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.