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Searching for a needle in a haystack

By Sophie Prendergast, 7th December 2023

Recovering non-invasive biologging packages in the heart of the North Atlantic


Archival biologging tags (a single sensor) and purpose-built packages (a collection of sensors deployed at the same time in a purpose-built housing) are important tools to assess the underlying mechanisms that influence a species distribution, space-use patterns and how they interact with the environment. However, unlike most satellite tags, which transmit collected data via satellite, multisensor high frequency archival tags, which collect massive amounts of data, must be recovered to access the information. This can present challenges when working with a highly migratory species such as the sicklefin devil ray, Mobula tarapacana, as the species can travel large distances within a short period of time. We try to avoid potential tag loss by deploying our archival biologging packages for between 1 and 10 days. Most of the time, the tags surface within a reasonable distance from the tagging location and it takes less than 24 hours to recover. However, on occasion the packages surface much further from the tagging location, which requires a multi-day recovery expedition.

Our first-time seeing land (left – Faial Island; right – Pico Island) after traveling over 120 nautical miles from port to recover 3 non-invasive iPilot biologging packages. Photo © Sophie Prendergast

In July the EcoDivePWN team deployed 3 non-invasive biologging packages on sicklefin devil rays at Princess Alice Seamount, located approximately 45 nm from Faial Island. After 4 days the tags released and floated to the surface, with two of the tags surfacing 90 nm and 120nm from Faial. The third tag surfaced closer to the tagging location. The ribs normally used for recovery do not carry enough fuel to journey 120 nm from shore and back, so my colleague Jan Faktor and I boarded a small sailboat to retrieve the tags. During the day we would take turns manning the rudder, nap, look for whales, and chat. At night we took steering shifts, rotating every 2 hours.

The team used a VHF radio to locate the tag and navigate as close to the tags location as possible before using large flashlights to search for the tag in the darkness. The second tag was recovered during the early hours of the morning, before sunrise, approximately 90nm from Faial Island. Photo © Jan Faktor.

After sailing through the first night and most of the next day we reached the furthest tag. With no time to waste we set our course back to shore and after 30 miles we reached the second tag in the early hours of the morning. However, it was still dark, which adds an additional challenge to the recovery. After narrowing down the search with the VHF antenna and sailing in many circles with our flashlights pointed to the sea, to detect the glow from the tags reflective tape, we found the second tag. The seas were calm for most of the morning so to celebrate collecting two of the three tags we stopped the boat for a few minutes to jump in and swim.

The final tag was collected 20 nm from Princess Alice Seamount around mid-day on 25/07/2023. This image, by Sophie Prendergast, shows Jan Faktor just after scooping the last tag from the sea making for a successful recovery expedition! Photo © Sophie Prendergast

We spent the third day sailing towards the last tag, escorted by numerous pods of spotted, striped, and common dolphins. During my steering shift on the 3rd night, I watched the moon set below the horizon, and counted the shooting stars to keep myself awake after the previous, sleepless, night. The lack of sleep was worth it though because the Azorean night sky is like nothing else and at sea, with almost no other light, it is even more impressive. As the boat cuts through the water the bioluminescent plankton sparkles mirroring the sky above. On the 3rd night the water was so calm, and the bioluminescence was so strong that you couldn’t tell where the sky met the sea. It was as if we were sailing through space.

On the 4th day we finally recovered the last tag and were able to return to Faial. We saw land for the first time later that afternoon and entered the Port on the evening of the 4th day eager to examine the data. With the right boat, proper equipment, and a little bit of luck we are able to retrieve tags no matter where they may be.

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