By Brittany Hoedemaker
The bluntnose sixgill shark (Hexanchus griseus), with its fluorescent green eyes and six (as opposed to the common five) gills can be found in temperate and tropical waters globally. The usually sluggish sharks live along the ocean floor, rendering them out of sight and out of mind for most people despite their wide distribution. However, these sharks still find themselves listed as Near Threatened by the International Union for the Conservation of Nature (IUCN) due to depletion from incidental bycatch and sport fishing.
Seen often in the Puget Sound in the 1990s and 2000s, the sixgill seems to be a less frequent visitor in the last decade. A 2010 study of movement patterns of 34 Puget Sound sixgills found that the sharks were resident to the area for at least four years before leaving for the outer coast, and displayed site fidelity while resident. This study and others by NOAA’s Northwest Fisheries Science Center, the Seattle Aquarium, and the Washington Department of Fish and Wildlife saw that Puget Sound sixgill sharks tend to cluster around Elliot Bay and are closely related. Such findings, along with diver sightings of sub-adult sharks, suggest the Puget Sound may serve (or have served) as a nursery for juvenile sixgills.
Demographic models, which are used to show transition and movement of birth and death rates, and studies have shown the importance of effective management strategies to protect various age classes of sharks to achieve population recovery. For some shark species, juvenile survival is more important in reversing declining population trends than survival at other age classes. This has potentially critical implications for management of the near threatened sixgill. However, not enough is known about the life history of the sixgill sharks, nor have researchers been able to satisfactorily confirm the hypothesis that the Puget Sound is in fact a nursery for the sharks to suggest new management strategies for the species.
Declining salmon populations, high levels of contaminants including polychlorinated biphenyls (PCBs) and contaminants of emerging concern (CECs), climate change, human activities including fishing, and other threats are impacting Puget Sound waters and the marine life that relies on a healthy ecosystem. It is possible that these threats have contributed to the disappearance of the sixgill in the area, but not enough research has been done to make any such conclusions.
Perhaps more likely, however, is that this “disappearance” is just natural variation in population dynamics for the species. Given how little we know about sixgills, it may be that they only return to the Puget Sound to give birth every so often (perhaps every 10 years). This would explain the gap in sightings in recent years. It would also explain why divers have reported more sixgill deaths in recent years as the cohort from the 1990s and 2000s ages.
With little known about these elusive sharks, it is difficult to determine why or if they have left the Puget Sound, and thus even more challenging to effectively manage or conserve populations in our waters and beyond. Perhaps this species will return to the spotlight and be the subject of more research and attention again. In the meantime, keep an eye out and hope for the return of the sixgill.