White shark census reveals rarity off California coast
White sharks off the central California coast are far fewer in number than researchers expected, according to the first-ever scientific census. But what that says about the health of the population is not clear.
A white shark tagged with both acoustic (front) and pop-up satellite (rear) tags. The acoustic tag is detected when the shark swims within 250 m of a listening station, while the pop-up satellite tag records information about location, temperature and depth – and relays it to the laboratory when the tag releases itself from the shark.
Perched atop the tip of the food chain, big, fierce predators are rare. Now it seems that white sharks may be the rarest of the rare among ocean predators, according to the first-ever scientific census of their numbers along the central California coast, by a group that included Stanford researchers.
The researchers estimate that there are only about 219 adult and sub-adult white sharks – commonly known as "great white sharks" – plying the waters in the region stretching from Bodega Bay to Año Nuevo.
"Although white sharks are protected in many areas, including the waters off California, before this study we had no real idea how many there were," said Barbara Block, the Charles and Elizabeth Prothro Professor in Marine Sciences at Stanford's Hopkins Marine Station. "Now that we have a baseline, we can start to look at the dynamics of the white shark population – to determine whether their numbers are growing or declining in the wild."
The sharks were identified by the distinctive patterns of nicks, notches and scars they accrue on the trailing edge of their dorsal fins, which are typically visible above the water when sharks are swimming near the surface.
Block is one of the authors of a study published March 9 in Biology Letters. There are at least two other genetically distinct populations of white sharks in the world – one around New Zealand and Australia, the other in the waters off South Africa.
"This low number was a real surprise," said Taylor Chapple, the study's lead author and a doctoral student at the University of California-Davis when the work was done.
"It's lower than we expected, and also substantially smaller than populations of other large marine predators, such as killer whales and polar bears. However, this estimate only represents a single point in time; further research will tell us if this number represents a healthy, viable population or one critically in danger of collapse, or something in between."
The census was conducted in 2006, 2007 and 2008 on adult white sharks that inhabit shelf waters along the central California coast each year, beginning in late July and lingering until late January. Satellite tagging has shown that these sharks return from their open ocean sojourns with remarkable precision, which is what makes it possible to do a reliable census, Block said.
Teams of researchers went out in pursuit of the white sharks around Tomales Point, near Bodega Bay, and to the Farallon Islands off the San Francisco coast, areas where the sharks are known to congregate. The teams worked from small vessels to get close enough to the sharks to take detailed photographs.
The researchers took most of the photos during tagging expeditions, when they were attaching tracking tags to the sharks.
Slowly pulling a seal-shaped decoy made of carpet or neoprene through the water, the researchers would lure a curious shark close in to their boat. They usually gave the decoy some added allure by dangling a chunk of seal or whale blubber over the side of the boat, to create a scent plume of "eau de marine mammal" in the water.
When a shark came alongside the boat, a pole was used to affix the tag and the designated shutterbug snapped profile shots of the dorsal fin.
From 321 photographs, the researchers identified 131 individual sharks. Using a variety of standard mathematical models to analyze these data, they calculated that the likely total population along the central California coast is about 219 adult and sub-adult sharks. "This is a surprisingly low number for an apex predator. Given their low genetic diversity, it is important to establish the census of our population of predators," Block said.
Although the census was conducted from 2006 to 2008, researchers have been photographing dorsal fins of white sharks off the California coast for 22 years. In another study by the same research team, published online in Marine Biology last week, they described how the jagged patterns on the dorsal fins persist over time. One individual shark that was first identified in 1987 was seen seven more times over the years, with the most recent sighting three years ago.
In addition to demonstrating the reliability of using dorsal fins to identify white sharks, the study provided the first evidence of white sharks surviving in the wild for at least 22 years.
Four other white sharks were identified repeatedly over spans of at least 16 years.
"We've found that these white sharks return to the same regions of the coast year after year," Block said. "That is what makes it possible to estimate their numbers using this relatively simple technique."
The persistent appearances of the white sharks along the central California coast, where all the photographs in the Marine Biology study were taken, further supports findings from previous satellite tagging studies that revealed that although the sharks make an annual migration from California to either the Hawaiian Islands or an area in the open ocean approximately halfway between the Baja Peninsula and the Hawaiian Islands, dubbed the "White Shark Café," they always come back to California after their open ocean odysseys. That makes having an accurate count of their population invaluable.
"This is a critical first step in maintaining the wilderness that exists just off our shores," Block said.