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Aquatic Mammals 2020, 46(6), 556-560, DOI 10.1578/AM.46.6.2020.556
Short Note
Intentional Stranding by Mammal-Hunting Killer Whales
(Orcinus orca) in the Salish Sea
Josh D. McInnes,1 Justine N. Buckmaster,2 Kelsey D. Cullen,1
Chelsea R. Mathieson,1 and Josh P. Tawse1
1Transient Killer Whale Research Project, 201-1002 Pakington Street, Victoria, BC, V8V 3A1
E-mail: jmcinnes@uvic.ca
214500 Admiralty Way, Apt. Q204, Lynnwood, WA 98087, USA
Killer whales (Orcinus orca) are cooperative
apex predators that have been documented forag-
ing on a wide array of prey, ranging from small
schooling fish to large cetaceans (Jefferson et al.,
1991; Baird, 2000; Ford, 2009). Foraging strate-
gies of killer whales that hunt marine mammals
are complex and vary globally (Hoelzel, 1991).
A high-risk and specialized form of killer whale
foraging behaviour is known as intentional strand-
ing. During this foraging behaviour, members of
a group of killer whales deliberately direct them-
selves towards pinniped prey, accelerate towards
the shore, and become temporarily stranded on
their ventral surface in the surf zone (López &
López, 1985; Guinet, 1991; Silber & Fertl, 1995).
In Patagonia, along the shores of the Peninsula
Valdéz, a small population of killer whales exhibit
intentional stranding by using channels between
reefs and steeply sloping beaches to partially beach
themselves to capture southern sea lions (Otaria
flavescens) and southern elephant seals (Mirounga
leonina) (López & López, 1985; Hoelzel, 1991; Vila
et al., 2008). Intentional stranding has also been
documented by killer whales on Possession Island
in the Crozet Archipelago in the sub-Antarctic
Indian Ocean. Unlike the steep beaches of Peninsula
Valdéz, the two prominent beaches on Possession
Island where killer whales use intentional strand-
ing have a low grade slope. Southern elephant seals
are their primary prey in this reg
ion (Guinet, 1991;
Guinet & Bouvier, 1995).
In the coastal waters off British Columbia
(Canada) and Washington (USA), Bigg’s (tran-
sient) killer whales are well-known predators
of marine mammals, particularly harbour seals
(Phoca vitulina) (Baird, 1994; Baird & Dill, 1995;
Ford & Ellis, 1999). While foraging for harbour
seals, Bigg’s killer whales frequently follow
the contours of the shoreline and enter shallow
regions where harbour seals are congregated on
or near exposed reefs, sand bars, and beaches
(Baird & Dill, 1995). The occurrence of foraging
for harbour seals tends to increase during the har-
bour seal pupping period from late July through
September as seal pups are easily captured while
taking their first swims (Bigg, 1969; Baird,
1994; Baird & Dill, 1995). On Protection Island,
Washington, a National Wildlife refuge for marine
mammals and birds (Figure 1), there is a signifi-
cant harbour seal haulout site where approxi-
mately 500 harbour seals have been documented
annually using the sand and cobble beaches for
resting, molting, and pupping (Hayward et al.,
2005; Cowles & Hayward, 2008). Due to this
abundance of seals, sightings of Bigg’s killer
whales in the vicinity of Protection Island occur
throughout the year (J. McInnes, unpub. data,
2011-2020). Prior to 1994, there was an anecdotal
report of killer whales intentionally stranding on
one of this island’s beaches while pursuing har-
bour seals (Baird & Dill, 1995). The only other
documented live-stranding events by Bigg’s
killer whales in the Salish Sea have been reported
as accidental events that most likely occurred
while the whales were foraging in shallow water
(Towers et al., 2020). Herein, we report the first
photo-documented record of Bigg’s killer whales
intentionally stranding themselves in pursuit of
harbour seals in the Salish Sea at a haulout site on
Protection Island.
On 14 August 2016 at 1545 h, two groups of
Bigg’s killer whales were encountered feed-
ing on unidentified prey in the Strait of Juan de
Fuca (approximately 48° 10' N, 122° 44' W).
The two killer whale groups were identified by
photographs as the T037A and T065A matrilines
(known as the T037As and T065As, respectively)
(Transient Killer Whale Research Project, unpub.
catalog, 2011-2020; Towers et al., 2019). When
the feeding event concluded, the T037A group
557Intentional Stranding by Mammal-Hunting Killer Whales
Figure 1. Bathymetry surrounding Protection Island, located in the southeastern portion of the Strait of Juan de Fuca, near the
entrance to Admiralty Inlet. The star identifies the location where the intentional stranding event took place. Contours are in
5 m intervals. Bathymetric data were acquired from the National Oceanic and Atmospheric Administration (NOAA) (NOAA
Office of Ocean Exploration and Research, 2014). Map was created using QGIS, Version 3.12.0 (‘Burcuresti’).
departed, while the T065As remained in the area
until 1630 h. The T065As are frequently observed
in the waters off Southern Vancouver Island,
British Columbia, and Puget Sound, Washington
(J. McInnes, unpub. data, 2011-2020). At the time
of the event, the T065A matriline was comprised
of five members (Figure 2; Towers et al., 2019).
The T065A group then travelled southeast
towards the western tip of Protection Island, reach-
ing approximately 48° 07' N, 122° 53' W at 1652 h.
The whales foraged nearshore at the south side of
the island, and a successful harbour seal predation
event was documented at 1742 h (Figure 3A).
The first documented intentional stranding
event occurred at 1756 h along Kanem Point, the
westernmost tip of the island, where the majority
of the harbour seals were hauled out (Figure 1).
T065A and T065A2 rushed the shoreline parallel
to each other and halted just prior to the edge of
the beach. Their dorsal fins oscillated as the full
weight of their bodies was not supported in the
water column, demonstrating that they had run
aground in the shallows leading up to the beach.
There was no immediate reaction from the harbour
seals to this first attempt at intentional stranding.
T065A then regrouped with T065A3, T065A4,
and T065A5 approximately 30 m offshore
(Figure 3B). T065A2 remained close to shore
and rushed the beach for a second time at 1757 h.
During this approach, the harbour seals reacted
with some fleeing up the beach and others enter-
ing the water. T065A2 arched his body and swung
his flukes to maneuver himself off the beach into
deeper water where he regrouped with the other
whales. At 1758 h, T065A and T065A4 moved
in to patrol the beach by closely following the
contour of the shoreline, passing within 3 to 5 m
of the harbour seals. Once T065A and T065A4
completed their pass of the beach and moved out
into the deeper water of the bay, T065A2 and
T065A3 performed the same patrol of the beach.
The third intentional stranding event took place
at 1805 h. T065A2 directed himself towards the
beach and accelerated towards the harbour seals.
Due to the shallow slope of the approaching
beach, his movement was impeded just metres
from shore. He became stranded on his ventral
surface at a point where it was not possible for
him to submerge more than three quarters of his
body, exposing his dorsal fin and the midline of
558 McInnes et al.
Figure 2. Genealogical schematic of the Bigg’s killer whale (Orcinus orca) T065A matrilineal group. Photographs show
identities and sex (if known) for each individual as of 2016. Representation of matrilineal descent is based on studies and
methods described in Ford & Ellis (1999), and identification images are by the Transient Killer Whale Research Project.
his saddle patch (Figure 3C). A few harbour seals
stampeded into the water, while others moved
higher up the beach. T065A2 suddenly began
swinging his body back and forth while undulat-
ing his fluke repeatedly as he worked to get off
the beach and back into deeper water. Low tide
occurred at 1448 h; the tide was rising throughout
the observation period.
At 1812 h, T065A3 followed the remainder of
the group, then quickly moved from the oppo-
site direction into the same shallow water where
T065A2 had previously stranded. The group
appeared to be chasing a harbour seal pup that had
stampeded into the shallow water moments after
T065A2 intentionally stranded. The group pur-
sued the seal pup along the shoreline, pushing it
out past the end of Kanem Point into deeper water.
When the harbour seal ran out of shallow water
at 1816 h, T065A, T065A4, and T065A5 disap-
peared under the surface. While no direct kill was
observed, it was concluded that successful preda-
tion occurred because of the presence of a surface
“oil” slick and the increased activity of marine
birds descending upon the area. As all members
of the T065As began to feed on the seal pup,
individuals from the group were seen rising verti-
cally out of the water (“spy-hopping”) and slap-
ping their flukes on the surface. This was the only
successful predation event documented during
the 20 min of foraging displayed by the T065As
in which they utilized intentional stranding as a
foraging technique. It should be noted that we
did not observe the T065As directly capture a
harbour seal while utilizing intentional stranding,
but, rather, they seemed to use this technique to
frighten their prey into the water.
Bigg’s killer whales have developed a unique
set of culturally transmitted foraging behaviours
that appear to be shaped by their marine mammal
prey (Ford & Ellis, 1999; Dahlheim & White, 2010;
Barrett-Lennard et al., 2011). To our knowledge,
this is the first recorded event of Bigg’s killer whales
using intentional stranding as a foraging behaviour
in the Salish Sea. The infrequency of such observed
events is likely due to a number of factors, including
water depth, bathymetry, haulout substrate, and the
associated risk of mortality (Baird & Dill, 1995).
This behaviour may be further limited to killer
whales that have acquired the requisite knowledge
and skills through generations of extensive practice
and training (Guinet, 1991). Pinniped haulout sites
are often in shallow water and difficult to navigate.
While the majority of pinniped haulout sites in the
Salish Sea are rocky exposed reefs, the Protection
Island haulout site where our observation took
place was in shallow water on a moderately sloped
cobble beach, similar to other regions where killer
whales have been extensively documented using
this behaviour (López & López, 1985; Hoelzel,
1991; Vila et al., 2008). The soft substrate may be
especially beneficial for minimizing physical injury
during temporary stranding events.
559Intentional Stranding by Mammal-Hunting Killer Whales
Figure 3. Photographs depicting behaviours of Bigg’s killer whales during intentional stranding event at Kanem Point,
Protection Island, on 14 August 2016: (A) Bigg's killer whales T065A and T065A4 making an exploratory pass near hauled-
out harbour seals (Phoca vitulina); (B) T065A, T065A2, and T065A4 approaching harbour seals hauled out on shore
moments before T065A2 rushed the beach intentionally stranding; and (C) male Bigg's killer whale T065A2 intentionally
stranding in an attempt to capture harbour seals. He was documented here undulating his flukes in an attempt to free himself
from being grounded in the surf zone.
Intentional stranding at Protection Island may
have developed as an extension of nearshore and
haulout foraging. However, with events only
photo-documented on one day, more research is
needed and more observations required of this
foraging behaviour to assess whether intentional
stranding is a foraging technique used with fre-
quency in the Salish Sea.
Acknowledgments
The authors would like to acknowledge and thank the
following people and organizations for their assis-
tance in shaping this manuscript and for their ongo-
ing support of the Transient Killer Whale Research
Project (TKWRP). Thank you to Dr. Lawrence Dill
for being TKWRP’s scientific advisor, as well as
for providing suggestions on how to improve this
manuscript. Dr. Marilyn Dahlheim, Jared Towers,
and one anonymous reviewer reviewed our manu-
script and provided suggestions on ways to further
improve it. And thank you to Puget Sound Express
for providing a platform aboard their vessel that
enabled us to observe this behaviour.
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