Abstract
The Bluntnose Sixgill Shark, Hexanchus griseus, is a large predatory shark, has a worldwide distribution and is listed as near-threatened by the International Union of Conservation of Nature (IUCN). The Seattle Aquarium collected observations of free-swimming Sixgill Sharks in Elliott Bay, Washington, under the aquarium’s pier in 20 m of water from 2003 to 2005 and again from 2008 to 2015 using the same methodology. Compared to total Sixgill sightings between 2003-2005 (273) fewer total Sixgills were sighted at the aquarium’s research station between 2008-2015 (33). The reason for the observed decline in sightings in unknown but based on data from other studies on Sixgills in Puget Sound during the same timeperiod the authors hypothesize the decrease may be due to natural variability of juvenile Sixgill recruitment to Elliott Bay.
Highlights
The Bluntnose Sixgill Shark, Hexanchus griseus, is one of the largest predatory shark species and has a worldwide geographic distribution including all tropical and temperate oceans (Castro, 2011)
Fewer shark were seen throughout the later study period with zero sharks seen on the majority, 63.6%, of the research days between 2008 and 2015 compared to zero sharks seen on only 10% of research days between 2003 and 2005 (Figure 1)
There was an observed decline in Sixgill sightings in Elliott Bay and Puget Sound starting in 2006 by all observers (Williams et al, 2009; Andrews et al, 2010, Unpublished diver sightings data 2018)
Summary
The Bluntnose Sixgill Shark, Hexanchus griseus, is one of the largest predatory shark species and has a worldwide geographic distribution including all tropical and temperate oceans (Castro, 2011). The Sixgill Shark is thought to be vulnerable to exploitation and was listed as Near Threatened on the IUCN Red List of Threatened Species and as a Species of Special Concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC, 2007; Cook and Compagno, 2009). Sixgills are known to inhabit depths ranging from the surface to 2500 m and they are typically observed or caught in deep waters in most parts of the world (180 –1100 m) (Cook and Compagno, 2009; Nakamura et al, 2015). For reasons not fully understood, sub-adult or juvenile Sixgills can regularly be observed in shallower waters (less than 40 m) at some locations, including San Francisco Bay, CA; Puget Sound, WA; and the inner and outer coast of Vancouver Island, Canada (Ebert, 1986). It is possible that Sixgill nursery areas where juveniles are found are spatially
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