NEW FOR 2021 acoustic tracking of Atlantic bluefin tuna

In 2021, Thunnus UK added to its successful tracking programme with an additional tracking technology – acoustic telemetry – thanks to funding from EU INTERREG as part of the FishIntel project. You can read more about FishIntel here: https://www.channelmanche.com/en/news-and-media/latest-news/new-project-to-use-cutting-edge-technology-to-enhance-the-habitats-of-key-fish-species/ Here’s more detail on the project from the lead for the University of Exeter on the project, Dr Lucy Hawkes:

What is acoustic telemetry?

Acoustic telemetry means using tags that emit acoustic (sound) signals underwater which can then be detected by receiver stations. Typically, receivers are underwater devices that are moored to the seabed in particular places of interest, and listen out the signals from acoustic tags that are carried by passing animals. You can read more about acoustic telemetry here (there are many more sources available): https://www.sciencedirect.com/science/article/pii/S0169534721002470

How does it work?

First, animals (in this case bluefin tuna) have to be captured, and the acoustic tag attached to them. In the vast majority of studies, acoustic tags are surgically implanted in fish, but we can’t do this in bluefin tuna because anaesthetising them for surgery is impossible owing to their huge size. We therefore attach the tags to the tuna using darts to hold them flush to the outside of the body. Each tag has a lifetime of approximately 5 years, and “pings” (sends acoustic messages) at intervals between 30 seconds and 2 minutes for the whole time it’s on the fish. You can find out more about the tags we are using here: https://www.thelmabiotel.com/transmitter/

Photo: A Thelma acoustic transmitter is mounted ready for deployment on an Atlantic bluefin tuna (black cylindrical tag on silver tagging pole), along with an ICCAT floy tag. Photo credit: Lucy Hawkes.

Then, separately, we install receiver stations in an area of interest. These are normally anchored to the seabed. While they are underwater, the receivers listen for any messages from tagged animals, which can be heard from a range of up to 500 metres (but this range depends on a number of factors such as the temperature structure of the water column, which can bounce acoustic signals in often unpredictable ways). The messages the receiver hears can be from tuna from our own project, or animals from anyone else who has deployed acoustic tags. The receiver stores the messages it receives within onboard memory until the receivers are serviced (we pull them up, change their batteries and download the data they have collected, then put them back again, usually once per year). Receivers that can transmit data have newly emerged to the market, which means that the need to service the receivers in future will be less frequent.

Thunnus UK, as part of FishIntel, have positioned six listening stations across the south coast of Cornwall (see map below), which will allow us to find out how long tagged bluefin tuna stay in the area before leaving, if and when they arrive back into the area and, because the tags and receivers work for many years, whether the tuna come back year after year. The FishIntel project is working with 12 partners across the UK, France and Belgium to install networks of receiver stations in sites along the channel, as well as work with existing networks in Lyme Bay and the Scilly Isles, which means that if and when bluefin tuna travel to these sites, they should be heard and logged on receiver stations there too. We also benefit from a new initiative called the European Tracking Network (http://www.europeantrackingnetwork.org), which is a project to join together all owners of receiver stations around European Atlantic and Mediterranean waters to share acoustic data between one another – this means that if bluefin tuna tagged in Cornish waters swim through straits of Gibraltar, for example, where a receiver station hears them, we will be notified.

Map: The 6 acoustic listening stations (shown as grey circles) that Thunnus UK have placed throughout Falmouth Bay. Inset: photo showing an acoustic listening station (black cylindrical object) ready for deployment at sea with a surface marker buoy. Map / photo credit: Matthew Witt, UoE.

What are the limitations?

Acoustic tags cannot tell us exactly where a fish has been or what route it took, only that a particular fish has been close to a receiver that we know the location of, on a particular day. If a fish moves away from receiver stations, we don’t hear it anymore, and we don’t know where it is…it might be really close and just out of range, or it might be far away! The tags themselves do not record any location information, and so acoustic telemetry is not good for answering detailed questions about movements away from listening stations.

So what’s the benefit?

Acoustic telemetry allows us to track a fish like a bluefin tuna for far longer than any satellite tracking device, and gives us a much more precise idea of what time of day they appear by receiver stations, because the type of data that satellite tracking devices that can be used on tuna provide give just one or two locations per day. The other benefit is that the data are collected by a shared infrastructure, enabling in detections in places where others are working with the same technology. Thus, by investing in a small network in the English Channel, we ‘buy into’ a larger network across the Atlantic for species we are interested in, but also help others who may be tracking highly migratory species, such as the European eel.

Why does ThunnusUK want to start using acoustic telemetry?

Our previous tracking work has used pop-up satellite tags on Atlantic bluefin tuna, which allow us to reconstruct their daily movements and diving behaviour. However, satellite tags are usually deployed for just one year, meaning that we don’t know anything yet about the inter-annual site fidelity of bluefin tuna to Falmouth Bay, whether the same fish return year after year. While we have started trialling 2-year PSAT attachments, acoustic telemetry is the perfect tool to answer this question!

How can I find out more?

You can find out more about the FishIntel project on Twitter: @fish_intel or on the European Tracking Network here @AquaticTracking You can of course also follow @thunnsUK for our updates and check this website for latest information. We are always happy to talk to interested parties and if you have questions about the project, or just Atlantic bluefin tuna in general then we’d love to hear from you (email is bluefin@exeter.ac.uk).