ANIMAL TELEMETRY (TAGGING) – A CRITICAL CONSERVATION TOOL

TAGGING AND TRACKING MOBULIDS

Manta and devil rays often undertake seasonal migrations, travelling tens, hundreds, and sometimes thousands of kilometres. This means that their habitat can encompass large areas, sometimes crossing national boundaries, where conservation management is often more challenging. Therefore, to effectively protect these animals, we must first understand what habitats they are using, when they are there, and what they are doing within it.

Photo identification (photo-ID) is one of the primary research methods favoured by Manta Trust scientists around the world and, over the last 15 years, it has provided great insight into the movements, size, and demographics of manta ray populations globally. It is a cheap and non-invasive data-collection technique that both trained researchers and members of the public can use. However, it alone cannot tell us everything that we need to know to adequately protect manta rays and their relatives around the world. Learn more about photo-ID and how you can contribute to our citizen science programme, IDtheManta.

One of the limitations of photo-ID research is that images can only be collected in locations (and at depths) that scuba divers and snorkellers can access; primarily restricting data collection to shallow reef systems. This means we have learnt a lot more about the spatial and behavioural ecology of the reef manta ray than their larger cousins, which live a more oceanic lifestyle. However, as this recent tagging study led by Initiative Manta En Nouvelle-Caledonie demonstrates, even with all the photo-ID data we have collected, there is still much we can learn about reef manta rays using other research methods. Photo-ID is further limited because most of the data is primarily collected from only a few aggregation sites where researchers know the rays regularly frequent to feed or clean, often for just a few minutes or hours at a time. Therefore, once the rays leave these sites, we have no idea where they are going, or what they are doing; knowledge which is critical if we are to effectively protect these species.

The Manta Trust utilises a wide range of research methods to gather knowledge on mobulid rays globally, which we then use to make informed management recommendations to governments and other stakeholders. These methods often include tagging studies. However, as tagging is an invasive technique, we only use it when it is required to fill vital knowledge gaps.

Types of tags and trackers

Coded ‘Passive’ Acoustic Tags

These small tags emit a numerical ID signal (somewhat like Morse code) at regular intervals which can be picked up by special underwater “listening” receivers. Passive acoustic telemetry uses tags which signal on average once every 90 seconds. These signals are recorded by acoustic receivers placed in the marine environment. Each time a tagged manta passes within ~200 m of an acoustic receiver, the receiver will record the tags unique numerical ID. It is only possible to get data if an animal passes close to a receiver, so it is necessary to have lots of receivers positioned throughout the animal’s habitat if a clear understanding of the animal’s wider movements are to be recorded. Acoustic tags are therefore a great tool for studying fine-scale manta movements, studying social interactions, and the importance of specific sites over time. However, passive acoustic tracking does not provide any data on the tagged animal’s diving behaviour, the temperature of the water it is swimming through, or where the animal goes once it leaves the detection range of the receiver array. These tags have a long battery life, often lasting for many years. However, externally attached tags usually fall off the animals before the tag battery dies.

Active ‘Pinger’ Acoustic Tags

Active acoustic tagging uses ‘pinger’ tags which send out their “Morse code” continuously. Like the coded tags, these ID signals can be detected within a few hundred metres. However, instead of deploying passive listening stations, the pinger tags is tracked continuously from the surface using a directional hydrophone, usually attached to a small boat. This allows researchers to actively follow a tagged animal, like a manta ray. The pinger tag also sends its depth each time it pings, allowing the trackers to not only record the movement of the animal, but also its depth. The downside to this method is that it is very time and labour-intensive, and if the tracked animal is lost (i.e., it moves outside of the detection area), the track will end unless the animals can be found again before tag battery dies (which is usually within a few weeks). This method has been used very effectively to study manta rays within lagoons, but generally this method works better with slower moving animals with smaller home ranges.

Satellite Tags

Satellite tags have an internal computer that can be programmed to continuously record lots of environmental data, such as temperature, depth, light levels, and/or position. However, the more data you record, more often, the quicker tag’s memory space and battery is depleted. Furthermore, global positioning systems (GPS) do not work underwater. Therefore, a manta fitted with a satellite tag does NOT allow researchers to track its movements in real time because manta rays spend most of their time at depths below the surface. Instead, many satellite tags are designed to archive their data. These archival satellite tags store the information collected as the animal swims through its environment and then transmit this information at the end of the programmed deployment time (usually after a few months to a year), when the tag releases from the animals and floats to the surface.  Once at the surface, the tag attempt to transmit all its archived data in batches to satellites orbiting overhead (the ARGOS network) before its battery is drained.  This usually means researchers receive only partial datasets of the information collected by tags during attachment (unless a tag can be located - e.g., it washes onto a beach where is can be recovered), which they then piece together to build a picture of the animal’s movement and habitat use during the deployment period.

In these archival tags, positional information of the track is often inferred through geolocation (i.e., dusk/dawn light curves) because the only GPS location points are the tag deployment location and the location where the tag floated to the surface at the end of the track. However, geolocation comes with a degree of uncertainty because there can often be errors of 10s of kilometres in the estimated positions given. Nonetheless, geolocation provide critical information on general movement trends, especially if the tagged animals travel large geographical distances (i.e., hundreds or thousands of kilometres) as shown in figure above from the TOPP programme.

More recently, technological advances have allowed the integration of archival tags with SPOT transmitting tags. SPOT tags allow fast accurate GPS locations to be detected, even if a tag is at the surface for only a few seconds. SPOT tags are traditionally most frequently used on species like sea turtles, which regularly come to the surface to breathe. These GPS locations can be transmitted to orbiting satellite, relaying this information to the researcher. These advanced satellite tags are often attached to manta rays and other large megafauna using a tether system which tow the tag slightly above the animal so that when it is close to the surface the tag can send its position (and some other data also). A towed SPLASH satellite tag therefore has all the benefits of the traditional archival tag, and the SPOT tag combined.

To reiterate, satellite tags can only transmit data when their antenna breaks the surface of the water, so data is not received consistently or immediately. And even when location data is transmitted to the ARGOS satellites, there is usually a lag of 24 hours before this information is replayed to the researcher. Researchers therefore cannot use these tags to remotely track manta rays in real time. This is a myth often perpetuated through fictional TV shows.

Satellite tags provide much more data than acoustic tags, but they are also a lot more expensive. To learn more about the different types of satellite tag, watch Dr. Mark Erdmann’s webinar ‘The Truth About Tagging’.

Animal-Borne Video Cameras

Whilst acoustic and satellite tags deliver data on the movement ecology of manta rays, and the environmental conditions the tagged animals experience, they do not tell us what the animals was doing. I.e., was the manta ray feeding when it dived to 500 metres, or was it migrating, or thermoregulating, etc. Animal-borne video cameras are a new technology that is being used to provide, for the first time, direct observations of manta ray behaviour at depth. With a manta’s eye view, researchers can observe social interactions, feeding behaviour, inter-species interactions, and even identify prey species. In this recent collaboration between the Manta Trust, the National Geographic Society, and the Scripps Institution of Oceanography, ‘Crittercams’ were attached to manta rays to capture feeding behaviour at depth. The team also took the opportunity to trial various applications methods. The minimally invasive method of using an active suction attachment proved most successful when peanut butter was used as a sealant on oceanic manta rays. However, in reef manta rays, a modified mount system was required.

Tag Attachment

Due to the tough and rough texture of manta rays’ skin (which is comprised of dermal denticles), long term suction tags are not viable. Acoustic and satellite tags are therefore usually attached externally to manta rays using a metal dart and a short tether. The tethers corrode in saltwater over time releasing the tag. Depending on the tag type deployed, the corrosion time is adjusted accordingly (i.e., the use of steel vs. stainless steel or titanium darts, or plastic vs. steel line). In many species, passive acoustic tags are now routinely inserted inside the animal, much like your pet has a microchip inserted inside it in case it gets lost. These internal tags can give tracks of up to a decade whereas external tags usually last on average one year.

Learn more about Tagging