The Inside Scoop on La Jolla Leopard Sharks
We featured the La Jolla leopard sharks here last week, which left some of us (i.e., me) with lots of questions about these amazing creatures. This week, we were lucky enough to get some answers from the expert on the sharks, Dr. Andrew Nosal. Andy has been researching sharks for 10 years, and he knows the La Jolla population better than anyone. Check out our question and answer session with Andy below for the inside scoop on the La Jolla leopard sharks. (And for even more shark related goodness, follow him on Twitter @AndrewNosal.)
Andrew P. Nosal, Ph.D., B.S.
- Assistant Professor of Biological Sciences at Saint Katherine College
- Visiting Assistant Researcher at Scripps Institution of Oceanography at University of California, San Diego (UCSD)
- Postdoctoral Researcher at Scripps Institution of Oceanography at UCSD
- Adjunct Professor of Biology at University of San Diego
- Ph.D. in Marine Biology from Scripps Institution of Oceanography at UCSD
- B.S. in Biology from University of Virginia
Q: Can you tell us about the group of leopard sharks that gather in La Jolla each year, and why they come specifically to this area?
A: The leopard sharks that gather in La Jolla are mostly (>95%) pregnant females. Based on acoustic tracking, we know these sharks stay mostly within a "divergence zone" of low wave energy that is caused by bathymetric wave refraction by the offshore submarine canyon. So, these pregnant females are hanging out in a very calm area, which also tends to be warmer than surrounding areas due to less vertical mixing.
It is hypothesized these pregnant females are incubating their developing embryos, kind of like a mother bird sits on her eggs. The icing on the cake is that there is food nearby in the adjacent rocky reefs, kelp forests, and especially in the canyon, where the sharks go at night to feed on market squid.
Q: Where do the leopard sharks go when they aren’t in La Jolla?
A: We do not know where they go when they leave La Jolla. We have been able to track about half of the sharks we have tagged going north along the coast until they disappear around Los Angeles, suggesting they move offshore, perhaps to Catalina Island. The other half just disappear from La Jolla and are not detected elsewhere until they return the following spring. Interestingly, the same sharks that go north and return one year, go north the next year. The ones that disappear one year, disappear the next year. So there are two distinct behaviors, but both groups seem to meet up in La Jolla each ~June-November.
Q: Following up on that, do you have any idea where the babies and males are? (Is there a known mating ground and/or nursery elsewhere?)
A: I have seen some pups and juveniles in the seagrass beds around La Jolla, and they are randomly caught up and down the coast by recreational fishers. I suspect there is no special pupping or nursery area, but that they roam the coastline. The males are found in colder, deeper water (~30-50 feet deep), in and around kelp forests.
Q: Why is it just the females that come to La Jolla?
A: I suspect females and males have different temperature preferences, which is supported by our observations of males at deeper depths. La Jolla has warm water that may be too warm for males. Plus, these females are already pregnant by the time they start aggregating in La Jolla for the summer, so there is no reason for the males to be there (from a mating standpoint).
Q: What months are the sharks usually in town, and has this changed recently with our warmer summer waters?
A: The leopard sharks usually start showing up in large numbers at the end of June and persist through November, sometimes even December. The peak months are August and September. That said, you can find leopard sharks sporadically even during the winter, but never in the numbers we see in summer.
This year and last year, we experienced long stretches during the summer when we saw very few sharks. This is likely because the water temperature all along this region was very warm (mid to upper 70s °F). When the water is warm everywhere, the spot in La Jolla is likely not so special, so the sharks are simply spread out along the coast. When temperatures are more seasonable (upper 60s and low 70s °F), the La Jolla spot is once again special because it is the warmest along the coast, so the sharks concentrate there.
Q: Do you think La Jolla will lose its shark aggregation as the climate continues to warm?
A: As ocean temperatures continue to warm, we would expect to see fewer and fewer leopard sharks in La Jolla because the warm temperatures they seem to prefer will be available all over. There will be less reason for them to concentrate in La Jolla.
Q: Is there a prime time of day to see the sharks?
A: The best times are during low tides in the afternoon, especially during a Santa Ana condition (where we have offshore winds that flatten out the surf), with average water temperatures (upper 60s to low 70s). That said, I have seen plenty of leopard sharks in many different conditions at all times of day.
Q: Just how “harmless” are leopard sharks to people?
A: Unless you are a crab or squid, leopard sharks are quite harmless. Nevertheless, they certainly have teeth (albeit small ones) and could really hurt you if they bit you. They are quite skittish however, and the only way one would likely bite you is in self-defense (for example if you grab its tail). Leopard sharks have predators, and if they think you are one, they could try to defend themselves.
You should never touch the leopard sharks, although it is often tempting. Not only is it disrespectful and potentially dangerous, it would also be illegal if you accidentally injured the animal because these sharks are inside a marine reserve.
Q: What are the sharks' natural predators, in this area or elsewhere?
A: Adult leopard sharks are preyed on by larger sharks, such as white sharks and sevengill sharks, as well as bull (male) California sea lions. These predators are found throughout the leopard sharks' range, including in La Jolla. Sevengill sharks have been observed charging leopard sharks against the beach to catch them. The male sea lions do the same thing and actually eviscerate the shark, eating only the internal organs (mostly consisting of fatty liver and yolky embryos) and leaving the rest of the meat.
Q: We have heard that female sharks can mate with multiple partners then give birth to a litter of pups with different fathers. Is this true? If so, how and why does this happen?
A: All sharks studied to date are capable of storing sperm, even for several years. When the female mates, the sperm travel up through the uterus and are stored in the oviducal gland (a.k.a. shell gland). Later, when she ovulates, the eggs have to pass the opposite way, through the oviducal gland and into the uterus. When the eggs pass through the oviducal gland, they are fertilized by whatever viable sperm has been stored there. If multiple males' sperm was stored in a mother, then her litter of pups can be fathered by different males.
Although there has been speculation that there is a benefit to having pups fathered by multiple males, none of the existing hypotheses are convincing. The most likely explanation is that it happens simply because the females have little choice about which (or even how many) males they mate with.
Interestingly, some shark species exhibit high rates of multiple paternity (the frequency of litters in a population that have multiple fathers), while other species are more monogamous. On the high end, some species have almost all litters (>90%) fathered by multiple males, while on the low end it can be <10% of litters. For leopard sharks, I found that ~36% of litters in La Jolla have multiple fathers, which falls to the low end of the spectrum. This may be because leopard sharks are not believed to form mating aggregations (as other species do). Instead, the female may be able to flee after mating once.
Q: Do hatched leopard sharks cannibalize each other prior to exiting the mother? We have heard other sharks do this.
A: This is termed "intrauterine cannibalism," and while some sharks do this (e.g., sand tiger sharks), leopard sharks do not.
Q: How do you catch a leopard shark and transport it to the experimental aquarium?
A: I catch leopard sharks using handlines and baited barbless circle hooks. Once I bring the shark close to the boat, I use a large dip net (a 3-foot diameter net on a pole) to scoop it up, take the hook out, and then transfer it into a 3-foot wide holding tank on the boat for transport.
Q: What happens to the animals that are brought in to the experimental aquarium for study?
A: Because the experimental aquarium at Scripps Institution of Oceanography is an open flow-through system (not a closed system with recirculation and filtration), it is possible to get permission to release captive animals. With permission from the California Department of Fish and Wildlife (CDFW), we have released adults as well as pups born in captivity. This is only done after a thorough external exam to be sure there is no evidence of infection. In a closed system like the Birch Aquarium, it is often not possible to release animals to the wild. If animals are born in closed-system captivity, they are kept and cared for. Often times they can be donated to other aquarium facilities.
Q: How did you become a shark researcher, and how long have you been studying sharks?
A: I have been studying sharks for 10 years. I have always liked sharks, but I became more interested in their biology and ecology while studying abroad at the University of Queensland in Australia during my college years. I got to take marine biology classes at field stations and do field work on the Great Barrier Reef, where I saw many sharks and thought they were so beautiful and interesting.
Part of my becoming a shark researcher was luck. My grades in high school and college got me off to a good start, as did my research experience as an undergraduate. (That was key to getting into graduate school.) It also helped that I earned a fellowship (luckily, I got two from the National Science Foundation) to pay for graduate school.
Q: Being a shark researcher must be very cool. What are some unexpected or especially demanding aspects of the job that people might be surprised to hear?
A: People might be surprised that most of my time is not spent playing with sharks and SCUBA diving. In fact, I do very little SCUBA diving for my research because most of the field work is done at the ocean surface, on a boat fishing for the sharks. Fishing probably sounds fun, and it is when the weather is nice and you are catching things, but often neither of those is the case. Doing field work in the ocean is very challenging because there are many things you cannot control (weather is a big one), and setbacks are fairly common. Also, I almost exclusively do field work during the summer months. The rest of the year, I spend my days on my laptop, analyzing data, running statistics, writing papers, and applying for grants.
Perhaps this means that the job of a marine biologist is not as glamorous as people might think. It could also be surprising that not all biologists are paid equally; for example, an academic marine biologist earns a lot less than a biomedical researcher working for a company.
So why do I love my job? Because I get to work with amazing animals and come up with research questions that I find interesting. I then get to brainstorm and figure out a way to answer that question, raise the money to answer that question, and then actually do the experiment. I get to take ownership of a project. The best part is when the data first come in and I am the only human on Earth to know the answer to that question. It's a very special and rewarding moment. Then I get to write the results up as a paper and share them with the world!
Q: Sharks are perceived as being pretty scary animals. What do you say to people who are scared of shark-infested waters?
A: First, there is no such thing as "shark-infested waters," because you cannot infest your own home. The ocean is the sharks’ home. So if anything, the water is infested with humans. When we enter the ocean, we are sharing that space with the sharks and always have to respect that arrangement.
Second, shark attacks are not a common source of danger. Worldwide, only about five people die each year from a shark bite, compared to the many thousands of people that die in car wrecks and from diseases.
Finally, much of our fear of sharks is taught to us. Think about the last time you saw a nature documentary that featured sharks. It probably had scary music in the background. Filmmakers are using music to set a tense tone and tell us how to feel about sharks (compare this to beautiful, uplifting music that usually accompanies footage of dolphins, whales, and turtles). While part of our fear of sharks is natural, it is exaggerated by sensationalistic news coverage of shark attacks and the negative tones set in shark documentaries. I like to point this out because when we can trace the source of our fear, we have more control over it. Sharks are wonderful and beautiful animals that deserve respect (which maybe means a little fear), but not hysteria. [Editor’s note: for more info on this, check out Andy’s latest study, which was recently covered by Forbes.]
Top photo courtesy of Andy Nosal. Bottom two photos by Lauren Shipp.