Have you ever seen a white shark at Point Reyes? How about one feeding on seal carcasses near the headlands? Would you like to know where and how often white sharks are seen, and what they are usually seen doing? This may be useful information if you dive or surf in open water here!
Most observations were clustered in three areas: Point Reyes Headlands, McClures Rock, and Tomales Point. White sharks are mostly seen close to shore, in water depths from 5 meters to 30 meters. This is likely because they need to keep sight of the surface where they capture their prey.
When are they seen?
White sharks are observed at Point Reyes most frequently in the late summer and fall (August, September, October), which coincides with the seasonal peaks of pinnipeds—California sea lions congregate in the area when males migrate north in the fall, and juvenile northern elephant seals haul out in the fall. When Scot Anderson used decoys to track the frequency of white shark appearances, he found that they were seen approximately once every six hours at Point Reyes National Seashore! However, this frequency is still less than the frequency in which they are sighted at the Southern Farallon Island: once every 1.9 hours.
What do they eat?
At Point Reyes, white sharks prey on pinnipeds that congregate onshore and they also scavenge on marine mammal carcasses (especially whales). At the Farallones and Año Nuevo, the pinniped population that white sharks primarily feed on are the northern elephant seals. However, this is not what Scot Anderson found at Point Reyes—the white sharks here seem to be feeding mostly on harbor seals and California sea lions.
White sharks also scavenge off of marine mammal carcasses. Large dead marine mammals such as sperm whales, baleen whales, and northern elephant seas can leave an odor trail that can be carried for many kilometers. White sharks can track these "odor corridors" and follow them to the source. In 2004, when a dead sperm whale drifted to Point Reyes, as many as four white sharks came to feed on the carcass at one time. A large white shark (~943 kg) can survive up to 45 days after feeding on 30 kg of blubber from a dead cetacean!
Should I be concerned about attacks on humans?
There has been five human attacks by white sharks at Point Reyes over 23 years (as of 2008), all of which occurred in the fall season close to shore in water less than 10 meters deep. All the victims survived. Those highest at risk appear to be people who dive for abalone, spearfish, or surf in open water in or near areas where sharks have been sighted, or near seal colonies. Please be careful, and report any white shark attacks to a park ranger.
This page has been adapted from Ocean Updates: Vol. 1, Issue 6: "White Sharks at Point Reyes National Seashore" published on May 20, 2008.
From 2007 to 2012, Pacific Coast Science and Learning Center Science Communication Interns produced a series of podcasts, videos, and audio-slide shows exploring science from Bay Area national parks. Two of these The Natural Laboratory multimedia products focused on white sharks. Listen to the podcasts below.
The Natural Laboratory Podcast: New Findings about Great White Sharks of the North Pacific
A The Natural Laboratory podcast produced by the Pacific Coast Science and Learning Center in 2010 in which Casandra Brooks interviews Scot Anderson, a local researcher who has studied white sharks off of Point Reyes Seashore and at the Farallon Islands for more than two decades.
Credit / Author:
Casandra Brooks / Pacific Coast Science and Learning Center
This is the Natural Laboratory, a podcast exploring science for Bay Area National Parks. I'm Cassandra Brooks.
[music]
Today, I'm with Scot Anderson, a local researcher who's studied great white sharks here off Point Reyes Seashore and at the Farallons for more than two decades. In the fall of 2009, Anderson and his colleagues with Stanford University's Tagging of Pacific Predators Program, UC Davis and others published a paper on the sharks. Their study revealed new information about where the sharks travel to, how they spend their time, and showed that the shark population here off California's genetically different from great white shark populations throughout the world.
I sat down with Anderson to find out more about the study and what it tells us about these iconic and revered animals.
[Scot Anderson Interview]
Cassandra Brooks: Maybe you could tell me a little bit about what at typical day is like for you out in the field.
Scot Anderson: Yeah, okay, so a typical day, uh, going out looking for sharks, let's say, on the Farallon Island runs, we leave out of San Francisco, now, on a big sailboat called the Derek M. Baylis. It has this launch and we, uh, go to the island and we launch the launch and we work out of that for six hours on the water looking for sharks. Um, usually what we do is we put a decoy out that's about the size of a small sea lion and a small piece of whale blubber next to the boat, that's tied to the boat. And what that does is provides a area of scent around the boat that gets the sharks interested in sticking around. And then, if the shark comes around, we videotape them first, try to document what the shark looks like, who it is, and what their sex is, and then we go ahead and either tag them.
CB: How do you actually tag a great white shark?
SA: [cross talk] tag a white shark. [Brooks chuckles] Um, it sounds like it'd be a complicated process, but it's actually quite simple. You wait for the right moment, which is when the shark is at a 90-degree angle to you, swimming by the boat. So, then, the tag is on the end of a long pole and it has a harpoon-like dart on the end. Once it's embedded in the shark's skin, it's going to stay there until it finally pulls out, in a year to two years, or something like that.
CB: And during that year to two years, it's collecting data the whole time about where the shark is going and...?
SA: Yeah…okay so there's two kinds of tags we use. The first kind of tag is a satellite tag and that we put on the shark and then it...it records data, like, uh, the depth of the water, the temperature of the water, and light levels, and things like that. And that's on the shark until a pre-programmed date that it's, uh, released. When it releases, it floats to the surface and starts downloading data.
The other kind of tag's called an acoustic tag and, um, it makes a sound that's a signature sound for each individual shark, comes out to a number. If they swim within a quarter mile of the receiver, it logs them in. Now these receivers are placed on the bottom and we have one at Tomales Point, one at Point Reyes, two at the Farallons, and two at Año Nuevo.
CB: Wonderful, so people will just have these underwater devices that are just constantly picking up these pings that are individual to each shark, and, then, you know when they are there and when they are not there.
SA: Yeah.
CB: I was hoping you could tell me a little bit about the recent study that was published in the Proceedings of the Royal Society. And you were co-author on this study.
SA: Well, it's a study that, sort of pulls together a lot of different kinds of data. And, so, what we did was we took DNA data and looked at that and what we found is that the sharks are very closely related. And then you start looking at where the sharks have migrated to and from and it's actually a...an area that's well defined. They don't go much past Hawaii—maybe 500 miles beyond that—and they don't go past Midway, and they don't go north of in a line with Canada, and they are pretty much in this zone.
CB: But why do they even migrate as far as past Hawaii?
SA: So why do they go there?
CB: Yeah.
SA: Well, that still remains a question to be answered. But it really looks like it has to do more with breeding than feeding, because this population, when they're on the coast, is feeding on an abundance of food. So, why would you go somewhere where there is very little food?
CB: You were talking that the study also shows that this population is genetically distinct. Does that...so it means that they’re not breeding with sharks from South Africa, obviously, or other areas...
SA: Right, yeah, because of the genetics, we know they're isolated up here and it's its own distinct population, so...
CB: And that seems like a very important finding in terms of understanding their role in the ecology here in the North Pacific.
SA: Yeah, you know, when you look at the role animals have in the environment, they're either going to be a producer or a consumer. Obviously, white sharks are apex predators; they're at the top of the game here. The role they play in the environment is...has yet to be totally understood, because we know they eat seals and sea lions—during certain times of year—and we know that they scavenge on dead whales and things and...but so do other animals. Whether they're actually, you know, keeping the environment healthy and all that, it's yet to be seen, but probably. They must play some role.
[Conclusion]
Looking out over the ocean from Tomales Point, it's surprising to think that great white sharks, among the most massive and mobile predators in the world, don't use the whole Pacific as their playground and that they don't mate with other white sharks in the world. Instead, they follow a strict and isolating migration path between California and the Hawaii region, which, as winter rapidly approaches, they are soon to embark on. Why, exactly, they make these vast migrations remains to be seen, but until then, Anderson and colleagues will be out every fall doing more observational and tagging studies.
[music]
With the Pacific Coast Science and Learning Center, I'm Cassandra Brooks.
The Natural Laboratory Podcast: White Sharks of the Northern Pacific
A The Natural Laboratory podcast produced by the Pacific Coast Science and Learning Center in 2008 in which John Cannon interviews Taylor Chapple, a graduate student at the University of California-Davis who has been studying sharks at the mouth of Tomales Bay.
Credit / Author:
John Cannon / Pacific Coast Science and Learning Center
The Natural Laboratory Podcast Transcript: White Sharks of the Northern Pacific
[Intro music]
John Cannon: This is The Natural Laboratory, a podcast exploring science for San Francisco Bay Area national parks. I'm John Cannon.
JC: There are worse places to live than the coast of Point Reyes National Seashore, 30 miles north of San Francisco, if you are a sea lion. [Sea lions can be heard barking in the background.] The cool waters support many species of fish, octopus, and squid that top the list of these noisy characters' favorite prey. And the weather is relatively nice, if you have a very layer of blubber and a fur coat that stays warm when it's wet. It's chilly at times, but rarely too hot There are lots of places to rest: sandy beaches, rocky outcroppings, and even the occasional weather buoy.
JC: But every fall, the waters around Point Reyes get a little more dangerous. From the open ocean, great white sharks make their way to the coast. They're on the hunt for food. And pinnipeds, the group that includes sea lions and their cousins, harbor seals and elephant seals, carry a nice ribbon of energy-packed blubber, making them an ideal source of nourishment for a hungry 1500-pound predator.
JC: I recently spent some time with a team of scientists studying sharks near the mouth of Tomales Bay at the northern tip of the park. I found out the great whites aren't always easy to find. And sometimes, they're preoccupied with other things. It was pretty clear that the first shark we saw wasn't at all concerned with us. So, we had to go after the shark.
Taylor Chapple: Um, yeah, so we just...we just had, basically, a pile of water that had some red color to it and I guess it was an attack.
JC: That's Taylor Chapple. He's a graduate student at the University of California-Davis and the group's leader here at Point Reyes. In four years studying sharks here most days each fall, this is the first evidence of an attack Chapple has seen out here. When the team first saw the blood in the water, they race the boat to the spot to find only a flash of fins in the bloody aftermath quickly dissipating in the churning water.
TC: [Unintelligible] was a sea lion 'cause the sea lions don’t float once they get chomped on. Elephant seals or harbor seals will float on the surface ‘cause they have a lot more blubber, but the sea lions, unfortunately, sink. So, we can’t really tell where the animal is [unintelligible] and it doesn’t bring the shark to the surface, obviously, if they don’t stay up.
JC: That's one of the challenges that all shark researchers run into. Unlike whales, sharks and other fish have gills and don't have to come to the surface to breathe. But, as this attack illustrates, it’s often where they come to find their air-breathing prey.
JC: So, to learn more about sharks, Chapple and his assistants have a few strategies to bring the sharks up from the depths.
TC: Sharks love Johnny Cash.
JC: While Cash’s crooning might not be the most effective or scientific tact, a sea lion-shaped decoy made of carpet stitched around the buoy usually does the trick. It sits bobbing in the water about 30 meters behind our stationary boat.
TC: And the idea is that we attract, you know, the shark to the surface just using that visual display. You know, we don't have to go through the whole process of chumming. We don’t do any chumming, or, you know, throwing blood or anything in the water.
JC: Instead, they rely on the shark's innate attraction to the decoy, an instinct hardwired into the shark's brain by millions of years of evolution. It's that instinct that tells the shark that this black object about four feet long with flippers and floating at the surface, just might make a tasty meal.
TC: So, once an animal comes up to the decoy, we shoot photos of it 'cause each of the dorsal fins of a white shark, the trailing edge of it is basically like a fingerprint so we can ID an individual start from that. So, we take photos of that and then reel the decoy towards the boat. As the decoy gets closer to the boat, we use an underwater video camera and take shots of the animal, trying to get sex and any more basically distinct markers on the animal. And once we feel like we have enough information that we can ID an individual, then we'll put a tag in.
JC: Those tags provide biologists with a host of information. When a tagged shark is in the area, a small receiver on the boat tips the team off to the fact that it is close, even if there's no evidence at the surface. And the receiver registers the tag's unique number, so Chapple can actually identify that individual shark. By correlating its number with photos and data from the past, you can tell how big that shark is, its sex, and the last time you saw it. Special satellite tags also provide scientists with information about the movements of sharks on a larger scale. These revelations allow Chapple and his colleagues to home in on the mysterious movements between areas like Point Reyes and places far out in the Pacific where many of these sharks aggregate. This effort involves researchers who study other species in the ecosystem with similar monitors placed at various locations along the coast.
TC: And what’s nice is that these monitors are present for a number of different species. So, they have these monitors for salmon, for sturgeon, for sharks, and for, you know, a whole list of other species. So, in a collaboration, we can all get information about our animals being at somebody else's array of stations. So, you don't...we don't necessarily have to put 5000 of these listening stations around. We can just collaborate with other researchers.
JC: The data collected by all of these researchers will be compiled into a much larger ten-year undertaking known as the Census of Marine Life. This project aims to catalog the quote diversity and abundance of life in the oceans unquote, looking piece by piece at this grand web of ocean life.
JC: For now, Chapple will continue researching the strand of that web he knows the best: the white sharks of the Northern Pacific. And right on cue, a sail-shaped dorsal fin appears next to the decoy. This shark, an untagged female just a few feet shorter than the boat we're on is inquisitive, but it's not the violent eruption of activity I'd expected. Chapple says this cautious behavior is more typical than the bite first, ask questions later approach that white sharks seem to be notorious for.
TC: You know, it's not this horrific attack scene like you, you know, you see in the movies or they put on the Discovery Channel. It's usually pretty...pretty mellow, like, she came up once just past the decoy, and then did a circle under water and then came up just kind of nudged it to check it out. Very rarely do you get them that they come up and they, you know, attack the decoy or even...even bite it. And it's pretty rare that that happens. It's usually really...uh...kind of a mellow interaction. I guess as mellow as you can be being 16 feet long.
JC: But mellow is not the word to describe the great white's effect on its environment, and us. As an apex predator, this species is a critical component of a functioning and healthy ocean ecosystem. The World Conservation Union has listed white sharks as vulnerable and they're likely impacted by climate change, overfishing, and other changes brought about by humankind. But the truth is: we know very few basic details about their lives—where they go to breed, the size of the population, and how long they live, to name just a few. So, we're just beginning to understand what must be done to protect this remarkable ocean hunter. These questions only make the research that Chapple and others have taken on that much more important as we begin to uncover the extent of our own interdependence with the world's oceans. For the Pacific Coast Science and Learning Center at Point Reyes National Seashore, I'm John Cannon.
Anderson, Scot D. 2011. "Long-term individual identification and site fidelity of white sharks, Carcharodon carcharias, off California using dorsal fins." Marine Biology 158: 1233–1237. Available at https://rd.springer.com/article/10.1007/s00227-011-1643-5 (accessed on 11 June 2022).
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