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Linking seasonal migratory
patterns with prey availability in
Steller sea lions
Jamie N. Womble1, Michael F. Sigler2, Mary F. Willson3
1National
Park Service-Glacier Bay Field Station
Fisheries Science Center-Auke Bay Laboratory
3University of Alaska Fairbanks-School of Fisheries and Ocean Sciences
2Alaska
Migration
• Occurs in many species
– Annual
– Seasonal
– Daily
Seasonal Migration
• Movements among two or more areas that
are occupied in different seasons during
the annual cycle (Dingle 1996).
Possible explanations for the
evolution of migratory behavior
• Reduces the risk of predation
• Enhances access to resources such as
breeding sites
• Enhances access to patches of highquality food and/or shifting patterns of
food abundance
Photo: Jeff Mondragon
Serengeti Migration
Objectives
• Assess the seasonal distribution and
migratory patterns of sea lions
• Classify seasonal distribution patterns
• Determine to what extent seasonal
distribution patterns of sea lions can be
explained by seasonal prey concentrations
Prediction
• Sea lions should aggregate at terrestrial
sites near where seasonal prey densities are
high in order to achieve energy intake to
meet seasonally changing energy demands.
Methods
•
Aerial Surveys (2001-2004)
-24 sites (23 haulouts, 1 rookery)
- monthly (n = 39 surveys)
•
Classification of distribution patterns
-Standardized data by computing the
proportion of maximum count for each site
-Hierarchical cluster analysis
•
Estimation of proportion of sea lions associated with
each pattern
Lynn Canal
Stephens
Passage
Icy Strait/
Cross Sound
Chatham
Strait
Gulf of Alaska
Frederick
Sound
6000
MEAN NUMBER OF SEA LIONS
5000
4000
3000
2000
1000
0
JAN
FEB
MAR
APR
MAY
JUN
JUL
MONTH
AUG
SEP
OCT
NOV
DEC
JAN
FEB
#
#
#
#
#
#
#
1 - 75
76 - 150
#
151 - 300
#
301 - 500
#
501 - 700
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
##
#
#
#
#
#
#
#
#
#
#
1001 - 1500
#
#
#
#
701 - 1000
#
##
#
MAR
APR
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
~30% in
Lynn Canal
#
#
#
#
#
#
MAY
~31% in
Lynn
Canal
#
#
#
1 - 75
76 - 150
#
#
#
151 - 300
#
301 - 500
#
501 - 700
#
JUN
#
#
#
#
#
#
#
##
#
#
#
#
#
#
#
#
#
#
#
#
701 - 1000
#
#
#
#
#
#
#
#
1001 - 1500
#
#
JUL
~38% in
Cross
Sound
#
#
#
AUG
~30% in
Cross
Sound
#
#
#
~60% in
Frederick
Sound
#
#
#
#
#
#
#
#
#
#
#
#
#
~50% in
Frederick
Sound
##
#
#
OCT
SEP
#
#
#
#
#
#
1 - 75
76 - 150
#
151 - 300
#
301 - 500
#
#
#
#
#
#
#
#
#
#
##
#
#
#
#
#
#
#
#
#
#
#
501 - 700
#
701 - 1000
##
##
#
#
#
#
#
#
#
#
1001 - 1500
#
#
NOV
DEC
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
# #
#
##
#
#
#
#
##
#
#
#
#
#
0.00
YA S H A IS L
LITTLE IS L
S A IL IS L
DO RO TH Y
P T C A RO LU S
MET PT
G RA V E S RKS
S W BRO TH E RS
M ID DLE P A S S RK
Observations
RO C KY IS L
N W IN IA N IS L
S M A RBLE IS L
M IS T
S U N S E T IS L
G RA N P T
F U N TE R BA Y
WE S T BRO TH E RS
TU RN A BO U T IS L
P T LE A G U E
Type 3
Type 4
Type 2
Type 1
3.91
TE N A KE E
RO U N D RK
P T LU LL
C IRC LE P T
BE N JA M IN IS L
Distance
Classification of seasonal distribution patterns
11.73
7.82
Month
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
FEB
JAN
% of Max. # of Sea Lions
BENJ
CIRC
TENA
PTLU
RDRK
PTLG
TURN
WEBR
FUBA
1.0
0.9 Type 1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
Month
0.8
0.6
0.4
DEC
Type 2
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
FEB
JAN
% of Max. # of Sea Lions
1.0
GRPT
SMAR
MIST
SUNS
NWIN
0.2
Type 3
SWBR
MIPA
YASH
Month
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
FEB
GRRK
JAN
% of Max. # of Sea Lions
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
Month
DEC
NOV
0.4
OCT
0.6
SEP
0.8
AUG
Type 4
JUL
JUN
MAY
APR
MAR
FEB
JAN
% of Max. # of Sea Lions
1.0
ROCK
SAIL
MEPT
PTCA
LIIS
0.2
0.8
0.8
Type 2
Type 1
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
0.0
0.8
0.8
Type 4
Type 3
Up to 78% in
summer
Up to 44% in fall
0.6
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
DEC
NOV
OCT
SEP
AUG
JUL
JUN
MAY
APR
0.0
MAR
0.0
FEB
0.2
JAN
0.2
FEB
0.4
0.4
JAN
0.6
Up to 56% in
spring
JAN
DEC
NOV
OCT
SEP
AUG
JAN
0.0
JUL
0.2
JUN
0.2
MAY
0.4
APR
0.4
MAR
0.6
FEB
0.6
FEB
Up to 55% in winter
Type 1
Benjamin Island (2001-2004)
Womble & Sigler (2006) 325: 281-293 MEPS
Type 1
BENJAMIN ISLAND
2001-2004
FEB (n = 210)
1.0
APR (n = 207)
0.9
0.8
DEC (n = 165)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Pacific hake
Eulachon
Arrowtooth
flounder
Sculpin
Northern lampfish
Cephalopod
Pacific salmon
Capelin
Pacific cod
Skate
Pollock
0.0
Pacific herring
FREQUENCY OF OCCURRENCE
OCT (n = 205)
PREY SPECIES
Womble & Sigler (2006) 325: 281-293 MEPS
Type 2
Gran Point 2001-2004
1600
2001
1400
2002
2003
NUMBER OF SEA LIONS
1200
2004
1000
800
600
400
200
0
JAN
FEB
MAR
APR
MAY
JUN
JUL
MONTH
AUG
SEPT
OCT
NOV
DEC
Type 2
APR
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
MAY
#
#
#
#
Spawning eulachon
#
#
#
Spawning herring
#
#
#
#
#
#
#
#
#
Spawning capelin
#
#
#
#
#
Womble et al. (2005) 294: 271-282 MEPS
#
PREY SPECIES
Arrowtooth
sp.
P. cod
Sandfish
Myctophids
Salmon
Cephalopods
Skate
Eulachon
Herring
Pollock
Capelin
FREQUENCY OF OCCURRENCE
Type 2
GRAN POINT (Sea Lion Rock)
2001-2003 (n = 218)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Type 3
SOUTHWEST BROTHERS
2001-2004
2000
2001
NUMBER OF SEA LIONS
1800
2002
2003
1600
2004
1400
1200
1000
800
600
400
200
0
JAN
FEB
MAR
APR
MAY
JUN
JUL
MONTH
AUG
SEP
OCT
NOV
DEC
Type 4
Sail Island 2001-2004
1400
2001
2002
NUMBER OF SEA LIONS
1200
2003
2004
1000
800
600
400
200
0
JAN
FEB
MAR
APR
MAY
JUN
JUL
MONTH
AUG
SEP
OCT
NOV
DEC
Type 3 and Type 4
JUL
AUG
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
##
#
#
#
#
#
#
SEP
Salmon migratory corridor
#
#
#
#
Pollock primary prey species with
FO of salmon up to 35% in
late summer and fall in Frederick Sd with
(Tollit, UBC)
#
#
#
#
#
#
#
##
#
#
#
#
Spring-spawning
forage fish
(herring, eulachon,
Capelin))
Summer-spawning
salmon
Fall-spawning
salmon
Over-wintering
herring
Pollock
(available year-round)
MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB
Conclusions
• Seasonal prey species available from a few
weeks to several months
• Pollock is available throughout the year and is
supplemented with seasonal prey species
• Migratory behavior of sea lions enhances
access to patches of high-quality prey and
shifting patterns of food abundance
ACKNOWLEDGEMENTS
• AFSC-Auke Bay Laboratory
• NOAA Fisheries-SSLRI
• Rasmuson Fisheries Research Center
• Jacques Norvell-Tal Air
• Julie Scott-Ashe, Kathleen White, Mervi Kunnasranta, Ben Williams,
Dave Csepp, JJ Vollenweider, Ben Williams, Karen Blejwas
• MMPA/ESA Permit No. 782-1532-02