Reports: Green, D.E., Grigg, E., Allen, S. and Markowitz, H. (2004)
Monitoring the potential impact of the seismic retrofit construction activities
at the Richmond-San Rafael Bridge on harbor seals (Phoca vitulina): May
1 1998 - Dec 31 2004. Final Interim Report (IHA 11/19/03 - 11/18/04),
November 2004. 91 p. This is the latest version of a series
of regular reports to NMFS, Caltrans and interested parties on the findings
of this survey. For a copy of this report, or the project final report
(including findings from post-construction monitoring of the Castro Rocks
haul out site), please contact:
Publications: Grigg EK, Green DE, Allen SG, Markowitz H, (2002) Diurnal and Nocturnal
Haul Out Patterns of Harbor Seals (Phoca vitulina richardsi) at Castro Rocks,
San Francisco Bay, California, California Fish and Game Journal 2002 88(1):
15-27.
Surveys of harbor seals, Phoca vitulina richardsi, at Castro Rocks, San
Francisco Bay (SFB), California, were conducted from May 1998 through April
2001. Surveys were conducted at all hours of the day, and disturbance
data and seal responses were recorded continuously during these surveys.
Harbor seals hauled out at Castro Rocks during the daytime and nighttime throughout
the year. Over the course of the study period, mean nighttime counts
were significantly higher than mean daytime counts. Maximum daytime
and nighttime seal counts were recorded during the fall season. Tidal
dependence alone was not a clear predictor of fluctuations in seal numbers
hauling out during the daytime compared to the nighttime. Seals at
this site experienced high levels of disturbance from a variety of sources
during the daytime, and significantly lower levels of disturbance at night.
We believe that, in this highly urbanized environment, high levels of daytime
disturbance contribute to the higher use of this haul out site at nighttime,
versus daytime. Given projected increases in the number of people
living and working around SFB, protecting the integrity of haul out sites
in SFB is an important facet of protecting the harbor seal population.
Grigg EK, Allen SG, Green DE, Markowitz H, (2003) Harbor Seal (Phoca
vitulina richardii) population trends in the San Francisco Bay Estuary, 1970-2002.
California Fish and Game Journal, 90(2): 51-70, Spring 2004.
Pacific harbor seals, Phoca vitulina richardii, have used the San Francisco
Bay estuary in California as a nursery area and foraging site for thousands
of years. Like other pinniped species, harbor seals in California
were intensively hunted in the late 1800’s and early 1900’s, resulting in
population declines obvious in the San Francisco Bay (SFB) by the 1920’s.
In 1972, the Marine Mammal Protection Act was passed, providing protection
and management of harbor seal populations. We examined historical
data (1970-1997) and our own ground counts at three primary SFB haulout
sites (1998-2002) in order to understand the degree of recovery of the SFB
harbor seal population. One of the largest estuaries on the west coast
of the United States, SFB today is highly urbanized and heavily impacted
by human activity. We documented a mixed response and recovery of harbor
seals in SFB, likely due to a combination of factors, including habitat alteration,
disturbance, pollution, and survey techniques. From 1970-2002, seal
numbers at all three sites increased slightly during the fall/winter season,
and increased at two sites during the pupping/molting season. At the
largest SFB rookery site, however, no change was seen in seal numbers during
the pupping/molting season.
Students Working With This Project (Includes Completed and Active Work)
: Galloway, M. (2000) Factors influencing scanning rates of harbor seals
at Yerba Buena Island, California. Master's Thesis, San Francisco
State University, Department of Biology, San Francisco, CA, 90 pages.
Bohorquez, A. (2002) Pupping phenology and haul out patterns of harbor
seals in San Francisco, California. Master's Thesis, San Francisco State
University, Department of Biology, San Francisco, CA.
Nickel, B. (2003) Pacific harbor seal (Phoca vitulina richardii)
distribution, movement and foraging activities in the San Francisco Estuary,
California. M. A. candidate, San Francisco, California, San Francisco State
University.
Grigg, E.K. (In Progress) Habitat suitability models, habitat use
patterns, and haul out site selection factors of Pacific harbor seals (Phoca
vitulina richardii) in the San Francisco Bay estuary. PhD candidate,
Davis, California, University of California, Davis, Graduate Group in Ecology.
Additional Data Contributions:
In order to assist in region-wide surveys and ongoing monitoring, the RBHSS
contributes data on harbor seal numbers and distribution in SFB to:
• the National Park Service
• the Gulf of the Farallones National Marine Sanctuary
• the Don Edwards San Francisco Bay National Wildlife
Refuge (U.S. Fish and Wildlife Service)
• Angel Island State Park
Posters: Grigg EK, Green DE, Allen SG, Markowitz H, Disturbances to Harbor Seals
(Phoca vitulina richardsi) in San Francisco Bay, California, an Urbanized
Estuary. Society for Marine Mammalogy Biennial Conference, Maui HI, November
26 – December 3, 1999
We monitored disturbances to two primary harbor seal haul out sites within
San Francisco Bay, from May 1998 through April 1999: Castro Rocks
(CR), and Yerba Buena Island (YBI). The major causes of disturbance were
human-related: watercraft, aircraft and automobiles. We examined
watercraft disturbances in order to evaluate the effects of distance on
seal reactions. Watercraft within 200 m of the CR and YBI haul out
sites elicited a response from seals 73.1% and 74.4% of the time, respectively,
and flushed seals off the haul out sites 29.0% and 10.8% of the time, respectively.
Despite the significantly higher occurrence of watercraft near the haul
out site at YBI than at CR (p<0.0001), animals at YBI were less prone
to flush off the haul out site (p<0.006). Watercraft close to the
haul out site were more likely to flush seals during the pupping season at
CR, and during the fall season at YBI. At both sites, watercraft disturbed
seals more frequently on weekends than on weekdays (CR: p <0.05;
YBI: p<0.006).
We suggest that the differences in seal reactions at these two locations
are due to 1) reproductive status, 2) varying disturbance levels, and 3)
habituation. CR is a major pupping site in San Francisco Bay and females
with pups may have an increased tendency to flush during the pupping season.
Only a few pups occur at YBI and seals were more prone to flush during the
fall at YBI, when counts were low and transient animals may be present.
Habituation to disturbance by YBI seals may account for the significantly
reduced tendency to flush (compared to CR, p<0.005). Furthermore,
habituation may explain why weekend and weekday seal numbers at YBI were
not significantly different, but were significantly different at CR (p<0.01).
Green DE, Grigg EK, Petersen HM, Galloway M, Bohorquez AS, Sanders AM,
Allen SG, Markowitz H, Trends in harbor seal (Phoca vitulina richardsi)
haul out patterns at Castro Rocks and Yerba Buena Island in San Francisco
Bay, California. Society for Marine Mammalogy Biennial Conference, Maui
HI, November 26 – December 3, 1999
We examined two primary harbor seal haul out sites within San Francisco
Bay, California, from May 1998 through April 1999, in order to determine the
effects of tide height and time of day on the haul out patterns of the seals.
The mean number of seals present at Castro Rocks (CR) and Yerba Buena Island
(YBI) varied with tide height. Mean number of seals on the haul out
declined as tide height rose at both sites, with the greatest number of seals
present when the tide height was 2 ft. CR is a mid- to low-tide haul
out site, and space on the haul out site is unavailable at high tides.
However, haul out space is available at YBI at much higher tides, and we
recorded seals at this site up to a 6.5 ft tide height.
Time of day also influenced the number of seals seen on both haul out sites.
At CR, there was a slight drop in mean number of seals using the site from
approximately 0500 until 1200, with high mean numbers of seals present on
the haul out during the night (2100-0300). At YBI, there appears to
be a stronger relationship between time of day and haul out site use, with
a sharp drop in the numbers from 1000-1200. Based on surveys taken
between 0600-2000, we suggest that there is a bimodal haul out pattern at
this site, with high numbers present in the early morning hours (prior to
0800) and in the mid-afternoon (1300-1700).
Tide height appears to be a stronger factor in haul out patterns at CR,
due to the geography of that site. The relationship between time of
day and haul out numbers may be due to the role of human-related disturbance
to seals at both sites.
Bohorquez A, Grigg EK, Green DE, Markowitz H, Allen S, Red Pelaged Harbor
Seals in the San Francisco Bay. Society for Marine Mammalogy Biennial Conference,
Maui HI, November 26 – December 3, 1999
Red pelage in the northern pacific harbor seal (Phoca vitulina richardsi)
in San Francisco Bay has been recorded in the bay since 1969. There have
also been sightings in Oregon and Washington, Japan, Ireland and Maine, but
not in the numbers seen within the San Francisco Bay. Allen et al. (1993)
reported that red-pelage resulted from iron oxide adherence to the keratin
surface on the shaft of the hairs, possibly related to flocculation of ferrous
iron in the water column. Red-pelage occurs in all sex and age classes,
except for pups. In May of 1999, a higher proportion of red pelaged harbor
seals were recorded in the San Francisco Bay seal population (42.9% at Castro
Rocks, 49.3 at Yerba Buena Island) using the highest proportion of red pelaged
animals in the maximum total count. Using the average proportion of red pelaged
animals over the month of May, however, we see similar proportion to those
previously recorded (28% in 1999, 27% in the 1980s; Allen et al. 1993). At
Yerba Buena Island, we also see a drop in the proportion of red pelaged animals
in the winter months (6.4% in December), this may correspond to the Herring
migration through the area surrounding the island, which may attract more
seals from the coastal colonies. In spring 1999, at Castro Rocks, a higher
daily proportion of red pelaged mothers were seen in April (67%). The daily
proportion of red pelaged mothers dropped significantly in the month of May
(18%). Comparison of daily proportions for the two months showed a significant
difference (p<0.02) for the two months. Individual mothers were identified
and followed throughout the breeding season. Many red pelaged mothers were
seen with late stage pups, suggesting successful pupping.
Galloway MJ, Grigg EK, Green DE, Markowitz H, Allen, SG, Differential
Scanning between Male and Female Phoca vitulina richardsi Hauled-Out at
Yerba Buena Island, California. Animal Behaviour Society Conference, Georgia,
August 5-10, 2000.
Previous studies have shown conflicting results in scanning behavior between
adult male and female harbor seals. Scanning refers to movements that
increase the seals visual field. This study compares differences in
scanning bouts by hauled-out seals at Yerba Buena Island, CA (YBI) between
October 1998 and September 1999. YBI is primarily utilized by males
at all times of the year. Up to three seals at different locations
of the site were selected. Seal behavior was recorded for 15 seconds
per minute for up to 8 10-minute observations during the 45 4-hour surveys.
The number of scanning bouts from focal male and female seals was analyzed.
Overall results suggest that females scan more frequently. The differences
between scanning bouts were significant when comparing seals near the water.
Females near the water scanned significantly more than males on surveys
with below average disturbance levels and above average seal counts indicating
that several factors may have an influence on female scanning behavior.
Green DE, Grigg EK, Markowitz H, Allen SG, Update on the population
status of harbor seals (Phoca vitulina richardsi) in San Francisco Bay,
California. Marine Conservation Biology Institute 2nd Symposium, San Francisco,
CA, June 2001.
Since May 1998, we have been surveying the San Francisco Bay harbor seal
population. We evaluated population numbers and percent pups at three
major Bay haul out sites (Castro Rocks, Mowry/Newark Slough and Yerba Buena
Island) and compared our current data to historical harbor seal counts for
the Bay. Our data support the theory of a stable Bay population, in
contrast to the dramatic rise in harbor seal numbers along the coast of
California. Although our maximum harbor seal counts at Castro Rocks
are significantly higher than seal counts at that site in the 1980's, a
nearby haul out site (Strawberry Spit) was abandoned by harbor seals in
the early 1980's. Consequently, some seals may have shifted from the
Strawberry Spit site to Castro Rocks. Seal numbers have remained stable
at Mowry Slough, the Bay's largest harbor seal rookery, since the 1970's.
Yerba Buena Island is not generally recognized as a harbor seal rookery,
and data at that site is limited prior to 1995. However, we have noted
stable seal numbers, with a slight increase in pup numbers, at Yerba Buena
Island. The lack of growth in the Bay seal population may be due to
one or more of the following: limited food availability, pollutants
in the Bay waters affecting female reproductive success, limited suitable
haul out space in the face of increased shoreline development, and increases
in human disturbance around haul out areas.
Bohorquez, AS, Nickel BA, Grigg EK, Green DE, Bouse RM, Jaffe BE, Allen
SG, Markowitz H. The high price of gold: possible effects of hydraulic mining
on harbor seals in San Francisco. Conservation Biology Conference,
Hilo, HI, July 2001.
The Gold Rush of the 1853 brought prosperity to California; however, the
costs of the methods used to extract that gold are still being assessed
today. The use of highly pressurized water washed over one billion tons
of sediment from the Sierra Nevada foothills, a portion of which remains
in the San Francisco Bay (SFB). The SFB also has the highest proportion
of harbor seals with a red discoloration of their pelage. The red color
results from iron oxide adherence to the keratin surface on the shaft of
the coat hairs. This iron may adhere while seals are foraging in sediment
contaminated by re-exposed hydraulic mining debris. In order to test this
hypothesis, a model will be developed from a collaborative approach integrating
behavioral, geological and chemical methods by, (1) synthesizing behavioral
information obtained from VHF radio and satellite tagged harbor seals into
a Geographic Information System (GIS); (2) classifying habitat from a digital
terrain model based on bathymetric and hydraulic mining sediment data; and
(3) chemically analyzing the red pelage for the hydraulic mining signature
and other metals that may be linked to the debris. We present this plan to
demonstrate the benefits of collaborative analysis for conservation issues.
Grigg EK, Green DE, Allen SG, and Markowitz H. Population Status and
Trends of Harbor Seals (Phoca vitulina richardsi) in San Francisco Bay, CA,
1970-2000, State of the Estuary Biennial Conference, San Francisco, CA , October
2001.
Numerous large-scale construction activities are scheduled for San Francisco
Bay (SFB) over the next decade, including two bridge retrofit projects and
the San Francisco Airport runway expansion. These projects have the
potential to affect the population of harbor seals, the only resident marine
mammals in SFB. Ground-based counts were used to evaluate population
numbers and population changes between 1970 and 2000. Since May 1998,
we have been surveying three major harbor seal haul out sites in San Francisco
Bay (SFB), California: Mowry Slough (MS), Castro Rocks (CR) and Yerba Buena
Island (YBI). We evaluated current data against historical harbor
seal counts for the Bay, including an additional SFB haul out site (Strawberry
Spit, SS) that was abandoned in the early 1980's due to development and
a shift in food resources. In addition, we developed a model of population
trends of seals in SFB using stepwise polynomial regressions on the natural
logarithm of maximum yearly seal counts. Although the SFB population
has remained stable over the past 30 years, there have been shifts in the
number of seals using each site during both the pupping and non-pupping seasons.
During the pupping season, maximum counts increased at both CR and YBI, while
counts at MS decreased. During the non-pupping season, maximum counts increased
at all sites. We believe that increases at these sites were influenced
by the abandonment of the SS haul out site and by increases at nearby coastal
sites. A cubic regression provided the best fit for data during both the
pupping and non-pupping seasons. We plan to use this model to compare
predicted and actual seal counts in SFB during future large-scale construction
activities over the next decade. Harbor seals are a top predator species
in SFB, and are faced with high levels of anthropogenic stress. Evaluating
long-term population trends is an essential component in understanding and
protecting this resident species.
Nickel B, Grigg KE, Green DE, Allen SG, Markowitz H, Pacific harbor
seal (Phoca vitulina richardsi) distribution, movement, and foraging activities
within an urban estuary: implications for the effects of seismic retrofitting
in San Francisco Bay, California. Society for Marine Mammalogy Biennial
Conference, Vancouver, CA, November 2001.
Continual urban development and human population growth in San Francisco
Bay (SFB), California, increases the possibility that harbor seals will
abandon preferred breeding habitat due to anthropogenic disturbance and
habitat degradation. A primary harbor seal haul-out site and rookery,
Castro Rocks (CR), is adjacent to the Richmond-San Rafael Bridge (RSRB)
in northern SFB. From January to May, 2001, we conducted a pilot study on
harbor seal movements and foraging activities using a combination of VHF
and satellite-linked telemetry. This study complements an ongoing
monitoring program investigating the effects of a large-scale seismic retrofit
of the RSRB on the SFB harbor seal population.
Four harbor seals were captured and tagged (VHF: 1 adult female,
2 subadult females; PTT: adult male) at CR in January 2001.
Three of four tagged harbor seals showed high site fidelity to CR and used
consistent foraging areas within the study area. An adult female and
one sub-adult female were recorded at CR 60% and 90% of study days, respectively;
an adult male hauled out at CR during 81% of haul-out site surveys.
Harbor seals consistently foraged in areas within a mean distance of 6 km
from the primary haul out site. Home range estimates varied widely
from 50 km2 (sub-adult female) to 638 km2 (adult female). These results
support previous studies indicating harbor seals exhibit high haul-out site
fidelity in and around the pupping season, and illustrates the importance
of CR as a significant site for resident seals of SFB. With preferred
habitat shrinking at an accelerated rate, the potential loss of important
SFB sites poses a threat to the sustainability of the resident harbor seal
population. Further research on the movements and foraging activities
of harbor seals in relation to the seismic retrofit is required to accurately
assess potential effects to the resident population.
Austin K, Bohorquez A, Green D, Grigg E, Markowitz H, and Allen S. Observations
of epimeletic behavior in Northern Pacific harbor seal mothers toward their
dead pups at Castro Rocks, San Francisco Bay, California. Society
for Marine Mammalogy Biennial Conference, Vancouver, CA, November 2001.
Occurrences of epimeletic behavior of mothers toward their dead offspring
have been frequently documented in cetaceans, especially prominent in bottlenose
dolphins, yet this phenomenon has rarely been observed in pinnipeds.
Further, the few existing records of this care-giving behavior in pinniped
mothers directed to their dead pups only lasted for short durations.
While unavoidable factors may cause the death of the offspring, the response
of the mother may be to continue nurturance regardless of the cost.
Data concerning mother-pup interactions of Northern Pacific harbor seals
were gathered at Castro Rocks, San Francisco Bay, California throughout the
2001 pupping season. During this time period, two adult females were
observed carrying their dead pups in the water surrounding the site, placing
the dead pups onto the site and following the dead pups back into the water
after they had been washed away with the rising tides. Unlike previous
observations, in both of our cases the mother-pup pairs were seen for extremely
prolonged periods of time. In the first instance, the pup survived
for 3 days although it was born with a partial lanugo coat. The mother exhibited
the aforementioned behaviors toward the pup for at least 4 weeks after it
had died. In the second case, the pup was not observed while alive
but the pair was seen together for at least 3 weeks. Photographic records
established the identity of the mother and pup and confirmed that these
were two distinct instances. We believe that this is the longest recorded
observation of epimeletic behavior of mothers toward their offspring in
pinnipeds.
Green DE, Grigg EK, Markowitz H, Allen SG, The Impacts of Preconstruction
Core Sampling at the Richmond-San Rafael Bridge, CA on Harbor Seal (Phoca
vitulina) Haul Out Patterns. Society for Marine Mammalogy Biennial Conference,
Vancouver, CA, November 2001.
Castro Rocks (CR), the second largest rookery in San Francisco Bay, is
located next to the Richmond-San Rafael Bridge. Prior to a large-scale
seismic retrofit of the bridge, preconstruction core sampling (PCCS) was
conducted from January 24 through February 14, 2001 near CR. We examined
changes in harbor seal site use during PCCS and summarized PCCS-related
disturbances. We have been monitoring harbor seals at CR since May
1998. During surveys, biologists recorded 1) total count of seals
present on CR and 2) behavioral data pertaining to disturbances to seals.
Mean number of seals on CR during PCCS was significantly less than during
the same time period in 1999 and 2000 (F=4.29, p<0.05, df=46), as well
as during the 3 weeks following PCCS work (t=2.75, p<0.05, df=15). The
frequency of disturbances/hr of field time was significantly higher during
the PCCS compared to the same time period in 1999 and 2000 (F=6.43, p<0.005,
df=46). In addition, the number of disturbances to cause a flush/hr
was significantly higher during the PCCS than during the same time period
in 1999 and 2000 (F=5.73, p<0.01, df=46). Mean number of disturbances/hr
and mean number of flushes/hr were greater during PCCS compared to the 3
weeks prior to and following PCCS, although not statistically significant.
Taken collectively, these data suggest that seal haul out patterns at CR
will be impacted when the seismic retrofit construction is conducted near
the haul out site. What, if any, long term effects construction activities
will have on harbor seal haul out patterns at CR remains unclear at this
time.
Grigg EK, Nickel B, Green DE, Allen S, Markowitz H. Spatial
Analysis of Habitat Use Patterns of Harbor Seals (Phoca vitulina richardsi)
in San Francisco Bay, California. Society for Marine Mammalogy Biennial
Conference, Vancouver, CA, November 2001, GIS Remote Sensing Workshop.
A Geographic Information System (GIS) was used to investigate relationships
between harbor seals and hydrographic features in San Francisco Bay (SFB),
California. From January to May 2001, we conducted a pilot study on
harbor seal habitat use in SFB, using VHF and satellite-linked telemetry.
Analysis was done on an integrated database of behavioral and environmental
data. Four harbor seals were captured and tagged (1 adult female,
2 subadult females; PTT: 1 adult male) at a major SFB haul-out site in January
2001. Using ArcView GIS, we overlaid harbor seal locations onto a
digital elevation model (USGS DEM) of bathymetric features. We calculated
distances traveled from primary harbor seal haul-out sites to foraging areas,
as well as the farthest distance traveled by each seal from its primary
haul-out site. Fixed kernel utilization distributions were estimated
in order to define individual home ranges and foraging areas. Additionally,
a spatial dive model (interpolated from point locations of mean dive length)
was created to investigate dive patterns within individual home ranges.
Three of four seals tagged in the pilot study used consistent foraging areas,
within a mean distance of 6 km from each individual’s primary haul-out site.
Maximum distance traveled by any seal tracked was 59.58 km, representing
a shift by the adult female to an alternate rookery. Home range estimates
varied widely from 50 km2 (subadult female) to 638 km2 (adult female).
There was some overlap of harbor seal foraging areas with prominent SFB
bathymetric features, such as major shipping channels. Mean water
depths in which seals were located ranged from 6-11 m. This study
complements an ongoing monitoring program investigating the effects of a
large-scale seismic retrofit of the Richmond-San Rafael Bridge on the SFB
harbor seal population. The use of GIS to analyze spatial patterns
of habitat use within SFB greatly enhances the ability to assess effects
on the resident population. Research continues on harbor seal movements
and foraging activities using VHF and satellite-linked telemetry. Future
analysis will incorporate vegetation and prey distribution, sediment type,
and primary productivity in order to accurately model environmental features
encountered by the SFB harbor seal population.
Bohorquez A., Markowitz, H., Allen, S. Factors Influencing harbor Seal
Pupping Behaviors at Castro Rocks, San Francisco Bay, California. Society
for Marine Mammalogy Biennial Conference, Vancouver, CA, November 2001.
The seismic retrofit of the Richmond-San Rafael Bridge (RSRB) in the San
Francisco Bay, poses many threats to the local colony of Pacific harbor
seals (Phoca vitulina richardsi) in San Francisco Bay (SFB). Effects
may be as obvious as site abandonment, or as subtle as slight changes in
haul out pattern. For mother-pup pairs, availability of optimal haul
out space is important for pup growth and survival. In this study,
haul out patterns of mother-pup pairs were monitored at Castro Rocks, a haul
out site adjacent to the RSRB. We sectioned Castro Rocks into different
quadrants to determine if there was a correlation between environmental variables
and use of each quadrant by the mother-pup pairs. Over three years,
we found a significant difference in quadrant usage by mother-pup pairs (F=15.6,
p<0.02, df=16). The reasons for these differences may have multiple
causes including weather, topography, depth, or proximity to human activities.
We considered the effects of environmental factors (temperature, rain, cloud
coverage, wind speed and direction) each day on the number of pups in each
quadrant, to determine if there was a significant relationship. From
this, we designed a model for the number of pups that we predict to be in
each quadrant, given existing environmental and anthropogenic factors.
In the upcoming years, if we see a change in quadrant use by mother-pup
pairs after the seismic-retrofitting of the RSRB begins, we can eliminate
environmental factors as a cause by using this model.
Nickel, B., Grigg, E., Green, D., Markowitz, H., Allen, S., Should I
stay or should I go?: Monitoring Pacific harbor seal (Phoca vitulina
richardsi) movement and site use in and around the San Francisco Estuary.
California and World Oceans Conference, October 2002, Santa Barbara, CA.
A large-scale bridge seismic retrofit project near a primary San Francisco
Estuary (SFB) Pacific harbor seal (Phoca vitulina richardsi) rookery and
year-round haul-out site, Castro Rocks (CR), has the potential to affect
activity and residence patterns of local harbor seals. The harbor
seal continues to be protected in California under the Marine Mammal Protection
Act of 1972 and monitoring in relation to the seismic retrofit is required.
To this end, the California Department of Transportation has funded a monitoring
project to evaluate the effects of seismic construction activities on the
local harbor seal population.
In this study, seals were fitted with VHF-radio and satellite-linked transmitters
to collect detailed information on their spatial distribution and movement
within the region. Tagged seals exhibited strong fidelity to haul-out
areas and discrete foraging ranges (<50 km from the primary haul-out
site) within SFB and the adjacent coast. Individual home range estimates
varied widely in size from 11 km2 to 808 km2. Most seal locations were
in or near SFB; however, five individuals moved out of the estuary to offshore
foraging grounds and coastal haul-out sites. Four of these five seals
returned at least once to SFB during the study period. Six seals
were recorded using CR as their only haul-out site. Eleven individuals
used multiple haul-out sites in and around SFB, four of which frequented
coastal or offshore haul-out sites, as far away as the Farallon Islands (~65
km from CR). The need to develop adequate management techniques for
seals continues as pressure from urban development persists in SFB.
Information on harbor seal space use and movement from this study has important
implications for the measures adopted for the effective management of harbor
seals in the San Francisco Estuary.
Galloway, M, Scianamblo, L, Grigg, E, Nickel, B, Green, DE, Markowitz,
H., Greig, D, Morton, C Movement and Dive Patterns of Two Rehabilitated
Harbor Seal Pups (Phoca vitulina richardii) Released Back Into An Urbanized
Estuary. Society for Marine Mammalogy Biennial Conference, North Carolina,
November 2003.
Post-release monitoring of rehabilitated animals is an important step in
determining wildlife rehabilitation success, particularly if animals are
rescued from and released back into a region characterized by high levels
of urbanization. Although release of rehabilitated seals near areas
where they are rescued is preferable, release near urban environments increases
the potential for further human interaction. In this pilot study, two
male Pacific harbor seal pups (Phoca vitulina richardii) were treated at
The Marine Mammal Center (TMMC); a rehabilitation facility located in Sausalito,
California, after being rescued from the highly urbanized San Francisco Bay
(SFB), CA on 28 February (seal G8319) and 30 March (seal W9218) because of
human interaction. Prior to release on 30 April (G8319) and 17 May
(W9218), both seals were fitted with head-mounted VHF radiotransmitters to
facilitate post-release monitoring. Typically, TMMC releases pinnipeds
into more remote coastal areas; however, these seals were released close
to their original stranding locations in order to monitor their movements
and measure the success of an urban area release. G8319 remained near
the release site, but was not found again after the first two days post-release.
W9218 was found near the release site for the first few days post-release
before traveling to various locations in SFB and the California coast.
G8319 had a mean surface interval of 0.41 minutes (SD +/- 0.19) and a mean
dive time of 2.26 minutes (SD+/- 0.90). W9218 had a mean surface interval
of 0.35 minutes (SD 0.14) and a mean dive time of 1.92 minutes (SD 0.68).
Our results suggest that movement patterns of these two rehabilitated seals
are comparable to wild young of year harbor seals radiotagged within SFB.
Further study is needed to gauge the success and survivability of rehabilitated
seals released into urbanized regions.
Grigg, Emma K., Green, Deborah E., Allen, Sarah G., Markowitz, Hal.
Creatures of Habit? Inter-annual Site Fidelity in the Pacific
harbor seal (Phoca vitulina richardii). Society for Marine Mammalogy
Biennial Conference, San Diego, CA. December 2005.
Haul-out space is an important and possibly limiting resource for harbor
seals in urbanized estuaries such as the San Francisco Bay (SFB).
Seals are generally considered to exhibit high site fidelity to established
haul-out sites, and when assessing population status in impacted areas like
SFB, it is important to understand patterns of local seal use of such sites.
Using a combined approach including vhf telemetry (2001-2005) and visual
resightings of tagged and naturally-marked seals (1998-2005), we assessed
inter-annual site fidelity and haul-out duration of seals at Castro Rocks
(CR), a major SFB haul-out and pupping site. Site fidelity and haul-out
duration of tagged seals varied widely, with 41% of seals using CR on more
than 50% of days tagged, and 24% using CR on more than 90% of days tagged.
When CR was used as the primary haul-out site, mean time spent hauled out
ranged from 4.39±0.47 hrs/day to 9.68±0.72 hrs/day. Of
6 seals (4 males, 2 females) tagged as weaned pups at CR in 2001, one female
was resighted until 2005, and one male was resighted until 2003. Three
of four females tagged as weaned pups in 2002 were resighted in 2005, and
one female tagged as a yearling in 2001 was sighted with a pup in 2005.
These resightings suggest at least some natal site fidelity in these animals,
although resighting rates of male weaned pups tended to be lower, with only
2 of 6 males resighted at CR more than one month post-tagging. Resightings
of naturally marked animals indicate that seals may continue to use CR for
at least 8 years. These results confirm site fidelity by harbor seals
in SFB; duration of time hauled out at CR each day may be limited by availability
of this low tide site.
Oral Presentations: Green DE, Grigg E, Allen S, and Markowitz H, Nocturnal Haul Out Patterns
of Harbor Seals (Phoca vitulina richardsi) at Castro Rocks, San Francisco
Bay, California. The Wildlife Society Western Conference, Monterey CA, January
1999
Night counts of harbor seals hauled out at Castro Rocks, located near the
eastern end of the Richmond-San Rafael Bridge, were collected from May 1998
through September 1998. This work was conducted as part of a project
monitoring the effects of Caltrans’ seismic retrofit of the bridge on this
seal population. This information serves as baseline data against
which future counts, collected during and following the bridge construction
work, will be compared.
Seals were consistently present in high numbers on the haul out at night.
The maximum monthly night count steadily increased over the five-month period
from 81 in May to 140 in September. These numbers are comparable to
maximum monthly daytime seal counts, which ranged from 114 to 134 during
the study period, with no obvious trend such as is seen with the night counts.
This site is subject to higher levels of human disturbance during the day
due to watercraft, and the higher volume of vehicular traffic on the bridge.
The consistent nighttime use of Castro Rocks by seals may reflect a response
to high daytime disturbance levels.
Bohorquez AS, Galloway MJ, Green DE, Grigg EK, and Allen SG, Markowitz
H, Differential Response of Pacific Harbor Seals (Phoca vitulina richardsi)
Towards Kayaks Compared to Other Watercraft. Animal Behaviour Society Conference,
Georgia, August 5-10, 2000.
Previous studies have considered the effects of various types of watercraft
disturbances on the haul-out patterns and behavioral responses of harbor
seals. We considered the variation in occurrence of kayaks within 200m in
comparison with other types of watercraft at two harbor seal haul-out sites
within San Francisco Bay, California. Since kayaks are more maneuverable
they often advance closer to the haul out site than other types of watercraft.
A higher proportion of kayaks elicit a disturbance response from the seals
than all other types of watercraft within 200m of the haul-out sites.
Kayaks within 200m also caused a higher proportion of flushes. This differential
response to the kayaks is a consequence of the proximity of the kayaks to
the seals. Kayaks are more likely to elicit disturbance responses from hauled-out
harbor seals because they are quiet and low to the water. These factors
may not allow the seals to detect kayaks until they are much closer to the
haul-out site and may lead to a higher startle response.
Grigg EK, Green DE, Allen SG, Markowitz H, An Analysis of Relationships
between Environmental Variables and Harbor Seal (Phoca vitulina richardsi)
Haul Out Patterns at Castro Rocks, San Francisco Bay, CA. The Wildlife
Society Western Section Annual Meeting, Feb. 22-24, 2001. Sacramento,
CA.
We surveyed a primary harbor seal haul-out site in San Francisco Bay, California,
May 1999 - August 2000, in order to examine seasonal relationships between
environmental variables and seal haul out patterns. Since tide height
was correlated with seal counts, we used a subset of our data to control
for this relationship while examining other variables. Multiple regressions
were run to identify relationships between total seal number and air temperature,
water temperature, solar radiation, wind speed and wind chill. In
addition, a t test was used to compare seal counts on rainy days vs. nonrainy
days.
As harbor seal haul out patterns vary by season, we examined data from
each season (pupping, molting, fall, winter) independently. During the pupping
season, all environmental variables, except for rain, influenced the number
of harbor seals hauled out: the higher the air temperature, the lower the
number of seals hauled out, whereas increases in water temperature, solar
radiation, wind speed and wind chill were all related to increases in seal
counts. During molting season, increases in both wind speed and wind
chill were related with increases in seal counts. In contrast, when
each weather variable was taken independently, there were no significant
relationships between weather variables and the number of harbor seals hauled
out during fall and winter. Overall, rain effected the number of seals
hauled out – with fewer seals present on rainy days. The influence
of weather variables should not be generalized across sites or seasons since
each location, season and even population has its own unique set of characteristics
to be considered.
Bohorquez, AS, Green DE, Grigg EK, Markowitz H, Allen SG. Current status
of red-pelaged harbor seals within the San Francisco Bay. Conservation Biology
Conference, Hilo, HI, July 2001.
A previous study of the San Francisco Bay (SFB) red-pelaged harbor seals
suggests a significant difference in haul out use by these seals at three
sites within the bay. Allen et al. (1993) reported that red pelage resulted
from iron oxide adherence to the keratin surface on the shaft of the hairs,
likely related to foraging behavior. This coloration tends to make the red
fur more brittle, leading to the loss vibrissae. Using several methods of
comparison, a significant difference in the number of seals with red pelage
was found between the central bay and the north and south bays. Continued
analysis of this population from June 1999 to December 2000 showed significantly
more of red-pelaged seals used the north and south bay haul out sites (p<0.05).
We found red-pelaged mothers gave birth earlier in both pupping seasons
(p<0.05). These results may indicate that red-pelaged mothers are older
or foraging in nutrient rich areas, presenting a possible balance between
foraging efficiency and the cost of the red-pelage. We recommend using this
demographic data and the red-pelage as an indicator of optimal foraging areas
and therefore identifying these areas from protecting.
Grigg, E.K., Green, D.E., Allen, S.G., Nickel, B., Markowitz, H. and
Gulland, F. Overview of current monitoring of harbor seals (Phoca vitulina
richardsi) in San Francisco Bay, CA, 6th Biennial Conference on Research in
the Gulf of Farallones. San Francisco, CA, October 2001.
Numerous large-scale construction activities are scheduled for San Francisco
Bay (SFB) over the next decade, including two bridge retrofit projects and
the San Francisco Airport runway expansion. These projects have the
potential to affect the population of harbor seals, the only resident marine
mammals in SFB. Ground-based counts were used to evaluate population
numbers and population changes between 1970 and 2000. Since May 1998,
we have been surveying three major harbor seal haul out sites in San Francisco
Bay (SFB), California: Mowry Slough (MS), Castro Rocks (CR) and Yerba Buena
Island (YBI). We evaluated current data against historical harbor
seal counts for the Bay, including an additional SFB haul out site (Strawberry
Spit, SS) that was abandoned in the early 1980's due to development and
a shift in food resources. In addition, we developed a model of population
trends of seals in SFB using stepwise polynomial regressions on the natural
logarithm of maximum yearly seal counts. Although the SFB population
has remained stable over the past 30 years, there have been shifts in the
number of seals using each site during both the pupping and non-pupping seasons.
During the pupping season, maximum counts increased at both CR and YBI, while
counts at MS decreased. During the non-pupping season, maximum counts increased
at all sites. We believe that increases at these sites were influenced
by the abandonment of the SS haul out site and by increases at nearby coastal
sites. A cubic regression provided the best fit for data during both the
pupping and non-pupping seasons. We plan to use this model to compare
predicted and actual seal counts in SFB during future large-scale construction
activities over the next decade. Harbor seals are a top predator species
in SFB, and are faced with high levels of anthropogenic stress. Evaluating
long-term population trends is an essential component in understanding and
protecting this resident species.
Green, DE, Grigg, EK, Allen, SG, Markowitz, H. San Francisco Bay Harbor
Seals. Monthly district-wide meeting for Caltrans, Oakland, CA, December
2001. Presentation on the current findings of the project for the monthly
district meeting. No abstract available.
Grigg, EK, Green, DE, Allen, SG, Markowitz, H. The Richmond Bridge Harbor
Seal Survey. 8th Annual Wildlife and Aquatic Animal Medicine Symposium,
University of California at Davis, January 2002.
One-hour presentation summarizing the work of the Richmond Bridge Harbor
Seal Survey. No abstract available.
Green DE, Grigg EK, Markowitz H, Allen SG, San Francisco Bay Harbor
Seals. Bishop O'Dowd High School, Oakland. Outreach Presentations.
April 2002.
Four one-hour discussion groups about harbor seals in San Francisco Bay,
California. No abstract available.