Tagging and tracking sharks: the how and the why
The different methods scientist use to track sharks and why it's important to think before you tag
Why Tag Sharks?
Sharks have fascinated people since the dawn of time. You can find references to them in the Bible, Aristotle and even wood carvings from the middle ages. Yet we’ve only been able to use electronic tags to track their movements since the late 1980s, and since then our techniques have come a long way. In this page, I’ll introduce some of the methods I’m more familiar with in my work with sharks, particularly those we’ve used in South Africa and California.
The famous Dyer Island white shark, Slashfin, carrying one of our active acoustic tags in 2012
To tag or not to tag?
Tags are wonderful things. They can tell you so much about where an animal goes and what it does there. But before you start to put instruments on animals, you have to have a question. What do I want to know about this species/system/region? And what can the tags tell me about them?
There are many different types and specifications of tag, and they will do and tell you different things, plus they’re not all suitable for every species. For instance, a SPOT transmitter needs to be above the surface to transmit, and would therefore be pretty useless on a deep-sea species. Likewise, a Pop-off Archival tag would likely be too big for a shy shark species, getting caught in the crevices they like to dig in between or worse being too buoyant for them get back down to the seafloor.
There is no such thing as a perfect tag (yet), so once you have your question you can think of what type of tag is best, how many do you need, how many can you afford, and how long will it take to deploy them and get usable data back. Once you have these plans, you can start looking for funding and applying for ethics to conduct the research.
No sharks (or other wild animals) should ever be tagged without first gaining ethical approval for the research. If you go ahead and do this anyway, you may be breaking the law.
If you can get to this stage, now it’s time to start tagging sharks!
Conventional Tags
There are several types of conventional marker tags. They are all designed to be read upon future sightings or captures of various species of fish, including many sharks and rays.
You may not need and electronics in your tags at all. Conventional tags work just fine in many studies. These involve labelling a tag in some way, attaching it to a shark, and waiting until someone catches that shark again and tells you they found a tag on it.
These tags work great in sports and recreational fishing, particularly in places like South Africa where many anglers are keen to contribute to conservation efforts of the species caught.
Two of the most popular species in these programmes are Bronze Whalers and Ragged Tooth Sharks (also called the grey nurse or sand tiger). You can read more about them, and other species tagged in South Africa on the Oceanographic Research Institute (ORI) website.
The problem with using these types of tag on a larger species, such as a white shark, is that the recapture rates are usually pretty low. Larger shark species are often protected too, so it’s both difficult and illegal to land them in most cases. People have tried colour coded tags, as seen on the whale shark above, yet a problem in temperate waters is that we often see a lot of biofouling on tags, and this quickly turns any colours into a brown/green mess of barnacles and other growth.
Fortunately, for white sharks, we’re pretty good at using their natural markings to ID them [1], so with this species at least, all we need is a photo of the dorsal fin, and we can use mark-recapture to find out where this shark has been [2].
You can read more about photo ID on the ‘Gauging the Threat’ page.
Acoustic Telemetry
But what about when the sharks aren’t close by to you? Or where they go when they leave your dive spot? If this is something you need to know, then it’s time to select some form of electronic tagging. The most common method in elasmobranch research is, without doubt, acoustic telemetry.
The main problem with tracking any animals underwater is that our most used method of tracking things on land, radio waves, don’t work through the water. That means no VHF or Satellite transmissions from any animals are heard once they dive beneath the waves.
Not to worry though, because much like submarines find their way with sonar, fish migrations and movement patterns can be mapped with ultrasonic telemetry. And the principle is quite simple. The tag pings and a receiver hears it. Tags are programmed to transmit at certain signals or with coded identifications, which are all deciphered by the receiver.
The receivers are placed in anchored positions to pick up transmissions from tags as they come into range, known as passive telemetry, or onboard a research vessel (or AUV) that can follow the transmissions, known as active telemetry.
There are strengths and weaknesses to both methods, and knowing your question will help you decide which one is best suited to your project.
I’ve worked on a few projects using passive acoustic telemetry, from monitoring the linkage and residency of white sharks in California with Taylor Chapple and colleagues at Monterey Bay Aquarium or Stanford University [3] to monitoring sawfish and bull shark activities in the Fitzroy River with Adrian Gleiss, Karissa Lear and colleagues at Murdoch University or the Nyikina and Mangala Rangers [4]. Most recently, I joined Evan Byrnes and colleagues at Murdoch University, to help them measure the home range and energetics of lemon sharks at Ningaloo Reef.
Dr Taylor Chapple of Hopkins Marine Station, Stanford University and Oregon State University prepares to tag a white shark with an acoustic tag at Año Nuevo Island, California
Evan Byrnes of Murdoch University fitting a juvenile lemon shark with an internal acoustic tag at Ningaloo Reef, Western Australia.
I’ve also spent many long hours listening to pings and actively tracking white sharks around all day and night in Mossel Bay with Enrico Gennari, Ryan Johnson and colleagues at Oceans Research [5,6], in Gansbaai with Alison Towner and colleagues at the Dyer Island Conservation Trust [7,8] and off of Tomales Point with Paul Kanive and Scot Anderson at Monterey Bay Aquarium.
The strength in passive tracking is that you do not have to be out at sea and following sharks at all times of the day or night to get your data. The weakness is that you rely on them swimming within range of a receiver to get any data, and if they swim just a little outside your array, you get nothing until they return. You also have to wait six months to a year before downloading your data and seeing what the sharks have been doing. You have to hope they all didn’t decide to leave a day after you tagged them!
With active tracking, you can get incredibly fine-scale movements of sharks, and you don’t need to define an array before you start. The weakness is that you have to focus on one animal at a time and you’re at the whim of where the shark wants to go. Shallow reefs off river mouths in the middle of the night… offshore pinnacles in swells… it’s not always fun to follow the sharks into those! You also need to have good weather and have to put in long hours to get your data. Again, all things to carefully consider when choosing your method!
This is a picture of me tagging a white shark as part of our active tracking studies in Gansbaai. The shark was named Victarian and was around 4.5 meters long.
Active tracking positions of the first 11 white sharks tagged in our Gansbaai programme (aprox. 420 hrs of data). Each colour represents a different shark tracked and each position is at least 5 minutes from the last. Victarian's movements are the teal coloured tracks and she liked a specific reef to the northern end of the bay. All these tracks were made in Arc Map 10 using the methods described in my Masters [9].
All these positions required a team to be tracking a shark within a range of approximately 50 - 250 m. Here, I am actively tracking a large white shark off the Point Reyes National Seashore (near Tomales Point) with Paul Kanive.
Satellite Tracking
When I guided cage diving trips, I was often asked if we could get the GPS of the sharks. And you can, but it’s really not that simple. If you want to know long-distance migrations of sharks and the full extent of their home ranges, then you almost certainly need to consider using some form of satellite tracking.
And of options readily available today, there are two main methods. The Smart Position Only Tag (SPOT), which is a type of fin mounted or towed, Satellite-linked Radio Transmitter (SRT) (there is a similar alternative called a Splash tag). Or, the Pop-off Archival Tag (PAT) (see also mini-PATs).
The main difference is that a SPOT tag will give you a position every time the tag is clear of the water, and there is satellite coverage available. The brilliant photo below from colleague Nicola Stelluto of Marine Dynamics demonstrates how a SPOT tag fits to a white shark.
The copper bolts form a ‘saltwater switch’ which creates an electrical circuit when the tag is submerged in seawater, which breaks when the tag is clear of the water. Once this occurs, the tag will begin to transmit in the hope ARGOS satellites are overhead and can position where the tag is.
PAT tags work in a similar way but remain on the shark for a set number of days, logging the depths the shark swims at, the temperature of the water, an estimated daily position and (in newer models) acceleration the tag experiences. After the days expire, the tag pops off the shark and has a similar saltwater switch inbuilt, meaning it will begin to transmit once it reaches the surface.
A SPOT tag in place on a sub-adult white shark in Gansbaai, sighted during a Marine Dynamics cage diving expedition.
Here I'm about to deploy a PAT tag to one of Gansbaai's favorite white sharks, Nemo.
The strengths in the SPOT option is that tags can transmit for up to 5-years, making incredibly detailed patterns in migration and return migration possible. The weakness of this method is that in most cases, the tags are fitted to the fins of sharks, and this involves catching the animal in all cases, for very large white sharks this can be a challenge.
The strengths of PAT tags is that they can be deployed to free-swimming sharks and if retrieved can give incredibly detailed records of the swimming depths and temperatures sharks experience while migrating. The weaknesses are that tags may pop up far out to sea, meaning only transmitted summaries of data are available, and the longer the tags remain on the sharks, the larger the chance of failure.
In fact, tag failure is a huge consideration to make with all externally attached shark tags. The main cause of this failure? Biofouling. Yes, those little barnacles and algae do love to attach themselves and begin to grow on tags, and in a temperate species like white sharks, it’s extremely common to see rapid growth beginning on tags within weeks.
This growth can stop SPOT tags from transmitting and sink PAT tags entirely and is a constant challenge in biotelemetry. Anti-fouling coats, such as PropSpeed® most certainly help, yet after time the biofouling organisms still find a way. The use of copper in the saltwater switches of transmitters also helps, organisms cannot attach to the coper, but by connecting either side of the bolts, fouling can still grow over the switches in some occasions.
Yet the sharks do sometimes have a solution for us. The extreme range of white sharks means that sometimes sharks that get biofouling in temperate waters, migrate to sub-polar regions, or the tropics. These conditions take the fouling organisms out of their thermal range, and they die. In some cases resurrecting tags, we thought were long dead!
Biofouling can become a serious problem for marine biologging research. Here, a white shark with a 'fouled' SPOT tag seen from a Marine Dynamics cage diving expedition.
However, white sharks are able to tolerate extreme ranges in sea temperature. Here a white shark that previously had a heavily fouled and dysfunctional tag, swam from Gansbaai to the Chagos Archipelago (the straight line, indicating no positions recorded in between) and her tag began working perfectly again. It's thought the biofouling organisms would not have handled the temperature change.
Biologgers
With the methods described above, you can address questions on large-scale movements, residency patterns and fine-scale migrations at aggregation sites. Yet, there’s still more we discover about the movements, and particularly the activities of white sharks, if we take an even finer-scale approach to our tracking.
This is where biologgers come in. The basic definition of a biologger is an instrument that attaches to an animal and logs data. It is downloaded later as appose to a transmitter, that gives out data but has no storage capacity. Technically a PAT tag is a bit of both.
The main uses we have for biologgers is to gain insight into kinematics (the mechanisms of locomotion), energetics (i.e. metabolism, energy expenditure/budget), and foraging (how much/what is an animal eating). And the effectiveness of making these measurements is again dependent on the size of your study animal, your budget, and what kind of biologger you select.
I’ve explained a bit more and provided some videos on biologgers on the pages on the use of kelp habitat by white sharks and putting cameras on sharks.
In short, the biologgers we use will usually have an accelerometer, that allows us to see activity in 3-dimensions at scales up to 100 times a second. A depth sensor that will enable us to see swimming depth and vertical habitat use. A temperature sensor, allowing us to measure the temperature the water the animal is swimming through, or its internal temperature.
Some will also have magnetometers, that are effectively a 3D compass. Gyroscopes that measure angular rotation and are great for reading tail beats or turning rates. Light sensors, allowing us to estimate turbidity and sunlight. And Cameras, that will enable us to see what the animals are getting up to and possible interactions with their prey.
As we need to get these instruments back to get our data and hopefully use them again, we also equip them with a location transmitter (either a SPOT or VHF tag) and write our emails and phone numbers very clearly on them in case they wash up on a beach [10].
I will explain more on the process we use to look at biologging data in future posts but for now, I’ll leave you with some images of the attachment, retrieval and a look at some raw data from the tags.
Marine Dynamics have been integral to our tagging expeditions. Here, I'm placing a CATS Cam on a white shark at Joubertsdam, Gansbaai, South Africa, during a cage diving trip.
A white shark swims past our research vessel with a CATS Cam in place at Dyer Island, Gansbaai, South Africa [10].
Jerry Moxley and I after a successful retrieval at Tomales Point. Once the biologger detaches, we can track it down using VHF or SPOT tag locations and it's always a relief to finally get your tags safely back in hand.
Sensor readings from a CATS Cam attached to an adult white shark during a dynamic event. Depth (blue line) remains similar but activity (defined by Vector Dynamic Body Acceleration or VeDBA, purple line) rises dramatically as tail beats (seen here in the form of dynamic sway, teal line) increase. It would not have been possible to visualise this burst, which corresponded to a prey interaction, using the earlier described tracking methods.
There’s often some overlap in the methods we use for our research, for instance, if the weather’s looking particularly good, I sometimes use an additional active acoustic tag in our biologging camera tag set up. There’s a slot in the back of the unit, and we can track the shark while the biologger records. This can help anchor our dead-reckoning steps (created from combining accelerometer and magnetometer data).
We also use SPOT and PAT tags to find our biologgers, sometimes the sharks can travel large distances during the days they have the units on or in them, so we attach a SPOT tag to each camera and a PAT tag to each blubber burrito.
We also have plenty of sharks who are already a part of our mark-recapture or acoustic monitoring programmes who then get a biologger for a few days on top, helping us confirm if the animals are regular visitors to the aggregation areas.
Multiple complementary tagging and tracking studies, alongside traditional methods, build up a bigger picture of what’s happening with sharks in their ecosystems. And that’s why in many of the places we work with sharks, we’ve been doing so for many years.
Stay tuned for more tagging and tracking stories or follow me on Twitter @JewellResearch for more from the shark tracking community worldwide, I’ll often share or like as many shark tagging tweets as I come across!
References