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(Phoca vitulina richardsi) in San Francisco Bay, California |
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(Phoca vitulina richardsi) in San Francisco Bay, California |
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Introduction
From January 7-9, 2001, 8 harbor seals
were captured at Castro Rocks for tagging under Scientific Research Permit
# 373-1575 from National Marine Fisheries Service. Individuals experienced
in tagging harbor seals were recruited to carry out the tagging activities
(Steve Jeffries of the Washington Department of Fish & Game, Jim Harvey
from the Moss Landing Marine Laboratory), and a veterinarian experienced
in marine mammal work was onsite during all captures and tagging work (Frances
Gulland, DVM, from the Marine Mammal Center in Sausalito, CA). Tagging
harbor seals will allow us to monitor movements of these seals within the
Bay, and follow the movements of any that leave the Bay. This will
help us to understand the current habitat use patterns of seals in the
Bay, and how the seismic retrofit influences seal movements and distribution.
Methods
A “tangle net” method was utilized for seal capture due to the rocky substrate
surrounding Castro Rocks. Nets which were approximately 20-40 m in
length and approximately 5 m in depth were set to the south of Castro Rocks
and seals were passively caught as they got tangled in the net. The
tangle net method reduced the potential of having a large number of seals
to process at once, thereby minimizing processing time for each seal captured.
Once captured, seals were weighed, sexed, and blood samples were drawn.
Three female seals, 2 subadult non-red coats and one red coat were fitted
with head mount VHF radiotags (Advanced Telemetry Systems). One adult
non-red coat male was fitted with a dorsally-mounted satellite tag from
Telonics. All seals captured were flipper tagged on both rear flippers
to allow for later identification in the field.
Seals were tracked from land, by boat, and using stationary receivers and
dataloggers at two haul-out sites.
Sighting location for radiotagged seals was
obtained using a biangulation method during land-based tracking and with
a handheld GPS (after visual confirmation) during boat-based tracking.
In analyzing data for radiotagged seals, depth information was obtained for each animal location using the ArcView GIS Version 3.2 Spatial Analyst Extension from the Digital Elevation Model of the San Francisco Estuary generated by the Special Projects Office of the National Ocean Service. Location and depth data for radiotagged seals was provided by Barry Nickel of the Richmond Bridge Harbor Seal Survey. Maps of seal locations were produced using ArcView GIS Version 3.2 (ESRI 1998).
Results and Discussion
According
to land-based tracking efforts, the radiotagged adult redcoat female hauled
out at three different sites; Castro Rocks, Mowry Slough and Brooks Island
(Map 1). While in the central SFB (i.e. discounting
the times when the seal was located at or near Mowry Slough), the average
distance the adult female traveled from Castro Rocks was 3.22 km (+
2.27 km SE). The maximum distance this seal traveled from Castro
Rocks was 59.58 km, which represents the seal moving from the Castro Rocks
area to Mowry Slough. The average water depth the adult female was
recorded in (does not refer to the dive depth) was –5.6 m (+ 3.83
m SE). The radiotag on the adult female was lost after six weeks;
prior to losing the tag, this seal spent the majority of its time in the
central SFB. The adult redcoat was sighted at Mowry Slough after
losing the radiotag, and has not been seen at Castro Rocks since that time.
It is possible that this seal moved to Mowry Slough for the duration of
the pupping season.
One subadult female was seen mainly in the central to north SFB, with an average distance from Castro Rocks of 3.29 km (+ 2.09 km SE) and in average water depths of –11.1 m (+ 6.49 m SE), according to land-based tracking. In addition to land-based tracking, several boat based tracking efforts were conducted to track the subadult female seal (Map 1). According to boat-based calculations, the subadult female was seen an average 2.98 km (+ 1.33 km SE) from Castro Rocks and in waters with average depths of –9.6 m (+ 3.55 m SE).
The second subadult female lost its radiotag
soon after tagging. Based on the limited tracking data we were able
to collect on this seal (Map 1), and on subsequent
sightings by field biologists, this seal appeared to primarily use the
YBI haul out site.
In contrast to the three females with radiotags, the adult male with the
satellite tag was generally located in the area of Castro Rocks and to
the north in San Pablo Bay (Map 2). Sightings
in San Pablo Bay were concentrated along the shipping channel and near
the mouth of the Napa River. The average distance from Castro Rocks
of the locations for the male was 7.249 km ( + 0.837 km SE), with
a maximum of 21.44 km from Castro Rocks. Castro Rocks appears to
be the only haul out site utilized by the male during January and February
of 2001. Depth and home range analysis for the male has not been
completed.
The tracking work in San Francisco Bay is ongoing.
Map
1: Radiotagged Harbor Seals in San Francisco Bay
Map 2: Satellite-tagged Harbor Seal in San Francisco Bay
Many thanks to the following people for their help and advice with the capture, tagging, photos, data processing and analysis for the RBHSS tracking study:
Jim Harvey, Moss Landing Marine Labs (MLML)
Steve Jeffries, Washington Dept. of Fish and
Game
Frances Gulland, The Marine Mammal Center
(TMMC)
Dyanna Lambourn, Washington Dept. of Fish
and Game
Tracy Goldstein, TMMC
Stori Oates, MLML
Denise Grieg, MLML
Brett Long, UC Santa Cruz
Matt Rutishauser, MLML
Chuck Morton, Caltrans
and the rest of the Caltrans crew
Jan Roletto, NOAA/Gulf of the Farallones National
Marine Sanctuary
Chris Stallings, SFSU
Ralph Larson, SFSU
Michelle Lander, TMMC
Barry Nickel, RBHSS
Amy Bohorquez, RBHSS
and the rest of the RBHSS field team (bridge
seal spotters!)
Greg Early, New England Aquarium
Toby Garfield, SFSU
Will Harrison, Service Argos, Inc.
Kathy Frost, Alaska Dept. of Fish and Game
