Over the past few years more and more Atlantic bluefin tuna have been sighted off the coasts of the British Isles. This includes off Shearwater II, and we have seen magnificent displays of feeding bluefin tuna during the late summer and autumn since 2015. As well as working at Marine Discovery I’m a postgraduate student at the University of Exeter and have been researching the movements of bluefin tuna that aggregate off the coast of northwest Ireland during this same period. The principal driver for this work is understanding where these fish originate from, and the precise spawning locations of these adult fish (the fish that we see off the British Isles are 5-13+ years old and classed as spawning size).

Figure 1. Schematic of how a PAT tag works on a bluefin tuna.

Figure 1. Schematic of how a PAT tag works on a bluefin tuna.

Studying the movements of wild fish is pretty hard work in general, and bluefin tuna are no exception. In 2016, I worked with Dr Matthew Witt and Dr Lucy Hawkes from the University of Exeter and Dr Barbara Block and her team from the Tag-a-Giant Foundation of Stanford university to deploy 16 Pop-up Archival Tags (PATs; wildlife computers’ miniPATs) on adult bluefin off the coast of Donegal, Ireland. These tags are mounted externally on a marine animal and record light, temperature and depth continuously at pre-determined intervals (between 5-15 seconds) for up to a year. On a set date, the tag detaches from the animal and floats to the surface, whereby it transmits a summary of the archived data back to us via the Argos satellite network (Figure 1.). The light data that the tag records is used to reconstruct the movements of the animal over the course of the tags deployment; done by calculating the day length to resolve latitude and solar noon to resolve longitude (Figure 2.). There is a fine balance between tag efficacy on a live animal and its operational lifetime. That is to say, we need to keep the tags small (they are about 40g) so as not to negatively impact the fish’s swimming ability, but that means that there isn’t room for large battery units. Couple that with the fact that the tag must send all the data back through a network of polar-orbiting satellite (these “see” a tag at the poles 14x per day – this rate decreases with latitude) and what you end up with is a lot of work for a small tag. Because of this, data from the temperature and depth sensors is sent back as daily summaries. BUT, and it’s a big but, if you physically retrieve the tag you can gather every piece of information present on the tag’s hard drive and look at the temperature and depth of that animal every 5-15 seconds for up to a year. This data is very, very hard to come by.

Figure 2. How light-based geolocation works

Figure 2. How light-based geolocation works

So, when the PTT 144410 (Platform Terminal Transmitter 144414 – essentially a unique ID number) reported from the west coast of Malta in August, the opportunity presented itself for me to recover a tag containing 298 days of high-resolution data from a 400lb bluefin tuna that we released off Ireland in October 2016.
PAT tags transmit data at a known frequency, and whilst a tag is trying to send its data to a satellite one can triangulate its position using the Doppler effect (Fig 4.). This method can provide a location to within ~250m (or 10km+ at worst!), but you may only get a couple of these good locations a day. To get you that final 250m to being on top of the tag you use a PTT locator, which is radio receiver with a speaker tuned into the frequency that the tag is transmitting data at with a directional antenna. Every 45-60sec the tag tries to send a ‘chunk’ of data to the satellite (the satellite can only receive data when it can “see’ the tag), which is audible when using the PTT locator. Essentially the louder the signal, the closer you are.

Figure 3. Getting a location from a transmitting tag using the Doppler effect

Figure 3. Getting a location from a transmitting tag using the Doppler effect

From the locations that we had already received from PTT 144410 we had deduced that it was likely in the hands of a person. The tag had been moving all around Malta for the past week. At 7:00am on Saturday the 19th I set off with Duncan (!) my AirBnb host, who I had persuaded to come on the tag-hunt, and we drove to the last overnight position of PTT 144410. I setup the PTT locator. “MEEEEEEEEP”, the tag was still within a few kilometres of me. Phew. I had worried that person who had the tag could have already left on his/her daily activities with the tag. After a couple of hours, I found myself stood by a Toyota Land Cruiser and a Volkswagen Golf with a very strong signal, I was pretty sure that the tag was in one of the cars. Not wanting to go door to door early on a Saturday morning I waited for someone to emerge. At about 10:15 a man emerged with his family. Upon being asked by Simeon Deguara (the Research Director at the AquaBioTech Group, who works on bluefin in Malta and was helping me with the recovery) if he knew anything of the tag, he wandered over to the Land Cruiser and produced PTT 144410 from the glovebox. Success! It turned out that the man, Kenneth Gauci, and Simeon were old colleagues, and Kenneth went on to tell us he didn’t find the tag and that a friend of his had found it floating and given it to him (he worked on the bluefin fattening farms off the coast). It is always a worry when a bluefin tag ends up in the hands of a person, that the fish has been caught and is now on a plane to Japan. We won’t know for sure what happened to this fish until we analyse the data properly, but for now it is at least nice to think that, that bluefin is still alive and well. I explained to the Gauci family, over a much-appreciated cold drink, how the tag worked and why I was on their doorstep on a Saturday morning, all the way from England and holding a contraption that would be a better fit for Area 51. It was nice to chat to the family and see their amazement at the fact that the object they were holding had been on a tuna sleigh-ride all the way from Ireland. After half an hour or so Simeon and I left and I began making my way back to the UK.

Back at the University of Exeter’s Penryn Campus, Matthew Witt and I booted up the tag this morning and downloaded 1.8 million bits of information from PTT 144410. Now, to start the process of making sense of it and looking at what that 400lb Atlantic bluefin tuna has been doing since we last saw it in Ireland last year.

Searching for PTT 144410 using the locater device in Had Dingli, Malta

Searching for PTT 144410 using the locater device in Had Dingli, Malta

Success! I’m holding the tag in my right hand. They are veritable needle in an urban haystack! They would be virtually imporssible to find without the locator.

Success! I’m holding the tag in my right hand. They are veritable needle in an urban haystack! They would be virtually imporssible to find without the locator.