Stop, Drop, and Roll: Rolling Over Acoustic Telemetry Receivers

Conservation efforts directed at protecting marine animals can be particularly challenging because humans often know relatively little about the species we aim to protect. For example, we can’t effectively designate marine protected areas to save a certain species if we don’t have sufficient understanding of things like local population dynamics, migration patterns, and predator/prey species diversity. To that end, acoustic telemetry tracking systems have recently emerged as a high-cost, but a low-effort tool to gather complex data about the behavior of different marine species.

At a high level, the technology relies on species with acoustic tags coming into the range of underwater acoustic receivers. When a receiver picks up on the acoustic pulses that are emitted from a tag, data about that individual is recorded and stored in the receiver. Eventually, when the receiver is retrieved, scientists gain access to information about every tagged individual which circulated the receiver during its time underwater.

Acoustic telemetry may be useful in isolation, but its true value comes when multiple receivers are connected as part of a network. If researchers can access data collected by an entire network of receivers, either located across a small bay or across hundreds of kilometers of the ocean, they can start to learn a lot about the residency and migration patterns of tagged species, which in turn can lead to significantly more effective conservation measures. In South Africa, this system is organized by a research platform known as the Acoustic Tracking Array Platform (ATAP). This platform coordinates and disseminates information on hundreds of receivers and tags to scientists around the country. ATAP covers over 2000 kilometers of coastline ranging from False Bay to Ponta do Ouro and includes multiple smaller networks of listening stations in Mossel Bay, False Bay, and Algoa Bay. Every six months, the Dyer Island Conservation Trust assists this platform with “rolling over” three of their receivers, meaning we retrieve the old receivers to access the data and deploy new ones.

Fortunately for me, the rollover fell on day 12 of my internship. So on a lovely Friday morning, I got to head out to sea with Ralph (MDA), Hennie (MDA), Stine (volunteer), and Louis (volunteer and future intern). We began by plugging in the GPS coordinates where the receivers were dropped six months ago and headed in that direction. But within minutes we were hit by a brief (but totally welcome) distraction when dolphins approached our research vessel, Lwazi, and began to bow-ride. One of the dolphins in the pod was a 3-5 day old baby who was swimming alongside her mother — a highlight of the internship experience so far!

As our excitement cooled and we
regained focus, we eventually made it out to our desired coordinates and began a rather unique retrieval process. Essentially, we placed another machine that emits an acoustic signal in the water and set it to connect with the acoustic release that is attached to the receiver. Once the connection is established, the acoustic release disconnects the receiver from the weight on the ocean floor. The receiver (and its buoy) become detached from the weight and it floats to the surface. This takes on average about ten minutes, after which the receiver pops up right next to the boat. We hauled it on board, then deployed a new receiver in the exact same location. We repeated the process at three separate locations, and within just a few hours we had completed a successful acoustic telemetry rollover. The old data is on its way to ATAP and new data is (hopefully) beginning to gather within the underwater receiver!