Encyclopedia of Puget Sound

Why are so many sixgill sharks washing up in Puget Sound?

Over the past year, the Washington Department of Fish and Wildlife has reported an unusually high number of sixgill sharks found washed up along Puget Sound's shoreline. Four dead sharks in all were spotted, alarming scientists who believe that the large predators use Puget Sound as a pupping ground. Sixgills are rarely seen in Puget Sound but are one of its largest fishes, reaching lengths of up to 16 feet. Some speculate that warmer-than-usual waters could be a factor in the deaths, but the cause remains a mystery. We spoke with Fish and Wildlife biologist Lisa Hillier.

Bluntnose sixgill shark (Hexanchus griseus). Photos courtesy of NOAA Ship Okeanos Explorer.
Bluntnose sixgill shark (Hexanchus griseus). Photos courtesy of NOAA Ship Okeanos Explorer.

Encyclopedia of Puget Sound: With all these strandings, should we be concerned about Puget Sound’s sixgill shark population?

Lisa Hillier: Well, we’re always concerned about sharks. They are top predators, and it’s like being concerned about grizzly bears or wolves. They are kind of those apex predators that tell us what’s happening in our system and in our environment. 

EoPS: Do you have a theory about why so many sixgill sharks are washing up?

LH: We don’t really know, but it is getting warmer. These are a very deep-lived species. They can be found at up to 8000 feet deep off the coast. And when they come into shallow areas in Puget Sound, they may be affected by the warmer waters. 

The largest shark we saw was 14 feet long and she was reportedly seen in [one of Puget Sound’s bays] for several days. She may have come in on a really high king tide and then just got stranded and got too hot. That was in May, and when I did the necropsy, I was sweating. It was so hot. It was like an 80-degree day. So, we don’t really know, but it seems like there are an awful lot of sharks washing up this year. Four in one year is very unusual. Typically, it's about one every other year. 

EoPS: Tell me more about these sharks. You mentioned one was 14 feet. That’s a big shark.

L.H.: That is a big shark. The maximum length that they have ever found is 16 feet, so she was very old and mature. It was very sad to see her dead. Sixgills can get up to 1300 pounds, and she was huge. We couldn’t even physically weigh her, so I am not exactly sure how big she was as far as weight.

EoPS: How common are these sharks worldwide and in Puget Sound?

L.H.: We don’t know what their total abundance is because the species is circum-global. We know that they migrate along our coast. They are also up in the Strait of Georgia. They are in the San Francisco Bay and Southern California. But we think these sharks come into Puget Sound to pup. And we did see that there was evidence that the large mom had given birth, which means that her genetics hopefully went into the system and hopefully her babies will survive.

EoPS: If Puget Sound is a pupping ground for sixgill sharks, does that raise the level of concern about the large number of mortalities?

L.H.: If these are their pupping grounds and their pupping grounds are getting warmer and more acidic — we do know that Puget Sound has ocean acidification just like other areas of the world — it means that we need to protect where they are giving birth to their babies. And the babies are more sensitive, right? The adults are more robust as they have more fat and are able to survive without food for longer, but the smaller sharks are the ones that are sensitive. A couple of our sharks that washed up were little. One juvenile was only 4.76 feet long or 145 centimeters. We know that babies tend to die more often in most species so it’s no shock that baby sharks would wash up on our beaches more than adults. We need to make sure the area that they are born in stays healthy. We should be doing anything we can to make it healthy and keep it healthy.

About the Author: 
Jeff Rice is managing editor of the Encyclopedia of Puget Sound.