Ecstatic is the right word to describe how we felt after deploying five pop-up satellite archival tags (PSAT’s) on reef manta rays early July. The first ever PSAT’s to be set on reef manta rays on the continent of Africa, these sophisticated devices were programmed to stay on the animals for approximately 100 days, collecting a wealth of information on the reef manta rays’ movements and diving behavior.
A reef manta with a satellite tag
I have been diligently checking my ARGOS satellite account for any hits every day since the tags were first deployed. These tags are only able to transmit when at the surface, so when the animal is in or in the unlucky event that a tag has shed prematurely. Not expecting any hits until the tags were programmed to pop off on the 10th of October, it was a bit of a disappointment to find a steady stream of hits of one tag after only 19 days! And another tag started transmitting only after 36 days on the animal. My ecstatic state was quickly turned into a somewhat disappointed and panicked demeanor.
A steady stream of hits from the ARGOS satellite system most likely means a tag has popped-off, as was the case with tag 128924
These tags are highly advanced, and this means a hefty price tag. With each tag having a total cost of around US$3500, you want them to perform well and get the most out of them, especially as the chances of seeing them ever again is pretty slim. The positions we get from these tags when floating at the surface are not that accurate, with errors of at least several 100 meters, making it quite literally looking for a needle in a haystack when it is bobbing around in an immense ocean.
However, if washed ashore, the chances of retrieval are increased tenfold. Disappointment soon turned into excitement, when it seemed that we could actually retrieve the two tags. If found back, I could access the raw data on the tag, revealing all the measurements of water depth, water temperature, light levels, magnetic levels and acceleration for every 30 seconds! Far more than what is normally transmitted through the satellite system, which is only a condensed version of the data, this will be highly beneficial to our study on the movement ecology of reef manta rays along the Mozambican coastline. And not only will we be getting this high-resolution data, we will also be able to redeploy these two tags, saving us thousands of dollars.
The track of the second tag after it had popped-off. It started about 30km north of Bazaruto Island, and got blown towards Bazaruto Islands by some fierce northerly winds over the course of four days.
The tags were found to be transmitting and floating offshore off Bazaruto Island, and within a few days they seemed to have been washed ashore on the island. Lodges on the island were contacted and flyers, with information on the tags and the reward for returning them, were passed out at the local villages. We quickly got word that the tags were found, and after some phone calls, I set out to meet with the lucky beachcombers in Vilankulos.
After picking up the tags, the download of the data could commence
Looking at the data, already some interesting and very different behavior between the two animals can be observed. Tag 128922 showed data of the animal diving down to a maximum of 116 meters deep, but spending his time at an average depth of 17 meters. Tag 128924 however not only dived deeper, up to 184 meters, but also displayed a much deeper average depth of 43 meters. With these deeper depths came also colder temperatures, as low as 15 degrees Celsius. Each animal however also spent time near the surface on a daily basis, showing that they are quite the vertical swimmers.
Another incredibly interesting aspect these highly sophisticated tags record is accelerometer data. Every 1.28 seconds it record the accelerations in G per second across the x, y and z axes. This will give us insight into how fast these animals can move, and possibly also when they display different types of swimming behaviors. It may additionally show us when they are feeding, cleaning, resting or simply on the move, information that we could previously not obtain.
The task that lies before us now is to analyze all this data. Obtaining the most probable tracks of the animals, performing dive behavior analysis and acceleration data analysis involved a lot statistical modeling. Not an easy task, but Fabrice Jaine, our data analysis guru, and I will be cracking away at it for the next few weeks, fueling our desire to know more about these enigmatic animals.
The large amounts of data coming from these tags will help us elucidate the movements of these large rays
But why have these two tags shed prematurely? Why did they not stay on the animals until the 10th of October? These questions we can also answer, or at least speculate. The first tag to pop-off, was actually initiated by the tag itself. The animal behaved so consistently for about a week that it set off the Constant-Depth-Release. A built-in feature, this enables the tag to release earlier than programmed if for example the animals has died and is lying on the seabed. Judging by the dive profile of the animal this definitely did not happen, but it was diving so consistent that the average depth was similar every day, tricking the tag into popping off.
The second tag seemed to have been bitten off by something. This is not something uncommon with tags, as they do resemble small fish swimming with the manta ray. Predators such as game fish or sharks can mistake the tag for a fish, have a go at it, and break off the tether.
In short, the data we get from these tags is invaluable in the conservation of reef manta rays in southeastern Africa. This is an incredibly lucky start, and let’s hope the other three tags will be retrieved as well! Many thanks to all the people involved in getting the tags back to us; Janneman Conradie of MMF and Big Blue Vilankulos, Luis, Tony, Antonio and Mapanzulo of Pestana Bazaruto Lodge, Anel of Indigo Bay Resort, and Thomas and Arnaldo for finding the tags, Muito Obrigado!
* Written by Daniel van Duinkerken. This post was originally posted on www.marinemegafauna.org.