Keywords: Fishes, Species of concern, History

A shark species the length of a bus was once common in the Salish Sea. Then it was labeled a "destructive pest" and nearly wiped out. Can the gentle and often misunderstood basking shark make a comeback?

Among the animals native to Puget Sound is a sizable sea creature who historically frequented the area during the summer months to feed on a superabundance of their preferred prey, was reviled and persecuted by humans during the last century, and is now celebrated as a fascinating social being who hangs out in family groups.

That description may call to mind the region's iconic southern resident killer whales, but the creature in question is a much more obscure and elusive one: the basking shark (Cetorhinus maximus). Basking sharks are the world's second-largest shark (and fish) species, and while they were once common in some parts of the Salish Sea, they are now so rare that several of the scientists working to better understand them and restore their numbers have never even seen one. Increasing public awareness of the species could be a key first step to recovering them, these scientists say.

One of three species of filter-feeding sharks worldwide, basking sharks are gentle giants who cruise slowly through the water, mouths agape, using their massive gill rakers to sieve grain-of-rice-sized crustaceans called copepods and other zooplankton from the water. The sharks can reach a length of 40 feet and weigh up to 7,000 pounds.

A pair of basking sharks seen swimming near the surface of the water.

Basking sharks are most commonly seen near the surface in temperate coastal waters. Photo: Irish Basking Shark Group

Basking sharks are found all over the world but are most commonly seen near the surface in temperate coastal waters. They're drawn to upwelling areas, inlets, or fronts between tidal masses where high concentrations of their zooplankton prey accumulate and can gather into aggregations of nearly 1,400 individuals. The animals are thought to congregate in part to feed, but the gatherings may also be an opportunity for mating and socializing. Recent research suggests that basking sharks seen in aggregations are more closely related than would be expected by chance, so there seems also to be a family component to the gatherings.

Aerial view of a group of basking sharks swimming in a circle.

An aerial view of basking sharks swimming in a circle, a display of courtship behavior. Drone image from the west coast of Clare County, Ireland. Photo: Irish Basking Shark Group.

Past basking shark hotspots include Barkley Sound and Clayoquot Sound on the outer coast of Vancouver Island. During the spring and summer months, "there were so many of them and sometimes groups of hundreds or more that they would impede the passage of coastal steamers," marine biologist Romney McPhie said in a virtual lecture for Western Washington University's Salish Sea Institute earlier this year.

In the first half of the 20th century, basking sharks around Washington and British Columbia were targeted by sport and commercial fisheries, the latter of which was focused on harvesting their enormous livers for oil. Meanwhile, the growth of commercial salmon fisheries exacerbated the conflict with humans: the slow-moving sharks often got tangled in salmon gill nets and seines, and their bodies are covered with a slimy mucus that damages fishing gear.

Historical black and white image of a dozen men on the beach next to, and one on top of, a large basking shark. Trees and fishing boats in the background.

A basking shark caught by fishermen in Rivers Inlet, BC, Canada, July 1901. BC Archives reference code D-02035.

A 2006 book, "Basking Sharks: The Slaughter of B.C.'s Gentle Giants," recounts the history of the shark's persecution off the West Coast of Canada. Canada's Department of Fisheries and Oceans (DFO) declared basking sharks a "destructive pest" in 1949, adding them to a list of animals that could be killed on sight. In 1955, the agency launched a basking shark eradication program — with the goal of killing every last one — equipping a fisheries patrol vessel with a giant blade on the prow that would slice straight through the sharks as they fed peacefully at the surface of the water. By the time the eradication program ended in 1969, 413 basking sharks in Barkley Sound alone had been killed with the device; 34 sharks perished on the single deadliest day of the effort.

Black and white illustration of the sharp blade mounted on the bow of a ship ramming into a shark.

Illustration of ship with a sharp blade mounted on the bow that was used during a campaign to eradicate basking sharks during the 1950s in British Columbia. Source: Popular Mechanics (1956) 

The basking sharks that previously summered in the waters off the Pacific Northwest — likely drawn there by the region’s massive seasonal zooplankton blooms — are thought to be part of a population that winters off the coast of California and Mexico. Scientists estimate the eradication program reduced the numbers of this eastern North Pacific population by up to 90 percent. That population numbers perhaps 300 to 550 individuals today.

The eastern North Pacific population of basking sharks was listed as endangered under Canada's Species at Risk Act in 2010 and DFO released a recovery strategy (PDF) for the species the next year. Also in 2010, the U.S. National Marine Fisheries Service identified the population as a "species of concern," meaning that it may be imperiled but there is not enough information available to permit listing under the federal Endangered Species Act. The species as a whole is also listed as endangered on the International Union for the Conservation of Nature’s Red List.

In recent years, just two or three basking sharks have been spotted off the B.C. coast each year. Sightings in Puget Sound are so noteworthy that they often garner media attention: one was spotted near Shelton in 1997, two were seen around the San Juan Islands in 2009, and another off Edmonds in 2014. Only about 30 sightings have been recorded in Puget Sound and on the B.C. coast since 1996.

It's not clear why basking sharks haven’t returned to Pacific Northwest waters in greater numbers since the end of the eradication program. “Our understanding of recovery is limited, and the life history of the basking shark is working against it,” McPhie told the Encyclopedia of Puget Sound in an interview. Basking sharks reproduce slowly, bearing litters of one to six pups every 18 months to three years. In addition, McPhie said, although their persecution in local waters has ended the sharks may face unknown threats in the open ocean, such as collisions with vessels, being caught as bycatch in offshore fisheries, and illegal harvest of their fins.

It’s possible, though, that there are more basking sharks out there than sightings data reflect, in part because of a lack of public awareness of the species. “People don’t know to look for them. They don’t even know they exist on our coast, so they may see them and just not even know what they’re seeing,” said McPhie.

Without knowing where basking sharks can be found, it’s very difficult to study them, and only a few research efforts have been conducted in recent years. A 2018 study reported on the movements of four basking sharks fitted with satellite tracking tags off the coast of California, revealing that sharks in the eastern North Pacific can travel as far away as Hawaii. In another study, researchers assembled a comprehensive set of observations on basking sharks on the Pacific Coast from Baja California to British Columbia during 1962 through 2018, finding that sightings declined beginning in the mid-1980s — well after the end of human persecution of the species on the West Coast — and the size of basking shark aggregations declined between the 1960s and 1980s.

McPhie is part of an informal group of about half a dozen would-be basking shark researchers in the Salish Sea region who have been in touch with each other since last summer. The group met in person at the Northeast Pacific Shark Symposium in Seattle in March along with basking shark aficionados from farther south, totaling about 20 researchers. One task is to compile information about the history of attitudes towards and exploitation of basking sharks in U.S. waters, akin to what has already been gathered for Canada.

The group's main immediate aim, though, is to raise public awareness of basking sharks and encourage them to notify DFO's shark sightings network or the Spot a Basking Shark Project, a collaboration between NMFS and San Jose State University, if they see one. "Our best source of information at this point given their rarity is citizen sightings networks," McPhie said. Eventually, such data could help scientists understand the current abundance, distribution, and migration patterns of basking sharks in the Northeast Pacific, which would then open up the opportunity for more studies as well as targeted conservation measures.

The overall vision is a simple one, McPhie said. "We would just like to see them in our waters again," so that people living on the West Coast could appreciate basking sharks as part of their local fauna, she said. "It would be pretty wonderful."

About the author: Sarah DeWeerdt is a Seattle-based freelance science writer specializing in biology, medicine, and the environment. Her work has appeared in publications including Nature, Conservation, and Nautilus.

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