As today’s researchers try to figure out what sevengill and soupfin sharks are doing in Puget Sound, even larger sharks, known as bluntnose sixgill sharks (Hexanchus griseus), have captivated scientists with their comings and goings for more than two decades.
This shark, which can grow to 16 feet long, is normally found in the depths of the oceans where darkness prevails. But studies over the past 20 years supports the notion that some female sixgills are coming into Puget Sound to give birth in relatively shallow waters, where their offspring spend their formative years in productive estuaries. After living in such “nursery” conditions for several years, young sixgills may instinctively migrate back to the ocean depths.
“We tagged them and were able to watch them over a five-year period,” said Kelly Andrews, a research fisheries biologist with NOAA’s Northwest Fisheries Science Center. “We were able to show that Puget Sound is an estuary where sixgill sharks grow up and leave. What we don’t yet know is when they will come back and start the process over again.”
Andrews was just out of graduate school at San Diego State University when he took a job with NOAA in 2003. With a degree in ecology, he was asked to study the movements of various species, and sixgill sharks became his first major research project.
Acoustic transmitters implanted into sixgills allowed the sharks to be tracked by fixed receivers on the bottom of Puget Sound as well as mobile receivers deployed from a boat. Researchers on the boat worked in shifts to track the sharks for 24 hours at a time. They recorded the precise location of the sharks, including depth, during daytime and nighttime travels.
“We expected them to move more than they did,” Andrews said. “Some sharks sort of cruised back and forth by Salty’s restaurant (in West Seattle). Others would go along the east side by the Seattle Aquarium and ferry boats, then travel up into the Duwamish Waterway. It was fascinating, and it was some of the most-fun research I’ve ever been involved with.”
Scientific interest in the sixgill sharks of Puget Sound took off in the late 1990s, when recreational divers began encountering a surprising number of these sharks. At that time, almost nothing was known about the lives of sixgill sharks in Puget Sound, even though sport and commercial fishermen had been catching them on occasion since the 1800s.
As divers and researchers began to recognize the increasing numbers of sixgills in the area, they were surprised by what appeared to be a congregation of these sharks swimming along the Seattle waterfront in the urban waters of Elliott Bay. In 2003, the Seattle Aquarium began studying these sharks in partnership with NOAA and WDFW.
Researchers at the aquarium, including Jeff Christiansen and Shawn Larson, put out bait under piers 59 and 60, where the aquarium is located. It didn’t take long for the sixgills to gather for a meal. Divers observed the sharks from within a protective cage. They watched the sharks’ behavior, attached physical tags for identification, and took tissue samples for genetic analysis.
Of all the sixgill sharks observed at the aquarium and elsewhere throughout Puget Sound during the early 2000s, the vast majority were significantly smaller than mature adults. By using acoustic tags to track some of these sharks, Andrews and his fellow researchers concluded that groups of these sub-adults were moving about together in Puget Sound, as described in a paper published in the journal Plos One.
While movements of the sixgills varied, the sharks tended to stay in deeper waters during the day, coming closer to the surface at night. Whether their depth was related to prey eaten by the sharks, their sensitivity to light, or some other reason has yet to be determined, Andrews said.
Seasonal movements within Puget Sound were generally north in winter and south in summer, returning to the same place each year. Thanks to this tagging effort, 19 of the 34 tagged sharks were detected on their way out of Puget Sound between 2006 and 2009. Three females returned and remained for varying periods before leaving again.
Tagged sixgills that left Puget Sound were detected as far south as Point Reyes, Calif., and as far north as Queen Charlotte Strait in British Columbia. So far, nobody has determined what physical or biochemical triggers influenced their movements within Puget Sound or their migration out to the ocean and beyond.
An abundance of sightings — by recreational divers, research ships and underwater video at the Seattle Aquarium — led to the conclusion that the number of sixgill sharks in Puget Sound reached a peak between 1999 and 2007, after which the number of sharks suddenly declined. One explanation is that more than the usual number of sixgill females gave birth in Puget Sound about the same time. That resulted in their offspring growing up and migrating from Puget Sound en masse. No sixgills have been spotted at the Seattle Aquarium since 2012, and few encounters have been reported elsewhere in Puget Sound by recreational divers, sport and commercial fishermen and researchers surveying marine life.
“Even though we gathered a lot of information on a lot of sharks, we still don’t know the full story about sixgill sharks in Puget Sound,” Andrews said.
If the burgeoning numbers of sixgills observed during the early 2000s were part of a natural cycle, one might expect the number of sharks to rise again at some point, Andrews said. But how long might such a cycle last? Clearly, it must be more than 10 years.
“We have no idea what the size of this population is and whether it is increasing or decreasing,” he added.
Recently, recreational divers have been reporting a slight increase in the number of sixgill sharks at Redondo Beach north of Federal Way and Three-Tree Point near Burien, both in Central Puget Sound, along with other locations where the sharks were frequently seen in the early 2000s. Researchers have been monitoring these sightings.
The lives of bluntnose sixgill sharks in Puget Sound seem to be typical of sixgills elsewhere in the world. At birth, they are about 2 feet long. As they grow, males reach maturity when they get to about 10 feet; females are nearly 14 feet; and both continue to grow a little larger. Before their numbers declined in Puget Sound, reports of their size ranged from about 3.5 feet to 10.3 feet, meaning that out of hundreds of sixgill sharks observed locally, almost none had reached maturity.
Genetic analysis suggested that all the sixgills in Puget Sound were from a single intermixing population. Sharks found together were generally full- or half-siblings, meaning that these groups were likely born from a single mother. In contrast, the familial relationship from one group to another was much lower, suggesting that siblings stayed together and somewhat apart from other family groups. These findings were reported in 2010 in the journal Conservation Genetics by Seattle Aquarium’s Shawn Larson and collaborators.
On Jan. 21, 2007, a pregnant female sixgill, more than 14 feet long, was found on the beach along Hammersley Inlet and died soon thereafter with 76 unborn sharks inside her and another four extruded onto the beach. While it was a sad event for all involved, the incident provided a rare chance to study the makeup of offspring from a single mother, according to Greg Bargmann, a fisheries biologist with WDFW who came to the beach that day.
Genetic samples revealed that at least six males had contributed to the reproduction of the pups in this one litter, although the vast majority were sired by only a few males, according to Larson’s report. This multiply paternity, called polyandry, is common in sharks and results from the female mating multiple times. The evolutionary benefits of polyandry have been widely discussed among scientists, who say the result is increased diversity within a litter, which increases the chances of survival when confronted with habitat changes. On the other hand, females face a greater risk of injury, which can occur during mating.
Bluntnose sixgill sharks, among the largest sharks in the world, are found in nearly every ocean, often in waters from 600 to 3,000 feet deep but reported as deep as 8,200 feet. Surviving and even thriving at such remarkable depths makes these sharks difficult to study.
Like the sevengills and soupfins, sixgills are considered generalists, capable of eating animals at the top of the food web as well as scavenging for a variety of creatures that die and sink to the bottom. They are known to feed on crabs, a variety of bony fish, sharks and rays, and marine mammals.
Washington Department of Fish and Wildlife classifies the sixgill shark as a “species of greatest concern” in the State Wildlife Action Plan. Directed fishing for sixgills is not allowed under state fishing regulations, and any that are caught accidentally must be released without removing them from the water.
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