 1952 to 1962, represented as biomass. This was a period of
Olyutorskiy and Anadyrskiy Gulfs in
increasing catches in the post-World War II industrial whaling era as the indus1936. Both of the animals he sampled
try expanded in equipment and range
were mammal-eaters (mostly walrus),
yet neither had large whale remains in
their stomachs.
50°N
By far the largest sample was the 409
killer whales taken by whalers in the
coastal waters around Japan between
1948 and 1957 (Nishiwaki & Handa
1958). Of the 380 stomachs that had
food items remaining in their stomachs,
102 (26.8%) contained marine mammal
parts. Of those 102, only 2 contained
30°N
large whale remains (sei whales).
Ivanova (1961) examined the stomachs of 21 killer whales, also taken by
whalers throughout the Kuril Islands
Removals in biomass, metric tons
from 1951 to 1955. Based on stomach
Whale catches,1963–1967
9–1030
contents, it seemed that all of the whales
1031–2573
10°N
2574–4914
Ivanova sampled were fish-eaters.
4915–8360
8361–14011
Rice (1968) examined the stomach
14012–31785
contents of 10 killer whales that were
140°E
160°E
180°E
160°W
140°W
harpooned opportunistically during a
BIWS/IWC Catch Data: February 2003 files
number of cruises along the west coast,
Fig. 4. Catches from 1963 to 1967, represented as biomass. This was a short era
from Kodiak Island, Alaska south to San
of extremely high catches. Note that there were large catches over the entire
Miguel Island, California, from 1960 to
northern North Pacific, including off Kamchatka, in the Bering Sea, the Aleutian
1967. Of the 8 stomachs that contained
Islands and the Gulf of Alaska. Whaling station catches off Vancouver Island
and off California were also very high during this period
food items, 6 (75%) contained marine
mammal parts; of those, 1 contained
be the best measure when looking for temporal and
large whale remains (minke whale Balaenoptera
geographic effects of large-scale removals from an
acutorostrata).
ecosystem. For catches 50° N and above, biomass
In summary, during the years of the development
removals were calculated for each species by year
and pulse of whaling (i.e. prior to 1968), less than 3%
(Fig. 11), and cumulative biomass was presented for
of the mammal-eating killer whales were found to
the entire range of years in the catch database
have large whale remains in their stomachs (Table 1).
Mizroch & Rice: Whaling and killer whale prey switching
239
them (our observations, and see also
Jefferson et al. 1991). It is only the latter 2 responses that are important in
our context. Baleen whales likewise
show a spectrum of responses to the
proximity of killer whales; usually they
ignore them or move away slowly, but
Andrews (1914) reported that if gray
whales Eschrichtius robustus ‘are not
30°N
paralyzed by fright they head for shore
and slide in as close as possible to the
beach.’ Jefferson et al. (1991) extensively reviewed the interactions beRemovals in biomass, metric tons
tween killer whales and other species
Whale catches, 1986–1972
3–974
of marine mammals and noted ‘… at
975–2666
10°N
least occasionally, healthy non-calf
2667–7066
7067–16237
baleen whales are fed upon’.
16238–35261
35262-69538
In an extensive (but not exhaustive)
review of the literature going back as
140°E
160°E
180°E
160°W
140°W
BIWS/IWC Catch Data: February 2003 files far as Bennett (1840) and extending to
the year 1968, we could find only 11
Fig. 5. Whale catches from 1968 to 1972, represented as biomass. Whalers
accounts of killer whales attacking
moved south of the traditional whaling grounds and switched to the rich
grounds along the Subarctic Boundary area (ca. 42° N)
large or mid-sized whales in the
North Pacific. These involved 2 fin
whales, 1 bowhead whale Balaena
50°N
mysticetus, 5 gray whales, 2 minke
whales and 1 unidentified whale; 5 of
these attacks resulted in the death of
the whale, 3 failed, and in the other 3
instances the outcome could not be
determined. These attacks are enumerated below.
Fin whale. Turner (1886) ‘witnessed
30°N
2 of these creatures [killer whales]
attacking a very large Finback Whale
in the vicinity of Tigalda Island in the
eastern Aleutians, sometime during the
Removals in biomass, metric tons
years 1874 and 1881. The latter was
Whale catches, 1973–1987
1–556
nearly exhausted by the persistent and
557–1472
10°N
1473–3281
impetuous lunges made upon it by its
3282–8943
8944–19387
enemies’. The outcome of this en19388–46479
counter is not known.
As related by Pike & MacAskie (1969),
140°E
160°E
180°E
160°W
140°W
BIWS/IWC Catch Data: February 2003 files
on 14 June 1960, near Marble Island, on
the west side of the Queen Charlotte IsFig. 6. Whale catches from 1973 to 1987, represented as biomass. Catches from
1973 and afterwards were nearly all south of the traditional whaling grounds
lands, the crew of a Canadian Fisheries
and effort was concentrated north and south of 30° N
Vessel logged the sighting of ‘killer
whales attacking fin whale’.
Observations of killer whales attacking large whales
Bowhead whale. According to Bailey & Hendee
(1926), Charles Brower of Barrow reported that killer
Encounters between killer whales and large baleen
whales were said to have once attacked and killed a
whales are not necessarily agonistic. Killer whales
bowhead whale off Point Barrow, Alaska.
show a whole spectrum of responses to the nearby
Gray whale. Scammon (1874) saw 3 killer whales
presence of baleen whales: ignoring them, following
attack and kill a gray whale calf accompanied by its
them, chasing them, actually biting them, and killing
mother in a lagoon in Baja California.
50°N
Mar Ecol Prog Ser 310: 235–246, 2006
240
North Pacific catch data (BIWS/IWC) by predominant species
12000
Blue whale
Fin whale
Humpback whale
Sei whale
Sperm whale
8000
4000
0
192
4
192
6
192
8
193
0
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6
193
8
194
6
194
8
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0
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6
195
8
196
0
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196
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6
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197
0
197
2
197
4
197
6
197
8
198
0
198
2
198
4
198
6
198
8
200
1
Catch in numbers of whales
16000
Year
Fig. 7. Whale catch for each species by year, entire North Pacific combined.
Note the peak in sperm whale Physeter macrocephalus catches in 1969, and the
increase in sei whale catches after 1966
Fig. 8. Sperm whale catches from 1946 to 1967, in numbers of whales caught.
These catches were concentrated off the Japanese mainland and off Kamchatka, the Bering Sea and the Aleutian Islands
Scheffer (in Rice & Wolman 1971) saw 6 killer whales
unsuccessfully attack a gray whale in Monterey Bay,
California on 9 March 1952.
Gilmore (1961) reported an unsuccessful attack by
6 killer whales on 2 gray whales at La Jolla, California.
Morejohn (1968) observed an unsuccessful attack by
7 killer whales on 3 gray whales, including a female
with a calf, at Moss Landing, California.
Pike & MacAskie (1969) reported
killer whales attacking a pair of gray
whales off the Queen Charlotte Islands,
British Columbia.
Minke whale. There are at least 2
accounts of North Pacific killer whales
attacking minke whales prior to 1968.
Both attacks resulted in the death of the
minke whale.
Hancock (1965) watched a pod of 7
killer whales attack and kill a minke
whale on 26 May 1964 in Barkley Sound
on the west coast of Vancouver Island,
British Columbia. The pod consisted of
3 adult males, 2 adult females, and 2
calves. According to Hancock, ‘very little happened above the surface to suggest what was going on below.’ The
next day, a minke whale carcass was
found floating about 4.5 miles northeast
of the site of the kill, and was towed to
shore.
In the same area, on 20 July 1964,
Hancock (1965) found the carcass of
another minke whale that almost certainly had been killed by killer whales,
because some of its skin and blubber
had been stripped from its torso in the
same way as on the whale found after
the 26 May attack.
Unidentified whale. Tomilin (1967)
gave the following second-hand
account of an attack on an unidentified ‘whale’, perhaps a large baleen
whale, near the Commander Islands
in 1912: ‘Two dead dolphins were
thrown overboard from the warship
‘Manchzhur.’ The carcasses were soon
approached by a pack of Killers. A
whale happened to appear nearby.
Having sensed the whale, the Killers
left the dolphin carcasses and attacked him. A few hours later, the
‘Manchzhur’ found this whale dead on
the surface of the sea.’
DISCUSSION
Development and collapse of post-war whaling
Springer et al.’s (2003) representation of the development and collapse of modern whaling in the North
Pacific is not very accurate, either temporally and
geographically. Based on the information in their
Mizroch & Rice: Whaling and killer whale prey switching
241
from 2 factory ships with 9 catcher boats
in 1953 to 7 factory ships with 67 catcher
boats in 1963 (Anonymous 1965).
Whaling spread throughout the Kamchatka, Bering Sea/Aleutian Islands and
Gulf of Alaska whaling grounds because
whale abundance was seasonally high
in broad areas throughout the North
Pacific, and whalers were able to maximize their catch by moving where the
whale densities were highest.
Based on our Figs. 2 to 5, the bulk of
the whaling that would have affected
killer whales was conducted throughout the traditional whaling grounds off
Kamchatka, in the Bering Sea and
around the Aleutian Islands, and in the
Gulf of Alaska, all of which are north of
50° N. Although mammal-eating killer
whales have been documented to
Fig. 9. Sperm whale catches from 1968 to 1987, in numbers of whales caught. By
travel long distances (Goley & Straley
1968, sperm whale catches were taken much farther south
1994), if killer whales were highly dependent on great whales as prey, we
would assume that large whale declines throughout the entire northern
area would have an impact on killer
whales starting in 1968 (i.e. if the theory of Springer et al. [2003] is correct).
We agree with Springer et al. (2003)
that the BIWS/IWC catch dataset under-represents true biomass removals
because it does not include recently revealed data on illegal whaling. This illegal whaling was conducted from 1961
to 1972, simultaneously with and on the
same grounds as the legal whaling, so
the temporal and geographic trends in
the whaling data should be accurate,
with only some of the biomass totals
assumed to be too low. As reported
in Doroshenko (2000a), the Soviets began illegal whaling for humpback
whales in the North Pacific in 1962, and
Fig. 10. Distribution of sei whale catches throughout the North Pacific, all years
misreported catches on a large scale
combined. Note that the primary zone of concentration of sei whales has always
through 1967. The total catch of humpbeen much farther south, in the Subarctic Boundary area (ca. 42° N)
back whales (including unreported
catches) peaked in 1963 and was at
Fig. 1A,B, Springer et al. (2003) stated that whalers
very low levels by 1966. Illegal whaling and unreported
moved eastward as whale stocks close to Japan and the
catches of blue, gray, bowhead and right whales (EuUSSR were depleted. Based on our revised figures
balaena glacialis; for taxonomic status, see Baker et al.
(Figs. 2 to 5), catches close to homeports remained high
2003) began in 1963, peaked in 1964, and were at much
(i.e. not depleted) as whaling expanded to other areas
lower levels by 1966 (Doroshenko 2000b).
throughout the North Pacific. The geographic spread
As noted above, it is likely that sperm whaling south
was due to expansion of the whaling industry, not to
of 50° N caused the second peak in Springer et al.’s
localized depletions. Pelagic whaling effort increased
Fig. 2, which leaves a large time gap between the ces-
242
350000
3500000
Catch in metric tons
300000
3000000
Blue whale
Fin whale
Humpback whale
Sei whale
Sperm whale
Annual removals
Cumulative removals
250000
2500000
200000
2000000
150000
1500000
100000
1000000
50000
500000
Fig. 11. Biomass removals and
cumulative biomass removed for
catches 50° N and above for the
entire range of years in the
BIWS/IWC catch database. Note
that that the last big year of
whale biomass removals from
50° N and above was 1967, and
subsequent removals from 1968
onwards were negligible
0
192
5
192
6
193
1930
5
193
6
193
7
193
8
193
1949
8
194
9
195
1950
1
195
2
195
3
195
1954
5
195
6
195
7
195
1958
9
196
0
196
1
196
1962
1963
4
196
5
196
1966
7
196
8
196
9
197
1970
1
197
2
197
3
197
4
197
5
0
Cumulative biomass removals in metric tons
Mar Ecol Prog Ser 310: 235–246, 2006
Year
sation of whaling in the Bering Sea/Aleutian Islands
and Gulf of Alaska regions and the declines represented in their Fig. 2.
The catch maps in Springer et al. (2003) mask the
nature of the slow development and precipitous
decline of whaling in the North Pacific. The extraordinary biomass removals in the early 1960s, even without including the newly discovered records of illegal
whaling, would lead one to assume that any whalingrelated prey shifting should have begun by 1968, not
by the mid-1970s as their Fig. 2 indicates.
The last big year of whaling in the northern areas
was 1967. Whalers left the traditional whaling grounds
and switched to the rich grounds along the Subarctic
Boundary after 1972, and whaling in the northern
whaling grounds really ended well before 1972.
If killer whales had been dependent on great whale
stocks as a major source of prey, the effects should
have been apparent from 1964, the year of the highest
biomass removals, and by 1968, killer whales would
have been exerting great pressure on nearby pinniped
populations.
Large whale biomass removals
Springer et al.’s (2003) Fig. 2 is described as representing ‘Great Whale Biomass’, but the caption states
that it represents ‘whale landings in biomass within
370 km of the Aleutian archipelago and coast of the
western Gulf of Alaska’. At first glance, without reading the figure caption, it appears that whale biomass
started at low levels, spiked twice, and plummeted.
The figure is on the same plot as curves meant to represent sequential declines in abundance of various
pinniped species. What is worrisome is that our analyses of catch curves and biomass removals, using the
same dataset, cannot replicate the curve presented in
Springer et al.’s (2003) Fig. 2. Also, it is not clear what
method these authors used to calculate the ‘biomass’
presented in Fig. 2.
Springer et al. (2003) stated that ‘combined current
biomass is estimated to be only ~14% of pre-exploitation levels’, based on ‘Pfister (in press)’. However, the
Pfister (2004) estimate as published was 18% (not
14%), and Pfister noted that the estimate was very sen-
Table 1. Summary of finds of whale remains in killer whale stomachs from the North Pacific
Locality collected
Years
collected
Olyutorskiy and Anadyrskiy gulfsa
1936
Coastal waters around Japanb
1948–1957
Kuril Islandsc
1951–1955
Kodiak Island, AK,
1960–1967
to San Miguel Island, CAd
Total during whaling years
Prior to 1968
Number of
stomachs
examined
Number
with
food
items
Number
with
mammal
remains
Number
with
whale
remains
Percent of
mammal
eaters with
whale
remains
2
409
21
10
2
380
10
8
2
102
0
6
0
2
0
1
0
1.96
0
16.7
442
400
110
3
2.72
Sources: a Zenkovich (1938), b Nishi-waki & Handa (1958), c Ivanova (1961), d Rice (1968)
Species and
number
of whales
Sei Minke
2
1
Mizroch & Rice: Whaling and killer whale prey switching
sitive to the inclusion of sperm whales. If sperm whale
biomass were excluded because the peak of the sperm
whale catch occurred south and west of 50° N (Figs. 1
& 7; see also DeMaster et al. in press), the estimated
current biomass of large whales is estimated to be 46%
of pre-exploitation biomass.
Killer whale prey choices
Based on analysis of stomach contents, the only evidence that North Pacific killer whales preyed upon large
or mid-sized whales during the industrial whaling era
were the remains of 2 sei whales in the stomachs of 2
killer whales taken in Japanese waters (Nishiwaki &
Handa 1958) and the minke whale remains in a killer
whale taken on 22 March 1965 off Point Conception,
California in the eastern North Pacific (Rice 1968).
The earliest serious student of marine mammals in
the North Pacific was Charles M. Scammon; during his
years as captain of whaling vessels (1852 to 1874), he
reported seeing killer whales attacking baleen whales
only once (Scammon 1874). He concluded that ‘the
Orca, however, does not always live on such gigantic
food, and we incline to the belief that it is but rarely that
these carnivora of the sea attack the larger Cetaceans,
but chiefly prey with great rapacity upon their young’.
In the North Atlantic, the Norwegian cetologist Åge
Jonsgård, in discussing the prey species of killer
whales, asserted that ‘with regard to cetaceans it is
obviously true that it is mainly small-sized porpoises
that have been the prey.’ He reported, ‘during the
many years of Norwegian whaling in the Norwegian
Sea and adjacent Arctic waters for minke whales and
other whalebone whales, we have had no reports from
the whalers about these animals being attacked by
killer whales’ (Jonsgård 1968b). He concluded that
‘there is no proof that baleen whales and other large
cetaceans in good health and under normal conditions
are attacked by killer whales’ (Jonsgård 1968a).
Recent observations of killer whales attacking
large whales
Recent observations confirm that minke whales and
gray whales (especially gray whale calves) may be at
least an occasional target for North Pacific killer whales.
Minke whale
Minke whales have been seen being attacked and
killed by killer whales on a number of occasions in the
North Pacific after 1968:
243
In the Gulf of Alaska just south of Yakutat Bay, at
58° 22’ N, 138° 21’ W, on 29 April 1976, Marsha Caunt
Schad of the Marine Mammal Division, National
Marine Fisheries Service, saw a small pod of killer
whales attacking and killing a minke whale (Fiscus et
al. 1976). This is apparently the same incident that was
said to have been observed ‘off Yakutat’ in ‘late winter’
1977, as related by Hall (1986).
Off the west side of Amaknak Island (53° 54’ N,
166° 53’ W) in the eastern Aleutian Islands, on 5 August
1975, Robert R. Nelson saw a pod of 7 killer whales
chasing a minke whale (Lowry et al. 1987) which was
later found dead. In Prince William Sound, Alaska, on
an unspecified day in 1982, Mehlberg (1986) observed
and photographed a pod of killer whales as they killed
a minke whale.
In Johnstone Strait, British Columbia, Dorsey et al.
(1990) photographed a minke whale bearing a series of
parallel scars on its left flank just anterior to the dorsal
fin. The scars had almost certainly been inflicted by the
teeth of a killer whale.
In Icy Strait in southeastern Alaska in 1996, a pod of
13 killer whales was seen to kill a minke whale
(Matkin et al. 1999).
In Ganges Harbour on Saltspring Island, British
Columbia, on 15 October 2002, Lance Barrett-Lennard
and about 200 spectators witnessed the killing of a
minke whale (Anonymous 2002).
In Alaska and British Columbia, 8 attacks of killer
whales on minke whales, 5 of which were successful,
were reported by Ford et al. (1998) and Matkin & Saulitis
(1994), but they provided no further particulars.
There has never been a commercial fishery for
minke whales in the North Pacific (except locally
around Japan), so there is no reason to believe that
their population has changed significantly during the
latter half of the 20th century, nor that their contribution to the diet of killer whales has changed. The
importance of minke whales as prey of killer whales
differs greatly in other parts of the world. In the
Atlantic sector of the Southern Ocean, Budylenko
(1981) documented that in the cold Antarctic waters
the closely related Antarctic minke whales Balaenoptera bonaerensis are the dominant prey of killer
whales. However, in Norwegian waters, both killer
whales and minke whales are commonly seen. In
his monograph on the mammals of Norway, Collett
(1911–12), who had a special interest in cetaceans,
did not include the minke whale among the prey of
the killer whale; the only mammals he listed as prey
were harbor seal, gray seal, and harbor porpoise.
Jonsgård (1968b) reported that whalers operating in
the Norwegian Sea and adjacent Arctic Ocean never
reported minke whales being attacked by killer
whales.
244
Mar Ecol Prog Ser 310: 235–246, 2006
Gray whale
Rice & Wolman (1971) found healed parallel scars
that were obviously the tooth marks of killer whales on
57 (18%) of the 316 gray whales that they examined at
the whaling stations on Point San Pablo, California
from 1959 to 1969. There are also a number of observations of killer whales successfully attacking adult
gray whales, but most attacks on this species are
directed at young calves. Most notably, at certain
places and times of the year, killer whales regularly
prey on gray whale calves.
Each year the gray whales with their young calves
migrate north along the coast from Baja California to
the Bering Sea, passing through the areas inhabited by
known mammal-eating killer whales. Around Monterey Bay, California, killer whales congregate every
year in the late spring when female gray whales with
their young calves are migrating north close to the
shore. Nancy Black and Richard Ternullo have studied
these killer whales for 15 yr (Black 2001, 2003, Ternullo
& Black 2002). During this period they have observed
84 predation events by the killer whales, of which 25
(30%) involved the killing of gray whale calves and
29 (35%) involved the killing of California sea lions
Zalophus californianus. The remaining prey included
lesser numbers of porpoises, dolphins, elephant seals,
and seabirds.
In the area around False Pass in the Aleutian Islands,
ongoing studies indicate that killer whales regularly
attack gray whales — mainly calves — during their
spring migration through this area (C Matkin & L.
Barrett-Lennard, North Gulf Oceanic Society and Vancouver Aquarium, unpubl. data). In the coastal waters
of the Chukotski Peninsula, during the ice-free seasons
of the years 1990 to 2000, Inuit hunters reported all of
their observations of killer whale predation on marine
mammals (Melnikov & Zagrebin 2005). Of 92 attacks on
marine mammals, 61 (66%) were on gray whales; of
these 23 resulted in successful kills, 6 were unsuccessful, and the outcome was unknown in the other 32. The
remaining 31 attacks were made on walruses, Steller
sea lions, belugas, bowhead whales (2), and fish (1). In
the same area, from 1993 to 2000, 69 stranded carcasses
of gray whales were found; 31 of these appeared to
have been killed by killer whales. Ten of 14 were
less than 8 m long, and judged to be less than 1 yr old;
2 others were less than 9 m, and were judged to be
less than 2 yr old; the largest was a 12 m long adult.
Gray whales were not a target of post-war industrial
whaling. At the end of the industrial whaling period,
the gray whale population was at a fairly substantial
level and continued to increase steadily. The estimated
number of gray whale adults ranged between 10 000
and 12 000 in the late 1960s (Reilly et al. 1983) and gray
whales have continued to increase to levels of 18 000 to
25 000 in recent years (Rugh et al. 2005).
Baird (2002) discussed prey choice in mammaleating killer whales, taking into account prey profitability and risk of injury, and noted that predation on
large baleen whales should be rare, on theoretical
grounds. There are no data to support the idea that the
large balaenopterids (blue, fin, and humpback whales)
or sperm whales were ever preyed upon by North
Pacific killer whales, except under rare and exceptional circumstances.
Springer et al. (2003) cited Budylenko (1981), using
data from the southern hemisphere, as corroboration
that killer whale attacks on large whales are fairly
common, stated that, based on Budylenko’s studies,
> 60% of sperm whales were found to have killer
whale scars. However, upon closer reading, Budylenko
(1981) indicates that although Antarctic minke whales
were the dominant prey species, no other great whale
remains were found in any of the killer whale stomachs
sampled. Furthermore, Budylenko (1981) states ‘as is
evident from direct observations, killer whales can
attack large whales … however many of these larger
whales manage to escape.’ Based on stomach content
data collected by Yukhov et al. (1976) in the southern
hemisphere, killer whales sampled in the Indian
Ocean sector of the southern hemisphere (53–55° S)
appear to feed exclusively on Antarctic minke whales
and he concluded that minke whales were the ‘main
item’ in the diet of minke whales near the Antarctic ice
edge. Shevchenko (1976) noted that in cold waters
south of 50° S, minke whale remains were found in
over 84% of killer whale stomachs and stated ‘not one
single stomach was found to contain the remains of
sperm whales or large baleen whales’.
Many researchers have noted that adult sperm
whales are almost never attacked by killer whales
(Beale 1839, Berzin 1972, Rice 1989) — notwithstanding one exceptional lethal attack witnessed off California (Pitman et al. 2001). Their large size and their ability to make deep and prolonged dives would make
them a difficult prey item. Killer whales have been
seen attempting to capture newborn calves in breeding schools of southern hemisphere sperm whales
(Berzin 1972, Yukhov et al. 1976, Best et al. 1984), and
1 dead sperm whale calf was found with wounds that
appeared to have been inflicted by killer whales (Best
et al. 1984). Sperm whale remains have been recovered from the stomachs of killer whales in the subtropical southern hemisphere (Yukhov et al. 1976).
Shevchenko (1976) and Budylenko (1981) claim to
have found scars inflicted by the teeth of killer whales
on the bodies of sperm whales, but it may be difficult to
distinguish between such scars and those which are
commonly inflicted by intraspecific aggression. Rice
Mizroch & Rice: Whaling and killer whale prey switching
found scars attributable to killer whales on only 1 of
637 sperm whales landed at the whaling stations on
Point San Pablo, California, between 1959 and 1970.
CONCLUSIONS
The catch maps in Springer et al. (2003) masked the
nature of the slow development and precipitous
decline of whaling in the North Pacific. The extraordinary biomass removals in the early 1960s (even without the newly exposed surreptitious illegal whaling
operations), would lead one to conclude that any whaling-related prey shifting should have begun by 1968,
not by the mid-1970s as their Fig. 2 indicates. The last
big year of whaling in the northern areas was 1967.
After that, the whalers left their traditional whaling
grounds and moved south to the rich grounds along
the Subarctic Boundary (ca. 42° N). If killer whales had
been dependent on great whales (the large rorquals
and the sperm whale) stocks as a major source of prey,
the effects should have been apparent starting from
1964, the year of the highest biomass removals, and by
1968, killer whales would have been exerting great
pressure on nearby pinniped populations. DeMaster et
al. (in press) reanalyzed the pinniped data in Springer
et al.’s (2003) Fig. 2 and found concurrent, not sequential, declines in abundance for fur seals and harbor
seals, followed by declines in Steller sea lions, starting
after the late 1970s.
Our review of the historical record confirmed that
great whales have never been more than a rare item in
the diets of killer whales, either before or after the
depletion of the great whale populations in the North
Pacific during the whaling era. Killer whales rarely
attacked healthy, adult individual great whales, and
such attacks were usually unsuccessful.
In the North Pacific, attacks on minke whales were
occasionally witnessed and such attacks usually
resulted in a kill. Minke whales were never a target of
industrial whaling in the northern North Pacific or
Bering Sea, and would have been available as prey for
killer whales before, during, and after the industrial
whaling period.
Adult gray whales are occasionally attacked by killer
whales, but the frequency of tooth scars indicate that
many targeted animals succeed in escaping the killer
whales. On the other hand, recent studies have documented that at certain times and places killer whales
regularly prey upon gray whale calves during their
northward migration. The gray whale population was
growing steadily through the 20th century, so gray
whales would have become increasingly available as
prey for killer whales even as the other great whale
stocks were being depleted.
245
If killer whales had been dependent on great whale
stocks as a major source of prey, the effects should
have been apparent starting from 1964, the year of the
highest whaling biomass removals. By 1968, killer
whales would have been exerting great pressure on
nearby pinniped populations. There is no evidence of
declines during this period in any of the trend data for
the pinniped populations in the region.
Acknowledgements. M. Cameron, National Marine Mammal
Laboratory (NMML), provided timely technical assistance with
ArcMap support, and A. Zerbini, NMML, provided insightful
scientific suggestions as the paper was in development.
R. Angliss, R. Baird, N. Friday and R. Reeves, K. Stafford, G.
Duker, J. Lee and an anonymous reviewer provided thoughtful
review comments that greatly improved the paper. C. Matkin
and L. Barrett-Lennard provided access to unpublished data.
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Editorial responsibility: Howard I. Browman (Associate
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Submitted: May 6, 2005; Accepted: September 3, 2005
Proofs received from author(s): February 18, 2006