White Shark Studies the Farallon Islands: 2003 A Report to U.S. Fish and Wildlife Service, Farallon National Wildlife Refuge
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
White Shark Studies the Farallon Islands: 2003
A Report to U.S. Fish and Wildlife Service,
Farallon National Wildlife Refuge
Adam Brown
Marine Ecology Division
PRBO Conservation Science
4990 Shoreline Hwy
Stinson Beach, CA. 94970
Scot Anderson
P.O. Box 390
Inverness, CA 94937
INTRODUCTION
There is much interest and concern over the population of white sharks in California. As
an apex predator, there are questions relating to the role of white sharks in marine
ecosystem dynamics, effects on the pinniped population dynamics (which in turn may
influence fish stocks), attacks on humans, and status as a rare species (Klimley and
Ainley 1996). Southeast Farallon Island (SEFI), located 42km west of San Francisco, has
proven to be one of best places in California to study the white shark (Ainley et al. 1985,
Klimley and Ainley 1996). Analysis of shark-bitten pinnipeds from SEFI and the coast
(Long et al. 1996), and continuous monitoring of predation frequency at SEFI (Ainley et
al. 1985, Klimley et al. 1992, Pyle 1992, Pyle et al. 1996a) suggest that white sharks are
increasing (Pyle et al. 1996a), but additional years of monitoring are needed to assess
white shark abundance. Moreover, possible disturbance to white sharks feeding at SEFI
from increased cage-dive eco-tourism activities in 1999-2003 justify continued studies of
predator-prey interactions at this site.
While several studies have shed light on the life history of seasonally resident white
sharks, little is known of their movement patterns or breeding biology. Distributions of
captured and beach-washed white sharks suggest that adults travel to Baja California to
breed in March-June (Klimley 1985), but recent data has shown a wider range than
previously recognized, including trans-Pacific migrations towards the Hawaiian
Archipelago (Bustany et al. 2002). Moreover, no pregnant white sharks have ever been
observed in the eastern Pacific Ocean. While white sharks are known to occur in remote
places in the Pacific (e.g., Chile, New Zealand, Australia, Japan, and Hawaii), it is not
known if these populations are isolated from one another.
The Shark Trust
4 Creykes Court, The Millfields
Plymouth, PL1 3JD
www.sharktrust.orgIn 2003 we continued to monitor white shark activity (predation events and marked
individuals) at SEFI during the fall (September through November). Specifically, the
objectives of this study included: (1) to determine the frequency of predatory attacks (no.
per 100 hrs of observation), (2) to determine the species and size/age composition of
white shark prey, and (3) to mark and re-sight (using photogrammetry) individuals. In
addition, we applied six satellite pop-off transmitters in collaboration with Stanford
University (Dr. Barbara Block), to determine white shark movement patterns and
possible breeding distribution.
METHODS
To investigate attack frequencies and prey characteristics we stationed an observer atop
Lighthouse Hill (LHH; 365m above sea level) during daylight hours (visibility and wind
conditions permitting) from 1 September to 30 November 2003. This protocol has been
used since 1987. The observer scanned the ocean surface for shark activity. When a
sighting or predation event was detected, the observer (1) pinpointed the location of the
attack with a theodolite, (2) attempted to identify the prey species using a 20x and 60x
telescope, (3) examined the shark(s) involved for scars or other marking for individual
identification, (4) communicated the event location to personnel in a small boat. We also
made weekly counts of pinnipeds hauled-out on SEFI to relate attack frequency to prey
abundance.
Beginning in 1993, we identified individual white sharks through the use of observation,
photographs, and underwater video. Once a predatory event had been observed from
LHH, we proceeded to the site in a small boat (Boston Whaler); except when we maked
animals, we remained >10 m from feeding sharks to minimize the potential affect of our
presence on behavior. Previously, we discovered that white sharks investigate boats or
decoys for up to two hours subsequent to predatory events (Anderson et al. 1996b). This
behavior allowed time for us to use underwater video recorders to identify and sex
individual sharks by the presence (male) or absence (female) of claspers (Pratt, 1996).
We identified individuals by size and unique markings such as scars, mutilated fins, and
natural pigmentation and notch patterns on the trailing edge of the dorsal fin (Anderson
and Goldman 1996, Klimley and Anderson 1996). To facilitate this research, we moored
our small boat in Fisherman’s Bay for most of the season (15 September to 14 November
2003). On days of favorable weather and tides we launched a zodiac from North Landing
and transferred equipment to the research vessel up to three hours prior to a high tide and
remained in “standby” mode in the boat for up to six hours (until three hours following
the high tide.
In collaboration with Stanford University (Dr. Barbara Block and Kevin Weng), we
deployed six archival pop-up transmitters onto white sharks. Transmitters were attached
below the first dorsal fin (in the dorsal saddle musculature) using an eight-foot dowel
pole with a titanium billfish-tagging spear tip. Once every 2-10 minutes the tag records
water temperature, water depth, and light volume. These data are stored within the tag for
up to twelve months (according to the programmed pop-off times). Once the tags release,
The Shark Trust
4 Creykes Court, The Millfields
Plymouth, PL1 3JD
www.sharktrust.orgthey automatically download these data, along with final position via satellite to
computers at Hopkins Marine Station, Stanford University.
RESULTS
Observations of Shark Predation
We completed a total of 605 hours and 45 minutes hours of observations from LHH from
September 1 – November 29. During September we completed 222 hours and 50
minutes of shark watch. During October we logged 202 hours and 25 minutes of
observations. During November we completed 180 hours and 30 minutes of shark watch.
Observations were cancelled during fog or heavy rains when the ocean past West End
was not visible.
Feeding Events
We observed 66 white shark feeding events. Twenty attacks were on juvenile northern
elephant seals, and 10 attacks were on California sea lions. For the remaining 35 attacks,
the prey species was not identified.
Individual Identification.
We identified 6 individual sharks.
Tagging
We placed six tags in white sharks in 2003. Three tags were programmed to release in 6
months and 3 are scheduled for release after 12 months. Of the three 6-month tags, two
were placed on males, while one was placed on a shark of unknown sex. Of the three
twelve-month tags, two were placed into males while one was placed on a female (Table
1).
Table 1. Identification, sex and age of White Sharks tagged during 2003 on Southeast
Farallon Island
Name of White Shark Sex of White Shark Six/Twelve Month
Satellite Tag
Joelle unknown Six
Gouge male Six
unknown female Twelve
unknown male Twelve
Cut Tail male Six
Rough Fin male Six
DISCUSSION
We observed an above average number of white shark attacks during 2003. Of these
attacks 30% were on Northern Elephant Seals, 15% were on California Sea Lions, and
The Shark Trust
4 Creykes Court, The Millfields
Plymouth, PL1 3JD
www.sharktrust.org55% were on unknown prey species. The high number of attacks on unknown prey
species is indicative of the visual absence of a prey item during observations.
Feeding events on northern elephant seals are characterized by very bright blood, a large
and very distinct slick on the water, and a floating carcass. In contrast, feeding events on
California sea lions are characterized by dark blood, a small indistinct slick, and absence
of a floating carcass. Therefore, we surmise that most of our unknown prey items were
California sea lions.
This season we also observed very large seabird feeding flocks off of the east side of
SEFI. It appeared that the seabird flocks were concentrated on krill. These krill swarms
were often over a mile wide, with thousands of seabirds, hundreds of sea lions, tens of
whales, and unknown numbers of albacore tuna and surface feeding rockfish taking part
in foraging groups. We were able to monitor what the Western Gulls were feeding on in
the flocks, using their droppings. Starting in early September, that the majority of
droppings on the island were squid. Soon after we discovered this, a fishery for squid
developed off of the east side of the island, including purse-seiners and light boats.
White Shark behavior was affected this activity. During most years, the majority of
feeding events are observed off of the west side of the island. However, in 2003, we
witnessed the majority of our feeding events off of the east side of the island. This shift
in distribution was most likely associated with the presence of prey (i.e. California sea
lions) feeding on squid off of the east side of the island.
ACKNOWLEDGEMENTS
We thank USFWS for permission to conduct this research from SEFI and PRBO,
Homeland Foundation and Baker Trust for financial support. Bill Sydeman edited and
provided comments on this report. This is PRBO contribution no. 1138.
LITERATURE CITED
Ainley, D.G., R.P. Henderson, H.R. Huber, R.J. Boekelheide, S.G. Allen, and
T.L. McElroy. 1985. Dynamics of white shark / pinniped interactions in the
Gulf of the Farallones. Memoirs South. Calif. Acad. Sci. 9:109-122.
Anderson, S. D., and K. J. Goldman. 1996. Photographic evidence of white shark
movements in California waters. California Fish Game 82:182-186.
Anderson, S.D., R.P. Henderson, and P. Pyle. 1996b. Observations of white
shark reactions to unbaited decoys. Pp. 223-228 in A.P. Klimley and D.G.
Ainley (Eds.), The Ecology and Behavior of the White Shark. Academic Press,
San Diego.
Anderson, S.D., A.P. Klimley, P. Pyle, and R.P. Henderson. 1996a. Tidal height
and white shark predation at the South Farallon Islands, California. Pp.
The Shark Trust
4 Creykes Court, The Millfields
Plymouth, PL1 3JD
www.sharktrust.org275-280 in A.P. Klimley and D.G. Ainley (Eds.), The Ecology and Behavior of
the White Shark. Academic Press, San Diego.
Anderson, S.D., and P. Pyle. In press. A temporal, sex-specific occurrence
pattern among white sharks (Carcharodon carcharias) at the South Farallon
Islands, California. California Fish and Game.
Boustany, A., S.F. Davis, P. Pyle, S.D. Anderson, B.J. LeBoeuf, and B.A. Block.
2002. Expanded niche for great white sharks. Nature 415:35-36.
Goldman, K.J., S.D. Anderson, J.E. McCosker, and A.P. Klimley. 1996.
Temperature, swimming depth, and movements of a white shark at the South
Farallon Islands, California. Pp. 111-120 in A.P. Klimley and D.G. Ainley
(Eds.), The Ecology and Behavior of the White Shark. Academic Press, San Diego.
Hare, S.R., and N.J. Mantua. 2000. Empirical evidence for North Pacific regime
shifts in 1977 and 1989. Progress in Oceanography 47:103-145.
Heneman, B. and M. Glazer. 1996. More rare than dangerous: A case study of
white shark conservation in California. Pp. 481-491 in A.P. Klimley and
D.G. Ainley (Eds.), The Ecology and Behavior of the White Shark. Academic
Press, San Diego.
Klimley, A.P. 1985. The areal distribution and autoecology of the white shark,
Carcharodon carcharias off the west coast of North America. Memoirs of the
Southern California Academy of Sciences 9:15-40.
Klimley, A.P. and D.G. Ainley, Eds. 1996. The Ecology and Behavior of the
White Shark. Academic Press, San Diego.
Klimley, A.P. and S.D. Anderson. 1996. Residency patterns of white sharks at
the South Farallon Islands, California. Pp. 365-374 in A.P. Klimley and
D.G. Ainley (Eds.), The Ecology and Behavior of the White Shark. Academic
Press, San Diego.
Klimley, A.P., S.D. Anderson, P. Pyle, and R.P. Henderson. 1992.
Spatiotemporal patterns of white shark (Carcharodon carcharias) predation
at the South Farallon Islands, California. Copeia 1992:680-690.
Klimley, A.P., P. Pyle, and S.D. Anderson. 1996a. The behavior of white sharks
and their pinniped prey during predatory attacks. Pp. 175-192 in A.P.
Klimley and D.G. Ainley (Eds.), The Ecology and Behavior of the White
Shark. Academic Press, San Diego.
Klimley, A.P., P. Pyle, and S.D. Anderson. 1996b. Tail slap and breach:
Agonistic displays among white sharks? Pp. 241-256 in A.P. Klimley and
The Shark Trust
4 Creykes Court, The Millfields
Plymouth, PL1 3JD
www.sharktrust.orgD.G. Ainley (Eds.), The Ecology and Behavior of the White Shark. Academic
Press, San Diego.
Lea, R.N. and J.E. McCosker. 1996. White shark attacks in the Eastern
Pacific Ocean: An update and analysis. Pp. 419-434 in A.P. Klimley and D.G.
Ainley (Eds.), The Ecology and Behavior of the White Shark. Academic Press,
San Diego.
Long, D.J., K. Hanni, P. Pyle, J. Roletto, R.E. Jones and R. Bandar. 1996.
Geographical and temporal patterns of white shark predation on four
pinniped species along central California, 1970-1992. Pp. 263-274 in A.P.
Klimley and D.G. Ainley (Eds.), The Ecology and Behavior of the White
Shark. Academic Press, San Diego.
Pyle, P. 1992. Sympathy for a predator: White shark studies at Southeast
Farallon. Observer 93:1-11.
Pyle, P., S.D. Anderson, and D.G. Ainley. 1996a. Trends in white shark
predation at the South Farallon Islands, 1968-1993. Pp. 375-380 in A.P.
Klimley and D.G. Ainley (Eds.), The Ecology and Behavior of the White
Shark. Academic Press, San Diego.
Pyle, P., S.D. Anderson, A.P. Klimley, R.P. Henderson, and D.G. Ainley. 1996b.
Environmental factors affecting the occurrence and behavior of white sharks
at the South Farallon Islands, California. Pp. 281-292 in A.P. Klimley and
D.G. Ainley (Eds.), The Ecology and Behavior of the White Shark. Academic
Press, San Diego.
Pyle, P., M.J. Schramm, C. Keiper, and S.D. Anderson. 1999. Predation on a
white shark by a killer whale and a possible case of competitive exclusion.
Marine Mammal Sci. 15:563-568.
The Shark Trust
4 Creykes Court, The Millfields
Plymouth, PL1 3JD
www.sharktrust.orgYou can also read
