Not Just on Vacation: Why Leopard Sharks Hang Out in La Jolla

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00:00:13.00 Hi, I'm Andy Nosal and I'm a postdoctoral researcher at Scripps Institution of Oceanology and the Birch aquarium, which are both located in La Jolla, California.
00:00:20.14 I'm really excited to be able to tell you about this amazing phenonmenon that occurs just a mile south of our two institutions.
00:00:30.00 And that's here at the southern end of La Jolla shore's beach
00:00:32.00 Where every year, hundreds, if not, thousands of leopard sharks come together
00:00:35.00 in only waist-deep water
00:00:37.00 and those rather omnious lurking shadows that you see lurking underneath the water
00:00:40.00 that's them, those are the leopard sharks
00:00:42.00 and I know what you are thinking, right, if there were ever time the cliche shark infested water would apply, it'd be here
00:00:48.00 but you'd be wrong
00:00:52.00 these waters are no more infested with sharps than they are infested with dolphins, or seals, or humans, for that matter
00:00:56.00 at least the sharks live here, this is their home
00:00:59.00 and you can't infest your own home
00:01:01.00 but even so we might call these waters "teeming" with sharks
00:01:05.00 and they have been for as long as anyone can remember, but we never knew why they were teeming with sharks
00:01:11.00 and understanding that question, why the sharks come to this spot
00:01:15.00 is what I set out to do for my dissertation
00:01:19.00 but as it turns out, answering that question "why" is not so easy, and let me illustrate why.
00:01:22.00 imaging that you are walking down a city street
00:01:27.00 and up ahead of you, you see a large group of people gathered
00:01:31.00 natural curiousity, you wonder why are those people gathered
00:01:33.00 so if you care to know, you try to investigate, you go up and try to see who these people are
00:01:37.00 what they are doing, maybe they are all facing toward the middle of the group, in which case, they could be watching a street performermaybe they are marching and holding up signs, in which case they could be protesting
00:01:47.00 so you could look for these outward clues, or you could just ask them unfortunately with these leopard sharks, you can't just ask them
00:01:57.00 so you can only rely on these outward clues to uncover whyDiscovering these clues is what I set out to do
00:02:05.00 what did i do first? I had to get in the water with these sharks. I had to see them for myself.
00:02:11.00 So this is the actual arial video of the sharks, tethered by a helium balloon. You see some tattered borders here
00:02:18.00 and that me right there, swiming with a bunch of what appears to be tadpoles, that are actually lthe eopard sharks
00:02:25.00 that are all about the same size as me. Now I was so brave just doing back strokes with the leopard sharks
00:02:32.00 I can actually remember the first I got in the water with these animals. And I was really apprehensive. The water was not so clear, as you can see it was actually very murky
00:02:40.00 and I remember the first time a shadow materialized and it was a shark. I was startled, but so that fear just melts away into relaxation. your ears are full of water all you hear is your breath resonating in the snorkel. And then you start to just watch.
00:03:01.00 the sharks slowly swimming by you. And then you see the sharks all have a unique spotting pattern as unique as fingerprints
00:03:07.00 they are beautiful, beautiful animals
00:03:10.22 but as you watch the sharks more, as you watch them swimming, looking for these outward clues
00:03:15.00 you don't see any, they are not doing anything, they are not feeding, they are not giving birth, they are not doing what you might expect a bunch of animals coming together to do
00:03:26.00 so the point here is that just direct observation is not going to be enough, and this was frustrating. so it sent us back to the original question
00:03:35.00 Why are these sharks here?but even more importantly, why does it matter? It matters because when groups of animals come together, especially to an area like this where it is so accessible to humans
00:03:46.00 look there is a beach, there is human on the beach. when animals do this, they are extremely vulnerable to being exploited by humans. Imaging how easy
00:03:56.00 it would be to just wipe all these sharks out with a big net. You could kill them in one afternoon.
00:04:01.00 imaging how vulnerable all these sharks are being in this one spot if there is habitat destruction.
00:04:08.00 It is like putting all your eggs in one basket.
00:04:09.00 and so knowning where, when and why animals come together at a particular site, it gives us the means to
00:04:17.00 the most effective means to protect these animalsso these animals are really great, they are very special. They are only found in the western coast of North America.
00:04:30.00 they are found no where else in the world. Even though their range is only from Samish Bay, Washington to Mazatlan, Mexico
00:04:36.00 Really the core of their range is in the state of California
00:04:40.00 And it is throughout the state that they form these hangouts
00:04:44.00 these aggregation. the La Jolla hangout is not the only one.
00:04:49.00 But what is interesting about all the other hangouts that have been previously described
00:04:53.00 they have been limited to sheltered bays, coves, and estuaries, which offer food and shelter
00:04:59.00 what is so neat about the La Jolla aggregation, or the La Jolla hangout
00:05:03.00 is that it's not in a sheltered bay, cove, or estuary
00:05:06.00 it is along a sandy beach, on an open coast, where there is surf
00:05:10.00 So here you can actually see the hangout, it's right in shore of that submarine canyon
00:05:15.00 and as I've mentioned before, we've known about these leopard sharks for as long as anybody could remember
00:05:20.00 but they were only officially documented in 1950 by Scripp's professor Carl Hubbs
00:05:26.00 He was a professor at the same institution where I got my PhD.
00:05:29.00 which is why this is so special to me.
00:05:31.00 and even more special is that I was able to find his original hand-written collection notes from that day, dated August 3, 1950
00:05:39.00 where he documented that catching three large adult triakis semifasciata, that is the scientific name for leopard shark.
00:05:45.00 so he caught them in a beach scene at that very sight
00:05:48.00 but even though Carl Hubbs knew about these sharks
00:05:51.00 since 1950, we still didn't know why they are here
00:05:55.00 so again, that's what I set out to do for my dissertation work
00:05:58.00 so get at that question, why are these sharks here, I first needed to know who are these sharks, demographically speaking.
00:06:06.00 so after catching hundreds of these sharks and releasing them after we took their measurements,
00:06:12.00 determined what sex they were, we found that all of these sharks were mature adults
00:06:17.00 their average length was 1.4 meters, the smallest shark we found was 1.1 meters.
00:06:22.00 and these mature at 1.0. So that's how we know that all of these sharks were sexually mature.
00:06:29.00 but even more interesting, 97% of these sharks were female
00:06:34.00 which amounts to of course, only 3 out of 100 of these sharks to being male
00:06:39.00 so right away we can exclude mating as the probable cause of this aggregation. If the primary cause of this aggregating were mating,
00:06:47.00 we would expect to see a more balanced sex ratio
00:06:49.00 but we don't see as many males as we expect otherwise
00:06:52.00 but not only are these sharks most female, they are also pregnant female
00:06:57.00 and we know they are pregnant females is because we actually conducted ultrasound on them
00:07:00.00 so here you see a snout and a mouth of a pup inside a shark, inside of a mother shark
00:07:07.00 here we have the head moving inside of another
00:07:09.00 each of these female sharks give birth to litters that average 20 pups each
00:07:15.00 they are really really cute. they are only about 8 inches long, they are about the size of a Snickers.
00:07:19.00 and they are born live with all of the skills they need to hunt. there is no parental care in sharks
00:07:25.00 So all of these sharks are pregnant females
00:07:28.00 so we can now amend our original research question of why are these sharks here, to why are these pregnant female sharks here?
00:07:37.00 So now we know who these sharks are. We need to look for more outward clues to why they are actually here.
00:07:45.00 so we need to know what they are doing. We need to know what their movement patterns are like. So we need to track and follow these sharks.
00:07:52.00 How are we doing to follow these leopard sharks for 48 hours. That's about how long we need to get a feel about what these sharks are doing.
00:07:59.00 We are doing to have to put something on the sharks, that's going to allow us to follow them
00:08:04.00 and maybe you've heard of putting a GPS tag on a shark
00:08:07.00 unfortunately that doesn't work for a leopard shark
00:08:10.00 because they never come to the surface, and the GPS signal cannot travel through the water to get to the tracking satellite
00:08:15.00 so unlike for a Great White, which comes to the surface, GPS is not going to work for us
00:08:19.00 But sound, acoustics, that's what's going to work because sounds travels very well in water
00:08:26.00 and we can follow to sound
00:08:27.00 so what we did here is we caught 8 leopard sharks
00:08:31.00 in the same way we normally do using a hand line and a dip net.
00:08:35.00 once we have the shark seriously. we are going to attach an acoustic transmitter tag
00:08:41.00 about the size of a AA battery or the size of your index finger
00:08:44.00 this transmitter, once it is underwater, emits a pinging sound
00:08:48.00 at a certain frequently we can tune into, and listen, and follow
00:08:52.00 also on these transmitters, there is a depth and a temperature sensor
00:08:56.00 incoded in those "pings" is information about how deep the shark is as well as the temperature of the water surrounding the shark is
00:09:05.00 we can get a lot of fine scale information from this
00:09:08.00 we release the shark, once it's swimming, we follow that sound
00:09:12.00 we follow that sound using a special device called a directional hydrophone, that's this device right here
00:09:21.00 it's like an underwater microphone, but it's only listening in one directionand so it's mounted at the bottom of the rotatable staff, we can rotate the staff and therefore rotate the hydrophoneto zero in on where the strongest signal is coming from and know that's the directional of the shark, and procedure to follow the shark for 48 hours
00:09:39.00 without interruption. We are on that boat day and night. It's a small boat, only 17 feet long.
00:09:45.00 there is no cover, no shelter, but there is always two people. We take turns, tracking 3 hours
00:09:51.00 sleeping 3 hours. It's hard work, but it's the only way we can get the fine scale data to what these sharks are doing. Let me tell you what these sharks were doing.
00:10:01.00 I'm first showing you what these sharks did vertically with the vertical column
00:10:05.00 What we have here is a very simple graph showing on the y-axis, depth below the surface, depth below of the water in meters
00:10:14.00 what you will notice is that the sharks spent the vast majority of their time in very shallow water. In fact, 70% of their time was spent in the top two meters of the water column.
00:10:22.00 but not only were they really shallow. For example, they were shallow outhere in the middle of the water They were always really close to shore
00:10:30.00 they were often at and sometimes inside the surf zone.
00:10:34.00 Now the question was, why were these pregnant females so shallow, and so close to shore.
00:10:43.00 So our first thought was maybe it was a way to avoid meeting predators
00:10:46.00 You can imaging if you are hugging the coastline, you have now eliminated one direction from you can be attacked, that would be from the shore.
00:10:52.00 So that would make sense. the only problem with that is the major predator for these sharks is the male California sea lion
00:11:02.00 and that is unfortunately for the sharks, very effective at catching the sharks, even when they are hanging out at the shallow waters up against the surf.
00:11:11.00 here you can see a picture of a male sea lion catching a leopard shark
00:11:18.00 You think sea lions are cute and cuddly. Not when they are hunting. they actually grab these leopard sharking just underneath the gills and thrust around them with such force
00:11:26.00 it actually eviscerates them. All their internal organs come out, and the sea lions eat only these organs. They leave the rest of the shark, which awashes to shore.
00:11:34.00 So pretty grissly, but that's life being a leopard shark. So getting back to the question, why they are shallow and so close to shore?
00:11:43.00 It doesn't seem like that strategy really helps them avoid predators. So there must be another reason for this conspicuous behavior.
00:11:48.00 the other thing we thought of, maybe these pregnant females are selectively occupying the warmest available water.
00:11:56.00 So in the absence of any point source of warm water, to which these sharks might be attracted to.
00:12:02.00 for example, a geothermal vent, or a river outflow, or power plant's effluent. Nothing of which are present at this site. So the warmest water is going to be at the surface.
00:12:12.00 due to solar insulation. that is the sun's rays beating down on the surface and warming the water from the top down. But not only is the warmest water near the surface,
00:12:22.00 but it's going to be close to shore, because of the prevailing on shore going breeze that's prevalent in California
00:12:26.00 that is just blowing the warm surface water shore-ward and piling it up in the surf zone
00:12:32.00 this is exactly where the sharks are hanging out. So it does seem they are occupying the warmest available water. And all this points to
00:12:41.00 the fact that these pregnant females are likely incubating their developing embryos. They are shortening the gestation period. The gestation period
00:12:50.00 is already 10-11 months, and you can imaging why these sharks might want to make that as short as possible.
00:12:57.00 so it's almost like because these animals are ecto-thermal, which means their body temperature is the same as the surrounding water temperature.
00:13:04.00 the warmer the water is, the warmer their body temperature, and the faster things like cell divisions and growth happen. So the pups are growing faster when the mom is in warm water.
00:13:13.00 so it's like a mother bird sitting them to keep them warm, and maintaining an optimal temperature for development.
00:13:21.00 But if that is the case, if these sharks are just near the surface and close to shore to get to warm water, why that particular spot in La Jolla? Couldn't they do that anywhere?
00:13:32.00 And the answer is no, it has to be at this spot. And let me show you why. So I've shown you the vertical movements.
00:13:38.00 Now we are going to look at the horizontal movements, so if you remember the aggravation spot is here, and we have the submarine canyon right off-shore.
00:13:48.00 this is where our actively tracked sharks, the ones we actively follow, spend the most amount of time
00:13:55.00 this is what we call the home range, where they spend 95% of their time. And this is the core area where they spend 50% of their time.
00:14:03.00 Sounds like a smaller number, but look at how much tinier that space is, so their movement pattern is much more concentrated here. So now we have a better idea of what the sharks are doing horizontally.
00:14:13.00 Now let's see why this site is so special. I'm going to superimpose a map of wave height over the home range and the core area. If you look at the graph right here.
00:14:27.00 Warm colors like reds and oranges corresponds to high waves, the blues and greens, those are small waves
00:14:36.00 look at where the home range is. it's nestled in an area of low wave energy, small waves. We think that's important for a number of reasons.
00:14:47.00 There is small waves right here is because of the shape of the canyon. As this swell propagates and it passes the canyon towards shore,
00:14:55.00 the wave energy actually gets diverted away from the canyon head where the sharks are, producing the small wave here and large wave here which is why if you are walking down the beach,
00:15:05.00 you will see surfers here but not down there where the sharks are, and it's not because of the sharks there, it's because there are no waves.
00:15:11.00 So having small waves seems important for two reasons. One, if the waves are very gentle, it's a lot easier for these pregnant female to access the warm shallows. Another words, it's easier to get warm and shallow if there are small waves than if you are being pummeled by big waves.
00:15:30.00 So if you have a choice, why would you be here if you could be down here. Another reason is there is less wave action, which means that, there is less wave mixing with the deeper colder water.
00:15:43.00 so on average, this area where the sharks, it's about a degree or two warmer than the surrounding water. It may not sound like a big difference. But if you have a choice, be in the warmer water.
00:15:54.00 So all of this is pointing to the idea these sharks are incubating their developing embryos. But as great as this sites is. These sharks are only here during the day. At night, they leave the hangout place. I'm going to show you what one shark did just as an example
00:16:12.00 So take a look at the map, this is what the shark did during the day. So you have got this yellow lines following the sharks' movement. So the sun goes down, now look at what these shark's night time movements are.
00:16:25.00 Going into the campon, off shore. But by sunrise, it's back at the hangout spot, and it's resume its normal day time behavior
00:16:35.00 So the sharks leave, they go into the canyon at night. So we wonder, why leave the aggregation spot and go into the canyon? We figure they were most likely feeding.
00:16:49.00 In the canyon, the best food source would be marked squid, very small, this long 8 or so inches. At night these squids spawn in the shallow reaches of the canyon.
00:16:59.00 But then during the day they retrieve back into very deep water, where they wouldn't be very excessible to the leopard sharks.
00:17:07.00 so to test whether these sharks are feeding on the squid, we had to do a simple diet study. In the past, when you did a diet study,
00:17:17.00 you had to kill the animal, and cut open the stomach, and see what was it the stomach. But we didn't want to do that with these leopard sharks because people liked these leopard sharks.
00:17:26.00 So we wanted to use a non-lethal method called gastric lavage. and this is what it looks like, we have insert a PDC tube into the stomach
00:17:35.00 flush the stomach with sea water, invert the shark, and allow the gravity to do the rest. Gravity pulls out everything that's in the shark's stomach
00:17:44.00 and we catch it with a spagetti strainer, and look what we found - mostly squid, but there were some other things too. they have got pretty good taste, there is other fish, crabs, lobster, octopus
00:17:54.00 but squid were the most important. So now we had direct evident that these pregnant sharks are leaving the aggregating sites at night, going deep into the canyon to feed on squid
00:18:06.00 So another good reason to hang out at this spot. Just to recap so far, who are these sharks - mostly mature pregnant females. Why are they here? They seem to be attracted to the exceptionally calm and relative warm water of that spot.
00:18:23.00 in La Jolla, which is due to the off-shore submarine canyon. Just icing on the cake, there is got to be food somewhere, and they don't have to go too far to get it.
00:18:32.00 at night when they are hungry, they go down to the canyon, feed, and they are come back up in the morning. So the pieces of the puzzle were starting to coming together, but there were still some outstanding questions.
00:18:43.00 Namely, when exactly are these sharks here during the year. Are they here year round or are they here only a portion of the time. Do the same sharks return, or are all these sharks just transient individuals?
00:18:56.00 Lastly, where do the sharks go when they are not at the hangout spot in La Jolla. So to answer these questions, we now had to amend our question - instead of how to follow a shark for 48 hours, how do you follow a leopard shark for more than 3 years?
00:19:14.00 Now I wasn't going to be following sharks around in a boat for 3 years. So we needed to think of a better way to do that. Instead of actively following a shark,
00:19:23.00 we are now going to be passively listening with the pinging tracts, called automatic acoustic tracking
00:19:32.00 A differs in a few ways, let me show you how that works, we caught sharks as we did before, but now we needed more. We caught 19 sharks for this purpose
00:19:42.00 we bring them onto the boat, and we put them onto their backs, this induces a phenonmenon called tonic immobility
00:19:48.00 it's almost a state of hypnosis, where a sharks almost goes to sleep, and it's what we use for anesthesia.
00:19:53.00 now we need anesthesia because we have to surgically implant the acoustic pinger track into the shark's body. You can't put the pinger tag on a shark, on the outside and expect it to stay on for more than 3 years. It's going to get caught on rocks, tangled in kelp
00:20:12.00 so much stuff is going to be growing on it. So the only way to be sure the shark stayed tagged and tractable, it is to surgically implant it. But you do see that it only is a very small incision, about one inch long
00:20:24.00 we actually coat the tag the pinger tact in beeswax. which is a natural product, which means there is less of a chance that the shark's immune system is going to recognize it as a foreign plastic body and try to attack it.
00:20:37.00 Then we suture up the incision and off the shark it goes. the whole process takes only 5-7 minutes. And we have 100% survival.
00:20:48.00 Now once the shark gets released as I said, we are not going to actively following the shark, we are passively listening to it
00:20:56.00 so instead of a directional hydrophone, now underwater omnidirectional hydrophone that's listening in 360 degrees. That's what this black device is right here.
00:21:08.00 It's about one foot long and they are all mounted onto these underwater moorings right here
00:21:12.00 So each of these has a detection radius of 300 meters, or about a thousand feet. So everytime a surgically implanted shark swims within that distance of the acoustic receiver, the receiver records the time, the date, and the code for that particular shark. So over the course of
00:21:29.00 of 3 or more years, we know exactly which sharks were present where and when. So every couple of months, we have to scuba dive onto these receivers,
00:21:39.00 and download the data with a special bluetooth upload cable.
00:21:43.00 Now we had 7 of these receivers, up and down the coast of San Diego County. Of course we had one down in La Jolla, and let me zoom in to show how that one was positioned
00:21:53.00 it was positioned so that its detection range, that 1000 food radius circle it encompassed around that entire home range
00:22:03.00 if the sharks were hanging out in La Jolla, we had a really good chance at detecting them at that receiver and therefore knowing they were there
00:22:09.00 But we also had receivers spanning from the U.S.-mexico border all the way up to southern Orange County
00:22:16.00 This was to see if the sharks really preferred La Jolla, and how often they were detected at other places along the coastline
00:22:24.00 Now over the course of the more than 3 yr study period. From July 2009 to September 2012. We had over three quarters of a million detections at these receivers.
00:22:35.00 Lot of detections. We put Excel to its limits with these. What I want to show you now are the main trends we were able to extract from all these raw data
00:22:46.00 So the first trend, first conclusion, is that these female pregnant exhibited strong daily fidelty to the La Jolla hang out spot; another words, they were detected more often at the La Jolla than any other site
00:22:59.00 here's what we are looking at, we are looking West here, here's where we have all of our receivers, with La Jolla here.
00:23:04.00 On average they were detected 288 of their days at Liberty, which was more than 10x more than the next receiver, which is here at Dike Rock, which was only 10 miles away.
00:23:19.00 So very strong preference for this particular site, which shows us how important this site is to the sharks. Not only did they show strong daily fidelity to this site,
00:23:28.00 but strong annual fidelity, which means it's the same sharks coming back year after year
00:23:34.00 So let me show you what this looks like. We have the four years of our study from 2009 to 2012.
00:23:42.00 We tagged 12 females into 2009, another 7 in 2010, and here is how many sharks returned to the La Jolla hang out spot after the first year.
00:23:53.00 14 out of 19 total, so about 74% returned after year 1. Now we are going to go to year 2. We are going to lose a few more sharks, maybe they got eaten or caught, maybe they went to some other place in California.
00:24:07.00 But still we've got 10/19, so over 50% came back consequtively in 2 years. What's really amazing is that in even 3 years, now we only focus on 2009 where we had 12 sharks, we had 6/12 of these sharks, so 50%. So 50% of these sharks were returning year after year after year.
00:24:30.00 And these numbers are likely underestimated, we know that at least 2 of these sharks were captured and killed by fishers, and consequently did not have the chance to return.
00:24:41.00 and it wouldn't be surprising if more sharks were being captured and simply not reported to us which has happened before.
00:24:48.00 so these numbers are high, they are coming back day after day and year after year. Now I want to talk about these 6 six females, the ones who returned after 3 years of our study.
00:25:00.00 Now I'm going to be showing you the full detection records for each of these sharks. And I want to show you what just one of these looks. This for just one sharks, each one spans 24 hours from left to right
00:25:14.00 Over three years, the length of the study period from top to bottom, let me just zoom in to show what this looks like.
00:25:19.00 So again, spanning 24 hours from midnight to midnight, and each line going down is a different day. Starting from 2009 with tagging to the end of the study period in 2012.
00:25:35.00 Every little black pixel you see there is a detection made at that receiver in La Jolla. So everywhere you see black, that means the shark was detected at the La Jolla hang out spot.
00:25:46.00 And right away, just to illustrate how to look at these graphs. There is a clear difference, the sharks are more present between 6am and 6pm, which is consistent with what we know that the sharks are leaving the hangout spot at night. But even so
00:26:01.00 Every though you see the daily patterns, you can also see seasonal patterns. Now I'm going to be putting up the 5 sharks that we're going to be focusing on.
00:26:10.00 All let all those come in. So if you look from top to down, which is what we are going to be doing for the seasoning time. You can see that there are times when the are present, and they are absent, when they are present, and then they are absent again.
00:26:23.00 There is an amazing synchrony within their arrive and leave, sometimes with days or hours of each other.
00:26:29.00 And we can distill these data down into the following graph. What we have on the y-axis is the average number of unique females detected in La Jolla per day. And these numbers are average.
00:26:43.00 In 24 month month bands, from January, all the way to January there is a clear seasonal pattern where we have this high season from June to December.
00:26:55.00 This is really interesting because this occurs after the known pupping season for these sharks. So leopard sharks give birth somewhere between April and May.
00:27:04.00 If you remember, they have a gestation period of 10-11 months, which is longer than a human, which is of course 9 months.
00:27:11.00 What I didn't say before is that these sharks give birth every single year; they have an annual reproductive cycle.
00:27:27.00 So if we do the math, they give birth if April and May, but in order to give birth the following April and May, after the 10-11 month gestation periodthere is a very narrow window after they give birth, only a month or two, when they can recover, and then mate
00:27:34.00 So by the time they arrive at the La Jolla hang out spot in June, they have already made it, the eggs are fertilizes and they are already in the early stages of pregnancy
00:27:44.00 And what was really amazing is that the seasonal variety of female abundant in La Jolla was strongly coorelated with sea surface temperature
00:27:53.00 So whenever the sea surfers temperature arose, the number of females that arrive to exploit those warm water also rose.
00:28:00.00 In fact, this variable only explained 86% of variation in female abundance. Just one variable only. So you can see this is all pointing to the same conclusion
00:28:13.00 these pregnant females are attractive to the warm calm water here to incubate those embryos. So I want to wrap up my talk here talking about what we didn't quite figure out.
00:28:25.00 And that is where the sharks go when they are not in La Jolla. Again they are mostly there from June to December, so where do they go during Winter and Spring?
00:28:34.00 So here is our study area, all those yellow dots were our receivers. So in La Jolla, what we found was that about half of our tagged females, they went north along the coast, we actually detected them sequentially along each of these receivers.
00:28:48.00 Even when they went past the San Clementi reciever, we still had a chance of detecting them. You see these red dots up here? These are other receivers that are operated by a different lab, the lab at Cal State Long Beach.
00:29:01.00 They are the same receivers, we can share data and collaborate. So even though these sharks went North, the receivers at Los Angelos never detected any leopard sharks.
00:29:12.00 which means one of two things, either these sharks were over-wintering somewhere south of Los Angelos
00:29:18.00 or their transitioning off shore outside the range of those little red receivers outside Los Angelos and either going North, which we have no evidence of, or perhaps going to Santa Catalina Island
00:29:29.00 you see there is little red dot there, there was one receiver there that detected 2 of our leopard sharks the first winter, and they returned to La Jolla the following year. Unfortunately as this was not our receiver
00:29:41.00 we didn't have any control when there was out. It was only there for the first year of our study. So we don't know if in subsequent years, leopard sharks went again to Catalina.
00:29:42.00 That's something that remains to be seen. But only half of the sharks did this. The other half simply just disappeared from La Jolla. They were not detected anywhere else by anybody's receiver. And it's still a total mystery where those sharks went
00:30:03.00 What was really neat is that those sharks just disappeared, they come back the following year, those same ones disappear the following year.
00:30:10.00 The ones that went North and came back, they went North the following year. So it seems these two different behaviors were all meeting in La Jolla every summer and fall
00:30:19.00 So we are finally going to answer this question this year. We are going to be attaching very simple pop up satellite tags to these sharks, and program the tags to disconnect from the shark and float to the surface, once they are at the surface, the signal can be transmitted to a GPS satellite
00:30:39.00 and then I get an email, open it up, and I see exactly where these sharks are going in the winter. So finally, we are going to figure this out, and I can't wait to tag these sharks and then figure out where they go.
00:30:49.00 So to come full circle, why are these sharks here. These are pregnant female sharks, they appear to be attracted to this hangout spot in La Jolla because of the exceptionally calm, warm water.
00:31:04.00 Likely to incubate their developing embryos, again like a mother bird sitting on her eggs to keep them warm
00:31:10.00 Again, just icing on the cake, there is a lot of food near by, they don't have to go far to get the food that they need.
00:31:16.00 Why does it matter? It matters because we have shown how absolutely essential this spot is to the leopard sharks, how irreplaceable this spot is to the leopark sharks, they spend so much time here
00:31:33.00 just to put that into perspective. let me show you the following map, so here is our study area, there is La Jolla there, those yellow circles are the detection areas to scale. If we zoom out to our study area
00:31:47.00 those yellow dots they are still to scale, but look at how tiny they look, these seemingly insignificant pin points
00:31:55.00 along an entire coastline where they could go. where the sharks could go. If we zoom out to our regional level
00:32:03.00 to southern california, you can't even see those yellow dots anymore. Again just to make the point,
00:32:08.00 if you remember these sharks are found all the way from Washington state to Mexico, they could go anywhere along the coastline, and yet they come back to this tiny little spot every single year. So we are so lucky, well,
00:32:23.28 the sharks are so lucky that that spot is protected by a marine reserve, where you are not allow to catch them. Now I say these sharks are lucky
00:32:35.00 because that reserve when it was established a few decades ago was not to protect leopard sharks, it was to protect abalone, the leopard sharks just caught a lucky break
00:32:44.00 what about all the other hangout spots in California and Mexico for that matter where they are not protected.
00:32:52.00 Again these are the leopard sharks in La Jolla. Can you imaging if these sharks were not protected. How easy it would be to catch these sharks.
00:33:01.00 I got to say you know something has change from the first time I got in the water, I saw these are sharks, now I see the video, I get in the water, and I see them as pregnant mom, as pregnant female sharks.
00:33:16.00 So now think about wiping all of those pregnant females out, you are not just killing the adults, you are killing the next generation
00:33:22.00 and maybe multiple generations, becuase those pups are going to grow up, and they are going to have their own pups.
00:33:28.00 So you can wipe out these animals in a very short period of time. So it's absolutely that we protect places like this, where animals like sharks come together. And especially for sharks, which are
00:33:39.00 absolutely essential for the environment, they are top predators, they maintain balance in the ecosystem. We have to protect these animals
00:33:47.00 especially given the fact that they have been here for so long. these is geological evidence where there fossilized leopard shark teeth in geological deposits that have dated about 120,000 years
00:33:59.00 before present, that's 10,000 leopard shark generations. Wouldn't it be a shame if after so long, we would wipe out these sharks. So we need to keep these around for future generations of humans to appreciate and enjoy as well
00:34:14.00 so that, I would like to thank all the people that have contributed to this work over the last 7 or so years. Most of these have been undergraduate volunteers who have given their time to me
00:34:27.00 spent many long hours on a boat, working with these animals, but also my colleagues and my advisors, over the years who have helped me.
00:34:38.00 And I want to thank you for your company and for tuning in, if you have any questions, feel free to email me at that address. Thank you very much.

This Talk

Speaker: Andrew Nosal

Audience:

General Public
Educators of H. School / Intro Undergrad
Student
Educators of Adv. Undergrad / Grad
Researcher
Educators

Recorded: May 2015

Talk Overview

Every year from June to December, hundreds to thousands of leopard sharks (Triakis semifasciata) congregate in this one particular spot along the southern California shoreline. Why are they here? Andrew Nosal answers this question and articulates why we should care. He carefully studied these sharks and their behavior over the course of several years. He discovered that most of the sharks are mature pregnant females. During the day, these pregnant females spend most of their time swimming in the warm, calm waters that are unique to the La Jolla area. At night, they forage for squid in a nearby marine canyon. Because sharks lack the ability to regulate their internal temperature (i.e., they are ectothermic), Nosal concludes the pregnant sharks are attracted to La Jolla’s warm waters to support the developing fetuses – much like hens sitting on eggs. He emphasizes the importance of protecting the leopard sharks, especially considering how they carry the next generation of sharks and could easily be wiped out by careless human activity.

This talk is part of the Young Scientist Seminars, a video series produced that features young scientists giving talks about their research and discoveries.

Speaker Bio

Andrew Nosal

Andrew P. Nosal is currently a postdoctoral researcher at Scripps Institution of Oceanography, where he conducts research on the behavior, ecology, and conservation of sharks and rays. Nosal is most interested in the causes and consequences of movement phenomena like aggregation (grouping) behavior, sexual segregation (spatial separation of males and females), and seasonal migration. He… Continue Reading