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!