Abstract
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.
Highlights
Shark populations are in decline worldwide due to overharvest from shark finning, by-catch, entanglement, habitat loss and environmental degradation [1,2]
The population status and the impact of fisheries on these sharks remains unknown prompting their listing as either data deficient, vulnerable, or near threatened by the International Union for the Conservation of Nature (IUCN) [5]
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 [16]
Summary
Shark populations are in decline worldwide due to overharvest from shark finning, by-catch, entanglement, habitat loss and environmental degradation [1,2]. 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]. Life history characteristics include slow growth, late reproductive maturity at approximately 20 years and unknown longevity [1,4,6,7]. Subadult sixgills are defined as males less than 3 m and females less than 4 m [6,9]
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