Download white shark research at southeast farallon island

WHITE SHARK RESEARCH AT
SOUTHEAST FARALLON ISLAND 2009
REPORT TO THE U.S. FISH AND WILDLIFE SERVICE
FARALLON NATIONAL WILDLIFE REFUGE
J. Tietz
James R. Tietz
Marine Ecology Division
PRBO Conservation Science
3820 Cypress Drive # 11
Petaluma, CA, 94954
January 2010
© 2010 PRBO Conservation Science
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Any reference to or use of this report, or any portion thereof, shall include the following citation:
Tietz, J. R. 2009. White Shark Research at Southeast Farallon Island, 2009. Unpublished report to the
US Fish and Wildlife Service. PRBO Conservation Science, Petaluma, California. PRBO Contribution
Number 1722.
For further information contact the director of the Marine Ecology Division at [email protected] or
PRBO Conservation Science, 3820 Cypress Drive #11, Petaluma, CA, 94954.
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INTRODUCTION
Over the past decade, it has become more widely recognized that targeted fishing,
trophy hunting, fishery by-catch, and persecution have taken their toll on shark
populations worldwide (Baum et al. 2003, Myers and Worm 2005, Shivji et al. 2005,
Ferretti et al. 2008). Natural history traits, such as low fecundity and late maturation,
indicate that these species are highly susceptible to over-exploitation and that recovery
efforts will be long and require targeted efforts (Smith et al. 2000, Myers and Worm
2005). White shark populations are especially vulnerable because their apex predator
status means they were probably not abundant before human exploitation, and today,
they are frequently targeted by trophy hunters (Compagno et al. 1997).
The top-down effect that marine apex predators have on their community has only
recently been documented (Myers et al. 2007, Frid et al. 2008, Heithaus et al. 2008).
Models have shown that the loss of a top predator from a North Pacific marine
ecosystem may lead to “serious and unforeseen consequences” (Stevens et al. 2000).
However, it has also been shown that top-down effects may be reduced in upwelling
regions, such as along the California Coast, where marine ecosystems are dominated
by strongly fluctuating planktivorous populations that control trophic stability from the
bottom up (Cury et al. 2000).
During late summer, white sharks return to the California Coast after completing
their annual migration to offshore focal areas, so they can find abundant pinnipeds to
replenish their fat stores and reenergize them for the trip back out into the open ocean
(Weng et al. 2007, Jorgensen et al. 2009). During the fall, white sharks are the top
predator in most near-shore communities along the California Coast. In addition,
tagging has found that these white sharks show high site fidelity and prefer to hunt
along the same stretches of coastline every fall (Jorgensen et al. 2009). Although white
sharks are distributed globally, recent research has discovered that white sharks in the
northeastern Pacific are genetically distinct from other populations (Jorgensen et al.
2009), which only enhances their conservation value and the need to monitor them.
Due to their high mobility underwater and the vastness of their environment, directly
monitoring shark populations is logistically difficult (Baum et al. 2003). However,
because mature white sharks show site fidelity and feed on the surface, it is possible to
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monitor the frequency of their predation events, and this may provide an index to
populations of sharks that utilize discreet foraging areas. The large pinniped haul-out
and rookery provided by the Farallon Islands has made this a world-renowned location
for viewing sharks. For the past 22 years, biologists have been conducting
standardized surveys from the lighthouse to record white shark predation events. Longterm datasets, such as these, are important for monitoring populations that can have
extreme annual variation due to numerous factors unrelated to the actual population
trend.
METHODS
Between 1-September and 30-November 2009, we conducted standardized shark
surveys from the lighthouse on days when weather permitted. Each day, every trained
observer typically spent 2 hours at the lighthouse. The total number of shark survey
hours per day depended on the total number of biologists on the island. Shark surveys
were cancelled if it rained or low clouds limited the visibility of the observer to less than
1 km of water around the island. Shark surveys resumed as soon as the weather
improved. During shark surveys, observers continually scanned the waters for shark
activity and circled the lighthouse at least once every five minutes as shark predation
events usually remain on the sea surface from 5-15 minutes. To confirm an attack, an
observer was required to see 3 of the 5 shark attack indicators: 1) gulls swarming over a
small circular spot on the water, 2) a circular slick of pinniped oil trailing away from the
carcass, 3) concentrated blood pool in the water, 4) pinniped carcass in the water, or 5)
a white shark thrashing on the ocean surface. Once an attack was confirmed, the
observer recorded the start time and used a theodolite to pinpoint the location of the
feeding event on the sea surface. After the location was recorded, the observer used ≥
30x scope to attempt to identify the prey item. Occasionally, though, this was not
possible when the carcass rapidly disappeared beneath the water’s surface, or the
shark consumed the prey too quickly. Whenever there was an attack, we immediately
communicated with the Stanford Research Vessel to notify them of the attack, if they
were not already aware of it, and afterwards we would discuss the identity of the prey to
improve our accuracy.
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Pinnipeds are the primary food source of mature white sharks, and our weekly
pinniped survey has been shown to be the best predictor of shark attacks around the
Island (Brown et al., in press). Once per week, when weather permitted a clear view of
the island, an observer at the lighthouse counted all California and Steller sea lions,
harbor seals, and northern fur seals. Because northern elephant seals typically haul out
in gulches where they are not visible from the lighthouse, a ground survey of all the
gulches took place immediately after the lighthouse survey. Due to some areas being
closed to avoid wildlife disturbance, some elephant seals were counted from the
lighthouse that could not be viewed from the ground.
RESULTS
During the 2009 season, biologists logged a total of 535.2 hours of standardized
shark surveys from the lighthouse. Due to inclement weather and fluctuating numbers
of biologists available to conduct shark surveys, the average number of observation
hours per day was not stable (Figure 1). Although we had 4-5 biologists on the island
that could have conducted shark surveys during the final 3 weeks of September, an
unusually persistent fog bank severely limited our observation time so that we were only
able to average 4.0 observer hours per day. Another dip in observer effort occurred in
mid-October when an unseasonably large storm lingered over the island for several
days.
Including incidental observations, a total of 15 attacks were noted around the island
from 25 August to 8 November (Figure 2). Using just standardized survey data, the
daily attack rate was zero for the first and last three weeks with a small peak during midOctober (Figure 3). Only ten predation events were documented from the lighthouse
during standardized surveys. Two of these attacks were unusual, which may result in
one or both of them not being used in future analyses. The first occurred in midOctober when a shark attempted to depredate a California sea lion, but only managed
to wound its prey. The other occurred on 8-November when an observer recorded an
attack that occurred ~3 km northwest of the island, outside of the normal range of
reliable observations.
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Pinniped surveys were conducted 13 times on a near weekly basis except for during
mid-September when nearly two weeks elapsed between surveys due to the fog.
California sea lion numbers were below average at the beginning of the year, but spiked
upward in mid-October, before dropping down to average numbers for November
(Figure 4). Northern elephant seal numbers gradually increased over the fall, as is
typical for this season (Figure 5). The increase was not as dramatic, though, as in past
years, and counts in October and November were 55% lower than compared to the
previous 3-year average (Figure 6). Counts of the other pinniped species were all
relatively low and near average (Figure 5).
Prey species were identified at ten of the attacks (Figure 7). California sea lions
were the most commonly identified prey species during September, but Northern
elephant seals greatly outnumbered sea lions during October and November.
DISCUSSION
The seasonal distribution of shark attacks this year showed a small spike in October
with very few attacks before or after. Historically, the beginning of September is the
time when sharks start arriving around the island, but over the past few years, we have
detected very few sharks during the first few weeks of surveys. It is difficult to speculate
about the reason for the paucity of sharks feeding around the island during this time of
year, though, because numbers of pinnipeds have been fairly consistent, with large
numbers of California Sea Lions and relatively few of the other species. Perhaps the
acoustical or GPS tagging that is being conducted by TOPP (Tagging of Pacific
Predators) will help determine whether white sharks are arriving at the California
Coastline later in the year, or are simply utilizing another part of the coastline during
September.
In 2008, white shark attacks increased dramatically in November to make up for a
slow start to the season, but this year, attacks dropped down virtually to zero at the start
of that month. There were three possible reasons that could have reduced shark
feeding around the island that probably were not mutually exclusive. The first is that
there were 55% fewer elephant seals than the 2006-2008 average during October and
November (Figure 6). Brown et al. (in press) found that the total number of elephant
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seals counted during weekly surveys is the most important factor in determining annual
variation in the number of shark attacks at SE Farallon Island. Over the last several
years, sand from haul-out beaches has washed out into the ocean and is not getting
replaced. This change in habitat, along with emigration, and demographic changes in
the population (Lee 2009) may be reducing the importance of this site as a haul-out and
a breeding colony for elephant seals. These factors may be forcing white sharks to find
prey elsewhere.
Another possible reason for the apparent disappearance of sharks from the Farallon
Islands may be the new tagging research, led by Michael Domeier, initiated in late
October. This operation hooked two sharks (one on 29 October and the other on 2
November) and pulled them out of water onto their boat where they drilled holes in their
dorsal fins and bolted on transmitters. Although this operation appeared highly
invasive, this technique has been used at Guadalupe Island off Baja California without
obvious adverse effects to the population around the island.
The final factor that may have driven the sharks away from the island was when 7
killer whales swam up to the west side of the island and depredated at least three
animals: the first was a large elephant seal; the second also appeared to be an elephant
seal; but the third occurred mostly below the surface, and although we have no way to
be certain of the prey species’ identity, it could have been a white shark. On 4 October
1997, a pair of killer whales was documented killing and partly consuming a white shark
1 km northwest of Southeast Farallon Island (Pyle et al. 1999). Following this event,
white shark sightings at the island were reduced by 97% below the previous 8 year
average (Pyle et al. 1999). A similar event occurred on 19 November 2000 when a
killer whale again killed a white shark. In 2000 during the first half of November, the
shark predation rate was ~1.6 attacks per day. Following the killer whale attack, white
shark sightings around the island ceased. Furthermore, one shark had a popup satellite
tag that indicated that it left the islands within an hour of the attack and headed west
into the Pacific (Adam Brown, pers. comm.). Although one attack this year occurred
after the killer whales showed up, it was ~3 km northwest of the Southeast Island by
Mid-Farallon, a small rocky islet well isolated from the rest of the Farallon Islands.
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Regardless of what caused the apparent exodus in early November, this event
ensured that 2009 would be the second worst ever for shark predation events at the
Farallon Islands (Figure 8 and 9). Although 1997 was worse overall than 2009 in total
attacks and attack rate, the shark predation rate in 2009 prior to the killer whales
arriving (2.4 attacks/100 hrs) was nearly half that in 1997 prior to that killer whale attack
(4.7 attacks/100 hrs). One possibility for the slow shark predation rate in 2009 is that
killer whales were also around the island on 20 August. This is typically the time,
though, when relatively few sharks are present at the Farallon Islands. A shark attack
on 25 August, though, would seem to indicate that the killer whales left the islands
without harming any white sharks.
Since 2000, white shark predation events, and presumably numbers, have been
trending downwards (Figures 8 and 9). The answer to whether this is just a natural
cycle, a spatial shift in white shark foraging, or an actual population decline in the
Eastern Pacific will need to wait until more data can be collected and analyzed. If this
really is a population decline, it will be interesting to learn in future years whether trophic
cascades are possible in an upwelling environment. At the Farallon Islands, pinnipeds
are attracted to the islands for purposes of resting, socializing, and reproducing, all
activities that occur out of the water. The degree to which they forage close to the
island is not known. If white shark numbers continue to decline around the Farallones
in the fall, though, pinniped foraging could increase around the island during that
season, further depleting near-shore fish stocks, which could cause a trophic cascade
to breeding seabirds the following year.
ACKNOWLEDGEMENTS
I thank the following interns and staff for spending numerous hours scanning the
ocean for distant shark attacks: Matt Brady, Sophie Chiang, Mark Dettling, Jenny Erbes,
Jill Gautreaux, Andrew Greene, Oliver James, Dan Maxwell, Kristie Nelson, Ryan
Terrill, Sage Tezak, and Pete Warzybok. I am also grateful to The Shark Trust for
funding this shark research at the islands. In addition, I thank the U.S. Fish and Wildlife
Service for permission to conduct these surveys from the lighthouse.
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LITERATURE CITED
Baum, J.K., R.A. Myers, D.G. Kehler, B. Worm, S.J. Harley, and P.A. Doherty. 2003.
Collapse and conservation of shark populations in the Northwest Atlantic.
Science 299:389-391.
Brown, A., D.E. Lee, R.W. Bradley, and S. Anderson. In Press. Copeia. Dynamics of
white shark predation in California: effects of prey abundance.
Compagno, L.J.V., M.A. Marks, and I.K. Fergusson. 1997. Threatened fish of the
world: Carcharadon carcharias (Linnaeus, 1758) (Lamnidae). Environmental
Biology of Fishes 50:61-61.
Cury, P. A. Bakun, R.J.M. Crawford, A. Jarre, R.A. Quinones, L.J. Shannon, and H.M.
Verheye. 2000. Small pelagics in upwelling systems: patterns of interaction and
structural changes in “wasp-waist” ecosystems. Journal of Marine Sciences
57:603-618.
Ferretti, F., R.A. Myers, F. Serena, and H.K. Lotze. 2008. Loss of large predatory
sharks from the Mediterranean Sea. Conservation Biology 22:952-964.
Frid, A., G.G. Baker, and L.M. Dill. 2008. Do shark declines create fear-released
systems? Oikos 117:191-201.
Heithaus, M.R., A. Frid, A.J. Wirsing, and B. Worm. 2008. Predicting ecological
consequences of marine top predator declines. Trends in Ecology and Evolution
23:202-210.
Jorgensen, S.J., C.A. Reeb, T.K. Chapple, S. Anderson, C. Perle, S.R. Van Sommeran,
C. Fritz-Cope, A.C. Brown, A.P. Klimley, B.A. Block. 2009. Philopatry and
migration of Pacific white sharks. Proceedings of the Royal Society B 1155:1-10.
Lee, D.E. 2009. Population size and reproductive success of northern elephant seals
on the Southeast Farallon Islands 2008-2009. Report to the US Fish and Wildlife
Service Farallon National Wildlife Refuge.
Myers, R.A. and B. Worm. 2005. Extinction, survival or recovery of large predatory
fishes. Philosophical Transactions of the Royal Society B 360:13-20.
Myers, R.A., J.K. Baum, T.D. Shepard, S.P. Powers, C.H. Peterson. 2007. Cascading
effects of the loss of apex predatory sharks from a coastal ocean. Science
315:1846-1850.
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Pyle, P., M.J. Schramm, C. Keiper, and S.D. Anderson. 1999. Predation on a white
shark (Carcharadon carcharias) by a killer whale (Orcinus orca) and a possible
case of a competitive displacement. Marine Mammal Science 15:563-568.
Shivji, M.S., D.D. Chapman, E.K. Pikitch, and P.W. Raymond. 2005. Genetic profiling
reveals illegal international trade in fins of the great white shark, Carcharadon
carcharias. Conservation Genetics 6:1035-1039.
Smith, S.E., D.W. Au, and C. Show. 1998. Intrinsic rebound potentials of 26 species of
Pacific sharks. Marine and Freshwater Research 49:663-678.
Stevens, J.D., R. Bonfil, N.K. Dulvy, and P.A. Walker. 2000. The effects of fishing on
sharks, rays, and chimeras (chondrichthyans), and the implications for marine
ecosystems. Journal of Marine Sciences 57:476-494.
Weng, K.C., A.M. Boustany, P. Pyle, S.D. Anderson, A. Brown, and B.A. Block. 2007.
Migration and habitat of white sharks (Carcharadon carcharias) in the eastern
Pacific Ocean. Marine Biology 152:877-894.
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Average # of obsveration hours per day
10
8
6
4
2
2No
v
9No
v
16
-N
ov
23
-N
ov
30
-N
ov
5O
ct
12
-O
ct
19
-O
ct
26
-O
ct
14
-S
ep
21
-S
ep
28
-S
ep
7Se
p
0
Date
Figure 1. Average numbers of standardized daily observation hours during 2009 averaged by
weekly periods. Standardized observations were conducted from the lighthouse on
Southeast Farallon Island, CA.
6
Number of attacks
5
4
3
2
1
-A
ug
3Se
p
10
-S
ep
17
-S
ep
24
-S
ep
1O
ct
8O
ct
15
-O
ct
22
-O
ct
29
-O
ct
5N
ov
12
-N
ov
19
-N
ov
26
-N
ov
27
20
-A
ug
0
Date
Figure 2. Weekly seasonal distribution of white shark predation events during 2009 at
Southeast Farallon Island, CA. This includes both standardized and incidental
observations.
11
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Average # of attacks per day
0.7
0.6
0.5
0.4
0.3
0.2
0.1
-N
ov
30
-N
ov
23
-N
ov
16
9N
ov
2N
ov
ct
-O
ct
26
-O
ct
19
-O
ct
12
O
5-
ep
-S
ep
28
-S
ep
21
-S
14
7Se
p
0.0
Date
Figure 3. Daily attack rate during 2009 based on average number of standardized observation
hours within a given week. Standardized observations were conducted from the
lighthouse on Southeast Farallon Island, CA.
Numbers of California sea lions
5000
4000
3000
2000
1000
20
-
A
u
27 g
-A
ug
3Se
10 p
-S
e
17 p
-S
e
24 p
-S
ep
1O
ct
8O
c
15 t
-O
c
22 t
-O
c
29 t
-O
ct
5N
o
12 v
-N
o
19 v
-N
o
26 v
-N
ov
3D
ec
0
Date
Figure 4. Numbers of California Sea Lions counted weekly during 2009 at Southeast Farallon
Island, CA. Sea lions were counted from the lighthouse.
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300
Steller Sea Lion
Harbor Seal
Northern Fur Seal
Northern Elephant Seal
Number of pinnipeds
250
200
150
100
50
20
-A
u
27 g
-A
ug
3Se
10 p
-S
e
17 p
-S
e
24 p
-S
ep
1O
ct
8O
c
15 t
-O
c
22 t
-O
c
29 t
-O
ct
5N
o
12 v
-N
o
19 v
-N
o
26 v
-N
ov
3D
ec
0
Date
Figure 5. Numbers of pinnipeds counted weekly during 2009 at Southeast Farallon Island, CA.
Steller Sea Lion, Harbor Seal, and Northern Fur Seal were counted from the
lighthouse, while Northern Elephant Seal was counted by walking the shoreline.
700
2006
2007
2008
2009
Numbers of Elephant Seals
600
500
400
300
200
100
ov
15
-N
ov
22
-N
ov
8N
4O
ct
11
-O
ct
18
-O
ct
25
-O
ct
1N
ov
6Se
p
13
-S
ep
20
-S
ep
27
-S
ep
ug
30
-A
23
-A
ug
0
Date
Figure 6. Numbers of Northern Elephant Seals counted during weekly surveys from 2006 to
2009 at Southeast Farallon Island.
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6
Northern Elephant Seal
California Sea Lion
Number of prey
5
Unknown
4
3
2
1
0
Sept
Oct
Nov
Month
Figure 7. Seasonal distribution of prey species identified at white shark predation events in
2008 at Southeast Farallon Island, CA.
80
70
Number of attacks
60
50
40
30
20
10
0
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
Year
Figure 8. Total number of white shark predation events seen at Southeast Farallon Island, CA
by year. This includes observations seen during standardized shark watches and
incidental observations.
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14
Predation events per 100 hours
12
10
8
6
4
2
0
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
Year
Figure 9. Frequency of white shark predation events at Southeast Farallon Island, CA by year.
Data used to create this figure were restricted to standardized shark watches
conducted from the lighthouse between September 1st and November 30th.