Observations on Abundance of Bluntnose Sixgill Sharks, Hexanchus griseus, in an Urban Waterway in Puget Sound, 2003-2005
2003-2005. PLoS ONE 9(1): e87081. doi:10.1371/journal.pone.0087081
Observations on Abundance of Bluntnose Sixgill Sharks, Hexanchus griseus , in an Urban Waterway in Puget Sound, 2003-2005
Denise Griffing 0
Shawn Larson 0
Joel Hollander 0
Tim Carpenter 0
Jeff Christiansen 0
Charles Doss 0
A. Peter Klimley, University of California Davis, United States of America
0 1 Life Sciences , Seattle Aquarium, Seattle , Washington, United States of America, 2 Department of Statistics, University of Washington , Seattle, Washington , United States of America
The bluntnose sixgill shark, Hexanchus griseus, is a widely distributed but poorly understood large, apex predator. Anecdotal reports of diver-shark encounters in the late 1990's and early 2000's in the Pacific Northwest stimulated interest in the normally deep-dwelling shark and its presence in the shallow waters of Puget Sound. Analysis of underwater video documenting sharks at the Seattle Aquarium's sixgill research site in Elliott Bay and mark-resight techniques were used to answer research questions about abundance and seasonality. Seasonal changes in relative abundance in Puget Sound from 2003-2005 are reported here. At the Seattle Aquarium study site, 45 sixgills were tagged with modified Floy visual marker tags, along with an estimated 197 observations of untagged sharks plus 31 returning tagged sharks, for a total of 273 sixgill observations recorded. A mark-resight statistical model based on analysis of underwater video estimated a range of abundance from a high of 98 sharks seen in July of 2004 to a low of 32 sharks seen in March of 2004. Both analyses found sixgills significantly more abundant in the summer months at the Seattle Aquarium's research station.
Funding: The authors acknowledge The Seattle Aquarium, a grant from Royal Caribbean Ocean Fund, and a grant from the Foley Frischkorn Conservation Fund
for financial support of this project. In kind contributions were provided by staff at The NOAA Northwest Fisheries Science Center, the Washington Department of
Fish and Wildlife, Ocean Systems Inc. (Splashcam), and City Fish in Pike Place Market, Seattle. The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
Competing Interests: The authors did receive in-kind financial support in the form of equipment and consumable donations from both "Ocean Systems Inc.
(Splashcam)" and "City Fish in Pike Place Market, Seattle." This partnership does not alter the authors adherence to all the PLOS ONE policies on sharing data and
Shark populations are in decline worldwide due to overharvest
from shark finning, by-catch, entanglement, habitat loss and
environmental degradation [1,2]. Many large sharks are wide
ranging occurring in most of the worlds oceans such as the
broadnose sevengill (Notorynchus cepedianus), spiny dogfish (Squalus
acanthias), the great white shark (Carcharodon carcharias), the blue
shark (Prionace glauca), and the bluntnose sixgill shark (Hexanchus
griseus) [1,3,4]. Yet in spite of the widespread distribution all of
these sharks, all are species at risk because of life histories that
include late maturity, low reproductive capacity and their
potential vulnerability to overharvest. The population status and
the impact of fisheries on these sharks remains unknown
prompting their listing as either data deficient (broadnose
sevengill), vulnerable (great white shark and spiny dogfish), or
near threatened (blue shark and sixgill) by the International Union
for the Conservation of Nature (IUCN) .
The bluntnose sixgill is found in tropical and temperate waters
ranging from shallow coastal waters to the continental slopes and
down to abyssal depths . Life history characteristics include slow
growth, late reproductive maturity at approximately 20 years and
unknown longevity [1,4,6,7]. Distinguishing physical
characteristics include six gill slits, a single dorsal fin located posteriorly on the
body, and a sub-terminal mouth with dimorphic tooth patterns in
the upper and lower jaws [4,6]. Females are ovoviviparous bearing
between 22108 pups with unknown gestation and reproductive
frequency . Newborn sixgill pups are typically 0.7 m while
adults may reach a maximum length of 6 m, with females larger
than males [1,8]. Subadult sixgills are defined as males less than
3 m and females less than 4 m [6,9].
The sixgills depth range is from the surface waters to 3000 m
[1,4,6]. Although they are thought to be primarily bottom and
deep dwelling, they have been reported occurring in shallow
estuaries in the United States such as Puget Sound in Washington
[7,10] and San Francisco Bay in California [1,8] as well as the
Georgia Basin in British Columbia, Canada . The sixgills
presence in the shallow waters of Puget Sound in the late 1990s
through 2000 led to an increase in anecdotal reports of encounters
between sixgill sharks and divers. These divers noted they were
more likely to encounter sixgills in summer months than in winter.
It was unknown whether this indicated a true seasonal change in
sixgill behavior or was an artifact of changing diver effort with
many divers only diving in the summer. In addition, in the
summer of 2000, directed fishing for sharks by local recreational
fishers resulted in the catch of several sixgills from an area in Elliott
Bay near Alki Point where divers had reported frequent
encounters. Underwater video and still photography revealed that
numerous sightings were made of the same individual sharks at
that location over several months. After this directed fishing
activity, diver sightings of sixgills in that area ceased for at least
three months, sparking questions about the abundance of sixgills in
This event also stimulated the Seattle Aquariums (SA) interest
to study this normally deep-dwelling shark using simple minimally
invasive techniques, such as SCUBA and underwater video
cameras, without incurring the expenses normally associated with
deep ocean research such as using a submersible. Prior to this
event, there were no catch limits for the sixgills in Washington
waters. However due to the publics, as well as the SAs, concern
over the potential overexploitation of local sixgills, Washington
Department of Fish and Wildlife (WDFW) regulators responded
by placing a temporary closure (later made permanent) on the
taking of sixgills in Puget Sound. State fisheries biologists pointed
to the lack of information on abundance, movement patterns, and
biological parameters of sixgill sharks in Puget Sound dictating a
cautious approach to their harvest.
To gather basic information about sixgill sharks in Washington
waters, specifically the inland waters of Puget Sound, a joint
research team was established in 2002. This team included
representatives of National Oceanic and Atmospheric
Administrations (NOAA) National Marine Fisheries Service (NMFS),
WDFW, University of Washington (UW), SA, and Point Defiance
Zoo and Aquarium (PDZA). During 20032008 these
organizations conducted three independent tagging operations on sixgills in
Puget Sound. Data acquired through these efforts included the
capture and tagging of over 300 sharks in and revealed many
aspects of their presence and movements in Puget Sound .
One of the most interesting findings of this collaborative
research was that all of the sixgills caught in Puget Sound were
found to be subadults [12,13]. Average size for males was 2.4 m
(range: 1.52.9 m) while the average size for females was 2.5 m
(range: 1.73.1 m), smaller than estimated sizes of maturity for
both sexes [6,9,12,13]. Acoustic tagging and tracking of a subset of
these sharks caught revealed both short term and long term
movement patterns. Short term movements of acoustically tagged
sharks monitored by active tracking of individuals for 2448 hours
revealed diel vertical migration patterns . These sharks
were found to make vertical migrations at dawn and dusk, being
found in shallower waters at night (25141 m) and deeper waters
during the day (42170 m) . Long term movement patterns of
sixgills using both active and passive tracking revealed sixgills
exhibiting sedentary behavior with restricted daily movements and
high site fidelity to the area in which they were caught and tagged
[7,14]. However, these sharks were not limited to just one area and
shifted seasonally every six months between summer areas and
winter areas [7,14]. These adjacent resident areas were
approximately 1025 km apart with the summer area to the north of the
winter area [7,14]. This apparent seasonal shift between resident
areas corroborated an earlier study of sixgills in the shallow (40 m)
inland waters of Georgia Strait in British Columbia, Canada .
Between 20012002, using video camera data, Dunbrack and
Zielinski (2003) documented strong seasonality of sixgill presence
with significantly greater abundance of sharks during the summer
months relative to the rest of the year .
Here we report the results of the SA research efforts from 2003
2005 using video analysis, external tagging, and mark-resight
statistical techniques in Elliott Bay. The null hypothesis was that
numbers of sharks observed at the SA research station did not
change over the study period or seasonally. Research questions
were as follows: Could we determine relative abundance of sixgills
in Elliott Bay using video analysis techniques and mark-resight
statistical methods? And were sixgills more abundant in Elliott Bay
during the summer than in the winter as suggested by local diver
Materials and Methods
The SA does not have an Institutional Animal Care and Use
Committee (IACUC). The SA has an equivalent in-house animal
research advisory committee, the Seattle Aquarium Research
Center for Conservation and Husbandry (SEARCCH), made up
of 10 outside researchers at the PhD level in addition to the SAs
director and curators. The SEARCCH committee approved this
research before tagging commenced in 2003. In addition a permit
from WDFW for the external tagging and biopsy of up to 50 sixgill
sharks per year was obtained annually between 20032005
(WDFW scientific Collection Permit numbers 03-040, 04-036,
Puget Sound is a large, fjord-like estuary comprising most of the
northwestern quarter of Washington State (Figure 1) [16,17].
Elliott Bay is situated on the east side of central Puget Sounds
main basin and the SA is centrally located in Seattles waterfront
on Piers 59 and 60 in Elliott Bay (Figure 1). The sixgill shark
research site is located underneath the west end of Pier 59 in 20 m
of water that leads to a steep drop-off in excess of 150 m .
A protected contact caged area under the SA was constructed to
provide a permanent protective barrier for research divers during
shark tagging operations. The cage was constructed by wrapping
vinyl-coated wire fencing around seven pilings. This area was
approximately 3 m2, and enclosed on five sides with openings on
the top and east sides to allow diver access and egress during video
observations and tagging/biopsy procedures. A bait station was
placed 1 m west of the cage. The research area was surrounded by
four fixed lights (2-Super-SeaLite and 2-Multi-SeaLite; Deep Sea
Power & Light, San Diego, CA), and five fixed cameras (3-Delta
Vision Industrial and 2-Deep Blue Pro; Ocean Systems Inc.,
Everett, WA) for video documentation of shark behavior.
Bait was used to attract sharks to the research station. The bait
was placed in an open top, fiberglass reinforced plastic grated box.
Divers placed up to 80 l of thawed bait in the bait box and one to
three chumsicles attached via 1 m stainless steel anchors. The
chumsicle was made using the same bait as that placed in the bait
box, but was frozen to a tether in 20 l buckets. The frozen
chumsicles floated suspended above the bait station by the tether
to provide a scent attractant higher in the water column with
continual dispersal of scent as it thawed. Bait typically consisted of
salmon (Oncorhynchus sp) and halibut (Hippoglossus stenolepis) carcasses
although other species were occasionally present including Pacific
herring (Clupea pallasi), Pacific spiny dogfish (Squalus acanthias),
skilfish (Erilepis zonifer), and giant Pacific octopus (Enteroctopus
The SA study used a consistent bimonthly level of effort
yearround from January 2003 through May 2005 on odd months
(January, March, May, July, September, and November) with
additional research events in June 2003 and April 2004, for a total
of 17 events. Research events comprised a four-day period with
site set-up and camera installation on day one, research activities
on nights two and three, and site take down on day four. Research
activities began on the evening of night two with bait placement at
18:00, with bait typically refreshed 24 hours later (18:00 on night
three). Research events were one to two nights in duration, with
only four of the total 17 events of one night duration.
This research involved videotaping the sharks and implanting
visual marker tags. Animals were tagged in situ by SCUBA divers
while free swimming by the research station. Sharks were tagged
Figure 1. Map of Puget Sound. The location of the Seattle Aquarium is marked with a red star.
using visual tags (Floy VM69 stainless-steel dart tags) extended in
length to 30 cm and modified to contain unique shape
combinations for individual identification of sharks. Four different plastic
shapes (circle, square, triangle, and cross), each approximately
2.5 cm square, were attached to the streamer in up to four
locations to yield 256 possible tag combinations. The tags were
also imprinted with SA contact information in the event of
retrieval by divers or fishermen. Divers attached the visual tags
using a pole spear as sharks approached during baiting operations.
The dart tags were inserted into the dorsal musculature anterior to
the dorsal fin and inserted at an angle towards the head end of the
shark using standard methodology (Figure 2) .
All shark observations and SCUBA diver activity (tagging and
biopsy attempts) were documented on video tape. Video was
recorded for 12 hours per night (from 18:00 to 06:00 the following
morning) and event dates were defined from start to end of video
recording. The video recordings were analyzed to note presence/
absence of sixgills, to identify individual sixgills, to determine sex
and to record tag status . The goal was to determine total
number of recognizable individual sharks per event to estimate
abundance and number of returns. Each shark observation was
documented and defined as the period of time when an
individually recognized shark was visible on the video (Table S1).
Individual sharks were identified using two methods; visual tag
shape sequence and unique morphological characteristics. Some
sixgills had numerous marks with distinctive patterns. One
challenge in identifying individual sixgills was that these marks
could occur anywhere on the body in contrast to photo ID
techniques used on great white sharks that tend to concentrate on
one part of the body such as the dorsal fin . In addition these
marks could change over time due to new injuries or healing of old
ones . For this study, the assumption was made that an
individual animals marks would remain static for the duration of a
single research event (i.e., 2 days). However we do not assume the
marks persist between events. Thus untagged sharks (UT, Table
S1) when seen between events were given a new id number even if
they may have been seen before. Only tagged sharks (T, Table S1)
were not given a new ID number and instead were counted as a
return of a previously identified shark.
To count individual sharks during the video analysis, sharks
were denoted either as tagged or untagged. The tag site, tag shape
sequence, sex, and unique markings were recorded from the video
sighting (see Video S1) on a line diagram showing three views of a
generic sixgill: Right side, dorsal side, and left side. The preferred
method for identifying an individual was the diver-implanted
visual marker tag (Figure 2). However, when a sixgill was untagged
or the tag shape sequence could not be seen, then additional
factors were used for identification such as tag location, sex, and
markings. While photographs of both sides of an individual sixgill
would be optimal, it was rare to view the entire sixgill in a single
video frame due to the sharks size and limited water visibility. If at
least one side of a sixgill, right or left, was seen and the sharks
markings did not match up with any known sixgills, then a new
identification number was assigned indicating a potentially unique
individual (Table S1) . Over the course of a research event,
these "orphaned sides" were resolved when subsequent footage
revealed both sides of an individual. Since the identifiability of a
shark varied based on factors such as lighting, distance from
camera, and the sharks direction of travel, each observation was
given an ID confidence variable with levels of confident,
tentative, or unidentifiable for an untagged shark. Sharks with
three or more identifiable markings are given a confident
identification level, and sharks with some amount of marking less
than three identifiable marks are generally tentative.
In addition to reporting absolute numbers of tagged and
untagged sharks seen for each research event, the number of
sharks was also analyzed using a Mark-Resight statistical model. In
the Mark-Resight paradigm, researchers introduce some
fieldreadable marks into the population (external visual tags in this
case) and then collect encounter data (via sightings) on both the
marked and unmarked individuals (i.e., tagged and untagged) in
the population . Of the three Mark-Resight (M-RS)
models described by White and Cooch 2012, only the
Zerotruncated Poisson log-normal model (ZPNE) does not assume we
know the number of tagged sharks in the population at all times
, a condition we knew we could not meet. This model is
robust in that the likelihood of encountering a tagged shark is the
product of two likelihoods, the open (primary) likelihood and the
closed (secondary) . For this dataset, the open or primary
sessions corresponded to research events (i.e. a two-day data
collection period) and the closed or secondary sessions
corresponded to unique dates within an event. The design is
based on the assumption that during a single primary session the
population is totally closed because the time period is too short for
temporary emigration (leaving the sampling area but with the
possibility of returning) or permanent emigration (deaths or
otherwise leaving with no chance of returning) to occur. However
during the time between primary sessions we expect the
population to be open during which permanent and temporary
emigration may occur.
Based on the nature of the sixgill observation data there may be
several violations of ZPNE model assumptions. The most
fundamental assumption of the model was that the tagged and
untagged sharks behaved identically and were thus seen
identically. This means that the divers did not selectively tag sharks in
some way such that the tagged sharks had different characteristics
than the untagged ones, and that the tags did not affect the
observability of the sharks on the video. The former assumption
seems to hold, but the latter does not. Tags clearly increase a
sharks observability on the video, especially if the shark did not
have other unique markings. Because this is an important
assumption of the model, we addressed it as follows: We revisited
all the videotape for tagged shark sightings, and recorded a new
variable, equivalency (short for tagged equivalency to
untagged) by evaluating the sharks identifiability based on other
unique markings as if it had been untagged. When we added this
data into the model, we excluded all tagged sharks that had
tentative or unidentifiable equivalency (resulting in four
sharks being excluded).
Next we had to deal with the sharks seen from a single side
(orphaned side sightings). Including them in the model could
lead to overestimates of abundance if, for example, a right-sided
orphaned side and a left-sided orphaned side were actually the
same individual, termed potential duplicate (PD) in Table S1. We
could have chosen to include left-sided orphaned sides only, but
for some events, there may have been more right-sided orphaned
sides. Instead we evaluated the orphaned sides in each event and
made individual determinations as to whether or not they were
potential duplicates of other orphaned sides. Because the dataset is
limited we opted to run the program twice, once including all
orphaned-sides, maximal, and again excluding the potential
duplicates, minimal (Table 1 and Table S1).
The significance of differences in the number of sharks seen
between seasons and years was evaluated using Mann Whitney U
tests with Bonferonni corrected significance level p,0.01. The
significance between observed and expected sex ratio was
determined using chi-square analysis.
Sixgills were seen at all SA research events via video from 2003
2005, except for during September 2004 (Table 2, Figure 3 and
Table S1). The total number of observations was 273 (45 tagged,
197 untagged, and 31 returning tagged-Table 2) excluding
orphaned side sharks which are potential duplicates (Table S1).
The daily count ranged from a low of 0 on September 23, 2004
and on November 17, 2004 to a high of 30 on May 21, 2005
Several sharks, both tagged and untagged, returned to the
research station. Tagged sharks returned 31 times at various times
at liberty (Table S1). Seventeen of the tagged sixgills returned one
or more times resulting in a 37.8% return rate, and recreational
divers submitted a sighting report for HGSA-0020 at another
location in Elliott Bay, increasing the return rate to 40.0%. Some
tagged sixgills never returned (N = 28) while others returned
multiple times (N = 9) (Table S1). Most of the tag returns occurred
the next research day (N = 12). The mean time at liberty between
initial tagging and the first return was 77 days. The longest time at
liberty between initial tagging and first tag return sighting was 249
days (Table S1, shark ID# HGSA-0136) and the longest time at
liberty between sightings was 354 calendar days (Table S1, shark
ID# HGSA-0015 for the third return). In addition there was some
evidence of external tag loss. Video review provided evidence that
shark ID# HGSA-0137 (initially tagged on June 23, 2003)
subsequently returned on November 13, 2003 without a visual
marker tag. This individuals markings were consistent between
sightings, and upon return, the sixgill had a puncture mark at the
former tag site.
The abundance of sixgills estimated using the Mark-Resight
model ranged from a low of 27 (95% CI range 1168) in March
2004 (excluding potential duplicates) to high of 98 (95% CI range
65146) in July 2004 (including potential duplicates, Table 1). Not
all 17 events contributed data to the model. Seven events were
excluded because either no tagging took place (events 13), the
events were less than the normal 2 days sampling duration (events
7 and 10) and thus there was no possibility for a shark to be
resighted, or because the paucity of observations would skew the
model (events 13 and 14, Table 2). Finally since the data exhibited
strong seasonality, we needed to incorporate that aspect into the
model. We did this by binning both secondary and primary
variables into summer and winter categories. The summer
category or high season occurred during events 4, 5, 6, 11, 12 and
17, and the winter category or low season comprised the
remaining events: 8, 9, 15 and 16 (Table 2).
Absolute and estimated numbers of sixgills showed significant
seasonal differences with more sixgills observed during the summer
months than in the winter months (Z value = 3.22 and p = 0.0012;
U = 34 and p,0.01, see Table 2, Figure 3). However there were
no significant differences between years (Z value ranged from
1.59 to 0.80, p values ranged from 0.11 to 0.42, and U ranged
from 43.5 to 24.5 and was non-significant, see Table 2 and Figure
Sex of the sharks was determined from the video. Of the 273
sharks, 137 were identified as females, 70 were males, and the sex
could not be determined for the remaining 66 sharks (76% sexed).
A clear view of the pelvic region was required in order to
determine the sex of a shark. The ratio of sharks with known sex to
unknowns was 3:1 and the sex ratio for females to males was
1.96:1. This sex ratio was significantly different from the 1:1
expected ratio with females more numerous at the SA research site
than males (chi-square = 10.85, p,0.001).
Table 1. Mark-resight model parameter estimates, and 95% lower (LCL) and upper (UCL) confidence levels for abundance
parameters for 10 primary sessions.
Note: Include PD = includes all orphaned sides noted in Table S1 including R-PD and L-PD. Exclude PD means includes only R and L orphaned sides that are not PD.
Table 2. Number of sharks observed at Seattle Aquarium by research date.
N of tags at liberty
Note: These numbers exclude potential duplicate sharks (See Table S1). N = total number of recognizable individual sharks per event. Untagged = number of untagged
sharks identified. New Tags = number of sharks tagged during a research day. Returns: number of sharks that were seen with tags placed on an earlier date. N of tags at
liberty: the number of previously tagged sharks at liberty which can return. Season = L = low season (November-March) and H = high season (April-September).
This is the first study to report abundance of sixgills within
Elliott Bay and Puget Sound. The levels of sixgill abundance at the
SA research station reported here is likely an underestimate
because it includes the population of marked sharks only. This is
because only sharks with a certain amount of identifiable markings
could be uniquely identified and included in the data. Sharks
without markings were in fact seen on video but were excluded
from our data because they could not be confidently re-identified.
Thus their numbers could expand or contract within the local
population without affecting our estimates. If we had an estimate
of the proportion of unmarked sharks to marked sharks then we
could use the model to estimate the abundance of all sharks in our
We do not know how far or wide we were attracting sharks to
the SA research station. However we do know most of the sharks
tracked in Puget Sound were found to make small daily
movements, 0.2 to 3.1 km, with the maximum displacement
between acoustic detections being 29.2 km . Since Elliott Bay
is 21 km2 and approximately 9.6 km wide at the mouth, we were
likely attracting sharks that are resident within Elliott Bay.
However residency in Elliott Bay was thought to be seasonal .
Andrews et al., 2010 documented sixgills acoustically tagged and
caught in Elliott Bay in the summer were found about 10 km to
the southwest in the deeper channels off of Alki Point in the winter
. If the sharks at the SA research site behaved the same way,
then perhaps the reason why we observed significantly fewer
sixgills in the winter or low season was because most of them
shifted to a winter home range far enough from the SA research
station such that they were no longer attracted to it.
This is not the first or only study to estimate sixgill abundance
from video analysis. A similar study was conducted from 2001
2007 at Flora Islets in the inland waters of southern Strait of
Georgia, Canada [15,24,25]. At this site only sixgills swimming
towards a time lapse camera or inbound were counted and thus
the data reported was number of inbound sharks per hour .
Similar to the data reported here, they documented seasonality
with many more shark sightings in the summer than at other times
of the year, with a high count of up to 7 sharks per hour in late
June and early July 2002 . Lengths were also estimated from
the video for the period of 20012002 when sixgills were most
abundant and all sharks observed were found to be subadults,
similar to the finding of sharks in Puget Sound . Dunbrack and
Zielinski (2005) identified a total of 35 individuals over the two
summer seasons, 20012002 . These abundance estimates in
the Flora Islets are relatively low compared to estimates reported
here in Elliott Bay: 80 individuals in the summer of 2003, and 55
individuals in summer 2004 (Table 2). In addition, over the seven
year study in the Flora Islets the frequency of sharks viewed in
2001 was significantly higher than in any other subsequent year
with steadily decreasing observations over time, with the sharks
observed in 2006 only 1% of those seen in 2001 .
The video analysis techniques used here had limitations and
unfortunately were not designed to accurately size sharks.
However sixgills sampled in Puget Sound during 20032007 via
longline by our research partners (WDFW and NOAA NMFS)
were found to be exclusively subadults, or males less than 3 m and
females less than 4 m [12,13]. Thus we assumed that the sharks
observed at the SA research station were also subadults. In
addition, genetic studies of sharks sampled at the SA research
station and by our research partners revealed a high degree of
relatedness (full and half siblings) among sixgills sampled at the
same time and place (such as one longline set or SA research
event), suggesting subadult cohorts may travel together . Thus
the sixgills sampled at the SA research station may have been the
resident cohort group that lived in Elliott Bay in the summer. This
cohort group likely then shifted to their southern resident range in
the winter resulting in the lower numbers of sixgills observed
during the low or winter season (Table 1, 2 and Figure 3).
Sex was easier to determine from video observations than size.
Female sixgills were observed significantly more often than males
at the SA research site. Sixgills have not been previously
documented as sexually segregating; although sexual segregation
has been reported in other shark species including the scalloped
hammerhead shark, Sphyrna lewini [26,27], the great white shark
, the lemon shark Negaprion brevirostris , and the blue shark
. However the sex ratio of sixgills sampled by our research
partners in Puget Sound did not differ significantly from 1:1 .
We do not know whether female sixgills were preferentially
attracted to our research site, if the resident sixgills in the area
were predominately female, or if the sex ratio was skewed due to
the difficulty of sexing young male sharks via video analysis.
More research needs to be done to determine if this apex
predators subadult residence to the inland waterways of the
Northeast pacific is unique. While the observed pattern of seasonal
shifts in sixgill abundance has also been reported in other
cowsharks, such as broadnose sevengills [9,31], the factors driving
these seasonal changes may differ. For example, Lucifora et al.
(2005) theorized that shallow bays serve as nursery areas for
sevengills in Patagonia, Argentina  while Williams et al. 2010
suggested that sevengills may use Pacific Northwest estuaries for
foraging . In Puget Sound, Andrews et al. (2010) proposed that
seasonality in sixgill abundance may be prey driven . Sixgills are
thought to utilize both scavenging and active predation, however,
the relative importance of these feeding strategies is unknown .
The spotted ratfish, Hydrolagus colliei, and Pacific spiny dogfish are
thought to be major prey items for Puget Sound sixgills .
Quinn et al. (1980) found H. colliei exhibiting similar seasonal shifts
to shallower waters in spring and summer followed by movements
to deeper water in fall and winter . In addition, Reum and
Essington (2008) found that Pacific spiny dogfish were more
evident in Puget Sound in the summer and fall than in winter .
Thus perhaps the seasonality of sixgill abundance at the SA
research site in Elliott Bay were driven by the resident sixgills
following their preferred prey.
Video of the right side of shark ID
#HGSAThe authors wish to thank the SA staff, volunteers, and the Aquariums
directors during the study, Robert Davidson, John Braden, and Bill Arntz,
for supporting this research. The authors gratefully acknowledge the
expertise and equipment donated by their collaborators at the WDFW
(Greg Bargmann, Debbie Farrer, and Dayv Lowry) and NOAA NMFS
(Phil Levin, Kelly Andrews and Greg Williams).
The Seattle Aquarium does not have an Institutional Animal Care and
Use Committee (IACUC). The SA has an equivalent in-house animal
research advisory committee, the Seattle Aquarium Research Center for
Conservation and Husbandry (SEARCCH), made up of 10 outside
researchers at the PhD level in addition to the SAs director and curators.
The SEARCCH committee approved this research before tagging
commenced in 2003. In addition a permit from WDFW for the external
tagging and biopsy of up to 50 sixgill sharks per year was obtained annually
between 20032005 (WDFW cientific Collection Permit numbers 03-040,
04-036, and 05-036a). Language specific to the sixgill research activity in
2005 was as follows: The Seattle Aquarium is currently undertaking
research on Sixgill sharks (Hexanchus griseus).
As a public institution the Seattle Aquariums data is publicly owned and
anyone can request copies of the raw video footage by contacting the
Seattle Aquarium or the corresponding author.
Conceived and designed the experiments: DG JH TC JC CD. Performed
the experiments: DG SL JH TC JC. Analyzed the data: DG SL JC CD.
Contributed reagents/materials/analysis tools: DG SL JH TC JC CD.
Wrote the paper: DG SL TC JC CD.
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