THE DISTRIBUTION, POPULATION DYNAMICS, AND BEHAVIOR
OF THE HARBOR SEAL (PHOCA VITULINA RICHARDI)
SOUTH
SM~
F~CISCO
IN
BAY, CALIFORNIA
A Thesis Presented to the Graduate Faculty
of
California State University, Hayward
0
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Biology
By
Lyman E. Fancher·
....
t
November, 1979
Monterey
---
------:-------
ACKNOWLEDGEMENTS
I would like to express my appreciation to
Dr. Samuel McGinnis, Dr. Edward Lyke, and Dr. Robert Main
for their comments and suggestions which were offered
during
preparation of this thesisi the Leslie Salt
th~
company and the personnel of the South San Francisco Bay
National Wildlife for ·~heir cooperation and for allowing
access to the Mowry Slough area during the course of this
studyi Doris Alcorn for her kind supporti and to Jack
0
Binder,drawbridge tender on Southern Pacific Railroad's
Dumbarton Bridge, for introducing me to the harbor seal.
iii
TABLE OF CONTENTS
Page
LIST OF TABLES .
v
LIST OF FIGURES
vi
Chapter
INTRODUCTION
l
MATERIALS AND METHODS
8
RESULTS AND DISCUSSION .
22
Census . .
22
Population Dynamics and Seasonal Movements
34
Distribution of Harbor Seals in Mowry Slough .
36
Individual Behavior and Behavioral Interactions Between Individuals
51
Diving Behavior
68
SUMMARY AND CONCLUSIONS
99
Suggestions for Future Research and Management
LITERATURE CITED .
.
.
.
.
101
104
iv
LIST OF TABLES
Page
Table
1.
2.
3.
4.
5.
6.
Population data for Mowry Slough area
from 1972-1974 and for 1976 .
'25
Census data on harbor seal numbers at the
various haulout sites in the San
Francisco Bay area in 1976
28
Frequency of vigilance postures displayed
by four harbar seals on South East
Farallon Isl~nd on June 26, 1974
62
Average duration of dives and time spent
on the surface between dives executed by
eight individual harbor seals and a female
with a pup
7~
Comparison of the average diving time of
seals of different age groups at the .05
confidence level
77
Comparison of four types of aquatic movement patterns displayed by seals in terms
of dives/hour and rate of travel in the
horizontal plane in meters per minute .
84
v
LIST OF FIGURES
Page
Figure
1.
Location of known harbor seal haulout
sites in the San Francisco Bay area .
2
2.
Detail map of Mowry Slough area .
9
3.
Map of the Grecko Island haulout site .
12
4.
Map of South East Farallon Island (largest
island shown in figure)
15
Location of harbor seal haulout site on
the northernmost tip of Ano Nuevo Island
17
Population dynamics for Mowry Slough area
from 1972-1974 using the highest number
of animals observed each month
23
Distributions of seals in the Mowry Slough
area during high tide on April 27, 1972 .
37
Distribution of seals in Mowry Slough area
during low tide, on April 27, 1972
39
5.
6.
7.
8.
9. ·Number of harbor seals hauled out inside
of Mowry Slough and on Mowry Slough
channel with respect to time of day and
tide condition, on September 27, 1973 .
10.
11.
42- -
Number of harbor seals hauled out on
Ano Nuevo Island with respect to time of
day and tide conditions on February 8
and February 9, 1974
42
Number of seals hauled out at low and high
tide during the 1972 pupping season,
Mowry Slough
44
12. ~size of the largest seal group observed in
the Mowry Slough area expressed as a
percentage of the total population on the
dates indicated in 1972 .
vi
52
LIST OF FIGURES (Continued)
Page
Figure
13.
14.
15.
16.
17.
18.
19.
20.
21.
~
Comparison of the frequency of vigilance
postures displayed by groups of harbor
seals observed on South East Farallon
Island 1 Ano Nuevo Island, and in the
Mowry Slough area
64
Dive duration histogram for 31 individual
harbor seals
69
Frequency histogram of the duration of
time spent .on the surface between dives
by 29 individual harbor seals .
74
Duration of time submerged and corresponding
time on surface between dives for harbor
seal number one (see Table 4)
80
Duration of time submerged and corresponding
time on surface between dives·for harbor
seal number two (see Table 4)
·82
Aquatic movements of seal number two on
May 7, 1973 between 1715 and 1911 Pacific
Daylight time .
86
Aquatic movements of seal number ten on
April 30, 1975 between 0853 and 0911
Pacific Daylight time .
88"
Aquatic movements of a mother and pup on
May 6, 1975, between 0917 and 0934 Pacific
Daylight time .
90
Aquatic movements of seal number eleven
on January 14, 1976 from 1425 to 1428
Pacific Standard time, seal number twelve
on May 3, 1974 from 1353 to 1401 Pacific
Daylight time, and seal number thirteen on
June 5, 1974 from 1420 to 1422 Pacific
Daylight time .
. . .
92
vii
INTRODUCTION
The range of the harbor seal subspecies Phoca
vitulina richardi, found in San Francisco Bay, extends
north along the Pacific coast of North America from
northern Baja, Mexico to Herchel Island, the east
Bering Sea, and the Aleutian Islands (Scheffer, 1968}.
Within San Francisco Bay harbor seals predictably come ashore at six different locations (Figure 1).
These are:
Strawberry Spit on the northwestern shore of
Richardson Bay; Castro Rocks, located at the eastern end
of the Richmond Bridge; Corkscrew Slough, which enters
Redwood Creek near its entrance to San Francisco Bay;
Guadelupe Slough, which enters at the southern most part
of the Bay; l'-1owry Slough, which enters on the eastern
shore ·of the extreme southern part of San Francisco Bay;
and Grecko Island, located just south of the entrance
to Redwood Creek.
Pups are born only at Castro Rocks,
Grecko Island, and Mowry Slough.
During my study the most extensive work was
conducted in the vicinity of Mowry Slough.
Available
archeological evidence indicates that Mowry Slough, which
is now known to be the site of one of the largest harbor
seal rookeries in northern California, has served as a
l
Figure 1.
Location of known harbor seal haulout
sites in the San Francisco Bay area:
A--Drakes
0
Estero, B--Double Point, C--Bolinas Bay,
D--Strawberry Spit, E--Castro Rocks, F--Corkscrew
Slough, G--Grecko Island, H--Mowry Slough,
I--Guadelupe Slough.
3
T
E
.:::
0
C')
l
4
.U 1
out site for harbor seals for quite some time.
Bones
, harbor seals have been found in Indian shell mounds
!ar the vicinity of Mowry Slough (Nelson, 1909) and
·ts at the bottom of the largest of these mounds
->DOSl
...
ve been dated at approximately 3000 years old (Kroeber,
1
PO)·
In.the early 1800's the harbor seal in San
-ancisco Bay was killed·in progressively larger numbers
r fur hunters.
By 1890 the once extensive harbor seal
)pulation of the Bay had been considerably reduced
3onnot, 1928).
~riodically
Harbor seals in Mowry Slough were
hunted for sport until the late 1940's and
irly 1950's according to a duck hunter interviewed in
)vember 1974.
By interviews with local duck hunters,
nployees of the Leslie Salt Company and Southern Pacific
iilroad, it was possible to document the continued
(istence of seals in Mowry Slough since 1920.
Mowry
Lough has recently become part of the newly created
Juth San Francisco Bay National Wildlife Refuge.
Bonnet (1928) described the seal population in
Mowry Slough area as extensive prior to 1890 and
.
1
0"
nner commented in 1962 that a number of pups were born.
the Mowry Slough area each year, but neither author
ncluded specl. f'lc flgures
.
in his report.
In 1967 Alpin
5
stated that the harbor seal population in Mowry Slough
consisted of 50 to 60 nonmigrating seals.
The harbor seal population in San Francisco Bay
bas received very little scientific attention in the
past.
Prior to the start of my observations in Mowry
slough in 1969 the only study that had been done was a
census conducted by Bartholowmew (1947) on a group of
harbor seals that hauled out near the east end of the
Oakland Bay Bridge.
Since the start of my observations,
other studies on the harbor seal in and near San
Francisco Bay have been completed or are presently nearing
completion.
0
From 1970 to 1971, Paulbitski and Maguire
{1972) conducted a census on seals using Strawberry Spit
and Castro Rocks.
In 1974 and 1976 Alcorn conducted a
study on the thermal ecology of the harbor seals in
Mowry Slough . . In 1976 the Bodega Bay Institute of
Pollution Ecology conducted a one-year study on the
population biology and pollution ecology of harbor seals
in San Francisco Bay (Risebrough et al., 1976).
From
1976 to the present Allen has been investigating seasonal
fluctuations in seal numbers and the effects of human
disturbances on harbor seals that frequent Bolinas Bay,
locat~d 16 km north of the Golden Gate (Allen, 1979),
Double Point, a sandy coastal beach located 18 km north
6
of the Golden Gate, and Drake's Estero, 20 km north of
the Golden Gate.
The objectives of my study were as follows.
A
three-year census (1972-1974) was undertaken to determine
yearly population fluctuations and whether the population
present during pupping season was increasing, decreasing
or stable.
Data on the location of seals on land were
plotted during all months of the year during low and
high tides in order to determine the manner in which the
distribution of seals changed in relation to season and
tidal changes.
Observations were conducted in order to
compare the following aspects of harbor seal behavior in
Howry Slough with work previously done in other regions:
interactions between females and their pups; maturation
of pup behavior; breeding season and breeding behavior;
interactions between individuals; and individual behavior
in terms of the frequency and types of activity a seal
exhibited while on land.
The habor seal is considered to be one of the most
alert species of-marine mammal on land, periodically
interrupting its sleep to raise its head and scan the
horizon, even in captivity.
This trait has been referred
•<~
to as vigilance behavior (Schusterman, 1968).
Observa-
tions were conducted to determine if seals in Mowry Slough
displayed vigilance to a greater extent than seals in more
7
remote areas because of periodic human related disturbances
in the past.
To determine this the vigilance behavior of
harbor seals on two nearby coastal islands, Ano Nuevo
and south East Farallon Island, were observed and
compared with the behavior of seals in Mowry Slough.
Although the diving physiology of the harbor seal
has been studied extensively in the laboratory (Anderson,
1966), detailed observations of free diving seals in the
wild have not been done.
During this study the following
characteristics of seal aquatic behavior were investigated:
length of dives; length of time spent on the surface
0
between dives; horizontal movements underwater in terms
of time and space; and, to a limited extent, food habits.
MATERIALS AND METHODS
Observations were conducted at five different
locations.
Within San Francisco Bay observations were
made at Mowry Slough, Grecko Island, and from the
vumbarton Railroad Bridge.
On the nearby open coast
observations were made on South East Farallon Island and
on Afio Nuevo Island.
A description of each of these
locations follows.
Mowry Slough, 180 meters across at its entrance,
is one of the larger sloughs in the south
B~y
(Figure 1).
Tidal waters flowing in and out of the slough maintain
a channel that runs west from the slough's entrance
across the mudflats to the Bay at low tide (Figure 2) .
Even during minus tides this channel is deep enough to
provide an escape route for seals hauled out near it or
inside Mowry Slough.
Along the initial portion of Mowry
Slough that runs in an east-west direction the slope of
north shore is much steeper than the south shore.
The shoreline of the Bay south of Mowry Slough to
Calaveras Point (Figure 2) consists of a steep bank that
rises approximately one and one-half meters above the mud
-a t
low tide, while the shoreline north of Mowry
Slough consists of a gradual slope.
8
Figure 2.
Detail map of Mowry Slough area.
the location of blinds used dUring the study.
X
marks
10
Winter high tide
haulout site
Mowry Slough channel
at low tide
Seal
Cove
0
N
1
\-"'-"-
1 km
Calaveras
Point
ll
Remnants of the once extensive saltmarsh habitat
that characterized the south Bay before large areas were
diked still exist between the shoreline and the levees of
adjacent evaporation ponds in the Mowry Slough area.
Along the north and south shore of Mowry Slough and along
the Bay shoreline north of Mowry Slough dominant plant
species consist of pure stands of Spartina foliosa at
the lower elevations while Salicornia ambigua and
Distichlis spicata occupy higher ground.
The Bay shore-
line south of Mowry Slough is covered mainly by Distichlis
spicata.
During all months of the year harbor seals
regularly haul out inside Mowry Slough and along the
mudflat channel at low tide (Figure 2).
During the pupping
season (late March to early May) seals haul out at
various sites on the shoreline of the Bay from the entrance
of Mowry Slough south to Calaveras Point and at the
entrance to a small slough, located two kilometers north
of the entrance to Mowry Slough (Figure 2) .
Levees of
salt evaporation ponds parallel the shoreline of the Bay
this area as well as Mowry Slough.
Grecko Island lies just south of the entrance to
Redwood Creek (Figure 3).
A group of about 17 harbor
haul out on Grecko Island at a point approximately
and one-half kilometers south of Redwood Creek at each
'igure 3.
Map of the Grecko Island haulout site.
13
N
haul out site
r--150
(
m
4,
14
high tide year round.
The Bay shoreline at the haulout
site consists of a steep bank that rises approximately one
meter above the mud flats at low tide, and is covered
mainly by Distichlis spicata.
Southern Pacific Railroad's Dumbarton Bridge
crosses south San Francisco Bay at its narrowest point
(Figure 1) .
The depth of the water at the bridge is
approximately fifteen
m~ters
at mean high tide and
thirteen meters at mean low tide.
The current at the
bridge during peak ebb or flood tides can reach two
knots (Alpin, 1967).
During pupping season as many as
30 to 40 seals can be observed diving near the bridge at
tide.
South East Farallon Island is a small island in
Farallon group located 49 km west of the Golden Gate
(Figure 4).
Harbor seals observed on South East Farallon
Island were hauled out next to a surge channel on a rocky
area on the south side of the island.
Ano Nuevo Island is a small island, eight acres
area, located 80 km south of the Golden Gate and
ly 750 m from the California coast.
The 23 to
seals observed on the island hauled out near
south end on a sloping shelf of rock approximately
meters in area, and located 22 m north of the
(Figure 5) .
Figure 4.
Mapoof South East Farallon Island
(largest island shown in figure) .
16
T
3
l
::r
Ill
1:
0
...
1:
Ill
II>
Figure 5.
Location of harbor seal haulout site on
the northernmost tip of ADo Nuevo Island.
18
~~'-... Haulout sites
-mmrr
Cliff edge
~30m~
Main
Island
19
Seals were censused in the Mowry Slough area
mon~hs
during most
of the year from 1972 to 1974.
Counts
were made from the levees bordering Mowry Slough during
both low and high tide.
Normally from late July to late
March 7 x 35 binoculars were sufficient for this purpose.
However, during the pupping season large numbers of seals
hauled out next to Mowry Slough channel at low tide were
at times as far as 1.7 km from the nearest observation
point, making counts with binoculars or spotting scopes
difficult.
Satisfactory counts during pupping season
were not obtained until a 'Quarter telescope (Questar 3 1/2,
Questar Corporation, manufacturer) was used starting in
1972.
Thirty-seven counts were made in 1972, 34 in 1973
and 21 counts were made in 1974.
Seals were counted as either pups or adults
because of the difficulty in distinguishing subadult from
t seals.
In late July it became increasingly dif-
t to distinguish pups from first-year subadults.
Only seven counts were made during the 1974
pupp~ng
because access to Mowry Slough was limited due to
Leslie Salt Company's levee maintenance program . . The
figures for adults and pups present during the
pupping season were provided by an aerial survey
in the area on April 25, 1974 by the California
of Fish and Game.
20
Data on the locations of
~eals
on land were
plotted with the use of a U.S. Geological Survey map of
the area and compass readings taken with an azimuthal
compass.
To observe the behavior of harbor seals, various
types of blinds were constructed.
blinds are noted in Figure 2.
The locations of these
During this study
approximately 400 hours were spent observing seals on land
and 136 hours were spent observing seals in the water.
In order to compare the amount of vigilance
behavior displayed by seals at different locations,
v
groups of seals were observed for periods of time ranging
from two to four and one-half hours.
Approximately every
30 minutes the total number of headlifts displayed by a
group of seals for a ten minute period were
The total number of headlifts per ten-minute
was then divided by the number of seals in the
On South East Farallon Island there were only
that were accessible when observations were
, thus results from these observations were
on an individual basis as well as a group basis.
I conducted observations on seal aquatic behavior
Southern Pacific Railroad Bridge.
The length
in terms of time was determined in the following
A seal with a distinctive scar and/or pelage
21
co l
or was chosen for observation.
When the seal dove the
time was noted and the general area was watched until it
surfaced again.
When the seal surfaced the time was
again noted, followed by a verification of the seal's
identity with binoculars.
The amount of time the seal
spent on the surface between dives was also noted by
using two stop watches.
seals diving near the railroad bridge could be
visually tracked for various periods of time by estimating
the seal's position in the water in reference to different parts of the bridge structure.
RESULTS AND :=_-: ::JSSION
Censu.::
The seal population of __:_-,~y Slough fluctuates
with t=
seasonally (Figure 6),
~~west
numbers
observed from August to Januar,- :=:=2 the highest numbers
observed at the peak of
puppin~
From 1972 to 1974 and. in 1976
present during the winter
highest number of seals
==3son in late April.
~~~
rang:~ ~om
presen~ ~~
season ranged from 306 to 319
from this study, and the one
the height of pupping
1).
~~~~e
The results
by the Bodega Bay
indicate that both
::..~-~,
the winter population and the
~~=ng
season population
s+=~- :.
of seals in Mov1ry Slough are
Stable populations of P.
to exist on Gertrude
(Newby, 1971) and
23 to 31 and the
cc-.~~=ted
tute (Risebrough,et al.,
coast of southern
2ighest number of seals
7.
l:.::,::..~d
OL -~~
Californi~
richardi have been
in Puget Sound,
Channel Islands off
7rey and Alpin, 1970),
a stable population of P. ~- ~tulina has been
in Shetland, Great Bri~ (Venables and Venables,
Th e ear 1·lest date pups
v~
observed in the Mowry
area was March 22, 1974 =-r= the latest date on
22
Figure 6.
Population dynamics for !-1owry Slough area
from 1972-1974 using the highest number of animals
observed each month.
The number of pups is not
0
reported after June due to the difficulty of
stinguishing pups from yearling seals at the
stances observations were made.
I
I
\
I
Total
\
~
''
'
''
'
''
population
1972
'\
'~
1973
\
1974
\
\
\
\
\
\
\
\
I
\
\
\
\
\
\
\
I
\
so25
--- --JAN
FEB
JUl
AUG
S EP
--- -- +
NOV
DEC
Table 1.
Population data for Mowry Slough area from
1972-1974 and for 1976.
Mortality rates were based
on the peak number of adult and subadult seals that
returned to the Mowry Slough area during pupping
season from the previous year.
26
Pupping season
Aug· Jan
Highest no.
seals
no
date
Year
_Highest no
adult- subadult
no
date
Highest no
pups
no
date
0
Mortality
ail
age groups
/0
0
Total
/ 0
Pups
()
1972
31
Augl4
.246
Apr21
63
Apr24
309
20
1973
29
Oct30
266
Apr25
53
May 8
319
17
14
197~
23 Aug 9 * 248
Apr25
70
Apr25
318
22
21
Octl8 **234
Apr22
80
May11
314
25
26
1976
23
I
Cal. Fish and Game aerial survey
Risebrough et al (1976)
27
Nhich a peak in the number of pups occurred was May 8,
L973 .
re
similar dates describe the length and time of
ar of the harbor seal pupping season in other parts of
the san Francisco Bay area and in Humboldt Bay, California
(Rosenthal, 1968; Knutsen, 1974).
Bigg (1969b) discussed clines in the pupping
3 eason
~orth
of P. v. richardi along the Pacific coast of
America, concluding that pupping tends to occur
?rogressively later as one goes south from Alaska to
30 uthern
Puget Sound, Washington, and earlier as one goes
from Willapa Bay, Washington to Baja, California.
The number of pups born in Mowry Slough from
l972 to 1974 and in 1976 ranged from 16.6% of the peak
total population in 1973 to 25% in 1976 (Risebrough,et al.,
976).
No observations were made in 1975.
In
:o:moarison, in 1976 pups born at the two other major
groundsin the Bay _area, Bolinas Bay and Double
(Table 2) represented 33% and 36% of the peak total
respectively (Risebrough, et al., 1976).
In
Bay, California, where the peak total harbor seal
during pupping season has increased from 304
1966 (Rosenthal, 1968) to 425 seals in 1973
'1974), the percentage of pups born each season
from 3.2% in 1966 to 20% in 1973.
Based on
of pregnant females found in a random sample
Table 2.
Census data on harbor seal numbers at the
0
various haulout sites in the San Francisco Bay area
1976.
29
Pupping season
Aug-Jan
Location
Highest no.
pups
date
no.
Highest no.
seals
no.
date
Highest no.
sea Is
no. date
)
)
'
Drakes Estero
Double Point
+200
Jan 24
Strawberry Spit
- 34
- 82
·Jan
Castro
- 51
Sep 10
-
Jan 13
Bolinas Bay
Rocks
Corkscrew Slough
1
Grec:ko Island
- 25
Mowry Slough
-
Guadalupe Slough
- Risebrough
et al
-
23
4
Jan 26
8
+
-
-
%Pups
408
49
4
Jul
7
Jun 25
Apr
4
t
-
85 May 7
20.8
16 May
32.6
3
0
0
Jun 17
-
4
Jun 17
8.5
Apr
4
-
3
Apr 4
35
Mar 29
-
37.5
3 Mar29
8.5
Oct 18 -306
Apr 22
-
80 May 11
26.1
Oct 18
Apr 22
Jan 22
1976
'Data not available
4 AIIen [personal communication]
-
-
47
8
0
-
0
0
30
of 245 harbor seals taken in the coastal watersof British
columbia, Bigg (1969b) estimated the productivity of
harbor seals in this area to be 20%.
Boulva {1971)
reported that Atlantic harbor seal pups {P. v. concolor)
on sable Island, Nova Scotia, in 1970 represented 18% of
the total seal population.
Based on aerial photographs taken by the Bodega
Bay Institute, subadults comprised 31% of the seal
population in Mowry Slough on October 18, 1975 and 27%
on April 9, 1976.
At Double Point, subadults represented
23% of the seal population near the end of pupping season
1976 (Risebrough, et al., 1976).
Harbor seals lack sexual dimorphism and it is
icult to determine the sex ratio of a population of
seals in the field.
Scheffer and Slipp (1944)
the sex ratio of 74 harbor seals, shot in
state by bounty hunters, was one to one.
In
sample of 50 harbor seals one or more years of
collected in Alaska, 46% were females
(Bigg, 1968).
I rarely found dead seals during this study.
One
was found in 1972 and one in 1973, both under a
In 1974 two dead pups were found, one that was
' found at the start of the pupping season, and
was badly decomposed, found toward the end
season.
Except for the decomposed pup, all of
31
the dead pups had no noticeable birth defects nor did their
physical appearance give any clue as to why they died.
occasional incidents of stillbirths have been reported
at Strawberry Spit (Paulbitski, 1972).
During the 1976 pupping season the Bodega Bay
Institute conducted a search in the Mowry Slough area for
dead seals for the purpose of analyzing tissue samples
for organic pollutarits and trace elements.
Between
April 19, 1976 and April 30, 1976 five dead pups were
found that were full-term and without visible birth
0
defects.
Since a total of 80 live pups were counted
during the 1976 pupping season in Mowry Slough (Risebrough,
et al., 1976), this would give an estimated pup mortality
5.8%.
Knutsen (1974)
saw no dead pups during the 1973
season in Humboldt Bay, California.
to the flushing action of the tides.
He attributed
Newby
(1973)
a mortality of 10% for pups born on Gertrude
in Puget Sound, Washington, during the 1970
season.
Three out of the eight dead pups found
had severe birth defects which he attributed to
ssue levels of polychlorinated biphenyls and other
pollutants.
Boulva (1971) reported a mortality
at least 12% during the first month of life for
32
P.
~-
concolor pups born on Sable Island, Nova Scotia in
1970.
Four dead adult seals were found during this
study, all at Dumbarton Point near the east end of the
southern Pacific Railroad Bridge.
One body was totally
decomposed and the outward appearance of the other two
(both females) gave no evidence as to the cause of death.
During the 1976 pupping season a dead adult lactating
female was found in Mowry Slough.
Again the outward
appearance did not indicate the cause of death.
The mortality for all age groups ranged from 14%
1973 to 21% in 1974
(Table 1).
However, since the
total population during pupping season appears to
been stable from 1972 to 1976, then the mortality
all age groups would also be equivalent to the
percentage of pups born each year which ranged from 17%
to 25%.
Mortality figures derived from both methods are
and have been increasing since 1973.
The annual
mortality for P. v. richardi in British Columbia and the
harbor seal
to be 20%
(P. v. vitulina) in Holland has been
(Bigg, 1969a; Van Bemmel, 1956).
Harbor seals in the Mowry Slough area presented a
of pelage colors and spot patterns.
Pelage
ranged from light cream through varying shades and
of tan, brown, grey and black.
Seals with a
33
dark red pelage were also observed.
Either the entire
seal would be red or the color would extend only from the
head to the chest area.
Red seals comprised approximately
five percent of the adult and subadult population present
during pupping season.
Other than Mowry Slough, harbor seals with red
pelage have been seen at Strawberry Spit (Paulbitski,
personal communication), Corkscrew Slough, Grecko Island,-Bolinas Bay and Double Point (Allen, personal communication) .
Red harbor seals have not previously been
describeB in the literature,but were seen in Puget Sound
Nevvby (Paulbi tski, personal communication) .
Full-term harbor seal pups are born in the San
sco Bay area with a juvenile coat, white with dark
, which is shed during late April and early May.
The
(the temporary fur coat of pups born in colder
is shed in this area before birth and expelled
the placenta.
During the pupping season in Mowry Slough, seals
a dense growth of algae down the middle of their
were commonly observed.
Samples of marine algae
from the pelage of harbor seals collected in
state included Enteromorpha, Xenacoccus, and
(Scheffer and Slipp, 1944).
34
Population Dynamics and
Seasonal Movements
starting in mid February, just prior to the onset
of pupping in late March, there was a seasonal build-up
harbor seal numbers in the Mowry Slough area, with a
yearly peak in total population occurring in late April
{Figure 6).
The decline in seal numbers coincided with
time at which those pups born in early March would
have been weaned (based on a nursing period of four to
weeks, Newby, 1973; Knutsen, 1974).
While the harbor
seal is not considered to be a migratory animal¥ as is
northern fur seal (Callorhinus ursinus) , many
instances are reported where local movements associated
reproduction create pronounced seasonal fluctuations
harbor seal numbers similar to that in Mowry Slough
and Slipp, 1944; Bishop, 1967; Rosenthal, 1968;
1
1974) •
Only 7 to 10% of the total
se~l
population present
pupping season remains in Mowry Slough between
and January.
Based on aerial surveys of the
isco Bay shoreline conducted by the Bodega Bay
(Risebrough, et al., 1976), and the information
Table
21
approximately 40 to 80% of the total
ation inside San Francisco Bay during pupping
res
s outside of the Bay.
35
some of those seals that leave Mowry Slough at the
end of pupping season may eventually end up at Strawberry
Spit by early January.
In 1976 peak seal numbers at
stra 1,1berry Spit represented 28% of the total number of
seals that left Mowry Slough after the pupping season
(Table 2) .
Information presently available indicates that
Bolinas Bay and Drakes Estero (Figure 1) may be coastal
haulout sites used.by those seals leaving San Francisco
after the pupping season.
Allen (personal cornmunica-
tion) noted an increase in seal numbers at these
locations following the decrease in seal numbers that
takes place at Mowry Slough and Double Point at the end
of pupping season.
Both residential and transient seal populations
within San Francisco Bay.
Seal numbers at Grecko
Island and Castro Rocks do not fluctuate in the drastic
manner in which numbers fluctuate at Strawberry Spit and
Slough, even though pups are born at these
(Table 2) .
Davies (1949) noted the existence of transient and
populations of grey seals (Halichoeurs grypus)
' and suggested that this difference in
was because, in small colonies, sufficient food
throughout the year so that seals did not
· large colonies the food problem was
' wher eas ln
36
such that only a few can remain behind.
Feeding behavior
may explain why seals return each year to Strawberry Spit
even though they are subjected to a much higher frequency
of human-related distrubances than in Mowry Slough or on
the coast.
Peak numbers of seals at Strawberry Spit
coincide with a seasonal run of Pacific herring (Clupea
pallasi) into Richardson Bay (Figure l)
to spawn and lay
eggs.
Distribution of Harbor Seals
in Mowry Slough
0
From late June to late January seals generally
hauled out together at one location inside Mowry Slough
following peak high tide
(Figure 2) .
As pupping season
approached there v1as a gradual increase in the number of
out sites used.
The widest distribution of seals in
Mowry Slough area was observed at the height of
s·eason near the end of April
(Figure 7) .
With the approach of low tide, most seals would
Mowry Slough and sites along the Bay shoreline
pupping season) and move out to sites adjoining
mudflat channel running west from Mowry Slough
2 and Figure 8).
The reason for these tidal-
movements was because seals preferred to haul out
water deep enough for escape.
7.
Distributions of seals in the Mmvry Slough
area during high tide on
Ap~il
27, 1972.
38
0
8.
Distribution of seals in Mowry Slough area
low tide, on April 27, 1972.
40
41
From August t·_
ary in Mowry Slough seals were
typically absent from
ou t site along the
:ime they abandoned their haul-
mu~·
channel
of the incoming tide,
they hauled out inside Mowry
Slough on the ebbing t. ·
to late July, unless
(Figure 2), because
Figure 9).
d: ·
From early April
-bed, seals were observed
----~r s during all tidal conditions
hauled out in varying :. .
(Figure 11) .
During all mon1:; -,f the year Bartholomew (1947)
detected no relationshj ;. :/,tween tide conditions and the
behavior ofc han/,· seals that hauled out near the
east end of the Oakland ;;._ J bridge.
Bishop (1967)
reported a similar obse,~~~ion for P. v. richardi on
~Jring pupping season.
Island in Alaskh
has been observed thili
Sound, California
However,
t::eals on Gertrude Island in
(Yrmtsen, 1974), were usually
during the higher l'';rtion of flood tides; these
with my
observatiu 1 u~
in Mowry Slough from August to
When, during a 2A
· '!
hour tidal eye 1 e, harbor seals
to haul out can
c) 1 , 1,1 ,nd
time of year,
out on Ana
on where they haul out as
The highest number of harbor
Nll~'vo
Island on February 8 and 9,
about four hour•·
·t
.
· h t1" de
-~
ollow1ng
pea k h 1g
10) •
At all tin"',," o f
t h e year, b o th day and
Figure 9.
Number of harbor seals hauled out inside
of Mowry Slough and on Mowry Slough channel with
respect to time of day and tide condition, on
September 27, 1973.
10.
Number of harbor seals hauled out on
Nuevo Island with respect to time of day and
condition on February 8 and February 9, 1974.
43
70
60
-;;50
0
Seals hauled
out on channel
Sedls hauled out on Mowry Sl.
~
111
4>
.::30
0
+
0
z
'\
20
'\
'\
'\
'\
'\
10
,·
'\
'\
'\
'\
\
1600
1200
1000
Time of
day
tide
70
I
Feb 8
I
60
I
f
Feb 9
I
I
I
I
I
I
I
I
I
-t
Start of wind and
I
j
I
i
I
I
f
1200
Time of
tide Feb 9
tide Feb 8
1400
day
1600
11.
Number of seals hauled
~ut
at low and
tide during the 1972 pupping season, Mowry
45
Adult- sub adults
j ' ,.,
II
I
I
I
1\
I \
I I \
I I
\
I I
\
I
1
. ",
'--..
',
'
' '
I
I
I
I: \
II
I
'
\
I
\
.-
\
\
\\
_,. _,. ------
/
I
I
------
I
''
''
'-
I
High
tide - -
low
tide
---
I
I
I
-
I
I
APR
MAY
JUN
46
night, seals were observed on Grecko Island only during
high tide, while seals haul out on Castro Rocks at low
tide.
seals hauled out on Strawberry Spit only at night
and early morning during high tide (Paulbitski, 1972).
Differences in the haulout behavior of harbor
s with respect to tides from one area to another are
attributable to differences in the physical features of
haulout sites.
At Castro Rocks and on Ano Nuevo
Island, the haulout sites are mostly submerged during high
, while the Grecko Island site is accessible only at
tide.
~~
Strawberry Spit, which is also accessible
during high tide, seals have been subject to
frequent human-related disturbances during daylight hours
the past and now haul out only at night and early
(Paulbitski, 1972).
·Two factors may explain the.seasonal change
in the haulout behavior of seals in the Mowry
area with respect to tides.
Females do not feed
nursing their pups (Fisher, 1952; Spalding, 1974~,'
females with pups would be expected to haul out
a wider variety of tidal conditions than would
females.
Because roughly 90% of the seals
Slough during the pupping season haul out at a
of locations outside q_f the vicinity of Mowry
M'~,;r
.'>
some seals may prefer to haul out at low tide and
47
some seals may prefer to haul out at high tide, depending
on the physical nature of their winter haulout site.
During the pupping seasonand at other times of
the year, seals consistently preferred the north bank of
Mowry Slough to the south bank (Figures 2, 7, 8).
Along
the initial east-west portion of Mowry Slough the deep
water channel runs closer to the north bank than the
south bank, providing quicker access to deep water for
s hauled out on the north bank.
From July to January, seals were observed to haul
out at the same site on the north side of Mowry Slough
()
2) thorughout the course of this study.
During the pupping season seals did not show a
preference for identical haulout sites along Mowry Slough
the mudflat channel.
However, seals were observed to
out at the same three sites on the east Bay shore,
and south of Mowry Slough at each high tide
season (from early April to early June) .
durih~
These
are shown in Figure 2 and are designated "Seal
, "Seal Cove", and Calaveras Point.
The winter haulout site on.the north bank of Mowry
{Figure 2) did not appear to differ physically
other portion of the north bank of Mowry Slough.
r
each of the three haulout sites on the Bay
north and south of Mowry Slough were characterized
48
by a distinct physical feature.
At "Seal Slough" seals
hauled out on either side of the entrance to a five meter
slough that cut a small channel across the mudflats
Seals remaining here long after high tide
at loW tide.
were seen using this channel to reach the waters of the
The highest number of seals seen at "Seal Slough" was
2.
At "Seal Cove" as many as 26 seals hauled out on the
north side of a small cove, and as many as 13 seals hauled
out on a point of the east Bay shoreline called Calaveras
With the aid of written descriptions and photoit was possible to determine that the same six
seals hauled out at
II
Seal Cove II from one observation
to the next during the 1973 and 1974 pupping
season.
A blind female seal with a pup was seen at
Point during 1974 and 1976 pupping seasons.
same two seals, "Scarback", an adult male, and
", an adult female, were seen at "Seal Cove" in
3 and 1974.
"Scarback" was seen at "Seal Cove" again
the 1976 pupping season by Alcorn (personal
)
.
A large red male seal that was first sighted on
?, 1972, at the winter haulout site on the north
Mowry Slough, was later seen at "Seal Cove" on
and June 1, 1974.
49
Peterson (1968) noted that the same fur seals
(Callorhinus ursinus) returned to the same hauling site
from one pupping season to the next in Alaska and
referred to this behavior as "site tenacity".
Peterson
concluded that the tendency of male and female fur seals
to return consistently to the same location simply meant
that the same individuals were on land near each other
at the same time.
·Sullivan (personal communication),
however, has documented the existence of a dominance
hierarchy among a group of harbor seals where individuals
were consistently seen togethe~ at a site on the northern
California open coast.
Although most seals in Mowry Slough during pupping
season did not appear to show a preference for a
particular site, some individuals were found to exhibit
"
tenacity".
Two explanations for this are offered.
of the seal population in Mowry Slough during pupping
is composed of smaller groups of seals from other
and social bonds developed at these locations may
when seals migrate to Mowry Slough during pupping
However, the sighting of the red male seal in a
100 seals in Mowry Slough on July 7, 1972, prior
seal's appearance in the small group of seals
"Seal Cove" on May 31 and June l, 1974, may
that the seals hauling out at "Seal Cove" were a
50
temporary subgroup of the pupping season seal population.
In this case social bonds based on mutual recognition may
not have existed within the group of seals that hauled
out at "Seal Cove".
The size, age and sex composition of seal groups
·
1n
the Mowry Slough area during pupping season varied
considerably.
Adult seals and females with pups were
frequently seen to haul out alone, and occasionally two
adults and one pup were seen together.
Groups of seals
observed on the east Bay shoreline south of Mowry Slough
consisted mainly of adult females with pups, but adult
males were also frequently seen in these groups.
On one
occasion a seal group consisting only of females with pups
was observed near "Seal Cove".
The larger seal groups
out in Mowry Slough consisted of adults, sub, and pups.
Reports of the age and sex composition of harbor
groups formed during the pupping season in other
also vary considerably.
P. v. richardi in Alaska
, 1967) and in Mugu Lagoon, California (Evans and
'1969), were seen to haul out in large mixed
, including adult males, females, yearlings,
s with pups.
Fisher (1954b) noted that females
seal (P. v. concolor) tended to
in certain areas, separated from other members
51
of the population during pupping season.
Newby (1973)
reported a similar observation for P. v. richardi females
on Gertrude Island in Puget Sound, as did Knutsen (1974)
for~·
~·
ricardi females in Humbolt Bay, California.
Although increased population density was a
factor, the dispersion of the seal population in the
Mowry Slough area during pupping season was also caused
a decrease in gregarious· behavior.
This was evidenced
the increased number of small seal groups, incidents
\·lhere adult seals hauled out alone, and by the relative
decrease in the size of the larger seal groups
(Figure 12).
This change in the seal's gregarious behavior may
been caused by the tendency for females with pups to
out apart from the rest of the population or in
groups, as was also reported by Newby (1973),
(1954b) and Knutsen (1974).
Male adult and sub-
' their gregarious nature remaining unchanged,
the smaller groups initiated by pregnant females
females with pups.
Individual Behavior and Behavioral
Interactions Between Individuals
One of the most dynamic aspects of harbor seal
is the nature of the relationship between the
and her pup.
Bishop (1967) has described this
Figure 12.
Size of the largest seal group observed
(j
the Mowry Slough area expressed as a percentage
the total population on the dates indicated in
2.
53
Feb
Mar
Apr
·
l
May
Jun
date of 1912 total pop. peak
Jul
54
relationship as
conditions.
affectio~~te
and tolerant under normal
Observation~ ~f
the Mowry Slough area
su~:ort
Venables and Venc
P. v. vitulina pups
in the water in Great Britain.
s~en
conducte~
Bishop's description.
es (1955) observed
nursi~g
Aquatic nursing was not
and observations
mother-pup interactions in
in the Mowry Slough area
in Alaska (Bishop, 1967),
Washington (Newby, .1971, :973), and California (Knutsen,
1974) indicate that
on land on the
nursi~J
northeaste~~
In the Mowry
SlQUS~
takes place almost exclusively
Pacific coast.
area females would normally
begin nursing their pups cS soon as they both had hauled
out on the mudflats or in the salt marsh areas.
If the
pair had been hauled out for a period of time, the pup
would elicit the female's nursing posture by a combination
of vocalizing, nudging the female's side with its muzzle,
and by scratching at the female's chest or side with its
flipper.
Knutsen (1974) reported that nursing bouts for ..
~· v. richardi in Humboldt Bay, California never lasted
than 13 minutes and averaged 6.6 minutes.
in the Mowry Slough area lasted from l
Nursing
to 34 minutes.
on observations of tagged pups, Newby (1971)
that the interval between feeding appeared to be
55
about three to four hours for harbor seal pups in Puget
sound, Washington.
Although harbor seal pups are capable of swimming
at birth (Klinkhart, 1967), they ride on their mother 1 s
back for the first few weeks of life when in the water.
1 frequently observed pups riding their mother's back in
Mowry Slough during pupping season.
This behavior is
common among harbor, seals and has been described in
detail by Knutsen (1974).
Generally by about May 6 most
pups in the Mowry Slough area were swimming on their own.
Females with pups were much more active than other
members of the Mowry Slough seal population.
Knutsen
(1974) reported a similar observation for harbor seals in
Humboldt Bay, California.
On several occasions females
were observed leading their pups ashore and remaining only
briefly before taking the pup back into the water.
At
other times the females would remain hauled out just long
enough to nurse their pups.
Seal counts taken in Mowry Slough at low tide from
mid May (Figure 11) revealed that pup
fluctuated less from day to day than adult and
numbers.
This indicates that females with pups
confined their movements to the immediate
of Mowry Slough.
Based on observations of a
Knutsen (1974) reported that in Humboldt Bay,
56
california, females with pups ?enerally stayed near their
haulout sites when they were swimming.
Females with pups were occasionally sighted
swimming in the middle of south San Francisco Bay east
of Mowry Slough and near the Southern Pacific Railroad
Bridge.
The fact that pups were heard calling near the
railroad bridge at night also indicates that journeys
into mid Bay waters were also sometimes undertaken at
night.
During the nursing period in the Mowry Slough area
p~ps
were constantly in the company of their mothers.
In
Alaska, where the harbor seal is hunted annually, Bishop
967) reported a high number of desertions early in the
pupping season.
Deserted unweaned pups were not observed
the Mowry Slough area.
Newby (1971) made the same
observation in Puget· Sound, Washington, as did Knutsen
in Humboldt Bay, California.
While in the water the females and pups repeatedly
noses.
This would occur each time the pup
on the female's back or after each dive the pair
If the pup surfaced before the female
swim about calling until the female surfaced.
When hauled out, the female would sometimes alert
to potential danger by moving close to the water
slapp·
lng the surface vigorously with her flipper
57
until the pup entered the water.
Newby (1971) described
similar behavior in Puget Sound, Washington, seals.
On
those occasions when the pup was very young and
reluctant to enter the water, the female would remain
beached even when boats passed very close.
Except during peak high tide, the steep banks of
the haulout sites between Mowry Slough and Calaveras
point proved to be difficult for pups to negotiate.
If
pups could not climb up the banks to the haulout site,
females would enter the water, allow their pups to climb
on their backs and then haul the pup ashore.
<-
When
abandoning these haulout sites at low tide, sometimes the
young pups would be reluctant to follow their mothers
down the banks to the water.
The female would then coax
the pup down by repeatedly moving back and forth near the
haulout site.
When other seals at the haulout site moved close
a female -and pup, the female would either move away
she would force the seal to move away with growls,
waving, and muzzle thrusts.
Pups moving too close
the haulout site would be dealt with in
ar manner.
On three occasions interactions were observed in
between a female and her pup that could only
play.
This behavior consisted of chases
58
of short duration and vigorous water splashing bouts.
NeWbY (1971) reported observing play behavior between
females and their pups in Puget Sound, Washington.
While in the water near the haulout site, pups
would engage in play behavior with other pups.
Knutsen
(1974) reported that pups ignored each other in Humboldt
Bay, California.
thernsel ves.
Pups were also observed playing by
On one,occasioh a pup near the "Seal Cove"
site would repeatedly lie on the shore parallel to the
incoming waves from the Bay and allow the wave action to
roll it back in to the water .
c
Harbor seal pups are nursed for approximately
to six weeks in Humboldt Bay, California (Knutsen,
} and in Puget Sound, Washington (Newby, 1971, 1973).
1973 the first pups were observed in Humboldt Bay by
(1974) on March 23, and the first weaned pups
observed there on May 6.
Lone pups were observed
Southern Pacific 1 s railroad bridge as early as
that were non-vocal.
Unweaned pups in a similar
would have been calling for their mothers.
By
, nursing behavior had ceased in the Mowry
1 coition by harbor seals has been observed
8
and Venables (1955, 1957, 1959) during a
v. vitulina in Shetland, Great Britain.
59
ACCO rd
ing to these workers, harbor seals mate in the
Preceding actual mating encounters, harbor seals
in courtship behavior that Venables and Venables
1957, 1959) have referred to as "rolling behavior".
Bishop (1967) observed adult male P. v. richardi
ska aggressively approach post-lactating females
water and on land, grasping and biting the female
the neck.
Bishop (1967) also observed the
" encounters described by Venables and Venables
, 1957, 1959) but he did not observe actual mating.
(1974) observed "rolling behavior" between harbor
Humboldt Bay, California, between February and
did not see this behavior lead to
In the Mowry Slough area five incidents were
in which two harbor seals of undetermined sex
"rolling behavior" during pupping season and
October and January, but mating was never
The aggressive encounters described by Bishop
Alaska were not seen during this study,
ionally adult harbor seals of undetermined
with prominent wounds in the neck
it was not possible to define inclusive
breeding season 1n
.
the Mowry Slough based
60
on observations of breeding encounters, some theoretical
artes can be deduced based on harbor seal reproductive
data.
According to harbor seal reproductive data (Bigg,
1 g69a) the breeding season in Mowry Slough lasted from
April 25 to July 8.
These dates coincide with a
of rapid population decline in the Mowry Slough
area (Figure 6) ; thus, it is possible that as August
approached, an
incr~asing
percentage of the seal
population mated outside of the Mowry Slough area.
With the exception of females with pups, behavioral
interactions between seals sJl-aring the same haulout site
not vary from one month to the next.
Except at the
of pupping season, hauled out seals exhibited a
gregarious nature which was balanced by the
of a personal distance between individuals.
aggressive encounters observed on land were a direct
of one seal moving too close to another seal, ·or
defending their pups.
An intruding seal was
with one or more of a series of postures th~~
growling, energetic waving or scratching with
foreflippers, and head thrusts at the intruding
Usually the intruder could be moved with growls
ipper waving; the head thrust seemed to be used
resort.
Actual contact during these encounters
resulted in injury to the participants.
61
The number c= these interactions increased within larger
groups of se.::.ls hauled out on Mowry Slough as the tide
rose.
This ,,·as due to the crowding that resulted because
seals were reluctant to move very far into the salt marsh
~rdering Mo~ry
Slough.
Bishop (1967) observed a
similar increase in the number of interactions among
v. richardi in Alaska as the tide rose.
Except for females with pups, the behavior of
on land in the Mowry Slough area consisted of the
following, listed in order of decreasing frequency:
sleeping, vigilance, maintenance behavior (such as
scratching, stretching, etc.), changing position on the
haulout site in· response to tides, and social interactions.
slept on either their left or right side or on
ventral side.
Observations conducted to compare the viligance
of seals on ADo Nuevo Island and South East _ ·
Island with vigilance behavior of seals in the
Slough area are shown in Table 3 and Figure 13.
3 shows the typical pattern of vigilance behavior
East Farallon Island.
In Figure 13,
behavior of seals on Ano nuevo Island, and South East
Island, and from the Mowry Slough area are
Two sets of data are included in Figure 13 from
data collected on the group of seals at
Table 3.
Frequency of vigilance postures displayed
four harbor seals on South East Farallon Island
on June 2 6 , 19 7 4 .
63
Average number of headlifts per
10 minute observation period
seai no.1
7
7
seal no. 2 seal no. 3
seal no.4
4
7
6
4
2
0
0
8
2
7
11
3
1
0
1
0
1
7
0
1
0
Total
1
42
y
s.d.
6.00
3.40
1
18
2.57
2.87
4
11
1.57
1.70
0
3
16
2.28
2.98
0
Figure 13.
Comparison of the frequency of vigilance
stures displayed by groups of harbor seals
observed on South East Farallon Island, Ano Nuevo
Island, and in the Mowry Slough area.
65
8
Ill
ClJ
/+' --
::::
t::
E
7
+'
c.
c.
--t
I
I
I
I
I
I
0
...ClJ
--
6
::::
...0
bll
I
I
I
I
I
5
l1l
ClJ
Ill
...
"'c.
....
4
Ill
"C
l1l
"'
\
3
\
..c:
-...
\
\
\
0
"'
.t:l
\
2
E
::::
+-- --+
+
t:
ClJ
bll
...
l1l
+. . .
I
+- --¥
"'>
<
j
2
3
Elapsed time in hours between observation
location
Average
number
of seals
--Ano Nuevo Island
12
4
- - - Farallon Island
--Mowry Slough
-
Seal Cove
34
I
11
periods
4
66
nseal Cove" during pupping season, and data collected on
the small group of seals that remain in Mowry Slough after
pupping season.
At all observation sites during this study some
seals displayed vigilance behavior more than others.
On
south East Farallon Island a large adult male (seal
number one) displayed vigilance behavior more often than
his three companions (Table 3) .
Knutsen (1974) reported
iliat vigilance behavior was displayed by some seals in
Humboldt Bay more than others and that adult males
appeared to be the most alert individuals.
The number of head-lifts per 10 minute observation
period displayed by individuals and by groups of seals
varied with time.
Generally, when seals first hauled out
the number of head-lifts per 10 minutes was high and
decreased the longer the seals remained hauled out
(Figure 13 for Farallon Island and "Seal Cove").
Just
before seals were about to abandon their haulout site,
Often the number of head-lifts would increase (Table 3,
seal numbers 2, 3, 4, and Figure 13 for Mowry Slough) .
The winter group of seals in Mowry Slough did not
..,display vigilance behavior to a greater extent than the
observed on Ano Nuevo Island (Figure 13).
Either
to which harbor seals display vigilance is an
characteristic, as suggested by Schusterman
67
(1968), or the present sources and frequencies of
disturbances to seals in Mowry Slough are not extensive
enough to have influenced the vigilance behavior of
the seals in this area.
The small group of seals at "Seal Cove" in the
Mowry Slough area and the seals on South East Farallon
Island displayed vigilance behavior to the greatest
extent (Figure 13).
The extremely alert male on South
Farallon Island contributed from 30 to 80% of the
head-life "scores" shown in Figure 13.
Most of the
at "Seal Cove" were females with pups.
Newby (1973)
that females with pups on Gertrude Island in Puget
Sound, Washington, were constantly alert and
ne~vous.
, Knutsen (1974) found that females with pups in
Bay, California, showed less vigilance behavior
adult males.
The "Seal Cove" site was more exposed
wave action and water traffic than haulout sites
the north shore of .Howry Slough.
Further, seals
the "Seal Cove" site may have come from another
(Strawberry Spit for example) where they were
to a much higher frequency of human-related
~ .... .u.....
eS.
should be noted that the large amount of
observed in the amount of vigilance behavior
among individual seals at the same haulout site
68
may make comparisons in the amount of vigilance behavior
displayed at different sites invalid.
The flight reaction of seals in the Mowry Slough
area changed seasonally.
Seals were much more tolerant
of human-related disturbances during the pupping season
than at any other time of the year.
However, from July
to the first of August, seals appeared to be the most
nervous and were often observed to scramble into the
water for no apparent reason.
Hickling (1962) noted that
the flight reaction of grey seals (Halichoerus grypus) on
the Farne Islands varied from one week to the next.
()
Peterson and Bartholomew (1967) reported that the flight
reaction of California sea lions (Zalophus californianus)
on the Channel Islands also changed markedly during the
pupping season.
Diving Behavior
The average length of dive observed was 4.9
minutes, ranging in duration from 1 to 8.5 minutes
(Figure 14).
Dives lasting from 4 to 5 minutes occurred
most frequently.
The longest dives, 6.7 to 8 minutes
Were made by two large adult seals (seal number 1 and
number 2 in Table 4).
The average duration of the time seals spent on
surface between dives was 33.8 seconds, and ranged
.<
Figure 14.
Dive duration histogram for 31 individual
harbor seals.
One hundred eighty-nine dives were
observed near Southern Pacific Railroad's Dumbarton
Bridge, south San Francisco Bay.
70
40
N = 189 dives
X=
4.9 min.
S.D.=± 1.4 min.
30
Range= 1.0-B . .S min.
31 individuals
c
10
0
l-0
1.5
2.0
2.5
3:0
3.5
4.0
4.5
5.0
Minutes
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Table 4.
Average duration of dives and time spent
on the surface between dives executed by eight
individual harbor seals and a female with a pup.
72
'
Dives(minutes)
Approx.
Sea I
age
no.
N
-X
Ladult
1
11
7.4
l.adult
2
22
S.adult
3
s.adult
I
Time on surface(seconds)
Range
N
-X
±0.4
6.7-7.4
10
46
±
1
26-56
6.0
±1.0
5.0-8.0
15
44
±
7
30-51
15
5.0
±0.9
2.8-6.4
15
36
±
9
15-47
4
10
4.9
±0.6
3.5-5.8
10
33
±
7
20-45
S.adult
5
12
4.'5
±0.2
4.0-4.8
11
27
+
6
20-35
Sub-ad.
6
10
3.5
±0.4
2.8-4.0
9
27
±
5
21-35
S.adult
7
*
9
4.7
±0.4
4.2-5.7
>
>
>
>
S.adult
8
*
5
4.3
±0.1
4.1-4.4
>
>
>
>
Motherp'up
9
12
0.8
±0.6
0.7-2.0
13
30
* observed
S.D.
S.D.
±16
in .Mowry slough, others near R.R. bridge
>data not recorded
Range
5-90.
73
from 5 to 69 seconds (Figure 15).
The most frequently
observed length of stay on the surface between dives
lasted from 25 to 35 seconds.
The principal details of pinniped diving
physiology were worked out by Scholander and Irving in
the late 1930's and early 1940's (Scholander, et al.,
1942) and have been reviewed by Anderson (1966).
When a
seal dives, muscular sphincters on the posterior vena
cava and colateral large veins contract, thereby closing
off the circulation from large areas of muscle and
viscera while maintaining a large supply of blood to the
0
brain and heart.
Thus, while submerged, the seal's
muscles and viscera must rely on the amount of oxygen
that remains in its tissues, and in its capillaries, and
on muscular myoglobin.
Despite the capacity to survive forced dives in
the laboratory, lasting from 25 to 30 minutes (Harrison
and Tomlinson, 1963), Scholander, et al.
(1942) concluded
that under normal conditions harbor seals would remain
submerged only as long as their muscles could operate
aerobically, i.e. from 4 to 5 minutes.
The average
diving time of 4.9 minutes observed in the current study
""'
supports Scholander's conclusion.
The average surfaced
time of 33.8 seconds lends even stronger support to
Scholander's hypothesis because the relative briefness of
Figure 15.
Frequency histogram of the duration of
time spent on the surface between dives by 29
individual harbor seals.
One hundred sixty sightings
from near Southern Pacific Railroad's Dumbarton
Bridge are represented.
75
40 -
N = 160 sightings
x= 33.8 sec.
S.D.=± 10.9 sec.
30
Range= 5-69
29 individuals
Ill
bll
c
...
·-
.c
Cll
I ll
-
(;
20 -
0
0
z
10 -
I
0
5
10
15
20
25
30
35
seconds
40
45
50
55
60
I
T
65
70
76
this average recovery period points to little or no
lactic acid accumulation during the dive.
Harbor seals
forced to dive from 15 to 20 minutes in the laboratory
required a 10 minute recovery period before their
ventilation rate returned to normal (Harrison and
Tomlinson, 1963).
The results indicate that the large seals dove
longer (Tables 4 and 5).
Experimental dives conducted
under laboratory conditions have also shown that the
larger a seal, the longer it could dive.
The female
that was observed diving with her pup obviously timed the
"
duration of her dives to coincide with the pup's lower
capacity to remain submerged.
Because of the distances involved, timing dives
in areas other than the railroad bridge was difficult.
However, successive dives made by two seals in the
vicinity of the entrance to Mowry Slough were timed and
are included in Table 4.
These seals were observed
diving in an area where the average depth of water was
from 3.8 to 4.1 meters at mean high tide while seals seen
diving near the railroad bridge were diving in water
that was approximately 15 meters deep at mean high tide.
The average length of divesmade by the seals near Mowry
Slough did not vary significantly from the average length
of dives executed by seals near the railroad bridge.
Table 5.
Comparison of the average diving time of
seals of different age groups at the .05 confidence
level.
Mean differences found to be significant are
marked with an X.
78
Small
adult
I~
adult
sea I
no.
3
4
5
6
7
1
X
X
X
X
X
Mother
pup
8
9
X
X
X
X
X
X
X
X
X
X
X
X
-
Large
adult
I
2
3
4
Small
adult
5
6
1--
X
7
-
Sub
adult
8
X
79
Though these data are not conclusive, they suggest that
within the range of from 3.8 to 15 meters the average
amount of time a seal spends submerged is independent of
depth.
I observed no direct correlation between diving
times and surface times
(Figures 16 and 17).
If
Scholander's (1942) hypothesis is correct and seals in
the wild dive only as long as their muscle oxygen supplies
last, then the length of time a seal remained submerged
would depend on the extent of muscular activity.
Thus,
active dives would be shorter but the seal would have to
spend as much or more time on the surface recovering than
for less active dives.
Also, seals may, at times, have
spent more time on the surface than was necessary before
making their next dive.
The aquatic movements of seven adult seals and one
pup were visually tracked for periods of time
rang~ng
seven minutes to one hour and fifty-eight minutes.
from
The
longest a particular individual was observed near the
bridge was five and one-half hours.
Numerical data from
these tracking observations are shown in Table 6.
The
movements of five adults and a mother and pup are plotted
in Figures 18 to 21.
Seals exhibited four different movement patterns
according to the spatial relationship between diving
Figure 16.
Duration of time submerged and cor-
0
responding time on surface between dives for harbor
seal number one (see Table 4).
81
8
7
6
Anim::~l submerged
Animal on
surface
Figure 17.
Duration of time submerged and cor-
responding time on surface between dives for harbor
seal number two (see Table 4).
83
6
5
4
Ill
@I
3
:::1
1:
E 2
1
.8
.6
.4
.2
Animal submerged
Animal on
surfoce
Table 6.
Comparison of four types of
aqua~:c
ment patterns displayed by seals in terms
c:
movedives/
hour and rate of travel in the horizontal ;:ane in
meters per minute.
85
Movement
type
Fig.
Dives
Rate of
repre-
per
travel
sented
hour
m/min
Seal
no.
1 0;5
2.9
1.5:0
2.7
2
8.3
2.5
2
,9.0
2.7
1
16.5
5.0
1
1 o:6
1.4
1
12.0
2.9
1
16.0
7.3
2
2
•(i
18
A
B
10
19
16.6
11.3
c
'9
20
52.0
12.0
11
21
1 7.5
133.3
12
21
23.0
133.3
13
21
23.0
1 30.0
D
Figure 18.·
Aquatic movements of seal number two on
May 7, 1973 between 1715 and 1911 Pacific Daylight
time.
Figures indicate the elapsed time in minutes
C·
from the initial observation that the seal dove at
the location indicated.
Dotted lines represent the
assumed route of travel while submerged.
87
,....
..
1.1')
\.,
I \
I \
I
\
E
\
1.1')
\
I
"'
I
I
I
\
\
i
~
1.1')
\
o::t
\)
'V
.1.1')
I
~
+-1
I
!V t
.•+-. II \ \
1
I .
I
I
'
0
I
0
1--
)
I
\
~
\
I
I
I
(+ '-)_
~
\
1
M
I
....
i
Figure
l~.
Aquatic movements of seal number ten on
0
April 3C
1975 between 0853 and 0911 Pacific
Daylight time.
89
0
0
0
\
0
\
I\
I
\
\
I
I
I
I
I
\
\
\
I
\
\
I
I
\
\
'V
'~
\
I
I
I
I
I
I
I
I
I
I
I
y,
I
I
I
I
I
I
I
I
I
I
I
I
I
I
1-\
\
\
\
\
~
\
llO
.....
0
\
T
E
\
\
\
\
\
\
\
\
\
\
\
I
I
I
I
I
I
I
?
l
0
...
m
Figure 20.
Aquatic movements of a mother and pup on
0
May 6, 1975, between 0917 and 0934 Pacific Daylight
time.
91
~-------
l
--+
A:,-
0
0
l
E
0
1
C!>
I
1
b!l
"C
-....
m
I
I
w
I
I
I
I
I
I
I
I
'
.... ....
+- -+
I I
0
M
~
0
~
0
.... k.
. . .+
I
0.
':':'
0
,..+
l
o:l
....
0
.....
0
Figure 21.
Aquatic movements of seal number eleven
on January 14, 1976 from
~425
to 1428 Pacific
Standard time, seal number twelve on May 3, 1974
from 1353 to 1401 Pacific Daylight time, and seal
number thirteen on June 5, 1974 from 1420 to 1422
Pacific Daylight time.
M
Ol
sea I no.12
sea I no. 11
/.o
~
I
I
I
I
I
I
I
I
I
I
I
I
I
I
0:01.76~\
seal no.l3
Bridge
o:oo-+
t
I
I
I
I
I
I
I
I
I
I
I
I
I
~.
I
Hetch-Hetchy
gate house
I
I
llllllllllllill
I
I
I
I
I
I
I
\
\
I
I
I
I
I
I
I
I
I
I
I
I
I
\
\
I
I
I
\
I
\
\
I
I
I
\
I
I
I
\
I
0:02.30-+
I
\
I
I
I
I
I
I
I
94
locations, differences in the rate of travel between
diving sites, and the number of dives executed per hour.
For the purpose of discussion these patterns will be
referred to as type A, B, C and D.
The movements of diving seals could usually be
described as type A or type B.
type A movement pattern.
Figure 18 illustrates the
Here the seal
(seal number 2)
spent one hour and 'twelve minutes diving within a
approximate area of 340 square meters on the south side
of the bridge and 33 minutes diving in an area of 630
square meters on the north side of the bridge.
At times
the seal made from two to four dives at the same diving
site.
Seals were commonly seen diving at the same site
both at the bridge and in Mowry Slough, in some cases
for as long as 40 minutes.
Some seals not only showed a
temporary preference for the s_ame diving site, but
returned to the same area from one day to the next.
Spalding (1964) also observed harbor seals maintaining
a stationary position while feeding near an inlet on the
Skenna River in British Columbia.
Figure 19 illustrates the type B pattern.
""
Although
seal number 10 made approximately the same
number of dives per hour as seals showing the type A
pattern, it never dove at the same site and covered more
distance between diving sites resulting in a higher rate
of travel, as shown in Table 6.
95
Both the type A and type B patterns represented
the movements of seals that were feeding.
I make this
assumption because of the average length of time seals
dove, the length of time seals remained in the area, and
because seals occasionally surfaced with fish.
Differences between type A and type B patterns
might be attributed to differences in hunting methods
and/or species of prey caught.
behavior that was
characteris~ic
The stationary diving
of the type A pattern
could be best explained if it is assumed that seals were
feeding on the bottom.
This would have provided the seals
with a reference point in order to maintain their
stationary position, especially in the face o£ the strong
tidal currents that are typical of the south Bay.
This
assumption is supported by observations of seals on the
surface with staghorn sculpins (Lep!ocottus armatus)
which is a typical bottom-dwelling species.
The type B pattern, on the other hand, might be
the result of seals pursuing fish nearer the surface of
the water.
·'
One of the three species of fish brought to
the surface was smelt (Atherinopsis sp.), a species that
tends to occupy the upper water levels.
The type C pattern (Figure 20) most likely
represents a "training mission" for the pup rather than
a feeding expedition for the mother, because of the
96
relative brevity of the dives.
Also, there is evidence
that females with pups fast while nursing (Fisher, 1952:
Spalding, 1964).
The comparatively high rate of travel and shallow
dives characteristic of the type D pattern (Figure 21)
indicates that the seals showing this type of behavior
were not feeding but were traveling from one point to
another.
Harbor seals feed on a wide variety of fish,
preferring those species most readily available and easily
caught (Scheffer and Slipp, 1944).
The numbers and
species of fish that seals brought to the surface were
as follows:
14 staghorn sculpin (Leptocottus armatus),
15 smelt (Atherinopsis sp.), and two unidentified species
of fish.
I assume that most of the time, prey caught
underwater were consumed underwater.
Out of more than
200 sightings, only 31 incidents in which seals brought
fish to the surface were recorded.
On two occasions the
seal dove soon after surfacing with the fish still in its
mouth and surfaced a short time later without the fish.
It is possible that seals brought fish to the surface only
when the fish were caught just before the seal surfaced
for air.
At times seals appeared to have trouble swallowing
fish brought to the surface.
During one such incident a
97
seal surfaced with a fish in its mouth head first.
After
appearing to experience some difficulty in swallowing the
fish, the seal dove briefly and surfaced again with the
fish still in its mouth.
After a brief stay on the
surface, during which the seal again failed to swallow the
fish, it dove and two minutes later surfaced without the
fish.
On another occasion a seal surfaced with a sculpin
by the tail and
proc~eded
to play with the fish, much in
the manner that cats play with mice, for about two
minutes before the fish was eaten.
Fisher (1952) observed
harbor seals struggling at the surface with salmon on the
Skenna River in British Columbia.
According to Spalding
(1964), sea lions eat small fish such as herring underwater; larger prey are brought to the surface and reduced
to small pieces by violent shaking.
Both day and night (during full moon) the largest
number of seals were observed feeding from two to three
hours before and after peak high tide.
During that time
that seals are not on land, it may be generally assumed
that they spend a relatively large portion of their time
in the water feeding.
Thus, i t appears that seals at
Ano Nuevo Island fed mainly at high tide.
The number of
""
seals hauled out on Ano Nuevo Island were lowest at high
tide (Figure 10) .
In contrast, seals at Grecko Island
98
may have fed mainly at low tide because the highest
numbers were observed there at high tide.
Harbor seals have been reported to feed mainly
during high tide in the Skenna River, British Columbia
(Fisher, 1952), in Puget Sound (Newby, 1971), and in
Humboldt Bay (Knutsen, 1974).
However, Bartholomew (1947)
could find no correlation to either time of day or tide
as to when harbor seals hauled out near the east end of
the Oakland Bay Bridge in San Francisco Bay; the same
author (1951) reported that harbor seals
~ould
be seen at
any hour of the day swimming just outside the surf off of
c
San Nicolas Island which is off of the Southern California··
coast.
SUMMARY AND CONCLUSIONS
The harbor seal population in the Mowry Slough
area between 1972 and 1974 and in 1976 fluctuated
seasonally from 23 to 31 seals during the months of
August through January, to a peak total population of
306 to 319 seals at the height of pupping season in
late April.
Pups were born in the area from late March
to late April.
Although the percentage of pups born each
year from 1972 to 1976 has increased from 16.6% to
25% of the peak total population, both the winter population and the pupping season population are stable.
Both seasonal and tide-related changes in the
distribution of seals were found to occur.
Tide-
related changes in the distribution of seals occurred
daily as a result of seal movements from higher ground
to the mudflats at low tide.
A seasonally-related
change in seal distribution occurred as a result of the
increase in population density during pupping season and
a possible decrease in the gregarious nature of the seal
population during pupping season.
Seals hauled out during all tide conditions from
early April to late July.
From August to January, seals
hauled out from two and a half to three hours following
high tide.
99
100
The winter population in Mowry Slough generally
hauled out at the same location at each high tide.
Haulout site tenacity was exhibited to a much less degree
during the pupping seasoni however, certain individuals
returned to the same haulout site during the pupping
season from one day to the next and from one year to the
next.
Behavioral interactions between individuals in the
Mowry Slough area did not vary from those of harbor seals
studied in other areas in California and Washington.
Aggressive encounters observed never resulted in injury
and were apparently caused from violations of a seal's
personal distance.
Neither mating behavior or the
existence of a dominance hierarchy among seals on land
were observed.
The winter group of seals in Mowry Slough
exhibited no greater vigilance behavior than that of a
more isolated group of harbor seals on Ano Nuevo Island.
However, a group of seals composed largely of females
with pups that hauled out near Mowry Slough during
pupping season showed vigilance behavior to a greater
degree than the winter group of seals in Mowry Slough or
"" seals on Ano Nuevo Island.
the
The flight reaction of
seals in the Mowry Slough area was the least intense
101
during pupping season and the most intense from July to
August.
Seals that were feeding dove for an average of
4.9 minutes and spent an average of 33.8 seconds on
surface between dives.
~he
These data support Scholander's
hypothesis that harbor seals diving in the wild stay
submerged only as long as their muscular oxygen stores
last.
No correlation was found between the amount of
time a seal remained submerged and the amount of time it
spent on the surface between dives.
Basically two types of diving behavior were
c
observed during feeding:
some seals dove repeatedly in
the same general area,indicating that they may be feeding
on the bottom; and some seals would cover considerable
distances between diving sites while submerged, indicating
that feeding occurred nearer to the surface.
seldom consumed their prey on the surface.
Seals very
In the Mowry
Slough area seals preferred to feed from two to three
hours before and after high tide.
Suggestions for Future Research
and Management
In the past, access to seal hauling grounds in
the Mowry Slough area was strictly controlled by the
Leslie Salt Company; thus, the frequency of human-related
102
disturbances was kept to a minimum.
Recently (1979) the
Mowry Slough area was incorporated into a National
Federal Wildlife Refuge, and it is my opinion that access
to this area should continue to be strictly controlled
in the future.
A yearly census of the seal population should be
instituted, with counts being made at least during the
population peak
i~
late April and again in midwinter.
A careful watch should be maintained during the
pupping season for incidents of birth defects and still
births.
These might indicate environmental pollution
qy
specific agents 1 such as heavy metals, which have been
reported to be responsible for birth defects in harbor
seals in Puget Sound (Newby, 1971).
The most frequent sources of disturbances to
harbor seals in the Mowry Slough area noted during this
study were from low-flying aircraft and water traffic.
It
is recommended that an altitude restriction be instituted
for this area and that water traffic be limited in Mowry
Slough through public information programs.
Further research is needed to determine more
conclusively
the destination of those seals that leave
...,
Mowry Slough at the end of pupping season.
The future
welfare of the transient portion of the Mowry Slough seal
103
population will, no doubt, depend on conditions at the
winter hauling grounds.
LITERATURE CITED
Alcorn, D.
(In prep).
Thermal ecology of harbor seals,
Phoca vitulina in south San Francisco Bay. M.S.
·
Thesis, California State Univ., Hayward, 52 pp.
Allen, S.
1978. Birth of a Harbor Seal.
issue num. 11, pp. 12-13 .
Oceans Mag.
. 1979. Assessment of harbor seal population of
Bolinas Lagoon. Marine Mammal Comm. control num.
MM 8 ACA 102.
---,---=--:-
Alpin, J. K.
1967. Biological survey of San Francisco
Bay (1963-1966).
State of Calif. Dept. of Fish and
Game Marine Resources Operations MR Ref. #67-4.
Anderson, H. T.
1966. Physiological adaptations in
diving vertebrates. Physiol. Rev. 46:
212-243.
(:
Bartholomew, G. A.
1947. A census of harbor seals in
San Francisco Bay. Journ. Mamrn.
34-35.
1950.
Spring, summer and fall census of the
pinnipeds on San Nicolas Island. Calif. Journal.
Mamm.
32: 15-21.
Bigg, M. A.
1969a. Clines in the pupping season of the
harbor seal Phoca vitulina.
Jour. Fish. Res. Board
Canada 26:
449-445.
1969b. The harbour seal in British Columbia.
Fish Res. Board Canada Bull. no. 172. 33 p.
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