Collisions between ships and whales are reportedregularly throughout the world’s oceans and for somespecies of endangered whales, ship strikes are amajor threat to their survival and recovery (Clapham
et al. 1999, Laist et al. 2001, Douglas et al. 2008,Berman-Kowalewski et al. 2010, Neilson et al. 2012),though some recent work has questioned the degreeof risk they pose at a population level (Monnahan etal. 2014). Often these collisions result in seriousinjury or death; however, it is highly likely that only a
Simultaneous tracking of blue whales and largeships demonstrates limited behavioral responses
for avoiding collision
Megan F. McKenna1,2,5,*, John Calambokidis2, Erin M. Oleson3, David W. Laist1, Jeremy A. Goldbogen4
1Marine Mammal Commission, 4340 East-West Highway, Suite 700, Bethesda, MD 20814, USA2Cascadia Research Collective, 218½ West 4th Ave., Olympia, WA 98501, USA
3NOAA-NMFS-Pacific Islands Fisheries Science Center, 1601 Kapiolani Blvd. Ste. 1110, Honolulu, HI 96814, USA4Department of Biology, Hopkins Marine Station, Stanford University, Pacific Grove, CA 93950, USA
5Present address: National Park Service, 1201 Oakridge Drive, Fort Collins, CO 80525, USA
ABSTRACT: Collisions between ships and whales are reported throughout the world’s oceans. Forsome endangered whale populations, ship strikes are a major threat to survival and recovery. Fac-tors known to affect the incidence and severity of collisions include spatial co-occurrence of shipsand whales, hydrodynamic forces around ships, and ship speed. Less understood and likely key tounderstanding differences in interactions between whales and ships is whale behavior in the pres-ence of ships. In commercial shipping lanes off southern California, we simultaneously recordedblue whale behavior and commercial ship movement. A total of 20 ship passages with 9 individualwhales were observed at distances ranging from 60 to 3600 m. We documented a dive response(i.e. shallow dive during surface period) of blue whales in the path of oncoming ships in 55% ofthe ship passages, but found no evidence for lateral avoidance. Descent rate, duration, and maxi-mum depth of the observed response dives were similar to whale behavior immediately after suction-cup tag deployments. These behavioral data were combined with ship hydrodynamicforces to evaluate the maximum ship speed that would allow a whale time to avoid an oncomingship. Our analysis suggests that the ability of blue whales to avoid ships is limited to relativelyslow descents, with no horizontal movements away from a ship. We posit that this constrainedresponse repertoire would limit their ability to adjust their response behavior to different shipspeeds. This is likely a factor in making blue whales, and perhaps other large whales, more vul-nerable to ship strikes.
small fraction of struck carcasses are recovered, andreported numbers are most likely underestimates(Williams et al. 2011). Understanding interactionsbetween ships and large whales has become a globalconservation issue owing to increases in commercialshipping traffic and predicted growth of the industry(Corbett & Winebrake 2007). A number of mitigationefforts have been initiated to reduce these impacts(Silber et al. 2012). To date they have focused onreducing spatial overlap between ships through rout-ing measures (Van der Hoop et al. 2012, Redfern etal. 2013) or reducing ship speeds in sensitive whalehabitat (Gende et al. 2011, Lagueux et al. 2011,McKenna et al. 2012, Silber & Bettridge 2012).
The incidence and severity of ship−whale collisionshas been linked to a number of factors. Analyses ofdocumented ship−whale collisions suggest that shipspeed may be one of the factors. The probability oflethality becomes less common at speeds below 7.2 to7.7 m s–1 (14 to 15 knots) and rare at speeds below5.1 m s–1 (10 knots) (Laist et al. 2001, Vanderlaan &Taggart 2007, Wiley et al. 2011). Explanations of theincreased lethality at higher speeds include greaterimpact forces with increasing ship speeds (Camp-bell-Malone & Barco 2008), an increase in accelera-tion forces as ship speeds increase (Silber et al. 2010),and whale avoidance (Laist et al. 2001). An analysisof lethal injuries from ships under different speedregulations found that ship speed limits can be aneffective tool for reducing mortality risk for certainspecies of whales (Conn & Silber 2013), particularlyfor right whales Eubalaena glacialis in and nearareas of restricted speed along the United States eastcoast (Laist et al. 2014). In an analysis of large-whalemortalities along the entire east coast of North Amer-ica, Van der Hoop (2013) reported no overall changein ship-strike mortalities after multiple mitigationefforts, but results are only applicable on the largerspatial scale and localized efforts appear to havebeen successful (Laist et al. 2014). Given that therelationship be tween lethality and ship speeddepended on reports of known ship−whale collisions,it remains unclear if the results are biased by a lowerdetection of collisions when ships are traveling atslower speeds (<7.2 m s–1 [<14 knots]).
Few studies have examined how whales react toclosely approaching large ships, yet this is likely akey to understanding the differing vulnerability ofspecies, the reduced incidence of ship−strike atslower speeds, and the effectiveness of mitigation.Possible avoidance responses may include verticalmovements to avoid ships by diving beneath them orremaining at depth until ships pass, or horizontal
movements and changes in swim speed to avoidoncoming ships. An approaching ship may elicit athreat-like response, which may include longer sur-face intervals, increased breath rate, or shallowerdiving. It is also possible that whales do not changetheir behavior in the presence of ships. Evidence insupport of any behavioral response of whales hasbeen limited by logistical challenges of gatheringhigh-resolution whale behavioral data in the pres-ence of large ships. Anecdotal information fromwhalers suggests whales are less responsive whilefeeding and response may be related to feeding tech-nique, specifically surface feeding would increasevulnerability (Laist et al. 2001). To date, the only sup-port for a ‘startle response’ consists of anecdotalreports of whales taking evasive action within a fewhundred meters of oncoming ships (Laist et al. 2001).Bio-logging tag data from whales occupying areas ofhigh shipping traffic, however, can provide detailedwhale behavior during close encounters with ships.This technique has been used to examine manateereactions to recreational boats and tourism vessels(Miksis-Olds et al. 2007, Ryck 2013).
In this study, we combined data from tagged bluewhales Balaenoptera musculus with automatic iden-tification system (AIS) ship-tracking data collectedoff the coast of southern California, a region of highshipping density (Redfern et al. 2013). The primaryaccess route into one of the world’s largest ports, thePort of Los Angeles-Long Beach (POLA) travelsthrough this region. POLA is the second busiest portin North America (CINMS 2009). Until recently, anestimated 75% of the vessels leaving, and 65% ofvessels arriving at, POLA and Port Hueneme trav-eled through the Santa Barbara Channel (SBC)(CINMS 2009). Commercial vessel traffic in the SBCis concentrated in designated shipping lanes, with anaverage of 18 ships transiting per day (McKenna etal. 2009). The majority of traffic is categorized ascargo ships (e.g. container ships, bulk carriers, andvehicle carriers), traveling at average speeds of 10 ms−1 (19 knots).
This region is also a primary feeding ground forthe Endangered eastern North Pacific (ENP) bluewhale (Calambokidis et al. 1990, 2000, 2007). Ag -gregations of ENP blue whales come to this regionto feed on dense patches of krill (Calambokidis etal. 2000, 2007). Recent estimates of the abundanceof this population of blue whales, using both line-transect and mark-recapture methods, showed noclear increase, despite decades of commercial whal-ing ending in 1971 (Barlow 1995, Calambokidis &Barlow 2004, Calambokidis et al. 2009). ENP blue
220
McKenna et al.: Whale behavior and ship-strike vulnerability
whale population estimates are around 2000 to 3000animals (Calambokidis & Barlow 2004). A recentstudy suggested that density dependence is likelythe key reason for the observed lack of increase(Monnahan et al. 2014). The only known source ofjuvenile and adult mortality for ENP blue whales isfatal collisions between ships and whales (Carrettaet al. 2012), although noise, chemical pollution, andfishery interactions likely also impact the popula-tion. Between 1998 and 2007, 21 blue whale deathswere reported along the California coast; 4 of thosedeaths occurred in the fall of 2007 in the SBC(Berman-Kowalewski et al. 2010). Because the SBCwas identified as an area of high ship-strike risk forthis species (Berman-Kowalewski et al. 2010), ourstudy targeted this region to understand how bluewhales behave in the path of oncoming ships tohelp inform management of ships in regions of highship traffic density.
Fine-scale ENP blue whale behavior (i.e. dive pat-tern and lateral movement) during close encounterswith ships transiting the shipping lane off southernCalifornia were analyzed to understand and charac-terize behavioral responses to approaching ships.Based on our findings, we offer a theoretical frame-work for evaluating the consequences of varying shipspeeds to mitigate ship−whale collisions.
MATERIALS AND METHODS
Whale behavioral data were collected on archivalsuction-cup tags attached to blue whales. Three dif-ferent tag types were deployed: 2 types of acousticrecording tags (Bioacoustic Probe and Acousonde:Greeneridge Sciences) and GPS Fastloc location tags(TDR10-F: Wildlife Computers). The Bioacoustic Pro -be records acoustic data and is equipped with 3 auxiliary sensors (pressure, temperature, and 2-axisacceleration sampling at 1 s intervals). The Acou -sonde, a more recent tag design, houses a 3-axisaccelerometer and a higher frequency hydrophone,and has higher sampling rates (10 kHz) for the auxil-iary sensors. The TDR10-F tag records hydrostaticpressure at 1 Hz, temperature, and GPS positionsduring whale surface periods.
Blue whales in or near shipping lanes off the coastof southern California (Fig. 1) were targeted for tagdeployment. In some cases, after successful tagdeployment, whales were tracked visually from asmall boat. The tagged whale’s position and ship pas-sages were monitored by observers onboard thesmall research boat. In other cases, whales weretagged and not visually tracked; instead the TDR10-F tag equipped with a Fastloc GPS collected whaleGPS positions with a reported accuracy of 20 to 75 m.
221
10 0 10 20 km
120.0° W 119.5° 119.0° 118.5°
33.5°
34.0°
34.5°N
California
Port Hueneme
Port of Los Angeles/ Long Beach
AIS station
AIS station
Inbound lane
Outbound lane
Whale #8Whale #2
Whale #5Whale #4Whale #3Whale #7
Whale #9Whale #1Whale #6
Santa Cruz Isl
Fig. 1. Locations of documented ship passages to blue whales foraging off the coast of southern California. Whale numberingcorresponds to Tables 1 & 2 and Figs. 2, 3, & 5. Black lines indicate the commercial shipping traffic lanes through the SantaBarbara Channel into the ports of Los Angeles/Long Beach. (Q) Locations of the automatic identification system (AIS)
receivers used to collect the ship-track information
Endang Species Res 27: 219–232, 2015
Only high-quality GPS locations, defined as whalepositions that were calculated from >3 satellite posi-tions, were used. When the tags were recovered, thespatial data were combined with ship movementdata from AIS to determine the time, location, andestimated minimum distance between the whalesduring ship passages.
AIS data were collected at multiple locations fromall commercial ships >300 gross tons transiting theregion. The shore-based AIS stations (Fig. 1) providedthe entirety of the ship passages through the region,and the AIS receiver on the tagging boat providedreal-time detail during ship passages. Large commer-cial ships are required to transmit information ontheir position, speed, and unique identification infor-mation via AIS (Tetreault 2005). AIS was developedto decrease the risk of ship collisions; therefore, thetransmitted positional information is highly accurate.Ship AIS signals are transmitted every 6 to 10 s, and,if some positions were not re ceived at the shore sta-tions due to atmospheric interference, ship trackswere interpolated to 10 s increments using the speedof the ship and the bounding positions. Whale posi-
tional data were also interpolated to 10 s increments,assuming straight-line travel between positions andconstant swim speed. The 10 s increments providedthe necessary resolution to identify the distance andtime of ship passages because a ship would not travelits length within in this time frame.
To define the closest point of approach (CPA) be -tween the tagged whales and passing ships, the in-terpolated whale positional data were combined withthe interpolated ship-track AIS data. First, the whaleand ship positions closest in time were identified, andthen the horizontal distance between the whale andship was calculated based on these positions. Thiswas defined as the CPA distance. The methodologyof combining TDR10-F tag data with AIS data wasverified through a comparison with known ship andwhale positions (Fig. 2). Custom-built functions inMATLAB (Mathworks, Version 2012b) were devel-oped to analyze and combine the whale-track andship-track data. We defined a close approach as aship passage within 1 km of a whale. This is an arbi-trary distance thought to be a possible distance atwhich whales might be expected to react to ships by
222
TDR10-F tag
Tanker travelling at 6.3 m s–1 inbound
118.35°W 118.33 118.31 118.29 118.2733.59
33.6
33.61
33.62
33.63°N
01:39 01:40 01:42 01:43 01:45
0
50
100
150
200
250
300
Dep
th (m
)
Whale trackShip trackClosest approach (distance = 900 m)
GMT (local +7 h)
Dive profile
to harbor
Surface movement
Fig. 2. Verification of methodology combining TDR10-F tag GPS positions of the whales with ship positions from AIS (auto-matic identification system) data to calculate CPA (closest point of approach). To calculate distance between the ship andwhale at CPA, the whale and ship positions closest in time were identified; then, the horizontal distance between the whaleand ship was calculated based on these positions. Top left panel: bird’s-eye view of the movement of the ship and the whale,including the interpolated track of blue whale (Whale 1) with TDR10-F GPS tag points (blue dots) and the ship track from AISdata stream (red stars). Lower panel: dive profile of the whale, with the red star indicating CPA identified from combiningTDR10-F and AIS data. The locations of the ship and the whale at the closest distance are shown as black circles. The rightpanel shows a photograph just prior to the close approach shown in the left panels (1 knot = 0.514 m s–1); the tag is visible on
the back of the whale
McKenna et al.: Whale behavior and ship-strike vulnerability
making some avoidance response. However,for the purposes of this analysis, weexamined possible responses up to 3.6 kmfrom ships given the small sample size oftagged whales near ships and uncertainty asto whether and what whale responses mightbe at any distance.
Analysis of tag data provided the followingwhale behavioral information based onmeth odology from Goldbogen et al. (2006,2011): (1) surface duration be tween deep di -ves and number of breaths; (2) descent time,angle, and speed; (3) bottom time, number oflunges, and maximum dive depth; and (4)ascent time, angle, and speed. These behav-ioral variables were collected for the entiretag deployment for each whale using cus-tom-built functions in MATLAB (Mathworks,Version 2012b). This allowed for compar-isons between normal diving behavior anddive behaviors during close ship passages.
Whale lateral movements during ship pas-sages were evaluated for evidence of horizon-tal avoidance at the surface. Using whale po-sitional data (either from the TDR10-F tag orsmall boat observations), the distance of thewhale’s surface positions, both before and af-ter CPA, to the ship position at CPA wasmeasured. The heading of the whale from itsCPA position to positions after CPA were alsocalculated. These measurements allowed usto evaluate if a whale moved away from theship. If so, we would expect the whale dis-tance from the ship’s position at CPA to in-crease after CPA, and the whale heading toremain fairly constant. On the other hand, ifthe whale’s distance from the ship did not in-crease after CPA and its heading changed, itwould indicate the whale did not exhibit di-rected movement away from the passing ship.
RESULTS
A total of 20 ship passages (within 3.6 km ofa foraging whale) were analyzed and in-volved 9 individual blue whales with bio-log-ging tags attached (Table 1). The ship pas-sages occurred in the commercial shippinglanes that service POLA in southern Califor-nia (Fig. 1): 3 ship passages occurred justnorth of Santa Cruz Island in the in boundshipping lane, 5 occurred off the coast of Port
223
Wh
ale
Tag
Tag
ged
Tot
al
Gro
up
T
otal
B
ehav
ior
du
rin
g s
hip
pas
sag
esS
urf
ace
seri
es
Res
pon
se
no.
typ
eD
ate
Tim
e (h
)h
ours
typ
esh
ip
Wit
h r
e-L
ocat
ion
No
re-
Loc
atio
nw
ith
res
pon
sed
ive−
tag
ta
g o
np
assa
ges
spon
se d
ives
spon
se d
ive
div
e (%
)d
eplo
ymen
t
1T
DR
10M
k-F
21 S
ep 2
011
07:4
2:28
18.6
Sin
gle
72
Su
rfac
e (2
)5
Des
cen
t (1
)12
Un
kn
own
Bot
tom
(2)
Asc
ent
(2)
2B
ioac
oust
ic P
rob
e16
Au
g 2
008
10:4
4:00
10.6
Lea
d o
f p
air
22
Su
rfac
e (1
)0
−13
Yes
Des
cen
t (1
)3
Bio
acou
stic
Pro
be
16 S
ep 2
009
16:4
3:00
6.98
Lea
d o
f p
air
10
−1
Des
cen
t (1
)0
No
4T
DR
10M
k-F
5 A
ug
200
907
:47:
006.
23T
rail
of
pai
r1
1B
otto
m0
−35
Yes
5B
ioac
oust
ic P
rob
e4
Au
g 2
009
14:0
3:00
5.15
Tra
il o
f p
air
11
Su
rfac
e0
–20
No
6T
DR
10M
k-F
21 S
ep 2
011
08:1
3:22
25.4
Sin
gle
31
Su
rfac
e2
Des
cen
t (1
)5
Un
kn
own
Bot
tom
(1)
7B
ioac
oust
ic P
rob
e5
Au
g 2
009
10:4
5:00
4.45
Sin
gle
22
Bot
tom
(1)
0−
27Y
esA
scen
t (1
)8
Bio
acou
stic
Pro
be
14 A
ug
200
814
:32:
001.
62T
rail
of
pai
r1
1B
otto
m0
−14
No
9A
cou
son
de
19 A
ug
201
109
:59:
352.
26S
ing
le2
1S
urf
ace
1A
scen
t14
No
Tab
le 1
.Su
mm
ary
of t
agg
ed w
hal
es, s
hip
pas
sag
es, a
nd
su
rfac
e b
ehav
ior
to s
hip
pas
sag
es. D
ista
nce
s b
etw
een
th
e w
hal
e an
d s
hip
ran
ged
fro
m 6
0 to
130
0 m
(se
e F
ig.5
).T
he
‘wh
ale
no.
’ col
um
n c
orre
spon
ds
to t
he
wh
ale
nu
mb
ers
in b
oth
Tab
le1
and
Fig
.5. I
n t
he
‘loc
atio
n’ c
olu
mn
s th
e n
um
ber
in p
aren
thes
es r
efer
s to
th
e to
tal n
um
ber
of
ship
pas
sag
es t
hat
occ
urr
ed f
or t
hat
sp
ecif
ic v
erti
cal l
ocat
ion
of
the
wh
ale.
‘Su
rfac
e se
ries
wit
h r
esp
onse
div
e’ r
efer
s to
th
e p
erce
nt
of t
he
tota
l su
rfac
e p
erio
ds
for
the
in-
div
idu
al w
hal
e w
hic
h h
ad a
res
pon
se o
r re
spon
se-l
ike
div
e. ‘R
esp
onse
div
e-ta
g d
eplo
ymen
t’ in
dic
ates
wh
eth
er th
e w
hal
e d
id a
res
pon
se o
r re
spon
se-l
ike
div
e d
uri
ng
the
surf
ace
per
iod
rig
ht
afte
r ta
g d
eplo
ymen
t. T
his
is u
nk
now
n f
or t
he
TD
R10
Mk
-F t
agg
ed w
hal
es b
ecau
se t
he
tag
doe
s n
ot s
tart
rec
ord
ing
un
til i
t re
ach
es a
sp
ecif
ic d
epth
Endang Species Res 27: 219–232, 2015
Hueneme in the outbound lane, and the remainderoccurred at the entrance to the ports of Los Angelesand Long Beach. The ship−whale encounters in-volved 6 ship types (bulk carriers, container ship, pas-senger, roll-on roll-off car carrier [RO-RO], tanker,and vehicle carriers), ship speeds ranging from 5.3 to15.5 m s−1 (10 to 30 knots), different positions of thewhales in the water column at CPA (surface, descent,bottom, and ascent), and closest distances of the pas-sages ranging from 60 to 3600 m, including 11 thatwere considered close approaches (Tables 1 & 2). Allthese factors may influence how whales react to ap-proaching ships; therefore, we provide general de-scriptions of both the lateral movements and dive be-haviors of the whales during the ship passages toprovide initial insight on blue whale behavioral reac-tions to close encounters with large commercial shipsand their potential vulnerability to ship strikes.
Whale lateral movement during close ship passage
Lateral movement of whales during ship passagesshowed little to no evidence of avoidance or move-ment away from the passing ship (Fig. 3). Four of the6 encounters that we could analyze for changes inlateral movement resulted in whales actually movingcloser to the ship track after CPA (Fig. 3a,c,d,f,h). Forthese encounters, the whales exhibited a largechange in heading (>50°), suggesting that the move-ment was not in any particular direction or in a direc-tion away from the passing ship (Fig. 3b). This wasobserved for whales at the surface, at the bottom, andon ascent during CPA of the oncoming ship.
For those whales that showed evidence of move-ment to a position further from the track of the ship,the difference in distance was small (<100 m)(Fig. 3a,c,g), and these 2 whales actually crossed thepath of the ship, resurfacing on the other side of theship track (Fig. 3c,g). The headings to the next posi-tion changed by >50°, suggesting the limited lateralmovement was not directed away from the ship(Fig. 3b).
Characteristics of a response dive
During a 24 h deployment of a TDR10-F tag on ablue whale, a tanker leaving POLA and traveling at aspeed of 5.7 m s–1 (11 knots) came within 100 m of thetagged whale at the surface. In this event, the whaleresponded by diving below the ship, resurfacing
224
Wh
ale
and
sh
ip d
etai
lsC
har
acte
rist
ics
of r
esp
onse
div
esF
orag
ing
div
eE
stim
ated
los
s W
hal
e L
ocat
ion
D
ista
nce
S
hip
S
hip
D
ive
typ
eS
ink
rat
e D
epth
D
ura
tion
B
ody
Asc
ent
Des
cen
t in
for
agin
g
no.
of w
hal
e to
sh
ip
typ
esp
eed
(0.3
to
(4 t
o (4
5 to
an
gle
rate
rate
ti
me
(min
)at
CP
A(m
)(m
s−
1 )1
m s
−1 )
40 m
)25
1 s)
(−26
to
14°)
(m s
−1 )
(m s
−1 )
6S
urf
ace
60B
ulk
car
rier
5.8
(1)
Res
pon
se d
ive
0.6
−27
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1.1
1.3
1.2
9S
urf
ace
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Con
tain
er s
hip
8(3
) R
esp
onse
-lik
ed
ive
(sh
ort)
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−9.
932
.0−
94.2
1.2
2.0
0.7
2S
urf
ace
700
Veh
icle
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rier
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ehic
le c
arri
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) R
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−1
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3(3
) R
esp
onse
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ive
(sh
ort)
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−7.
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−1
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rfac
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rier
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(1)
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pon
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ive
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11.
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02
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cen
t20
0C
onta
iner
sh
ip11
.8(2
) R
esp
onse
-lik
e d
ive
(slo
w)
0.2
−14
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2.0
−2.
41.
31.
85.
14
Bot
tom
60C
onta
iner
sh
ip9.
1(2
) R
esp
onse
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(slo
w)
0.2
−19
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7.4
−1.
40.
5−
7B
otto
m30
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onta
iner
sh
ip9.
1(2
) R
esp
onse
-lik
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ive
(slo
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0.2
−19
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9.0
2.1
0.8
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8B
otto
m13
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ehic
le c
arri
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12.
10.
17
Asc
ent
3600
Veh
icle
car
rier
7.2
(1)
Res
pon
se
0.3
−13
.714
9.9
1.6
1.4
1.1
−
Tab
le 2
. Su
mm
ary
of r
esp
onse
div
es d
uri
ng
sh
ip p
assa
ges
. Th
e n
um
ber
of
the
wh
ale
corr
esp
ond
s to
Tab
le 1
an
d F
ig.5
. Th
e n
um
ber
in
th
e ‘D
ive
typ
e’ c
olu
mn
ref
ers
toth
e ty
pe
of r
esp
onse
div
e—
(1):
res
pon
se d
ive
wit
h e
xpec
ted
des
cen
t ra
te, d
ura
tion
, an
d b
ody
ang
le; (
2) r
esp
onse
-lik
e d
ive
wit
h a
slo
wer
th
an e
xpec
ted
des
cen
t ra
te; (
3)re
spon
se-l
ike
div
e w
ith
a s
hor
ter
than
exp
ecte
d d
ura
tion
. Th
e ca
lcu
lati
on o
f b
ody
ang
le is
on
ly p
ossi
ble
wit
h t
he
Bio
acou
stic
Pro
be
and
Aco
uso
nd
e ta
g t
ypes
. Est
imat
edlo
ss i
n f
orag
ing
tim
e is
th
e d
iffe
ren
ce b
etw
een
su
rfac
e ti
me
at c
lose
ap
pro
ach
an
d a
vera
ge
surf
ace
tim
e in
th
e re
mai
nin
g d
ive
pro
file
. If
nu
ll, s
urf
ace
tim
e is
no
lon
ger
th
an a
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ge.
CP
A: c
lose
st p
oin
t of
ap
pro
ach
; 1 k
not
= 0
.514
m s
–1
McKenna et al.: Whale behavior and ship-strike vulnerability
1 min after the ship passed, and then made a normalforaging dive about 1 min after resurfacing behindthe ship (see the Supplement at www. int-res. com/articles/ suppl/n027p219_ supp/). The whale’s reac-tion began at a distance of 250 m, and the whaledescended at a rate of 0.6 m s−1 to a depth of 30 m asthe tanker approached the whale’s position. Thisdive type was categorized as a ‘response dive’. Addi-tional whale dive data were examined to determine ifthis type of dive was present in other whale dive pro-files and under what circumstances.
We found that response dives were commondirectly after successful suction-cup tag deploymentfrom a 5 m rigid-hull inflatable boat and used thesedives to define the characteristics of response dives.
Descent rate, duration, depth, and body angle(Fig. 4a) were measured because these characteris-tics are relevant for evaluating the effectiveness ofresponse dives. We analyzed dive profiles directlyafter tag attachment for 33 tag deployments to quan-tify these characteristics. For these same whales, wealso measured the characteristics of normal foragingdives. Descent rates were calculated as change indepth over time from the surface to the bottom of thedive. Body angle was derived from the 2-axisaccelerometer (Goldbogen et al. 2006), but this wasnot possible from the TDR10-F tag data.
Duration of response dives after tag attachmentranged from 45 to 251 s (μ = 115.5 ± 69.8) and descentrates averaged 0.6 m s−1 (0.3 to 1.0 m s−1) at body
225
–500
–400
–300
–200
–100
0
100
400
500
(c) (d) (e) (f) (g)
Diff
eren
ce in
dis
tanc
e of
w
hale
to
ship
CP
A p
ositi
on (m
) [b
efor
e–af
ter]
Cha
nge
in w
hale
hea
din
g (°
)
–300
–200
–100
0
100
200
300
(h)
at surface at bottom on ascent
200
300
(c) (d) (e) (f) (g) (h)
at surface at bottom on ascent
a b
123
CPA
119.2°W
34.02°N
whale trackWhale#4
ship track Container ship 9 m s–1
e
12
3
CPA
whale trackWhale#7
ship track Container ship 9 m s–1
f
1
2
118.3°W
33.6°N
ship track Bulk carrier 6 m s–1
whale trackWhale#6
CPA
c
3
3
12
CPA
whale trackWhale#9
ship track Passenger ship 16 m s–1
g
33.6°N
1
2
3
CPA
h 118.3°W
whale trackWhale#1
ship track Bulk carrier 6 m s–1
1
2
CPA
ship track Container ship 8 m s–1
whale trackWhale#9
118.3°Wd
CPA
3
118.3°W
33.6°N
33.6°N
34.06°N
119.23°W
Fig. 3. Summary of horizontal surface movements of whales during ship passages that occurred at distances <300 m. (a) Differ-ence in distance of the whale to the ship’s closest point of approach (CPA) position; negative values indicate the whale movedcloser to the ship’s CPA position after CPA, and positive values indicate the whale moved further from the ship’s CPA position.(b) Change in the heading of the whales before and after CPA. Headings were calculated as the direction (in degrees) from theCPA surface position to the position of the next surface period. The differences between headings were calculated. Valuesclose to zero indicate little change in heading or, possibly, directed movement away from the path of a ship. Positive and neg-ative values indicate the headings were not similar and, therefore, the whale was not traveling in a particular direction. Thedesignations ‘at surface’, ‘at bottom’, and ‘on ascent’ indicate where the whales were during the CPA; the letters on the barscorrespond to the lower panels (c–h) showing the surface movements of whales. (c–h) Details of whale movement at the timeof CPA (indicated by red dot for whale positions and red triangle for ship positions). Data points indicate the order of surfaceperiods — 1: before CPA; red: CPA; 2: after CPA; 3: 2 surface periods after CPA. In (c,e,h) the light gray lines are the borders of
the shipping lanes; in (d,f,g) the shipping lanes are outside the resolution of the graphic. 1 knot = 0.514 m s–1
angles between 5 and 20° (Fig. 4b). This was consid-erably less than typical deep foraging dives (1.6 m s−1
descent rate at a 20 to 40° body angle (Fig. 4b). Thesedive parameters provided a basis for identifying anyobserved whale surface responses during the pas-sage of large ships. The entire dive profile of eachwhale possibly affected by passing ships was exam-ined for the presence of response dives. This pro-vided a context for assessing how common responsedives were in an individual dive profile (Table 1). Aresponse dive during a surface period was confirmedif the duration, descent rate, and body angle fellwithin the ranges measured from the response diveafter tag deployment. If some but not all of the char-acteristics fell within the ranges, the dive was labeleda ‘response-like’ dive. If no characteristics matched,the dive was labeled a ‘no surface response’.
Ship passages when whales were at the surface at CPA
For the 6 observed approaches whose CPA oc -curred when whales were at the surface after a for-aging dive (Fig. 5a, Table 2), all whales exhibited aresponse dive similar to those seen after tag deploy-ments (Fig. 5a). One of the response dives wasslightly shorter than typical response dives (Fig. 5a,
Panel 2, Whale #9), but the response-like dive waswithin the expected depth, body angle, and descentrate limits.
Ship passages when whales were on descent of a foraging dive at CPA
Of the 4 whales descending on a foraging dive atCPA only 1 showed evidence of a response dive
Fig. 4. Blue whale response to suction-cup tag deployments. (a) Example of a whale response dive during successful tagattachment. The response dive is followed by deep foraging dives (bottom of dive profile is not shown). (b) Characteristics(body angle, descent rate) of response dives compared to a foraging dive. Response dives have a significantly slower descentrate and lower body angles. Box plots along the x-axis show average descent rates for all response dives compared to foragingdives; a statistically significant difference is found between the dive types. For each box, the central mark is the median, theedges of the box are the 25th and 75th percentiles, the whiskers extend to the most extreme data points not considered
outliers, and outlier is plotted individually as a plus sign. The mean value (μ) is shown for each dive type
Fig. 5. Dive records of whales during ship passages. Therows of plots are organized by the location of the whale atthe closest point of approach (CPA): (a) at the surface, (b) ondescent, (c) at the bottom of a foraging dive, and (d) onascent. Each panel in the rows represents a differentship−whale encounter, ordered from closest distance to fur-thest. The red dots in each panel indicate when the closestdistance between the whale and ship occurred. Informationon the ship (type, speed, and distance) is provided in theheading of each panel. The whale number at the bottom ofeach panel corresponds to Tables 1 & 2 and Fig. 3. An arrowindicates an observed response or response-like dive. Arrowlabeled (1): dive with expected descent rate, duration, andbody angle; (2) slower than expected descent rate; (3)shorter than expected duration. If no response dive wasobserved, the panel is labeled ‘no surface response’. Aster-isks next to either the descent or ascent of a dive profile indicate a change in rate that is faster (*) or slower (**) than1 standard deviation from the mean of all rates in the dive
record. Time is local time. 1 knot = 0.514 m s–1
McKenna et al.: Whale behavior and ship-strike vulnerability 227
(a) A
t su
rfac
e
(b) O
n d
esce
nt
Depth (m) Depth (m) Depth (m) Depth (m)
(c) A
t b
otto
m
(d) O
n as
cent
21:5
321
:56
21:3
021
:33
21:3
621
:38
21:4
121
:44
21:4
721
:50
0 50 100
150
200
250
300
Bul
k ca
rrie
r (s
pee
d: 5
.8 m
s–1
, dis
tanc
e: 6
0 m
)
(1)
WH
ALE
#6
10:3
310
:40
10:4
810
:55
11:0
211
:09
0 20 40 60 80 100
120
140
160
180
200
Con
tain
er s
hip
(sp
eed
: 8.0
m s
–1, 2
00 m
)
WH
ALE
#9
16:1
216
:19
16:2
616
:33
16:4
016
:48
16:5
517
:02
17:0
917
:16
17:2
4
0 50 100
150
200
250
Veh
icle
car
rier
(sp
eed
: 8.9
m s
–1, 7
00 m
)
(1)
WH
ALE
#2
15:2
415
:27
15:3
015
:33
15:3
615
:38
15:4
115
:44
15:4
715
:50
0 50 100
150
200
250
300
Tank
er (s
pee
d: 6
.3 m
s–1
, 110
0 m
)
(3)
WH
ALE
#1
WH
ALE
#5
16:1
916
:26
16:3
316
:40
16:4
816
:55
17:0
217
:09
0 50 100
150
200
250
300
Veh
icle
car
rier
(sp
eed
: 6.8
m s
–1, 1
000
m)
(1)
tag
off
WH
ALE
#5
22:4
222
:45
22:1
922
:22
22:2
422
:27
22:3
022
:33
22:3
622
:39
0 50 100
150
200
250
300
Bul
k ca
rrie
r (s
pee
d: 5
.9 m
s–1
, 140
0 m
)
(1)
WH
ALE
#1
16:1
216
:19
16:2
616
:33
16:4
0
0 50 100
150
200
250
16:4
816
:55
17:0
217
:09
17:1
617
:24
Con
tain
er s
hip
(sp
eed
: 11.
8 m
s–1
, 200
m)
(2)
WH
ALE
#2
15:4
415
:47
15:5
015
:53
15:5
615
:59
16:0
116
:04
16:0
716
:10
0 50 100
150
200
250
300
Pas
seng
er s
hip
(sp
eed
: 6.2
m s
–1, 6
00 m
)
no s
urfa
ce r
esp
onse
WH
ALE
#6
18:2
118
:28
18:3
618
:43
18:5
018
:57
RO
RO
shi
p (s
pee
d: 8
.8 m
s–1
, 900
m)
no s
urfa
ce r
esp
onse
0 50 100
150
200
250
WH
ALE
#3
11:0
211
:09
11:1
611
:24
11:3
111
:38
0 50 100
150
200
250
300
Con
tain
er s
hip
(sp
eed
: 9.1
m s
–1, 6
0 m
)
(2)
11:4
5
WH
ALE
#4
11:0
211
:09
11:1
611
:24
11:3
111
:38
11:4
5
0 50 100
150
200
250
300
Con
tain
er c
arrie
r (s
pee
d: 9
.1 m
s–1
, 300
m)
(2)
WH
ALE
#7
01:3
201
:35
01:3
701
:40
01:4
301
:46
01:4
901
:52
01:5
501
:58
0 50 100
150
200
250
300
Tank
er (s
pee
d: 6
.3 m
s–1
, 90
0 m
)*
no s
urfa
ce r
esp
onse
WH
ALE
#1
15:2
415
:27
15:3
015
:33
15:3
615
:38
15:4
115
:44
15:4
715
:50
0 50 100
150
200
250
300
Tank
er (s
pee
d: 6
.3 m
s–1
, 110
0 m
)
no s
urfa
ce r
esp
onse
WH
ALE
#6
15:3
615
:43
15:5
015
:57
16:0
4
0 50 100
150
200
250
Veh
icle
car
rier
(sp
eed
: 10.
8 m
s–1
, 13
00 m
)
(1)
16:1
2
tag
off
WH
ALE
#8
11:0
911
:16
11:2
411
:31
11:3
811
:45
0 20 40 60 80 100
120
140
160
180
200
Pas
seng
er s
hip
(sp
eed
: 15.
5 m
s–1
, 200
m)
no s
urfa
ce r
esp
onse
WH
ALE
#9
14:4
414
:47
14:4
914
:52
14:5
514
:58
15:0
115
:04
15:0
715
:10
0 50 100
150
200
250
300
Con
tain
er s
hip
(sp
eed
: 6.1
m s
–1, 1
300
m)
no s
urfa
ce r
esp
onse
WH
ALE
#1
0 50 100
150
200
250
300
12:0
712
:14
12:2
112
:28
12:4
312
:50
12:5
7
Veh
icle
car
rier
(sp
eed
: 7.2
m s
–1, 3
600
m)
(1)
12:3
6
WH
ALE
#7
15:4
715
:50
15:5
315
:56
15:5
916
:01
16:0
416
:07
16:1
016
:13
0 50 100
150
200
250
300
Pas
seng
er s
hip
(sp
eed
: 6.2
m s
–1, 9
00 m
)
WH
ALE
#1
no s
urfa
ce r
esp
onse
21:2
121
:24
21:2
721
:30
21:3
321
:36
21:3
821
:41
21:4
421
:47
0 50 100
150
200
250
300
Bul
k ca
rrie
r (s
pee
d: 5
.7 m
s–1
, 500
m)
no s
urfa
ce r
esp
onse
WH
ALE
#1
20:4
920
:52
20:5
520
:58
21:0
121
:04
21:0
721
:10
21:1
221
:15
0 50 100
150
200
250
300
Con
tain
er s
hip
(sp
eed
: 5.3
m s
–1, 1
000
m)
no s
urfa
ce r
esp
onse
WH
ALE
#1
*
**
**
**
***
Sur
face
ser
ies
resp
onse
div
e ty
pe
(1) R
esp
onse
div
e p
rese
nt
(2) R
esp
onse
-lik
e d
ive
but
slo
w d
esce
nt r
ate
(3) R
esp
onse
-lik
e d
ive
but
sho
rt d
urat
ion
* A
scen
t/d
esce
nt r
ate
fast
er t
han
1SD
of m
ean
**A
scen
t/ d
esce
nt r
ate
slow
er t
han
1SD
of m
ean
(3)
ll
ll
ll
l
ll
ll
l
ll
ll
l
ll
ll
Endang Species Res 27: 219–232, 2015
(Fig. 5b), specifically a series of res ponse dives at thesurface period after CPA (Fig. 5b, Panel 1, Whale #2).In the latter case, a container ship traveling at fullspeed (11.8 m s–1 [23 knots]) came within 200 m of thewhale and the acoustic record contained distinct shipnoise throughout the bottom portion of the dive. Theother 3 whales whose CPA occurred while on des -cent showed no evidence of a response dive at thesurface before or after the CPA. In these cases, shipdistances were greater (600 and 900 m) and shipswere traveling at slower speeds (Fig. 5b, Panels 2−4;Whales #6, #3, #1).
Ship passages when whales were at the bottom of a foraging dive at CPA
Six whales were at the bottom of foraging diveswhen the ship passed overhead (Fig. 5c). Three ofthe 6 whales exhibited response dives or response-like dives at the surface after the ship passed and in2 of those cases whales dove at a slower rate(0.2 m s−1) than typical response dives (Fig. 5c, Pan-els 1 & 2; Whales #4 and #7). These 2 whales(Whales 4 & 7) had the highest proportion responsediving during surface periods (35 and 27%; Table 1),making it less clear if these dives were related tothe passage of ships. However, the descent rate forWhale 4 (Fig. 5c, Panel 1; Whale #4) was atypicalcompared to all other dives in this whale’s profile.Whale 8 also exhibited a response dive at the sur-face after the ship passage (Fig. 5c, Panel 5; Whale#8) and had a lower proportion of surfaces withresponse dives (14%), suggesting it may be morelikely that it was responding to the ship’s passage.The ship in that approach was also traveling faster(10.8 m s−1 [21 knots]) than those in other casesinvolving whales that were at the bottom of theirdive when ships passed (Fig. 5c).
Ship passages when whales were on ascent from a foraging dive at CPA
For whales that were on ascent at CPA, only 1 of 4(Whale 7) exhibited a response-like dive to an on -coming ship (Fig. 5d, Panel 4; Whale #7). Becauseits response dive had a slower than expecteddescent rate, this whale had a high percentage ofresponse dives in its profile, and the ship in thiscase was 3.6 km away at CPA, the observedresponse-like dive may not be related to the shippassage.
Changes in ascent and descent rates of foraging dives
In addition to the presence of response dives as evi-dence for a behavioral response to the ship passage,we examined ascent and descent rates during deepforaging dives before and after CPA by comparingthem to all other dives in the individual whale’s diveprofile. Seven of the 20 ship−whale encounters show -ed a change in ascent or descent rates that was atleast 1 standard deviation different from the mean ofall other ascent and descent rates in the individual’sdive profile (Fig. 5). These changes in ascent anddescent rates were observed for all whale locations atCPA (surface, bottom, ascent, and descent). During 3ship passages, descent rates were faster thanexpected on the foraging dive after CPA (Fig. 5a,Panel 2, Whale #9; Fig. 5c, Panels 3 & 5, Whales #1and #8). In 2 of the 20 ship passages, whale descentrates were slower than expected on the foraging diveafter CPA (Fig. 5b, Panel 1, Whale #2; Fig. 5c, Panel1, Whale #4). The ascent rate was faster during 1 en -counter (Fig. 5d, Panel 4; Whale #7) and slower foran other ship−whale encounter (Fig. 5d, Panel 1;Whale #9). The differences in ascent and descentresponses may relate to the distance or the speed ofthe ship at CPA. Additional ship−whale encountersare needed to understand the relationship betweenthese observed changes in behavior and passingships (e.g. ship speed, ship type).
Ship−whale collision model
Observed response dive behavior were consideredrelative to different ship speeds and whale reactiondistances to estimate the time needed for a whale tosuccessfully perform a response dive and avoid anoncoming ship. The following equation was used:
where WDi,j,k is the depth of the whale for a givendescent rate (DR), reaction distance (RD), and shipspeed (SSPD). Only whale reactions at the surfacewere considered in this equation. Input parametersinclude: whale descent rates between 0.6 and 1.6 ms−1 and whale reaction distances from 50 to 500 m.Modeled ship speeds ranged from 5.2 to 12.9 m s−1
(10 to 25 knots). The model assumed a ship draft of8 m and a zone of hydrodynamic risk beneath theship’s hull equal to 2-times the draft (Silber et al.2010), in this case 16 m.
WD DRRD
SSPDi j k ij
k, , = ×
228
McKenna et al.: Whale behavior and ship-strike vulnerability
The results of the simulations using the parametersof the response dive, with different ship speeds andwhale reaction distances, showed that a responsedive could result in a collision when whale reactiondistances are short and ship speeds are fast (Fig. 6a).Based on this model, in the observed case where a
whale at the surface initiated a response at a distanceof 250 m from the approaching ship and dove at 0.6 ms−1 (Fig. 5a, Panel 1; Whale #6), the whale would nothave been able to avoid the hull and hydrodynamicforces of a ship if the ship was traveling at 7.7 m s–1
(15 knots) or greater (Fig. 6). If the tanker was travel-
229
Whale reaction distance (m)
(a) Response dive (0.6 m s–1)
1.0 m s–1 descent rate
(b) Foraging dive (1.6 m s–1)
Wha
le d
epth
(m)
Whale reaction distance (m)
a) Response dive decent rate (0.6 m s–1) b) Foraging dive descent rate (1.6 m s–1)
Whale reaction distance (m)W
hale
dep
th (m
)
c) Summary
100 200 300 400 500
0
10
20
30
40
50
60
70
80
90
100 6
9
13
17
20 24
100 200 300 400 500
0
10
20
30
40
50
60
70
80
90
100
17
20
24
6 9 13
ship draft
propeller suction
effect depth
Shi
p s
pee
d (k
nots
)
Shi
p s
pee
d (k
nots
)
50 100 150 200 250 300 350 400 450 5005
10
15
20
25
Max
imum
shi
p s
pee
d (k
nots
)
rate (1.6 m s–1)
"not safe""not safe""n t f ""
"safe"
ent rate (0.6 m s–1)
ship dr
pellctiopp
t d"not safe"
13
17
2024
propsuc
p p
effect"not safe"ot f
"safe"
Fig. 6. Modeled depth of whales consid-ered safe from collisions based on reactiondistances and ship speeds for descent rateof the (a) response dive and (b) foragingdive. Curves represent different modeledship speeds (in knots), shown on right ver-tical axis. (c) Modeled distances at whichwhales would need to react to ‘avoid’ colli-sion with the ship as a function of the dif-ferent des cent rates shown in (a) and (b)
and for 1.0 m s−1. 1 knot = 0.514 m s–1
Endang Species Res 27: 219–232, 2015
ing at its normal operating speed (8.8 m s–1 [17knots]), the whale would have had to initiate its dive275 m or more before the ship to avoid a collision.
In general, a ship traveling at 6.7 m s–1 (13 knots)would require a whale to react with a response diveat a distance of at least 200 m to avoid collision. Forships traveling at 10.3 m s–1 (20 knots), whales wouldneed to begin their response dive at a distance of atleast 300 m. At the faster descent rate of foragingdives, the risk of collision is reduced (Fig. 6b). If weassume the reaction distance of a whale is 250 m,then the maximum speed in order for a response diveto be effective is 8.8 m s–1 (17 knots) (Fig. 6c).
Additional behavioral effects of close ship passage
In addition to the limitations of the response divesas an avoidance mechanism, the presence of a re -sponse dive during a surface period increases thetotal surface time, directly reducing the amount oftime whales spend foraging. For example, the pas-sage of a container ship when Whale 2 was at the sur-face resulted in an additional 5.5 min spent at the sur-face, when compared to the average surface timefrom the entire dive record. For this whale, the aver-age frequency of lunges per dive was 0.77 lungesmin−1 (3 lunges dive−1; 3.9 min bottom time). Increas-ing the time spent at the surface by 5.5 min mayresult in as many as 4 fewer lunges (Table 2). Giventhat there is a tradeoff between the response diveand lost foraging time, whales may be choosingwhen to react based on the perception of the threat.Whales foraging in the shipping lanes only showedconsistent reactions when at the surface during CPAand for faster moving ships.
DISCUSSION
Our study region was identified as an area of highship-strike risk (Berman-Kowalewski et al. 2010,Redfern et al. 2013), and our analysis of blue whalebehavior showed that whales do not appear to avoidareas of heavy ship traffic (e.g. commercial shippinglanes), nor, according to the results of our study, dothey move laterally away from oncoming ships (Fig.3). Our study did find evidence that blue whales arecapable of reacting to avoid approaching ships(within 3.6 km) using a response dive when at thesurface. Response dives were observed in 55% of theship passages, and were most common when whales
were at the surface and passing ships were at theirclosest distance. However, the response of bluewhales to oncoming ships was of limited effective-ness in avoiding collision, inconsistent across situa-tions (e.g. ship types and whale locations), and likelyto be incapable of adjusting to varying ship speeds(Fig. 6). Although recent evidence suggests ship-strikes are not an immediate threat to the survival ofthis population of blue whales (Monnahan et al.2014), the levels of mortality from ship-strikes arestill above legal limits set by the USA. The limitedeffectiveness of re sponse dives and lack of evidencefor lateral movement in reaction to ap proaching shipsshows that blue whales are vulnerable to collisionswith ships, especially in regions of high ship traffic.Mitigating the risk of ship strikes remains a legal pri-ority in these regions.
Slower ship speeds have been identified as a miti-gation strategy for reducing ship−whale collisions.The primary evidence for this benefit has been thereduced lethality of collisions when they occur withslower ships (Laist et al. 2001, Conn & Silber 2013,Laist et al. 2014). Unfortunately, most ship−whalecollisions are not documented at the time of occur-rence. Thus, the ship types and speeds most fre-quently causing collisions remain unknown. Mostship-strike deaths are primarily documented inreports of stranded whales with blunt force trauma orother injuries indicative of ship-strike. However,these underrepresent the true scope of the issue,since only a small portion of whale mortality resultsin carcasses washing ashore (Williams et al. 2011).The lack of information on the factors contributing toship-strikes makes it difficult to determine how shipspeed alters the incidence and lethality of ship-strikes. Only if whales respond differently to slowerships or make use of the added time to respondwould slower ship speeds reduce the incidence ofship-strikes and not just their lethality. Prior to thisresearch, data on whale responses to ships in colli-sion situations have been limited.
The response dives we describe would be moreeffective in avoiding slower-moving ships than fastones, assuming whale responses occur at the samedistance to the oncoming ship. For a response dive tobe effective when ships are traveling at 8.8 m s–1 (17knots) (Fig. 6c), whales would need to initiate aresponse dive with a descent rate of 0.6 m about 250m in front of an oncoming ship. We evaluated howwhale reaction distance, whale descent rates, andship speeds interact to affect the likelihood ofship−whale collisions. Our methodology assessedcollision risk based on measured parameters of a
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McKenna et al.: Whale behavior and ship-strike vulnerability
response dive during ship passages (Fig. 6). How-ever, given our limited sample size, it remainsunknown how different ship speeds affect the dis-tance at which whales react. If whales react to fasterships at greater distances, then it would somewhatreduce the risk, while if the reverse were true, itwould increase the vulnerability to faster-movingships.
The sound or other signal that caused the whalesobserved in this study to react to the approachingships is unknown. Previous research that broad-casted ship sounds to right whales did not elicit aresponse (Nowacek et al. 2004); however, the dis-tances tested were greater than the response dis-tance observed in this study. At closer distances inthis study, ship noise could sometimes be detected onthe acoustic tags. There is a possibility that the reac-tion to the oncoming ship may be a visual response.Blue whales are known to use visual cues to optimizeprey intake by diving under the prey patch anddirecting foraging lunges on back-lit prey patches(Goldbogen et al. 2013).
The ship approaches documented in this studyrevealed distinctive short-term shallow dive avoid-ance responses that did not prevent the whale fromquickly resuming normal foraging dives. This re -sponse was comparable to dive behavior measureddirectly after tag deployments, and possibly repre-sents a general reaction to perceived threats at thesurface. In contrast to response dives, foraging divesafter surface recovery intervals were characterizedby faster descent rates at steeper body angles (Gold-bogen et al. 2011), suggesting that whales at the sur-face may not typically use high-energy rapid descentdives to avoid surface threats.
The presence of response dives results in longersurface periods, which may increase whale vulnera-bility to collisions with other ships traveling in thesame lane and after a previous ship passage, particu-larly in regions with high levels of ship traffic. Fur-thermore, the lack of lateral avoidance keeps whaleswithin the shipping lanes during this extended sur -face period. Ship behavior around ports frequently re-sults in stacking up of ships depending on schedulesat the port (McKenna et al. 2009), possibly resulting incertain regions and time periods of increased risk ofship strike.
Differences in ship size, ship speed, bow design, orwhale activities at the time of ship encounters likelyinfluence whale response behavior (Ellison et al.2012). If behavior during ship encounters varies byspecies, it may explain the apparent differences incollision rates for different species (Douglas et al.
2008). The approach of this study offers a method toassess such variables and determine what shipspeeds may reduce, not just the lethality of collisions,but also how speed may alter the incidence of shipstrikes. The integration of high-resolution data frombio-logging tags with ship AIS data provides the nec-essary information to evaluate whale behavior in dif-ferent scenarios, and we encourage similar studies toexpand sample size.
Acknowledgements. Tag data were collected under NOAA-NMFS Permit 540-1811 issued to J. Calambokidis. Theauthors gratefully acknowledge the efforts of all involved incollecting these data, especially: S. Katz, K. Stingle, S. Ke -rosky, A. Friedlaender, B. Southall, E. Falcone, G. Schorr,C. Garsha and numerous field volunteers. We thank SteveKatz of the Channel Islands National Marine Sanctuary forcontinued support of the project, securing time onboard theRV ‘Shearwater’, his assistance with field logistics, and hisinsight on data analysis. The Channel Islands National Mar-ine Sanctuary provided assistance in field operations, andT. Shinn, M. Davis, and C. Lara are thanked for their help.This work was supported by ONR, Award NumberN000140811221 to E. Oleson and J. Hildebrand. Additionalfunds were provided by a NMFS Marine Mammal Conser-vation Division, National Academy of Science, postdoctoralfellowship awarded to the senior author and ChannelIslands National Marine Sanctuary.
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Editorial responsibility: Clive McMahon, Darwin, NT, Australia
Submitted: June 23, 2014; Accepted: December 29, 2014Proofs received from author(s): April 11, 2015
