A little while ago, you may have read my blog post on ‘the challenges of saving ocean giants (part I)’ describing some of the underwater challenges that make marine research, well, challenging.

You want to find out where your large, far-roaming, pelagic animal goes. Following it around is just not feasible due to low underwater visibility and the fact that humans are simply not made for being submerged for long periods of time.  Luckily there are a number of technologies that might just do the trick for you.

Tags allow for the identification or even tracking of individual animals. In the past, researchers wanting to know where a fish species, for example Bluefin Tuna, roamed, would catch them, quickly put a little plastic tag with an ID-number and contact details, get some basic information such as size and weight, and throw the fish back. By working closely with fishermen, hopefully in the next few weeks, months, perhaps even years, someone else would catch the same fish, see the tag, and contact the researchers, providing valuable information such as migration paths and growth rates. One great example of this is an Atlantic bluefin tuna tagged at two years old and just 14 lbs in 1997, which was caught again after sixteen years, now weighing in at a whopping 1,200 lbs!  But obviously you won’t be re-catching all the fish you tag, so you have to put out an awful lot of tags to get a decent amount back.

Nowadays, there are electronic tags, and they don’t necessarily need to be retrieved to get your data. Two common methods are acoustic telemetry and satellite telemetry. For the acoustic tags to work, you need to put out an array out of acoustic receivers, or ‘listening stations’. These stations listen out for an inaudible acoustic signal that the tags emit every minute or so. By downloading the data stored on the stations, we can see which tagged animals have been around.  And you can do this long-term. If you have a way of attaching the tag securely to the animal, or performing minor surgery and fitting the tag inside the animal (such as the abdominal cavity), you can monitor the animal’s presence near listening stations as long as the tag’s battery last. And this can be long, as long as five years.


actagThe acoustic receiver on the left listens out for the acoustic tags (seen on the right)


But you can only see if the animal is present in places where you put your stations, and of course the marine environment isn’t going to make it that easy. Environmental noise caused by waves or other organisms (such as snapping shrimp. Yes, you read that right, tiny little snapping shrimp) can produce so much noise that the listening stations cannot hear the acoustic tags properly. You also have to contend with all sorts of marine organisms trying to make the listening stations or tags their home, covering every inch of them.  Swell and strong currents might make your stations do a little travelling by themselves, leaving you without the device and, even worse, the data stored on it. 


taggedmantaA reef manta ray with an acoustic tag


Enter satellite telemetry: You aren’t dependent on other devices such as listening stations, and you can track the animal’s day to day movements as well as its depth and the temperature of the water it is in. These sophisticated tags archive data every 30 seconds to several minutes or so for several months, maybe even a year. After this pre-programmed period the tag pops off the animal and transmits daily summaries of all this information to an online database through the ARGOS satellite system. Pretty neat, hey?! 



A pop-up satellite archival tag, or ‘PSAT’


But again, there are still challenges to be overcome. GPS does not work in the underwater realm so we have to do with simpler, cruder ways of getting a daily position of the animal. Noontime shifts as you move around the earth longitudinally.  The length of the day gets shorter or longer dependent on the time of the year as you move between the equator and the poles. So you can estimate daily longitude and latitude by recording light levels. This all takes quite a bit of computing however, once you get the transmitted information, as it is all dependent on time of the year as well. And light levels aren’t always accurate, as marine mammals tend to dive up and down quite a bit, resulting in inaccurate light level readings. Recently there have been more accurate ways of telling latitude by looking at geomagnetic levels of the earths crust, which can give much more precise latitudinal positions.


Tagging_I8A4325Deploying a satellite tag on a reef manta ray in southern Mozambique


The data you get through the ARGOS satellite system is only a daily summary of the archived information. If you are lucky however, you can find them back. For my studies on reef manta rays I have been lucky enough to do so and you can read about it here!


taggedManta_I8A4301A reef manta ray with a fitted satellite tag


And of course there is the matter of biofouling, the growth of marine organisms on the tag, and tag retention. Any biofouling happening on the tag attachment might cause the tag to remain attached to the animal when it is supposed to pop off to transmit all it’s archived information.  If this happens, you might never hear from your tag again! Tags also occasionally tend to get eaten. Resembling a little fish swimming alongside your tagged manta ray, whale shark or great white, other predatory fish might gulp it up. And as these devices tend to be several thousands of dollars, this is obviously not something you would like to happen!

 As technology progresses, we will likely have better ways of tracking the movements of ocean giants. But as for now, we will have to do with the current options.

* Written by Daniel van Duinkerken. This post was originally posted on

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