sharks were once so abundant along the California coast that a thrill-seeking
trophy hunter reportedly harpooned a half-dozen in under three hours in
Monterey Bay. That was in 1947.
the big fish are so rare that it’s taken a team of scientists between San Diego
and Santa Cruz eight years to put tracking tags into just six animals. Their numbers
are so low, in fact, that researchers, working with tiny sample sizes, can
scarcely study them at all or draw firm conclusions about population trends,
threats to their survival, typical behavior, or how global warming may affect
is even concern that the sharks—filter feeders that may grow to 40 feet, the
size of a small whale—are so widely scattered through the ocean that they may
not be able to locate one another to mate and reproduce.
Van Sommeran, founder of the Pelagic Shark Research Foundation in Santa Cruz
and a collaborator in the ongoing tagging project, put a pop-off satellite tag
into a Monterey Bay basking shark in 2011—a shark that produced migration data
for a paper published last year in the journal Frontiers in Marine Science.
He rejects the notion, sometimes floated in the media, that basking sharks are
making a comeback.
“There’s more boat and air traffic today than ever before, and there are fewer sightings [of basking sharks] than ever before,” he says.
fish were listed in 2010 as a “species of concern” by the National Marine
Fisheries Service—a designation that vaguely calls for a better understanding of
the animals but offers no clear research or conservation framework.
Sommeran thinks the animals should be granted a higher level of federal
protection. “The criteria for endangered species status is pretty well accepted
to be depletion to 10 percent of historic abundance,” he says. “Basking sharks
are there, so why are they in this nebulous sidebar category of ‘species of
Dewar, a researcher with the National Oceanic and Atmospheric Administration
and the lead author on the recent paper, says listing a species as threatened
or endangered is a complex process that depends on a species’ absolute
population size, its potential for population rebound, and trends in population
size. While there is no doubt that basking sharks are far less abundant than
they once were, there is no evidence that the population, whatever it may be,
is growing or shrinking.
just haven’t been able to study enough animals,” Dewar says. She says her team
takes to the water in power boats when they receive eyewitness reports of the
filter-feeding sharks cruising at the surface. Successful outings, though, have
been few and far between. About four times out of five, she says, the sharks
are gone by the time they arrive.
low success rate makes each fruitful venture, with a tag jabbed with a long
spear into a shark’s back, a cause for celebration. “It’s kind of like Wimbledon—we hug each
other, throw our arms in the air, scream, celebrate,” Dewar says.
also a high-stakes gamble. The tags, says coauthor Owyn Snodgrass, another NOAA
scientist, cost several thousand dollars each. “It’s like throwing a laptop
into the ocean for a few months and hoping it will work later,” he says.
tags the scientists use are designed to break free from the fish and float to
the surface after a preprogrammed period of time—usually a few months. At the
surface, the tags transmit data skyward, and passing satellites bounce the
signals to the researchers’ computers.
always a thrill when you open your computer and see that email telling you your
tag has surfaced,” Snodgrass remarks.
the researchers, the data generated by the tagged basking sharks has provided a
glimpse into the ocean-wide migrations and day-to-day behavior of what is
certainly one of the least-understood large marine animals. One of the six
sharks tagged so far left the California coast, swam to Hawaii, and spent most
of its time at depths of 800 to 1,500 feet beneath the surface. Another shark
traveled south along the Baja California peninsula and came to the surface on a
daily basis. Two more tags essentially malfunctioned, releasing after just nine
days and 51 days, respectively, not far from the point of deployment. (Two more
sharks were tagged after the paper was written.)
tags from the longer migrations showed intriguing patterns of vertical movement
through the water column. The fish tended to remain near the surface while in
nearshore waters—probably taking advantage of coastal upwelling and the
abundant plankton it helps generate. When they moved farther offshore, the
sharks descended into deeper water.
study solved few mysteries while laying a foundation for further investigations
into where the fish go and whether these migrations place them in harm’s way.
want to know where they’re going and what fisheries they interact with,” Dewar
says. She says the sharks, when caught in nets in international fisheries, are
often killed for their large and valuable fins. She says this rarely, if ever,
occurs in United States waters, where finning is prohibited.
Skomal, a biologist with the Massachusetts Division of Marine Fisheries who was
not affiliated with Dewar’s research, says the California scientists’ paper
provides “insight into the biology of the world’s second-largest fish.” (The
whale shark is the world’s largest fish.) Skomal has studied basking sharks in
the Atlantic Ocean. There, he says, “you can consistently locate them.”
notes that the scarcity of basking sharks in the Pacific Ocean doesn’t just
make it difficult for scientists to study them; it can also have consequences
for the very survival of the species.
just hope they haven’t become so rare that they can’t reproduce,” he says.
accounts tell of fishermen and mariners seeing hundreds of basking sharks in a
single day, and the fish—navigational hazards for boats—were considered pests.
Off the coast of British Columbia, they were actively culled by the Canadian
government by fitting boats with large blades on their bows designed to fatally
injure the sharks in collisions. This eradication effort, which began in the
1940s and ended in 1970, removed somewhere between 1,000 and 2,600 basking
sharks from the ocean.
were treated similarly in California, where harpooning them for sport was a
niche tourist attraction in Monterey Bay for several decades. This vulgar
pastime, described by writer and historian Tim Thomas, persisted from the 1920s
until the early 1950s—about the time when the sharks slipped into scarcity.
According to the Frontiers in Marine Science paper, fishermen in
California may have taken 700 to 800 basking sharks in the first half of the
20th century. So depleted is the West Coast’s basking shark population that
only three have been incidentally caught by the Canadian groundfish trawl fleet
the history of the basking shark’s interactions with humans is not quite the
typical linear storyline of a species driven from abundance to near extinction.
In their paper, Dewar and her colleagues, referring to historical records,
describe periods in the 19th century in which basking sharks would go unseen
for 20 years at a time—a pattern indicating global migrations and other behaviors
still not understood.
now, scientists at the leading edge of studying the basking shark know
virtually nothing about them: where they mate, where they give birth, why they
leave productive coastal zones, to what degree regional populations intermingle,
and so on.
much about basking sharks remains mysterious,” Skomal says. “Their age at
maturity, their size at maturity, how long they live, how fast they grow—there
is a tremendous amount we don’t know.”
and Snodgrass say they have plans to address some of these persisting mysteries
in future research. But due to the difficulty of tagging and studying basking
sharks, progress will come slowly. “It could be 10 years before the next paper
is published,” Dewar says.
A native to San Francisco, Alastair Bland is a freelance journalist who writes about water policy in California, rivers and salmon, marine conservation and climate change. His work has appeared at NPR.org, Smithsonian.com, Yale Environment 360 and News Deeply, among many other outlets. When he isn't writing, Alastair is likely riding his bicycle uphill as fast as he can.