Sixgill shark pups are using Puget Sound as a nursery for 4 to 6 years

Young bluntnose sixgill sharks appear to spend about 4 to 6 years growing up in Puget Sound, and some may stay longer, according to Kelly Andrews, a research fisheries biologist at NOAA’s Northwest Fisheries Science Center. He is careful about the number. Growth measurements are the basis for the range, and as he put it, no one has been able to age a sixgill shark yet, so the figure is a suspicion rather than a count.

The estimate sits on top of a firmer finding. After 16 years of sampling, NOAA and its partners conclude that Puget Sound is a multigenerational nursery, a place females return to over decades to give birth and where their pups grow up.

274 tissue samples and a stranded mother with 70 pups

The evidence is mostly genetic. Researchers analyzed 274 tissue samples collected between 2003 and 2019 from sixgills in the Salish Sea, the inland sea spanning Washington and British Columbia. The resulting paper, published April 23, 2026 in Frontiers in Marine Science with Shawn Larson of the Seattle Aquarium as lead author, found no genetic structure across space or time inside the Salish Sea, which points to a single breeding population, with an estimated effective size of 1,063 to 1,287 individuals.

The strongest single sample came from 2019, when a pregnant female washed ashore in British Columbia’s Strait of Georgia. Scientists took tissue from the mother and from her 70 pups. The pups had multiple fathers, and the paper reports that two stranded pregnant females carried litters sired by four males and three males respectively. Some of the newly sampled sharks were close relatives of animals caught more than a decade earlier, including half siblings, Andrews said in NOAA Fisheries’ feature on the work, and that return of kin across years is what the nursery conclusion rests on.

Growth measurements behind the 4-to-6-year range

Because aging is not yet possible, the residency period is inferred. The Northwest team combines growth measurements with long-line sampling, visual and acoustic tagging, and isotopic signatures in muscle tissue that reveal what the sharks have been eating. The agency’s own wording is a suspicion, and the “or even longer” clause means the true figure for some sharks could run past six years.

Washington’s wildlife agency describes the same pattern from the state side. Its species page for the bluntnose sixgill says females use Puget Sound’s shallow waters as a nursery, that litters range from 22 to 108 pups, and that juveniles show high site fidelity there. That fidelity is the stated reason the young sharks could be vulnerable to warming or low oxygen, which would affect their prey.

A rare pupping ground and a recent history of surprise

Adult sixgills are deep-water animals. The Washington agency lists them as typically living below 300 feet and recorded as deep as 6,152 feet, with adults reaching at least 18 feet and 1,300 pounds, and recreational fishing for them is closed in all Washington waters. Andrews describes the pattern this way: pregnant females come from deep offshore water into Puget Sound and the rest of the Salish Sea, as far north as British Columbia’s Howe Sound. “It’s definitely a pupping ground and a rearing ground for this population,” he said, “similar to the role the Salish Sea plays for other species, like salmon.”

The research began almost by accident, and the Salish Sea turned out to hold one of the few pupping grounds for the species that scientists have identified. In 2003 almost nothing was known about the species, Andrews said, when recreational anglers started hooking them from piers in Seattle and Tacoma. OPB has reported that a sudden influx appeared in Puget Sound in the late 1990s and early 2000s, that the sharks vanished in 2008, and that sightings in 2015 and 2016 raised the possibility of a return. The Northwest Fisheries Science Center, which is headquartered in Seattle, ran the work alongside the Seattle Aquarium and the Washington Department of Fish and Wildlife.

Feeding habits, lifespan and other open gaps

NOAA’s story lists what the team still cannot say. Feeding habits are poorly known. How long sixgills live is unknown too, with comparisons to similar species suggesting very long lives. Andrews acknowledged there is still a lot that is not known about the animals. Aging methods are the most obvious missing tool, since the range for juveniles rests on size and growth alone, and the feature story does not report a plan to validate it with age data. Because sixgills cannot yet be aged, the 4-to-6-year window for pups in Puget Sound stays the center’s best estimate rather than a measured residency time, and the paper’s 1,063 to 1,287 effective breeders is the only population figure on the table.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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