Posted: Wednesday 4th October 2017 by The Isles of Scilly Wildlife Trust

When does a little black tag become a needle in a haystack? When it "falls" off a Tuna, drifts into Scilly and ends up on a boulder beach full of seaweed and Portuguese Man O'War. But, no amount of needles and haystacks will prevent Tom Horton from discovering his prize!

Guest Blog written by Tom Horton

Over the late summer and autumn months, visitors to the western coasts of the British Isles might see splashing at the surface of the water. While dolphins, seals and basking sharks could be spotted, Atlantic bluefin tuna are now being increasingly reported, bursting out the water whilst feeding on shoals of small silvery fish.

Bluefin tuna are one of only a handful of fish (one of a group of 30, of ~25,000 fish species) that are “warm blooded”, or endothermic, meaning they can exploit food-rich, but cold, regions like the northeast Atlantic. To stay warm, they need to feed often and on high quality food, such as herring, mackerel, sardines and sprat. It is this constant need to feed that brings them to the waters of the northeast Atlantic at the end of every summer and drives the frenzied feeding behaviour that make them so conspicuous. 

There are three species of bluefin tuna: Pacific, Southern and Atlantic. All three species’ have been heavily fished for decades. The global bluefin fishery is driven by an insatiable market demand and a seemingly limitless price-tag in the Japanese sushi-sashimi market, in which bluefin tuna is the most highly-prized delicacy (a single 222kg fish fetched £1.3 million at the season opening auction in Tsukiji fish market, Japan in 2013). All three bluefin are now listed as endangered, or critically endangered, by the World Conservation Union.

Studying the movements of wild animals in the marine realm is hard work, and studying bluefin tuna is no exception. They are highly migratory and are capable of circumnavigating the North Atlantic Ocean in a single year. Their migration patterns vary greatly, which poses major issues for those charged with their conservation and exploitation (this largely comes down to managing fishing pressure across their distribution).

Studying Tuna

At the University of Exeter, we have been researching the movements of bluefin tuna that aggregate off the coasts the British Isles for the past three years. 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 hence can breed). In addition to our own work, the International Commission for the Conservation of Atlantic Tunas (ICCAT; the body charged with managing the bluefin tuna fishery and conducting stock assessments) have been running the Atlantic-wide research program for bluefin tuna, since 2008.

The principal method for studying tuna migrations is by attaching pop-up archival tags (PATs) to bluefin and releasing them back into the ocean. These tags are mounted externally and record light, temperature and depth continuously every 15 seconds for up to a year. On a set date, the tag detaches from the animal and floats to the surface, when it then begins to transmit a summary of the archived data back to the tag owner via the Argos System satellite network (Figure 1: Schematic of how a PAT tag works on a bluefin tuna.). 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: How light-based geolocation works).


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 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,if you physically retrieve the tag you can gather every piece of information present on the tag’s memory and look at the temperature and depth of that animal every 15 seconds for up to a year. This data is very, very hard to come by.

So, when tag ‘16P2234’ reported from St Marys, Scilly on the 11th of September I received an email from our colleagues at ICCAT asking if we could collect the tag.

The tag, which was deployed on a bluefin tuna in a Portuguese tuna trap on the 11th of July 2017, had now detached and contained 60 days of high-resolution data. 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 physical principals of the Doppler effect (Figure 4: Getting a location from a transmitting tag using the Doppler effect).

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 tag locator, which is radio receiver tuned into the frequency that the tag is transmitting data at with a directional antenna. Every 45-60sec the tag tries to send data to the satellite (the satellite can only receive data when it can hear the tag), which is audible when using the tag locator equipment. Essentially the louder the signal, the closer you are.


From the locations that we had already received from ‘16P2234’ we knew it was most likely on the northwest coast of St Marys, after having detached from its host-fish about 50 nautical miles northwest of Scilly on the 8th of September and drifted onto the island.



A couple of hours after receiving the email I was on a flight from Land’s End airport to the St Marys and it just so happened I was sat next to Sarah Mason, the Chief Executive of the Scilly Wildlife Trust. We exchanged a few words on the flight (although I was fairly busy staring out of the window at the blustery passage between the mainland and Scilly) and then shared a taxi when we got to the Island. I explained what I was doing and she very kindly said that she would put the word out. I continued onwards with Paul from Toots taxis the best taxi driver I have ever met, he even delivered my bag to my hotel so I didn’t have to trudge around with it) to the Telegraph, where I continued on-foot. I walked to the last overnight position of ‘16P2234’ and setup the tag locator. “MEEEEEEEEP” was loudly heard from the speaker the tag was still within a few kilometres of me.

It’s always a worry that the tag could have moved from its last reported location due to large tides and storms that we’ve experienced recently. After a couple of hours, I found myself stood by a giant stack of weed that had been washed up by the storm, complete with Portuguese Men-o War (see Nikki’s previous blog on these) with a very strong signal, I was sure that the tag was in this pile. After a second check, it turned out the strongest signal was at a slightly more favourable patch about 50m away.

The beach is granite boulders and the signals seemed to be reverberated, making the tag tough to locate. I had had a very welcome offer of a second pair of hands and eyes from Nikki Banfield, Communications Officer at the Scilly Wildlife Trust and as I started digging around in the weeds in the area I had decide that the tag was in she arrived on the beach with her young spaniel Eysa in tow, brilliant!

Now the race was on to find the tag in the rocks and storm weed before dark and the tide coming back in. After a few location refinements and an hour and half of sifting through weeds and moving granite boulders I had us zeroed in on a small area. Just as we thought it might never happen Nikki pulled the tag out from a crevice a foot from my head. There it was, the tag in all its glory!


We won’t know for sure what happened to this fish until the data is analysed properly, but for now it is at least nice to think that, that bluefin is still alive and well. 


This is the first bluefin tag to wash up on an English coast and it is the first tag to provide a clue as to the life histories of the fish that have returned to our shores in recent years. We are in the process of establishing a research project to study bluefin tuna off the coast of Southwest England and hope to learn more about these enigmatic creatures. So, watch this space for news on that project and we’ll be sure to share some 16P2234’s secrets as soon as we learn them.

Sincere thanks to the Sarah and Nikki (and Eysa) from the Isles of Scilly Wildlife Trust and Dr Matthew Witt for logistical support though the process.

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