Follow the Food: Reading the Diary of Lemon Shark Movements and Behavior (9/1/2014)

 Accelerometry, Active Tracking, and Prey Abundance Surveys 

Photo of Rob and volunteers deploying a 109 cm lemon shark with 'tag package'

Lab PI Rob Bullock studies lemon shark behavior and habitat use here in Bimini. We attach tag packages (acoustic transmitter-bound accelerometers) on the dorsal fins of sharks to learn more about where they go and how they spend their days. The acoustic transmitters allow us to actively and passively track the sharks to determine locations, and the accelerometers record fine-scale movements in three dimensions (x, y, z), which we translate into behavior. A small sensor in this device deviates from the center with increasing amplitude as the shark accelerates, logging even the subtlest of movements. Presuming we recapture the tagged individuals after releasing them into the wild, we download this data so we can learn more about what these sharks do on a day and night basis.

 

Photo of two mojarra skulls in lemon shark stomach contents (left), and lemon shark trailed by mojarra (right), both taken by Chris Lang

Lemon sharks have eviscerable stomachs, which, as unusual as this may sound, we have used for our advantage to determine their preferred and predominant diet. Yes, we have inserted forceps into the mouths of anesthetized sharks to evert their stomachs and proceed with the analysis. A study back in 2010 by former Sharklabber, Stephen Newman, found out that despite the wide abundance of juvenile and adult grunts, mangrove snappers, barracuda, needlefish, and parrotfish that live within the local lemon shark refuge, more than 50% of a juvenile lemon’s diet is made up of the Yellowfin mojarra (Gerres cinereus) by weight and percentage index of relative importance. Given this undeniable preference, we felt it obligatory to assess the prey distributions for our little ones.

Still-shot of BRUVS recording near mangroves, note the abundance of snappers and lurking lemons in the background

To supplement his project, Rob has supervised two Master’s students to deploy BRUVS, apparatuses used to monitor the abundance of marine biodiversity, in the lagoon. Baited Remote Underwater Video Stations (BRUVS) are GoPro-mounted frames made of rebar that survey the proximate and passing species. Overall, more than 125 BRUVS were deployed. Rob and his students, Henrietta and George, placed these frames in a variety of sub habitats (sandy flats, sea grass beds, mangrove fringes) within, around, and beyond the known lemon shark nurseries. It took a lab-wide effort to watch every video and record the number of passing individuals for each species, which we used to extrapolate their population abundances in the lagoon. 

Slide photograph of North Bimini Marine Reserve's assessed faunal distribution taken from Rob Bullock's AES presentation

Here (above) is the summary of their findings. We divvied up the fish into their respective families, read top-down from left to right: mojarra, snapper, jack, needlefish, grunt, and parrotfish. The location of each circle represents a BRUVS deployment, and each diameter corresponds to the abundance determined from the video assessments (bigger circle-->more fish of this species found). It’s worth mentioning the apparent increase in diversity AND abundance near the mangrove-fringed land, marked by green. We hope developers on the islands look at this BRUVS study to understand exactly what is at stake when they decide to cut down these trees to build a house, or even a golf course...

Photo of 'tag package' comprised of acoustic transmitter (top) and accelerometer, taken by Chris Lang 

The focal point of Rob’s research, however, comes from tracking and accelerometry. Rob has to locate and net lemon sharks of a particular size range, and then tag them with the fused instruments (above). These function separately, but ultimately provide us with invaluable data, allowing us to record both the generic locations and fine scale movements of the sharks.

Photo of Matt Larsen (a Sharklab volunteer) actively tracking lemons outside the nursery entrance with hydrophone in hand

With receivers set up inside and around the lagoon, along with hydrophone-wielding trackers (above), we can both passively and actively track these sharks. Sonotronic Receivers are strategically deployed in these areas, and they pick up low frequency pulses emitted by the acoustic transmitter within a small range (~300 m). This is called passive tracking. These two methods complement one another to inform us on where they are going throughout the desired duration. 

Photo of Brianna Hall (Sharklab Volunteer) performing ethogram and observing accelerometer-tagged shark behavior from watchtower, taken by Chris Lang'

But what role do the accelerometers play in this? The aforementioned results are only based off data taken from acoustic transmitters. Accelerometers measure something very different. Interpreting the data from one of these is like translating a language written in seismic wave magnitudes into a shark’s behavior. Saying this is difficult is an understatement. 

Rob and our volunteers perform an ‘ethogram’ to observe a tagged shark in captivity, scribing its actions under various simulated circumstances (chasing prey, avoiding predation, resting, shaking head to ease consumption) with their respective times, and then interpreting these results using the behavior key he just created. 

Results of tagged lemon shark movements in relation to fish abundance in North Bimini Marine Reserve, taken from Rob Bullock's AES presentation

From Rob’s 12 tagged sharks so far, results suggest that juvenile lemons are indeed following the food, so to speak. As the water depth drops with every low tide, juvenile lemons are pushed outside the nursery and into lagoon areas that were once inhabited by larger sharks. This picture shows the logged locations from three of Rob's tagged sharks during low tide, whose paths are color-coded for viewer convenience. These findings overlap seamlessly with our BRUVS-determined abundances for juvenile lemon preferred prey species, the Yellowfin mojarra and grunts.

Example of z-axis data recording lemon shark tailbeats, the red box is a prey capture

This 12 second clip is 1/36000th of the data Rob analyzes using software for the deployment of just one shark's accelerometer. The z-axis measures the amplitude of each tailbeat from the shark. 

Every time a shark successfully captures prey, it proceeds to pump its caudal fin in speedy rhythm while shaking its head back and forth. This creates a unique output that can be seen through the high frequency, high amplitude waves as shown on the z-axis recordings within the red box above. The final set of tailbeats, shown here, are only applicable to sharks that have successfully seized prey. Across the first 12 tagged sharks, Rob has observed 84 total prey handling events, which averages out to be roughly ~1.4 successful prey captures per shark per day.

When the sharks aren’t foraging, they are resting, and he estimates this to make up about 10-20% of the lemon's daily time. To put in Rob’s words, “lemon sharks live on an energetic knife-edge,” requiring a sensitive caloric intake to sustain their metabolic requirements. 

Photo of lemon shark lining snapper and mojarra-filled mangrove roots, taken by CJ Crooks

Mangroves are essential nursery grounds for so many of the local species in Bimini, as evident from our BRUVS deployment results.  Many migratory fish we find in adulthood develop in this protected area, such as the barracuda, which can move to nearby coral reefs later on. Mangrove roots quell water turbulence and provide a settlement ground for free-swimming larvae of countless lifeforms to populate. Humans in Bimini sustain themselves primarily on sealife, and all of it is dependent on the health of the mangrove ecosystem. This interrelatedness must be addressed and thought through. 

The best tool for conservation is understanding. Observing juvenile lemon shark behavior and understanding that their foraging patterns mirror their prey distribution, we can make more informed decisions when determining areas to protect. Our lemons follow the food. But what happens when the habitat for their food source is removed? Because the status of the North Bimini Marine Reserve is still under dispute for development, we believe these results speak for themselves as to why the mangroves must stay rooted in.

Help us preserve the North Bimini Marine Reserve by signing this petition!  

In Honor of Motherhood: Birthing in Bimini (5/10/2014)

In the photo above, taken last April, our former lab manager, Jill Brooks, inserts an acoustic transmitter into the body cavity of an adult, female lemon shark (total length--231 cm). She then took a genetic sample from this female’s dorsal fin. 

One month ago, we downloaded our receiver data near the tip of South Bimini. This same female was acoustically detected by receivers on April 10th, almost a year later, and has lingered around these past weeks since, just in time for lemon birthing season...
Is this lemon pregnant? Last year she straddled the fence of sexual maturity, ultimately proving not to be.  After a year of further development, maybe this is her big debut. Soon we shall find out. And we don’t have to recapture her or observe her birthing to know for sure.

This is because, in June of this year, the lab will perform its 20th annual PIT project, a twelve night ordeal where we use gillnets to capture, then tag and measure almost the entire population of baby and juvenile lemons in the Bimini mangrove nurseries. Recaptured juveniles from past years already have electronic PIT microtags (also called RFID tags with unique ID codes) inserted, so we can measure their growth and weight gains.  But the newborns, called neonates, will all be captured for the first time, PIT-tagged, and DNA sampled.

The above photograph shows a fin-clipping, which will soon be shipped to the states for genetic analysis. It is placed in a DMSO-filled vial to prevent DNA denaturation.

With our increasing archive of genetic material, each year we expand the family tree of Bimini’s lemon sharks to piece together a pedigree of interrelatedness. This pioneering research project can open the door for understanding the behavior and ecology of sharks down the line, paving the way for possible studies in social preference for familiars and kin. None of this, of course, could be done without the magic of the mothers, bringing a new batch of neonates to Bimini with each year. Just wanted to acknowledge the marvel at this relevant time. Happy Mother's Day!

Hammering Home the Missing Link: Tagging & Tracking S. mokarran (4/10/2014)

This past winter, the Sharklab blazed yet another trail in the shark-tracking world. This time, by acoustically-tagging a Great Hammerhead sharks, Sphyrna mokarran. Acoustic telemetry has been used in ecology for years, yet this particular feat required a bit of ingenuity, and a fair amount of natural talent as well. 

Here lies the problem: hammerheads are sensitive. They require constant motion to flow water over their gills, a breathing process known as obligate ram ventilation. For this reason, capturing them with the typical hook-&-line method is like knocking on death’s front door, and not for ourselves. S. mokarran is an endangered and declining species, so to study these creatures with minimal impact, we would need a new solution. Our answer: pole-tagging.

With the help of The Waterman Project (www.thewatermen.org) and their founder, free-diving world-record holder William Winram (www.williamwinram.com/), we now have placed an acoustic tag beneath the dorsal fin on 17 of these enigmatic predators. First, we mounted an acoustic transmitter on the tip of a modified spear gun. Next, we tagged. Hammerheads are a particularly skittish species, so in our experience, free diving has been the most effective way to meet them underwater. William used his unique breathing superpowers to wait on the sea floor as the hammerheads approached...

Pole-tagging insertion is minimally invasive in comparison to the hooking alternative. And the prospective data we receive from these deployments may uncover so much about the otherwise uncharted life cycle of great hammerheads. For the past ten years, we have regularly spotted these charismatic predators during our winter months off the western edge of the islands. But where are they coming from, and where are they headed towards on this journey passing beside Bimini?

That’s what we’re trying to find out. For those who don’t know, Vemco is a manufacturing company that specializes in underwater tracking products used on a variety of migrating marine species. In 2006, a group of sturgeon researchers created the Acoustic Cooperative Telemetry (ACT) network. Around the same time, Sharklab founder, Dr. Gruber, and his colleagues started up an independent FACT array surrounding Florida’s coastline. What began as a small collaboration between researchers has now vasty expanded to include over 100 groups, with roughly 9500 transmitters deployed on more than 75 species. Because any Vemco receiver logs data from any passing animal equipped with a Vemco tag, the doors have been opened for cooperative data collecting. 

In January of this year, we deployed these Vemco receivers on our own turf, Bimini, Bahamas...

Around Bimini, we have installed 21 unique receivers to understand more about the great hammerheads’ relationship with the islands. We believe they migrate alongside west Bimini via the cooler Gulf Stream waters, but they have been spotted off the eastern edge in the shallow waters too, often hunting on rays (see Chapman & Gruber 1999). When a tagged hammerhead swims within 500m of any of our receivers, the acoustic signature is recorded, as well as the date and time. At the end of every four months, the Sharklab team will download the receiver data and inform us all on the hammers’ movements around the islands, both collectively and individually.

Two weeks ago, Dr. Tristan Guttridge indulged his curiosity and downloaded data from one of the 21 receivers (the one at our tagging site, located about 800m west of Shell Beach, South Bimini) as a preliminary assessment. The results were astounding. In 10 short weeks, this single receiver logged over eight thousand pings, including at least several from all 17 hammerheads we’ve tagged. Some left almost immediately after tagging, still yet to return. However, others came back repeatedly, thus proving Bimini to be more than a mere pit stop but an explorable foraging ground...though the increase in provisioning ecotourism has very likely contributed to this fidelity.

But great hammerheads aren’t the only species our receivers can track...

After downloading this data, Tristan noticed our West Grate receiver picked up two blue fin tuna (Thunnus thynnus) from Stanford’s Tuna Research and Conservation Center, tagged way up north in the Gulf of St. Lawrence, Canada. To reach Bimini they would have had to trek 2000 miles south and cross a 1000m deep Gulf Stream. Talk about some tough tuna! (Not to overlook, they were measured at 2.5m long, each).

On the very same receiver, a large tiger shark (~3.5m) from Tiger Beach, tagged by Dr. Neil Hammerschlag, spanned the Northwest Providence channel to ping us loud and clear. Apparently, all waters lead to Bimini. No wonder Ernest Hemingway found his way here.

The excitement has only begun. We have exposed a small sliver of what’s yet to discover, and time will surely work in our favor. These arrays represent so much more than a ping on a receiver. They represent a collaboration of scientists, of conservationists, of countries even, connecting data; lighting up migratory routes that have existed for eons, filling in the knowledge gaps that so often triumph over our terrestrial limitations. For this reason alone, we can celebrate a shared responsibility of ocean stewardship. Looking forward to hammering this one home. Stay posted!

We’d like to acknowledge our appreciation to the Save our Seas Foundation for making these advancements possible through their funding, helping us purchase these aforementioned receivers and temperature loggers. Participants of the Hammerhead course and research experiences have also crucially contributed to get this project off the ground. Shout outs to William Winram and The Watermen Project crew for their obvious inputs, as well as to our local dive operator, Neal Watson’s Bimini Scuba Center, for their continued generosity and supply of dive gear and tanks. 

Recapping a Recapture: Lauran's Accelerometer Deployment (4/2/2014)

One week ago, with the help of Eckerd College students, Principal Investigator Lauran Brewster tagged three juvenile lemon sharks with accelerometers in Bonefish Hole, an entranceway into one of the shark nurseries of the North Bimini Marine Reserve. These accelerometers monitor directional movements of a shark, giving us a refined look into their behavior and energetic consumption over time. Are they predominantly resting? Chasing down food? Being predated on by bigger sharks?

Each accelerometer is bound to an acoustic transmitter that pings off receivers we’ve deployed throughout the island, allowing us to track shark movement and location at the same time. As long as the battery lasts, that is. Our accelerometers measure 30 data points per second across 3 axes (x,y,z), while gauging temperature and depth every second. Such scrutiny drains the device’s battery life after five short days. So it’s no surprise that our number one task after the fact is to track down these wild sharks, retrieving our expensive equipment and the invaluable data they contain.

Before April 1st, Lauran had deployed accelerometers on 18 lemon sharks, of which 15 were successfully retrieved with usable data. I can already hear some doubters thinking: “But you just said you know their location, how hard can recapturing really be?” Certainly not as easy as it sounds. Keep reading to find out why...

On the first day of recapture, if we catch over fifty percent of the deployed sharks, we’ll call it a success. But there are the escape artists, the exploratory nomads, the ones that inevitably bypass our seine nets by the day. As a last resort, we can remove the underwater receivers in the most likely shark-traversed areas, download the data, and see if our tagged sharks made the local news. This at least gives us an idea of where to start looking. Welcome to the wonders of passive tracking.

The exact location of these sharks, however, can only be determined through active tracking. By positioning hydrophones underwater, we can pick up frequencies emitted by the shark’s acoustic transmitter. But these only have an 80 m range. The lagoon area alone spans ~ 3 x 4.5 km, and they can leave it as they please. Sounds like a needle in a haystack, or a baby shark in a really big ocean.

Fortunately, to increase our odds of success, we can rely on a bit of science. During high tide, when water levels in the lagoon rise, these juvenile sharks normally retreat into the protected mangrove nurseries. And when the tide drops, they leave their nurseries to explore and search for food. If we can intercept these juveniles at the mouth of their nursery during departure, we can hopefully find the ones equipped with accelerometers and remove them before it’s too late.

Two days ago, April 1st, 2014: With a bit of luck and tidal foresight on our side, we recaptured all three accelerometer-tagged lemons at Aya’s Spot. As the tide receded, 21 sharks vacated the channeled nursery in predictable fashion, some in sharky solitude and others in their friendly groups. It was remarkable. We tagged the sharks at high tide when they were trolling about the channel entranceway, ignorant but hopeful of their nursery preference deeper in the mangroves. It was entirely possible they had converged from different nurseries, prolonging our recapture effort, but fortunately, on this April Fool's, the joke was not on us. 

And now we can analyze the data.