It has been two weeks since I came back from a field trip to the Dutch Caribbean and now its time to look back and reflect on how everything worked out. The goal of the trip was to expand the existing network of acoustic receivers in the Dutch Caribbean EEZ and to catch and tag additional sharks in the area to study their movements. Please read my trip reports of St. Maarten and Statia and my initial post about this project for more information, in this post I will focus on the island where I was stationed for the longest time during the trip: Saba.
Unlike the work on St. Maarten and Statia, the work on Saba was a bit more complex. This year we wanted to place receivers and tag sharks on the immense Saba Bank. To give you an idea about the size of this bank compared to the islands we worked at, the Saba Bank is the dark blue area Southwest of Saba. With a total surface area of 2200 square kilometers (for comparison, Saba is 13 square kilometer), we were pretty sure we needed a bit more time to get our work done here.
But before we went to the Saba Bank it was time to retrieve the receivers that are in the waters around the island itself. Since October 2014 8 acoustic receivers have been recording the acoustic signals emitted by tagged sharks, my job was to retrieve this year-round data and to replace the batteries of each receiver. This means a full day of tracking down the 8 receivers around the island, dive them up, race back to the office, download the data, replace the batteries, race back to each location to place the receivers back underwater before sunset. Since the receivers had been in the water for a year, I had to clean some of them because of growth on the housing. There were even some little crabs, brittle stars and bivalves on living on some of them! So, this is what a new receiver (left) and a receiver that has been in the water for a year (right) look like next to each other. (side-note: imagine bringing two of these new receivers through airport security, yes.. that was an adventure!)
So after some diving around Saba, it was time to prepare for the job on the Saba Bank: place 8 receivers and catch and tag nurse sharks (Ginglymostoma cirratum) and Caribbean reef sharks (Carcharhinus perezii) with acoustic tags. Right from the start we knew this was going to be the most difficult area to work with and to get everything done in time. The positions of the receivers on the Saba Bank ranged between a 30 minute to a 2.5 hour boat drive, making it impossible to place more than three receivers on one day. Since you don’t have any protection from nearby land, sea conditions on the Bank can be significantly worse compared to the waters off Saba. Maybe the most important challenge was dive safety: since the Saba Bank is a marine park we didn’t want to anchor the boat, since that is going the destroy precious corals. That means that the persons on the boat should always have an eye out for the divers in the water to rule out the possibility for a diver to get lost at sea. I kind of had that feeling once, after coming back to the surface and not seeing the boat or any of the nearby islands (Saba or Statia).
The thing that makes the Saba Bank very special is that there’s probably only a handful of people that ever dove on those dive spots and the fact that nobody knows exactly what is down there. There’s only 6 fishermen who fish on the immense Bank and there is no recreational diving allowed. So when jumping off the boat you get this sudden feeling of extreme excitement and curiosity, but nothing in the world couldn’t prepare me for such a beautiful reef system!
Reefs as far as the visibility reached (>40 meters) and huge schools of fish, some of the dive spots probably even had one the highest fish abundance I’ve ever seen. Barrel sponges and brain corals of more than two meters across and all the reefs were looking incredibly healthy, further underlined by the diversity of fish living on and in the reefs.
There was only one harmful fella on the reef: the invasive lionfish. Ever since there were some individuals (accidentally) introduced in Florida in the 1990’s the lionfish is spreading extremely fast throughout the West Atlantic and Caribbean Sea, out competing other reef predators and decimating reef fish. To protect the reefs divers start hunting them with a spear, but the fast reproductive nature and their depth ranging much deeper than divers ever can dive to allows this fish to thrive in these waters. Coming across a couple of lionfish we tried to attract some sharks by hanging over the drop-off of the reef (a sudden drop of the bottom into great depths) with the speared lionfish still on the spear. Unfortunately without any luck.
After quickly placing the receivers, we always took our time to look around and to enjoy the beauty of the marine life surrounding us, after all I think that is the most important part of the job! One of the best dives was during an early morning, we set out at 06.00 to place three receivers and try to catch the first sharks on the Bank. On the third dive me and Dahlia placed the receiver on the most beautiful reef system ever, close to the drop-off. Just at the end of the dive we noticed that this was a perfect spot to start fishing, we just saw two juvenile Caribbean reef sharks cruising by along one of the smaller drop-offs! So after resurfacing we quickly prepared our rods and immediately started fishing. Not long after one of the rods went off, and we worked the shark up. Almost at the surface we could see it was a small Caribbean reef shark!
Once the shark was close to the boat it was time to bring him/her up on the boat for a quick and smooth tagging procedure. As soon as the shark was on the boat it is key that the shark is turned on its back as quickly as possible to initiate the trance-like state called ‘tonic immobility‘ and to limit stress. This state acts as a natural anesthetic and is used by shark scientists around to world. Once the shark was calm the procedure would start with taking out the barbless hook, applying the acoustic tag and quickly take notes as sex and total length. This whole procedure would only take about 5 minutes before the shark is placed back in the water.
That day turned out to be a huge success, as we tagged 4 sharks: 3 juvenile Caribbean reef sharks (~100cm) and 1 nurse shark (206 cm). The next day we left port even earlier (03.30) as we wanted to fish further away from Saba. We started fishing under a beautiful sunrise and that day we caught two more juvenile Caribbean reef sharks which were also close to 100 cm in total length. Without a doubt the best birthday I ever had!
What surprised me is that we only caught juvenile Caribbean reef sharks all around 100 cm, possibly indicating that these sharks were all of the same age and maybe even related (we happened to catch all of them in one certain spot). This surprised me because I didn’t expect juvenile sharks to spend their years growing up in an area which is known for its bigger predatory sharks (tiger sharks and hammerheads) that form a potential threat for smaller, juvenile sharks. However, we caught the juveniles on a shallow reef area, and normally the bigger sharks stay in deeper waters or near the drop-off. The fact that we manage to tag a certain age group of this species is very exciting for future studies.
Once the work on the Saba Bank was finished I spend some more days on Saba helping out other students during some dives and pre-analyzing my data. The reefs of Saba and the Saba Bank are extremely beautiful, as is the marine life that makes these reefs their home.
Now back in The Netherlands I am back behind a desk, making sense of huge R codes, analyzing the shark data from Saba and writing a paper about it. Once my analysis are finished I will post some preliminary results here on my website, after all it is the first acoustic shark tagging data from Dutch waters!
My awesome time and the work wasn’t possible without the help of the Saba Conservation Foundation and all its awesome crew: Dahlia, Koki, Jelle, Leslie, Kai, Pouchie, Gia. People that helped us out on the boat (and with a boat!), Laszlo, Jarno, Serena, Kenji and Bradley. And my awesome Dutch friends: Céline, Jonas, Youp, Rick, Rimco and all the rest of the people that I’ve met during this awesome field trip! Special thanks to Youp, who made some stunning photos of the tagging days.
See you next time Saba.