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.
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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
materials.
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) [5].
The bluntnose sixgill is found in tropical and temperate waters
ranging from shallow coastal waters to the continental slopes and
down to abyssal depths [4]. 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 [6]. 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 [7]. 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 (...truncated)