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.