Fieldwork in the Western Atlantic

Aidan Donnelly

University of Exeter

All marine teleost fish produce calcium carbonate in their intestine – this is a byproduct of their osmoregulation and occurs so that they can stay hydrated in a salty environment. It is estimated that fish could be producing up to 9 billion tonnes of calcium carbonate in the ocean annually! In the Atlantic, these fish carbonates have been found in sediment traps off the coast of Bermuda as part of the Oceanic Flux Program run by the Bermuda Institute of Ocean Sciences (BIOS). I had the privilege of being able to go and visit them in May 2025 to further investigate these particles and try to sample the fish populations that could be responsible for them. This was the first research cruise I have joined and it was definitely an experience – setting the bar high for future cruises too!

Now if I asked you to think of Bermuda and think of cruises your mind might first think of the huge American and European party ships? Fortunately, this was not the case for my trip, I visited BIOS to board the RV Atlantic Explorer (below left) thanks to the travel grant from The Challenger Society. BIOS is a small research station, based in the North of Bermuda, and owned by Arizona State University – it is a very closeknit group of scientists who do fascinating work on all things oceanic - with multiple cruises leaving every month on board their research vessel.

On the day of mobilisation, I found my (very cosy) cabin, met my shipmates and the lab group I was joining, and enjoyed the final moments of standing on a floor that wasn’t moving… We departed Bermuda at 0630 travelling out to the middle of the ocean, this is where things started to go downhill for me. It was very humbling trying to walk along a corridor, following the captain who was very nonchalant about the movement of the boat, and being thrown from wall to wall – this was also not great for the seasickness. No seasickness medication was a match for the fight the Western Atlantic was putting up. Fortunately, the swell eventually began to settle and in the evening of the first day I had some work to do – we deployed a Reeve net, a 1 m ring that funnels down to a cod-end, which was sent down to 200 m then winched back up to the boat. Once the net was back aboard it was look in a bucket time to search for what we caught! From this tow we caught a few juvenile fish and some mesopelagics, brilliant! Now onto the hard part – it was time to dissect the smallest fish I’ve ever attempted while also on a moving boat, brilliant. The fish were very delicate and it was tricky to start with, but I soon realised the easiest way is to dissect was between the rolls of the boat. I successfully removed the intestines from the fish we caught and placed them in some bleach to remove any of the organic material surrounding the carbonates that were present inside the intestine. That same day we deployed a Multiple Opening and Closing Net with an Environmental Sample System (MOCNESS) at 2330, this was sent down to 1 km deep, then winched back up over 4 hours. While reeling the net in it is possible to open and close different nets (hence the name) to separate fish across different depths, in this tow I managed to capture more juvenile fish which were dissected and preserved. It was now well past 4 am so it was time for bed to rest for the next day - regardless of the rough start, day 1 was a success!


Day 2 was more relaxing with no fish work scheduled during the daytime which meant I got to catch up on sleep from the late night before! At 2100 we deployed a MANTA tow – this gets pulled along the surface of the ocean and will target epipelagic fish who are present closer to the top of the ocean. We towed this for 45 minutes, eagerly watching to see if we could spot any fish entering the net, however, all we saw were two (very clever) flying fish that managed to jump over the opening of the net and escape! We did manage to catch two other flying fish and a sargassum fish which were dissected and preserved. Later that evening we deployed our 2nd MOCNESS tow of the cruise – this time we only sent the net down a few hundred metres then slowly winched back up while we patiently monitored (drank lots of coffee, ate lots of cookies) and hoped for fish to be caught. In this tow we caught a 20 cm dragonfish (below), it was the first time I’ve ever seen one of these in real life and it was freaky! Being the first time I’ve ever seen one it also meant I didn’t know the anatomy of its gut – which ended up spanning the entire length of its body! These were all the fish that were caught on day 2, so another success!


On day 3 we deployed our final MOCNESS tow during the day to compare to our nighttime tows. In this I caught my most fascinating sample: another dragonfish (left), however this time it had food in its stomach (potentially from its dinner the night before?). This unique sample means that we can begin to infer the kinds of diets these fish are eating, when they are eating / excreting and how they might be moving inorganic carbon between depths in the ocean.


Overall, the cruise was very successful for our first investigation into fish produced CaCO3 in the open ocean. Future work will hopefully allow us to fully elucidate the effect that fish have on the marine inorganic carbon cycle – we hope that our data will be able to better inform ocean models so that we can better predict how the oceans will change in the future. I was also fortunate enough to have time to explore the island after completing my lab work and enjoy the endless beaches and snorkelling that Bermuda has to offer! The trip taught me many lessons and has opened the door to many future opportunities which I am incredibly grateful for and hope to explore. When I arrived back in the UK I was able to give a talk to fellow researchers at Exeter about the work I carried out and our hope for future research. I would like to thank the crew onboard the RV Atlantic Explorer, the researchers at BIOS who were incredibly welcoming and brilliant in assisting me in my lab work, and who also introduced me into the fascinating world of plankton (a sentence I never thought I would be saying!), and to The Challenger Society who without, my fieldwork would not have been possible.


Profile:  I am a second year Masters by Research student at the University of Exeter, where I am supervised by Professor Rod Wilson. My main research aims to investigate the factors that affect calcium carbonate produced by marine fish. In the laboratory I have looked at the effects of different diets on the amount and biogeochemical properties of gut carbonates, and in the field I have begun to investigate the makeup of fish carbonates from wild fish. I have always been fascinated about how animals work and their physiology – my research allows me to investigate how marine animals are adapted to their environments and how they respond to potential future change or different scenarios like aquaculture.