D-tag radio listening from Cape Leeuwin’s lighthouse. Photo Credit: Victor Alvarez
Welcome! This is the third of seven posts in our blog series: “Citizen science and social media impact of ecological research: Study on southern right whales in South West Australia“. In the blog series, we give a citizen science perspective of our collaborative study, with the Marine Bioacoustics Lab from University of Aarhus, Denmark, on southern right whales to help you understand the work done by marine biologists in the field. Please see bottom of this blog post for an overview of future blogs in the series. This third post below describes how we use acoustic tags and the challenges to monitor the behaviour of southern right whales. The overall aim of developing this blog series is to explore possibilities for communication of marine research activities and study the role of social media and citizen science in ecological research.
(3/7) What are D-tags?
A D-tag (Digital Acoustic Recording Tag) is a device of approximately 10x5cm plus a 25cm radio antenna used by researchers to monitor the behaviour of marine mammals. The D-tag can be attached to the back of whales and dolphins to obtain recordings of their movements and sounds (communications). These recordings produce a dense data-set that can be later analysed by the research team.
A D-tag is a very specific device that is not commercialised by any company. The D-tag was first created in 1999 by electronic engineers at the Woods Hole Oceanic Institution. These devices combine different electronic elements: sensors to record audio, movement, displacement and depth, several gigabytes of memory, a system to download the data, over a day-long battery life, suctions cups for attachment, a programmable auto-release system, and a radio antenna to facilitate the localisation of the device after the release. This complex and complete combination of electronics, together with the lack of a more industrialised production system, makes each D-tag highly valuable.
D-tag recording and tracking
The autonomy of a D-tag recording can last for a period of 22-24 hours. During this time, all movements and communications between the mum and the calf, and other whales in the area, are being recorded and stored in the device’s memory. When the recording time finishes, the system allows air to enter the suction cups, making the device release from the back of the whale and dropping it off at any location in the bay where the whale happen to be at that moment.
The radio transmitter of the D-tag emits a continuous signal that can be detected with a radio antenna receiver. In this way, the scientists can periodically check the position and movement of the radio signal to determine whether the D-tag is still attached to the whale or it has been prematurely released and left floating in the ocean. These periodic checks of the position of the D-tag imply that a member of the team sets up an alarm at different times during the night to verify the recording is going well, as we will explain in a following post.
How a D-tags gets attached to a whale for monitoring
One of the main concerns of our research team is the safety and well-being of the whales and scientists. We mentioned in the previous post how we always approach the whales very slowly from the vessel. During this approach, the tag is situated at the end of a 9 meter carbon fibre pole that is used to reach the whale from a safe distance. Only when the whale is within reach, an expert from the research team uses the pole to give a tap on the back of the whale, releasing the D-tag that gets attached to the whale using a non-invasive suction cap system (see photo above).
The D-tag suction system is harmless and the whale only notice a gentle tap on the back. This tap produces a small reaction from the whale. The reaction can last up 5 minutes (normally 1-2 minutes) and it is always monitored by the team with a video recording of surfacing patterns and breathing. The tag allows researchers to monitor mother and calf behaviour while the tag is attached and recording, and the team takes again a safe and respectful distance to keep monitoring surfacing and breathing intervals from the boat.
The following video shows a complete sequence of a D-tagging on a southern right whale:
Video: A mum whale is tagged and turns around curious before continuing her normal behaviour, followed by her calf.
Early release and damage of D-tags
There are multiple reasons why the heavy weight, movements and unpredictable behaviour of the whales can provoke a D-tag to be released or damaged before the end of the recording. If that happens, the team needs to organise an early recovery to prevent loosing the D-tag and all the data it has recorded.
The causes for a premature release of the D-tag can be as simple as a whale rubbing its back on the ocean bottom or a (3-tons) baby calf rubbing against her mum. The later is a common behaviour in every mammal species; not only the baby looks for mum’s protection, but rubbing against the mum is also the most usual way of letting her know it’s feeding time.
During our research in Augusta, one of the D-tags was damaged when a calf rubbed against her mum’s back on the exact position where the D-tag was attached. As a consequence the antenna of the D-tag was broken and the device prematurely released and left floating on the ocean (see video below).
Video: A calf rubs against her mum’s back and the D-tag is prematurely released.
From the boat, the radio receptor allowed the team to detect the early release of the D-tag. This situation was very lucky; when the radio antenna was detached from the D-tag device, it limited the reach of the radio signal to approximately 1km, and the detection and recovery of the device would not have been possible if the team had left the area.
D-tags are not only expensive devices that are difficult to acquire, but they are also complicated to repair. Once a D-tag is damaged, only the persons who made it know how to repair it. In this case, the D-tag was luckily recovered, but it had to be send back to repair and it couldn’t be used for the remaining days of fieldwork.
Retrieving of D-tags
When all the tracking goes well, the D-tag is auto-released after 22-24 hours of recording. At this point, the ocean currents determine if the D-tag will be found on the shore or floating in the ocean. A local knowledge of the currents in the area can be very helpful, as we explain in a following post. Alternatively, the experience of consecutive days of tracking and retrieving of D-tags can provide a reliable indication about where the tags will be found.
The radio system will provide a first pointer for the location of the D-tag. The tag is emitting a beep that can be detected with the radio antenna receptor. The longer the distance between the radio transmitter and receptor, the less audible becomes the signal, and the person listening needs a good degree of expertise to be able to detect and point at the right direction and distance where the device may be found.
In that circumstance, elevation becomes very important. The higher you are situated, the better reception you get for the radio signal. Augusta’s Cape Leeuwin has a lighthouse overlooking the bay that is immensely valuable for this task. Every morning, very early, the lighthouse keeper would wait for the team and open the gates to allow us climbing to the top of the lighthouse to do the radio reception. Once back on the road, pulling over and jumping on the roof of the car to make additional listenings, allows for a more precise estimation before boarding the vessel.
When we got a more precise estimation of the direction and distance to the D-tag, we boarded the vessel and left port to retrieve the tag. During the D-tag retrieval the boat increases the speed and we do a few corrections and stops along the way to listen again to the radio signal before finding the tag. Although the D-tag has bright colours, it’s size makes it challenging to see it from the boat, and it’s not until the final approach has been made, and with the help of several the trained researchers, that the D-tag, containing all the scientifically valuable recordings, can be seen and successfully recovered to the boat.
In the following GPS track recording, the vessel successfully finds and recovers the D-tag, and takes it back to port in less than one hour, with a total distance of 16 nautical miles and maximum speed of 30.9 knots.
Video: The vessel used to find and retrieve the D-tags in Augusta.
Soon you will be able to navigate through the other sections of the blog series:
- (I) The research location and its importance in the migration of southern right whales
- (II) Watching whales
- (III) D-tags: acoustic monitoring of southern right whales (this post)
- (IV) Using a drone to measure behaviour and health
- (V) Homework: downloading research data and getting ready for next day
- (VI) A first-person perspective of a student researching with MUCRU
- (VII) Informing and engaging the local community