643
Diving behavior in relation to anlbient water temperature in northern elephant seals
HIROSHI HAKOYAMA 1
Department of Fisheries, Faculty of Agriculture, Kyoto University, 606 Kyoto, Japan
BURNEY 1. LE BOEUF
Department of Biology and Institute of Marine Science, University of Cal�fornia, Santa Cruz, CA 95064, U.S.A.
YASUHIKO NAITO
National Institute of Polar Research, 9-10, Kaga 1-chome, Itahashi-ku, Tokyo 173, Japan
AND
WATARU SAKAMOTO
Department of Fisheries, Faculty of Agriculture, Kyoto University, 606 Kyoto, Japan
Received September 17, 1993
Accepted February 1 6, 1994
HAKOYAMA, H., LE BOEUF, B.l., NAITO, Y., and SAKAMOTO, W. 1994. Diving behavior in relation to ambient water
temperature in northern elephant seals. Can. 1. Zool. 72: 643 - 651.
Our aim was to describe changes in ambient water temperature during the course of migration by northern elephant seals
(Mirounga angustirostris) and to examine evidence for the seal using abrupt temperature gradients for locating prey. During
migration in the post breeding season, the diving patterns of 10 adult females and 7 breeding-age males from Ano Nuevo,
California, were recorded with time-depth recorders in 1989-1991. Recorded sea surface temperatures declined from
11-13°C to a low of 3-9°C as the seals moved north and increased as they returned. Depth of diving was not closely linked
to sharp thermal gradients. A thermocline was evident only at the beginning and end of the migration in less than 100 m of
water, where less than 2% of diving takes place. There were sex differences in the temperature range at the depths where
75% of diving and foragi"g occurred, owing in part to habitat separation. The temperatures were lower and the range narrower
for females (4.2-5.2°C at 388 - 622 m) than for males (5.3- 6.0°C at 179- 439 m). We conclude that the northern elephant
seal habitat does not provide abrupt changes in temperature that might serve as important cues for locating prey.
HAKOYAMA, H., LE BOEUF, B.l., NAITO, Y., et SAKAMOTO, W. 1994. Diving behavior in relation to ambient water temperature
in northern elephant seals. Can. 1. Zool. 72 : 643-651.
Nous avons etudie les changements de temperature de l'eau au cours de la migration de l'Elephant de mer boreal (Mirounga
.angustirostris) et nous avons eprouve l'hypothese selon laquele cet animal se sert des-gradients marques de temperature pour
localiser ses proies. Les plongees et les temperatures ambiantes ont ete mesurees au moyen de sondes enregistreuses de
temps-profondeur au cours de la migration ulterieure a la reproduction chez 10 femelles adultes et 7 males en age de se
reproduire, a Ano Nuevo, Californie, au cours des annees 1989-1991. La temperature de l'eau en surface est passee de
11-13°C a un minimum de 3-9°C au cours de la migration vers Ie nord et a suivi la tendance inverse au cours de la migration
de retrour. La profondeur des plongees n'etait pas reliee de tres pres a des gradients thermique marques. II n'y avait pas de
zone de thermocline tres evidente, sauf au debut et a la fin de la migration, a moins de 100 m de profondeur ou moins de
2% des plongees ont ete observees. L'etendue des temperatures etait differente chez les males et chez les femelles aux
profondeurs ou 75% des plongees et des quetes de nourriture avaient lieu, en partie a cause de choix differents d'habitats.
Les temperatures etaient plus froides et l'etendue moins grande dans Ie cas des femelles, 4,2-5,2°C a 388-662 m, que dans
celui des males, 5,3- 6,9°C a 179- 439 m. Les resultats demontrent que l'habitat de l'Elephant de mer boreal ne comporte
pas de gradients marques de temperature qui pourraient favoriser la localisation des proies.
[Traduit par la Redaction]
Introduction
Water temperature and light intensity are the most important
The aim of this study was to determine the relationship
between diving behavior of northern elephant seals,
angustirostris,
and
water
temperature.
hypothesis of Boyd and Arnbom
We
Mirounga
tested
the
(1991) that elephant seals
use temperature discontinuities between water masses to help
locate prey whose density is associated with these abrupt
temperature gradients. It is important to test this hypothesis
because it is based on observation of only one female southern
elephant seal,
Mirounga leonina, swimming in Antarctic
waters with sharp discontinuities in water masses that are
colder than those experienced by northern elephant seals.
Since northern elephant seals migrate northward for long
distances (Le Boeuf
1994; Le Boeuf et al. 1993), the foraging
factors affecting vertical distribution of pelagic prey (Kinne
1963;
Paxton
Thermoclines
1967).
the
vertical
squids appear to use light intensity as a diel vertical migration
indicator (Pearcy et al.
1977; Willis and Pearcy 1982). The
deep scattering layer is most obvious in highly productive,
fertile areas (Tait
1980). Horizontal distribution of prey
species is also restricted by horizontal water temperature
distribution.
gradient
Thus,
during
measurement
the
foraging
of the ambient thermal
migration
of the
northern
elephant seal is important for understanding diving and
foraging behavior in relation to prey distribution.
Some studies on the diving behavior of other marine
environment and prey species change gradually during transit.
mammals (Croxall et al.
I Present address: Department of Biology, Faculty of Science,
Kyushu University, Fukuoka 812, lapan.
the
Printed in Canada / Imprime au Canada
restrict
distribution of pelagic fishes. Many mesopelagic fishes and
1985; Gentry and Kooyman 1986)
1992a, 1992b) address
and gentoo penguins (Williams et al.
relationship
between
distribution. Hindell et al.
diving
and
potential
prey
(1991) showed that the water
CAN. J. ZOOL. VOL. 72. 1994
644
160
170
140
150
120
130
60
50
50
40
40
b..
30
0
0
Renee
Joe
Pico
0
a.
30
-b
160
150
140
130
120
FIG. 1. The migratory paths of seals Pico, Joe, and Renee represent three categories of tracks, i.e., coastal, oceanic-coastal, and oceanic, that
characterize the tracks of 15 of the seals in the sample. Tracks are not available for one female (Rae) and one male (Weed). Three seals (Pico, C508,
and 0318) took the coastal path (shaded area a), five seals (Joe, Tyke, Zilla, Quake, and Lilly) took the oceanic-coastal path (shaded area b), and
seven females (Renee, Sydney, YBI43, D197, Quebec, CII5, and Glori) took the oceanic path (shaded area c).
temperatures where male and female southern elephant seals
foraged were
different.
From a
comparison of ambient
temperatures recorded by means of time-depth recorders
(TDRs) and sea surface temperatures (SSTs) determined by
satellite, they estimated that males foraged over the Antarctic
Continental Shelf and females foraged just off the Antarctic
Continental Shelf or near the Antarctic Polar Front.
This paper addresses changes in ambient temperature
experienced by free-ranging, migrating northern elephant
seals. The specific aims of the study were to
(i) describe
changes in ambient water temperature during the course of
migration, distinguishing between SST and water temperature
at depth;
(ii) examine changes in diving behavior in relation
to changes in ambient water temperature; and (iii) determine
whether there are sex differences in the relationship between
diving behavior and ambient temperature.
fishes, and invertebrates that live in diverse habitats from the
coastal region to the open sea. Predominant among these
species are mesopelagic squids, epipelagic Crustacea, and
neritic teleosts; benthic elasmobranchs and cyclostomes are
also consumed, especially by males (Condit and Le Boeuf
1984; Antonelis et aI. 1987; DeLong and Stewart 199 I ;
Antonelis et aI. 1994).
Diving studies of northern elephant seals using TDRs reveal
an average dive depth in the range
300-600 m, with
1000-1500 m (Le Boeuf et aI.
1988, 1989; Naito et aI. 1989; DeLong and Stewart 1991).
maximum dives in the range
Diving records suggest that dive depth varies with vertical
distribution of prey. The adult female diving pattern is
characterized by a diel periodicity that appears to be affected
by diel vertical migration of mesopelagic prey. Males, in
contrast,
most
frequently
exhibit
continuous,
uniform,
flat-bottomed dives near continental margins, suggesting the
Background information
Northern elephant seals reproduce on land and feed at sea.
Recent studies have begun to reveal their foraging habits.
They spend 8-10 months of the year foraging at sea (Le Boeuf
1994). Prey species have been identified by analysis of
pursuit of benthic prey (Le Boeuf
1994; Le Boeuf et aI. 1993).
Diving data and the migratory paths of all but two subjects
in this study were described in Le Boeuf
et aI.
(1994) and Le Boeuf
(1993). Summarized diving statistics revealed that the
by
mean duration, mean depth, and mean surface interval of
observation of prey captured. The diet is composed of squids,
breeding-age males and adult females were 21.25 and 20.82 min,
stomach
contents
of
living
and
dead
animals
and
HAKOYAMA ET AL.
645
(a)
14
--- C115
--+--6---+--0---0----A-+-
0197
0318
Glori
Lilly
Quebec
Rae
Renee
Sydney
- YB143
2 �----�----'---�--r--.---.-4
24
4
14
5
15
25
3
13
23
May
April
March
February
(b)
14
Adults
- 12
(,)
�
� 10
�Q)
-+-
Quake
_
Tyke
-&-
Weed
-tr-
Zilla
16
Subadults
c..
E 8
Q)
Q)
�
't:
:::J
en
as
Q)
en
6
-e-
C508
-+-
Joe
.......- Pico
4
2
9
19
29
March
8
18
April
28
8
18
May
28
7
June
FIG. 2. Mean daily sea surface temperatures (SSTs) during the migration of 10 adult female (a) 3 subadult male and 4 adult male (b) northern
elephant seals from Afio Nuevo, California. The arrow marks the boundary between a steep thermal gradient and relatively stable SSTs for the males
Weed, Tyke, Zilla, and Joe.
330 and 509 m, and 2.73 and 2.08 min, respectively. Males
migrated north toward the northeastern Pacific continental
margins, traveling thousands of kilometres from the rookery,
females moved to the open sea.
Methods
The results were obtained from 17 northern elephant seals during
the postbreeding migration in 1989 -1991. The subjects were 10 adult
females (4-10 years old) and 7 breeding-age males (4 adult males of
8-12 years old and 3 subadult males of 5 years old). One adult male
was sampled in 1989, 6 adult females and 3 adult males were sampled
in 1990, and 4 adult females and 3 subadult males were sampled in
1991.
TDRs (Wildlife Computer Mk3, Wildlife Computers, Woodinville,
Wash.) were attached and deployed on the animals at Afio Nuevo,
California (37.1°N, 122.2°W) in late February and March and
recovered in April, May, or June. The TDR and a radio transmitter
(Advanced Telemetry Systems, Bethel, Minnesota) were glued with
marine epoxy to the pelage on the dorsal midline of the seal behind
0
9
March
11
March
21
5
31
April
C115
15
April
1'0
10
25
20
April
�=- ....
20
5
May
30
1
-,
May
1
3�
100
0
800
700
600
500
27
19
29
8
18
April
28
800
17
February
800
700
7
6001
700
500
600
500
300
400
D197
�3
Feb ary
0
100
200
400
300
0
100
200
800
31
6001
700
800
600
700
400
400
......
5
.
17
February
-
28
•
10
22
27
L:f"1n
�
12
February
11
t""II
20
4
21
9
.F'4l �
March
March
March
1
�
19
30
-�
14
March
31
15
March
��l .-....1 h�1 -L...,,.,....-l
23
1"""\
Adult Females
J��
13
February
V".......,
I......" :.
19
February
0
100
200
300
400
---
500
19
26
21
�
27
"7fV'1l
500
February
16
March
March
17
April
���v�
-,
......,.�rly-.II
200
300
9
6
11
'
..-"\�3......
18
March
8
28
y-(� nJ4
u
300
200
100
�
�
-----1-n
800
24
ebruary
700
0. 500
Q.) 600
+-'
1
800,
19
February
0
100
700
600
500
400
200
300
200
- 300
..c 400
E
_
G
700
Y
800
26
February
0
100
600
500
200
300
400
0
100
Adult Females
29
9
24
10
25
Y8143
April
Sydney
Rae
April
Glori
D318
8
19
20
4
April
18
29
13
April
30
14
April
:0
'Ci
.j:>.
-..l
.N
0
r
r
<:
�
N
0
0
�
n
;J;>
Q'\
+::Q'\
200
600
500
400
300
30
9
April
lc� 29
Q,j
19
,
9
,
May
19
,
29
,
,
8
June
100
500
,
10
.
20
March
�:1
800
700
600
300
400
500
200
100
I
9
1
2
<
,
19
Marcil
29
3
? 77) r
23
QUake
14
March
o
800
700
600
400
500
111n...
o t::." I
400
600
Tyke
t arch
1
1
800
700
600
300
400
500
200
200
,
�9��V'{J\!I\J
_
300
,
-. __
200
Joe
__
300
700
J"'...
O�..;;:J
f)
April
8
23
22
18
April
13
April
12
28
8
7
cf
Adult Males
8
18
May
23
22
lJl::A:
May
13
May
12
[::f
28
2
7
12
11
6
June
June
June
June
FIG. 3. Changes in vertical distribution of water temperatures during the migration of 17 northern elephant seals from Ano Nuevo, California. The numbers on the contours represent temperature
in degrees centigrade.
Cl
500
0..
Q)600
..r:::::
E
'-"'"
-
°P/"'.l�
100
Subadult Males
0\
.j::.
-....l
»
r-
�
::r::
»
�
o
-<
»
3:
»
CAN. 1. ZOOL. VOL.
648
72,1994
the shoulders (see Le Boeuf et al. 1988, 1989, 1993). Instruments
were programmed to record depth every 30 s and water temperature
every 5 or 10 min. These data-sampling regimes allowed for
recording periods of approximately 35 - 90 days, sufficient time for
most seals to reach the farthest point in their migration from the
rookery. The entire period at sea, or nearly so, was obtained for 3
females (Quebec, Renee, and Lilly); approximately 70% of the time
at sea was sampled for 4 males (C508, Joe, Pico, and W eed).
Light-level data from the TDRs carried by the seals, adjusted by
comparisons of SSTs from the TDRs with SSTs from satellites,
provided estimates of position (DeLong et al. 1992; Le Boeuf 1994;
Le Boeuf et al. 1993). SST was defined as the mean water temperature
from 0 to 20 m. Mean daily SST was calculated throughout migration
for all animals. Vertical distribution of water temperature was
assessed by determining the mean water temperature at 20-m depth
intervals. A frequency distribution of dive depth and mean
temperature was calculated for each record to examine the
relationship between diving and the vertical temperature profile.
. -0.24, -0.27, and -0.28°C, respectively, for males Weed,
Results
Meteorological Center of the National Weather Service,
Joe, Tyke, and Zilla
(r2
=
0. 985, 0.962, 0.947, and 0.932, F
1742. 3, 677.9, 611.8, and 452.2, one-tailed test, P
=
<0.0001).
=
Males Weed and Joe stayed in the northern cold waters for
I 1/2 months, from early April to late May, before beginning
the return trip to the rookery.
Males Pico and C508 only migrated northward to waters
with SSTs of 8-9°C (Fig. 2b). Both males reached waters
of this temperature during the last week in March. From
this time to late May and early June, the SST increased to
I I-12°C. This increase in SST was due either to the seals'
returning south toward the rookery or to a seasonal increase
in SST in the area where they were foraging. Increasing
SSTs
in
the
temperate
waters
off
the
coast
of
the
northwestern United States during spring and summer are
well documented (information from the Japan Fisheries
Information Service Center, 1989-1991, and the National
Asheville, N.C.).
Migratory paths
The migratory paths of 6 males and 9 females in this study,
calculated from light levels and SST data by Le Boeuf et al.
Vertical distribution of water temperature
Figure 3 shows changes in the vertical distribution of water
(1993), can be arranged in the three general categories
temperature
depicted in Fig. 1. The coastal category refers to seals that
migration. During the first days of migration from the rookery
migrated
Oregon,
all seals were exposed to a steep thermocline, a vertical
Washington, and Vancouv r Island, British Columbia (Pico,
thermal gradient greater than 2°C/l 00 m, between the sea
north
to
50°,
the
coastal
�
waters
off
C508, and D318). The oceanic-coastal category applies to
seals that traveled farther from the coast in the open ocean to
sites near the coast above 50° (Joe, Tyke, Zilla, Quake, and
Lilly). The oceanic category comprises seals that migrated
across the open ocean in a northwesterly direction between
40 and 50° (Renee, Sydney, YB143, D197, Quebec, C115,
and Glori).
(Fig. 2), and decreased to as low as 3 -9°C during migration.
The lowest temperatures were obtained around mid-March for
females and in April and May for males. These records
correspond to when the seals had reached the farthest point
in their northward migration. Every individual migrated from
the relatively warm coastal waters of central California to
colder, more northerly waters before returning to the rookery.
The SST records suggest three distinct periods in the
migration: (1) moving north, characterized by a rapid decline
in mean daily SST; (2) a middle period of staying north,
characterized by relatively stable low mean daily SSTs; and
south
on
the
return
leg
to
the
rookery,
characterized by an increase in SSTs. The beginning of the
last period is only evident in the longest records e.g., those
for Quebec, Renee, Lilly, Joe, and Pico.
All adult females migrated northward to 6-9°C waters
(Fig. 2a). Two of the females that were recorded for long
periods,
Quebec
temperature
course
of
the
seals'
northward
surface and 100 m. As the seals migrated northward, the steep
thermocline disappeared and the vertical temperature gradient
and water temperatures declined. All seals were out of the
steep thermocline area in less than 3 weeks of northward
travel, i.e., before mid-March. There was no clear thermocline
below 100 m
'!t
any point in the migration of all seals. The
diving records (Lilly, Quebec, and Renee), reflecting their
SSTs at departure from Ano Nuevo ranged from II to 13°C
moving
the
steep thermocline reappeared for seals with sufficiently long
Surface water temperature
(3)
during
and
range
for
Renee,
stayed
approximately
in
10
waters
and
in
30
this
days,
respectively. Seven of the females provided data only during
the first segment of the migration, so it is not clear how long
they stayed in these cold waters. The youngest female (Lilly)
remained in 5°C waters from 30 March to 15 April.
return to the same location on their southward migration.
For those seals that reached locations with SSTs of 3- 4°C
(Weed, Tyke, Zilla, and Joe), there were periods with virtually
no changes in temperature throughout the water column to
500-600 m. Some seals (Lilly, Weed, and Joe) migrated to
locations with SSTs of 4-5°C, which have a local maximal
layer of 5-6°C at 200-400 m.
A plot of mean water temperature against diving depth
for each seal (Fig. 4) makes several points:
(i) there is no
(ii) the
evidence of a significant thermocline below 100 m;
temperature gradient and variability in temperature at depth
decrease with depth: at 500 m and deeper, the decrease in
the temperature gradient and the variability in temperature
are markedly attenuated;
(iii) the thermal gradient at depths
(iv) there
where seals do most of their diving is very small;
is a sex difference in the water temperature at which males
and females do most of their diving. The mean water
temperature ranged from 4.2 :±: 0. 3 to 5.2 :±: 0.2°C at the
depths where females did 71% of their diving (388 :±: 139
to 662 :±: 34 m). In contrast, the mean water temperature
ranged from 5. 3 :±: 1.2 to 6.9 :±: 1.5°C at the depths where
males did 74% of their diving (179 :±: 128 to 439 :±: 128 m).
Thus, females dived in waters with significantly lower
mean temperatures than did males at both the low end of
the temperature range
(t
that were colder than those frequented by adult females
high end
=
(3-4°C; Fig. 2b). The decrease in mean daily SST from
Pico had a second mode of dive depths at 975-1200 m,
departure to the first 29, 29, 36, and 35 days was -0.23,
where water temperatures ranged from 3 to 3.5°C.
Three of the males (Tyke, Zilla, and Joe) migrated to waters
(t
=
3.51, df
=
2.39, df
=
15, P < 0.05) and the
15, P < 0. 05). Note that the male
600
800
!I'I"""m:p-
,
10
1�
1000
2
1
10
1000 1�
2
10
Rae
N=2293
8
1"",,"'1'''''
6
2
1
1
_
12
10
8
6
\4
2
12
10
8
6
'I
12
10
8
6
4
2
._0
"
.n
1
I
9
14�
12
10
8
6
4
2
o
140
12
10
8
6
4
2
Depth (m)
5
8
7
6
5
4
2
1
'''''f'""+b
10
600 aOO 1000 1�
0
Zilla
8 12
N=2155
7 10
6 8
4
3
2
1
lii'iili:J=
10
600 aOO 1000 1�6
Quake
14
N=1466
12
10
8
6
4
2
F0
200 400"m---,600 aOO 1000 1200
Tyke
N=2043
Adult Males
200
400
�
, I " '=P-q
-n=F"
200
aOO
600
1000
N=5064
Joe
''fiH'.,
800
5
15
10
20
f 25
c:
Q)
:J
CJ
Q)
'-
10
1200
2
3
7LL
6?fl
5
4
r0
1000 1�
8
Pico
7
N=5069
6
4
3
>.
-I1"ft2 0
r--....-�
..
___
s60
''''' "--
C508
N=6469
Subadult Males
FIG. 4. Mean water temperature (±SD) plotted against dive depth (20-m intervals) for 17 northern elephant seals during migration from Ano Nuevo, California (dives greater than 1200 m are
included in the last interval). N is the number of dives.
"
4
2
lliil'1l111tu:y-..,
0
1
600
800 1000 1�
6
14
12
Renee
N=3696
12
10
10
8
8
6
6
4
4
2
2
0
1..!""'"""I""ilii"I"",iiiil"ii,n'xJ
,
1
o 200 400 600 800 1000 1�
I
I
8
Sydney
12
N=2691
7
10
6
8
5
4
6
3
4
2
1
'''n:p--., , ., 10
600
800 1000 1�
6
YB143
14
12
N=2252
12
10
10
8
8
6
6
4
4
2
2
01_'.,..,"'"''1'''
to
o 200 400 600 Uur
800 1000 1200
12
10
8
6
4
2
Depth (m)
Lilly
12
N=4238
6
10
5
8
6
3
4
2
2
0o1�IIIIIIIIIIIIIIIIIWN�l
200 400 600 800 1000 1200o
5
12
0197
N=1819
10
r6
5
8
4
6
4
2
2
01.mndlJlWIIIIIIIIIIIIIIIIIIII�... , 10
140' 200 400 600 800 1000 1�
, 8
16
�12
0318
14
N=2323
10
Q)
12
':J 8
10
8
� 6
Q)
6
a. 4
�
4
E
2
2
Q)
I, 1�
10
..",800 1000
8
Glori
12
N=2424
7
10
6
8
4
6
3
4
2
2
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Discussion
. top
1994
100 m when they are near the continental shelf, but the
thermoclines disappear as they move out over the open ocean.
Changes in ambient temperature during migration
The seals experienced a wide temperature range over the
Sex differences
course of the migration, characterized by increasing cold and
The diving patterns of males and females differed with
declining variability in temperature with depth during the
respect to ambient temperatures experienced. Evidently this
northward migration and the reverse on the return south to
was related to differences in the location and depth of diving
the roockery. From temperature data alone, it is clear that the
and in the foraging patterns employed by males and females
seals migrated from the North Pacific Central Water Mass to
(Le Boeuf et al.
the Pacific Subarctic Water Mass, an extensive water mass
in the water column, the depth of most of their dives varying
1993). Females appear to forage exclusively
with low salinity
(33.5-34.5%0 ) and relatively low water
(2-4°C) (Favorite et al. 1976). These are more
with the diel vertical migration of prey in the deep scattering
temperatures
layer. The majority of their dives were in the mean range
productive waters in terms of mesopelagic fish biomass, and,
388 -662 m, characterized by a narrow mean temperature
it is also suspected, of benthic prey density, than the adjacent
range, 4.2-5.2°C. In contrast, the majority of male dives were
water mass to the south and the coastal waters near the rookery
(Gj0saeter and Kawaguchi
1980; Okutani et al. 1988).
Most females moved steadily north to northwest across the
northeastern Pacific from the coast to as far as 150oW, in the
range 44-52°N. Males migrated to areas along the continental
margins off the state of Washington, and north and northwest
to as far as the northern reaches of the Gulf of Alaska and the
eastern Aleutian Islands. Similar migratory paths have been
observed for northern elephant seals from San Miguel Island
in southern California (DeLong et al.
DeLong
1992; Stewart and
1994).
of males appears to be the result of a large proportion of male
dives
being
bottom-limited.
Moreover,
females
did
not
experience the extremes of temperature that males did at the
preferred depths (Fig.
4). This may retlect greater prey
1993).
specialization in females (Le Boeuf et al.
In conclusion, our data suggest that changes in ambient
water temperature do not provide foraging northern elephant
seals with a precise cue to the vertical location of prey. Steep
habitat to the extent that they are observed in Antarctic waters.
The dive depth pattern of the seals in this sample was not
closely linked to steep thermal gradients in water temperature.
In all records, a steep thermocline was evident only in the top
100 m early in the migration, and again on the homeward return
of some seals. Most diving and the greater part of the migration
occurred where no thermocline was present. Moreover, the vast
majority of dives of adults and juveniles exceed
Many other variables that may be involved in finding prey
require further investigation. These include but are not limited
to spatial and bathymetric location, water currents, dissolved
oxygen
distribution,
1988, 1993; Le Boeuf 1994).
1993), the thermocline observed was evidently
restricted to the period when they were moving along the west
coast of the United States. Indeed, there was no difference in
water clarity,
prey bioluminescence
pattern, acoustic reception of prey sounds, and the presence
of predators.
100 m in depth
From our knowledge of the movements of these seals (Le
Boeuf, et al.
179- 439 m, where mean temperatures
5.3 to 6.9°C. The shallower diving
thermal gradients are not found in the northern elephant seal
Dive depth and thermoclin;s
(Le Boeuf et al.
in the mean range
were higher, ranging from
Acknowledgements
We
thank
D.
Crocker,
P.
Morris,
P.
Thorson,
and
numerous others for assistance in the field, R.D. Hill and
M. Braun of Wildlife Computers for help in data analysis,
and S. Blackwell, I. Boyd, D. Crocker, and C. Lydersen for
500 m midway
(5, 3, and 4°C for males Weed,
comments on the manuscript. This research was supported
Thus, northern elephant seals do not appear to use abrupt
from George A. Malloch, executor of the G. Macgowan
temperature throughout the water column to
in the migration of some males
Tyke, and Zilla, respectively).
changes in water temperature as a cue for finding prey as was
suggested for southern elephant seals by Boyd and Amborn
(1991). This difference, the link between diving and ambient
temperature and the foraging strategy employed, may be
habitat related. Southern elephant seals from Macquarie Island
(Hindell et al.
1991; Slip et al. 1994) and South Georgia
1991; McConnell et al. 1992; Fedak et al.
1994) feed mainly on squid (Rodhouse et al. 1992) between
(Boyd and Amborn
the
Antarctic
Polar
Front and
the
continental
shelf
of
Antarctica, waters characterized by low temperatures, steep
vertical gradients in temperature, and temperature inversions
(Whitworth
1988). Southern elephant seals from Patagonia,
however, do not enter these southerly waters to forage but
remain
in
Subantarctic
(Campagna et al.
Waters
east
of
their
rookery
1994). Subantarctic Water is the southern
hemisphere analogue to the Pacific Subarctic Water Mass
(Favorite et al.
1976), where northern elephant seals forage.
Plots of ambient temperature versus dive depth profiles for
northern elephant seals (Fig.
Patagonia (Fig.
4) and elephant seals from
4 in Campagna et al. 1994) are remarkably
similar; both species dive through a steep thermocline in the
in part by grants from the National Science Foundation and
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