Pupping Season Means More Great White Sharks
Monday, July 5, 2010
April through August is pupping season for great white sharks. Marine biologist, Dr. Michael Domeier has been studying white sharks in waters off the coast of California and Baja California. We find out about the migratory patterns of great white sharks, where they live, feed and breed.
MAUREEN CAVANAUGH (Host): A warning from the National Park Service about a potential threat from great white sharks to swimmers and surfers off the California coast is nothing to take lightly but it may not be as dramatic as some of the headlines suggest. It stems from a normal cycle of birthing and growing for great whites. Joining us now for more information is my guest, Dr. Michael Domeier. He is a marine biologist who is president of the Marine Conservation Science Institute. He’s currently featured in the series “Expedition Great White” on the National Geographic Channel. And, Dr. Domeier, welcome to These Days.
DR. MICHAEL DOMEIER (President, Marine Conservation Science Institute): Hey, good morning, Maureen. Thanks for having me.
CAVANAUGH: You’re very welcome. Thanks for being here. How common are great white shark sightings off our coast?
DR. DOMEIER: They’re actually, for juvenile great white sharks, they are relatively common in certain areas, like off San Onofre and off from Malibu and in Santa Monica Bay. But for adult white sharks, it’s very rare actually to see one.
CAVANAUGH: Now what – We just talked about – I just talked about this warning from the National Park Service, what is that about?
DR. DOMEIER: Well, there were, within a pretty short time, three sea lions killed off Santa Barbara, and I think that just – they’re just being cautious. This time of year adult, mature pregnant white sharks come into the shallow waters to give birth and at least that’s a hypothesis that I’ve been working on. And it seems pretty clear from incidental gill net captures of young of the year white sharks that that is, in fact, the case, that from about April to August we’re getting baby white sharks dropped off here and then the females turn around and take off again.
CAVANAUGH: I see. So are either the mothers or the babies a threat?
DR. DOMEIER: The babies are not a threat at all. They’re just eating fish and crustaceans and skates and rays. The adults, of course, are always a threat because they may sometimes make a mistake and think that a surfer or a swimmer is actually a seal. And this is a time of year we probably have more adult white sharks in our waters than any other time of year.
CAVANAUGH: I see. I’m speaking with Michael – Dr. Michael Domeier. He is the president of the Marine Conservation Science Institute. Dr. Domeier, you said something interesting and that is we think that’s what happens. Are you able to track the offshore movements of great white sharks?
DR. DOMEIER: Yes. In fact, I’ve been tracking the movements of adult white sharks for over 10 years. And in the last 3 years, I’ve been able to do it at really an unprecedented scale because I’ve been capturing these adult white sharks and attaching a transmitter to their dorsal fin so every time they come to the surface, I know exactly when and where they are. And these tags last multiple years. The juveniles a little bit more difficult to track. You have to use very small tags, they don’t last very long, only a few months at best for the battery life.
CAVANAUGH: Now that must be something, tagging a great white shark. How does that process work?
DR. DOMEIER: Yeah, and actually it is quite a process. We catch it sort of like you saw in the old “Jaws” movie with, you know, big rope, giant hook, and a big, heavy plastic floats to tire the shark out. And then a small boat is used to tire the shark out while on this line and it pulls it onto a platform that’s on a ship and the platform has heavy hydraulics and just lifts the shark right out of the water like an elevator.
CAVANAUGH: I see. And how are you able, once you get them tagged, to track their movements?
DR. DOMEIER: Well, by attaching this transmitter to their dorsal fin, each time the dorsal fin breaks the surface of the water it sends a signal off into space and if there happens to be the right satellite overhead, the satellite can pinpoint exactly where it is. And then I actually get that information over the internet.
CAVANAUGH: And what have you been able to find out so far?
DR. DOMEIER: Well, the – I started using a different type of tag, oh gosh, 10 or 11 years ago at Guadalupe Island in Mexico and those tags only lasted 9 or 10 months.
DR. DOMEIER: And we also were taking pictures of every shark we ever saw there, and we, over the next 10 years, my associate, Nicole, and I identified over 100 individual sharks. From those two early projects, we realized that mature females don’t come to Guadalupe Island each year. They disappear for a year or maybe two years at a time, and that became a big mystery. Well, where are they going?
DR. DOMEIER: So that’s what kicked me into this next level of tagging, which required me to capture the sharks.
DR. DOMEIER: We have now figured out where those sharks go. They’re going off into the middle of the ocean. In fact, all the sharks go off into the middle of the ocean from about January until August. But the females stay there for over – well over a year. The males come back each year to Guadalupe Island and then I believe it is the pregnant females are staying off shore and the ones that are not pregnant are coming back to Guadalupe Island. And what I believe is that they’re coming there to mate.
CAVANAUGH: I see. So do we know how long the gestation period is for the pregnant females?
DR. DOMEIER: You know, there’s been enough specimens that have been caught in commercial gill nets and stuff and been cut open that scientists think it’s more than a year, probably about 18 months.
DR. DOMEIER: And that’s what makes the females have a two-year reproductive cycle, and because of that they have a two-year migration cycle.
CAVANAUGH: I’m speaking with Dr. Michael Domeier and we’re talking about great white sharks. Recently a warning from the National Park Service was issued about a potential threat from great whites to swimmers and surfers off the California coast. Where, Dr. Domeier, is the largest population of great white sharks found?
DR. DOMEIER: Well, actually I think that would be in South Africa.
DR. DOMEIER: But in our part of the world, it would be north of Point Conception, around the Farallon Islands and in Nuevo for adult white sharks. It’s the highest population. Of course, and south of our border, Guadalupe Island has a population that we’ve done an estimate now, it’s probably around 135 adults at any given time during the season when they’re there, which is, again, like from August to February.
CAVANAUGH: Right. We do have a caller on the line. Chris is calling us from San Diego. Good morning, Chris, and welcome to These Days.
CHRIS (Caller, San Diego): Hi.
CHRIS: I was just wondering if the increased seal population around La Jolla might cause an increased chance of shark attacks on people.
CAVANAUGH: Thank you for that, Chris. What do you think, Doctor?
DR. DOMEIER: You know, I probably not. I mean, it is a possibility but Southern California is not a – it’s not a favorite habitat for adult white sharks. It seems that they just pass through here or they come here to give birth and then they leave. The adult white sharks really prefer that area around the Farallon Islands and Guadalupe Island. And what I actually believe is that what they really want to be is out in the middle of the ocean. That is their preferred habitat, that they are really an oceanic shark and they come to shore only to give birth and to mate. Of course, this – I’m talking to you very freely about an hypothesis I have, and you’ll find other scientists that will probably disagree with me.
DR. DOMEIER: But over the last few years, I have started to gather quite a bit of evidence to support this.
CAVANAUGH: Now, as you said, one of the reasons that the National Park Service warning was issued was because of sharks killing some sea lions in Santa Barbara?
DR. DOMEIER: Yeah.
CAVANAUGH: Was that it?
DR. DOMEIER: Yes.
CAVANAUGH: And is that typical?
DR. DOMEIER: Oh, yeah, that’s typical. I mean, white sharks, when they are near the coast, definitely eat pinnipeds, which are seals, marine mammals and they – their favorite’s probably elephant seals because they have a lot of blubber and they’re quite large. And the next down the list would be sea lions, and then those seals off – from San Diego are even smaller. Those are harbor seals. They certainly eat them. I mean, I have a picture of one, a harbor seal on the beach that has a big bite taken out of it, and that picture was taken probably six years ago.
CAVANAUGH: Do we know how large the population of great white sharks is in Southern California waters?
DR. DOMEIER: Not in Southern California waters. There’s just not enough to study off from Point Conception. I mean, north of Point Conception another team has done a population estimate for the ones around the Farallon Islands and it, again, was under 200 adults.
CAVANAUGH: Right. Is that population decreasing or just – do we just not know enough?
DR. DOMEIER: We don’t – You know what, this is the first year that my team has been able to come up with a population estimate for Guadalupe Island, and it’s the first year that this team from Stanford has produced an estimate, so we won’t know that until we continue to do these estimates year after year.
CAVANAUGH: Now when you tag the great white sharks to keep a – to be able to track their movements as they go out into the deep ocean, do you also take any DNA samples?
DR. DOMEIER: Yes, we take DNA samples. I take blood to look at reproductive hormone levels. And I examine the males for the presence of, excuse me, sperm in their claspers. Claspers are a modified pelvic fin that the males use to inseminate females.
CAVANAUGH: Right. And so what information do you get from those DNA samples?
DR. DOMEIER: Well, I have found that the females that I’ve sampled at Guadalupe Island have very low progesterone levels so they’re – they were not pregnant.
DR. DOMEIER: I had one of the females that I sampled quite late in the year, in December, had elevated estradiol, which means she was probably ovulating. And I have found live sperm in several of the males at Guadalupe Island.
CAVANAUGH: You know, it’s fascinating Dr. Domeier, excuse me, that you are just making these observations and, of course, great white sharks have been with us for such a long period of time. Are these sharks particularly—I guess that I know the answer to this—difficult to study?
DR. DOMEIER: Oh, yeah, they’re – they’re very difficult to study. For one, they’re quite rare…
DR. DOMEIER: …and then, of course, they’re huge. So they’re very difficult to handle and to capture. And it’s only been in the last couple of years where I’m really the only person in the world who’s capturing adult white sharks and releasing them alive again.
CAVANAUGH: Yes, I – Yes, I can imagine that you are. Well, do we know why they choose the California/Baja, California coast as their preferred location to give birth?
DR. DOMEIER: Well, it happens to be the habitat that the young need to survive. They need the shallow coastal waters and there’s probably some element of temperature that’s just right for them. So the females travel a long ways, you know, they’ll probably come 1500 miles from offshore to give birth in a specific location before they’re just turning around and going back offshore.
CAVANAUGH: And as you say, once the females give birth, they just take off?
DR. DOMEIER: Yeah, there’s absolutely no parental care.
CAVANAUGH: Wow. Okay, and so the babies are just left to fend for themselves.
DR. DOMEIER: Yeah but, you know what, they’re a pretty big fish. I mean, they’re babies but when they come out they’re, you know, at least 3 feet long and they already weigh about 40 pounds. So they’re a force to be reckoned with in that particular habitat.
CAVANAUGH: I see. How is that they – how long does it take them to transition from baby/juvenile behavior to adult behavior?
DR. DOMEIER: That probably takes about 10 years…
DR. DOMEIER: …and that’s a guess.
DR. DOMEIER: I mean, our understanding of age and growth in white sharks, again, is quite poor. You’d have to kill quite a few of them and take a vertebrae out and you can look at the rings on the vertebrae, like rings of a tree. Now white sharks are protected and so it’s really not a ethical thing to do and go out and kill them just for that. But I’m guessing probably 9 to 10 years and when they – reaching, you know, close to 9 to 11 feet, they start eating – instead of eating fish, they start eating mammals and they start this new – completely new life history pattern of instead of traveling up and down the coast, which is what the juveniles seem to do, all of a sudden they start going offshore and then accumulating a couple of specific adult aggregation sites. And that’s the next project that I really want to get underway, is looking at the sub-adults and how and when they make that transition from a juvenile behavior to adult behavior.
CAVANAUGH: Are they a threat to swimmers and surfers when they’re still juveniles?
DR. DOMEIER: Not as juveniles but, you know, when they start to transition to sub-adults they can be a threat. In fact, it’s interesting. In February, I held an international white shark symposium in Hawaii and pretty much all my colleagues from around the world came. And it was interesting for me to learn that in South Africa and Australia, where you often hear about white shark attacks, those are all a result of sub-adults. They don’t even know where their adult white sharks are. They rarely see adults in those two locations. So they’re talking about sharks that are in the 9 to 12 foot range.
CAVANAUGH: Now when the Park Service warning includes the words ‘the public to enter waters at their own risk,’ that can be a little disturbing. Why that wording, Dr. Domeier?
DR. DOMEIER: Well, I think because they don’t want to take any responsibility. They don’t want you to enter the water at their risk. It’s just a precautionary note. And it was a little bit surprising because nothing’s changed. This happens every single year, and I’ve been living in California, well, since 1992 and it’s the first time I’ve heard the Park Service come out with a statement like that.
CAVANAUGH: What – If you were going to be wording this warning, what, perhaps, would you say instead?
DR. DOMEIER: I think I would say the same thing. In fact there was – A few years ago, a helicopter pilot from Camp Pendleton started sending me pictures of sharks that he was seeing right outside the surf at San Onofre and it took me a few days to look at them because I get all kinds of crazy e-mails but when I did look at them they were, indeed, white sharks and I contacted the LA Times and I put out a very similar statement.
CAVANAUGH: I see.
DR. DOMEIER: And I came, actually, under a lot of criticism but just two weeks earlier a woman had been killed up in central California and I just thought it was the prudent thing to do.
CAVANAUGH: Okay, the public to enter waters at their own risk. I want to thank you so much for speaking with us today.
DR. DOMEIER: Thank you very much.
CAVANAUGH: Dr. Michael Domeier is a marine biologist who is president of Marine Conservation Sciences Institute, and you can see him because he’s currently featured in the series “Expedition Great White” on the National Geographic Channel. If you’d like to comment, you can go online, KPBS.org/thesedays. And stay with us for hour two of These Days coming up just in a few minutes here on KPBS.