Thunnus UK: The first two years of Atlantic bluefin tuna tracking

In the summer and autumn, Atlantic bluefin tuna can now be seen off the coast of the United Kingdom feeding on seasonally abundant stocks of prey fish. The last time these apex predators were a regular feature in UK coastal waters was over 50 years ago, although it is not yet clear why they have recently re-appeared.

Thunnus UK is a collaborative research programme between the University of Exeter, the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and the Tuna Research and Conservation Center at Stanford University. Thunnus UK has been funded by Defra, the European Maritime and Fisheries Fund, Welsh Government and EU-Interreg. The aim of the project is to investigate the ecology of Atlantic bluefin tuna off the United Kingdom, including how much of each year they spend in UK waters, where they go when they leave, and how their behaviours fit into the broader picture of Atlantic bluefin tuna ecology?

In 2018 and 2019, Thunnus UK research scientists working with recreational anglers deployed 33 pop-up satellite tags on Atlantic bluefin tuna. The tags record the environmental conditions that the tuna experience (depth, temperature and light), and store the data until the tags detach up to one year later. A summary of these data is then transmitted by the tag and is used to reconstruct their movements and behaviour. Waiting for the tags to detach and transmit data is necessary because tags that use real-time GPS tracking (like a mobile phone or car satnav) cannot be used because bluefin tuna spend most of their time below the surface of the water, where GPS signals cannot be received or sent. Once the data from the pop-up tags is received, work can then begin on analysis and quality control.

The Thunnus UK team is excited to be able to share some of the Atlantic bluefin tuna migrations recorded by tags deployed in 2018 and 2019, which are detailed in the figure below. After leaving waters of the English Channel and Celtic Sea, bluefin tuna migrate to the Bay of Biscay, where they spend a large proportion of their time, before some head out to the Central Atlantic or the Azores. After a period of foraging in these distant waters, approximately half of the fish that were tracked head to the Mediterranean Sea during the summer. This is significant because bluefin tuna are known to spawn (reproduce) in the Mediterranean. The scientific team are currently looking deeper into the data to see if the diving behaviour of the tuna there reveal if these fish did indeed spawn once they arrived. Occasionally PATs detach early from study animals or do not transmit all their data (the red triangles on the map). To ensure a high quality of data, we screen such datasets to ensure only the most accurate locations are included in migration reconstructions.

After a year of wondering where all these tagged fish were, tags began to pop-up and communicate their data summaries. Often, as we suspected, the bluefin we tracked returned to the waters they had been released in a whole year earlier, making tag recovery possible in many cases. Getting the tag back means all the raw data can be accessed providing even greater level of information – it is not possible to satellite relay the raw data. Over the past two years we have recovered 7 tags at sea and the public, reaching across the UK and France have returned 6 tags that washed up on beaches.

The next steps for Thunnus UK are to continue analysing this dataset, and in due course we expect to produce more in-depth analysis of these incredible data. We are always happy to talk to interested parties and if you have questions about the project, this dataset or just Atlantic bluefin tuna in general then we’d love to hear from you (email is bluefin@exeter.ac.uk).