After months of radio/blog silence (I did not want to bore you with another photo of my computer screen) I finished the manuscript of my PhD thesis just before the deadline of 1th of October, Woehoe! The last weeks I lived a life comparable to a burrowing shrimp: I lived inside my office, only shoveling the manuscripts over to all my supervisors on a regular basis.
I will defend my PhD-thesis titled “Seagrass systems under nutrient loads, hydrodynamics and green turtle grazing – Do green turtles rule the seagrass world?” at the Radboud Universiteit Nijmegen on March 22nd 2013 at 13:00. Now it is time to go on seagrass expedition to Bank D’Arguin, Mauretania.
Underwater meadow. Seagrass beds, like this one teeming with silverside fish off of Indonesia’s Bone Batang Island, are key habitats for young fish as well as the occasional larger mammal.Credit: Marjolijn J.A. Christianen
In total now we captured and measured more than 800 green turtles around Derawan and tagged 500 of them. The WWF guys (Udin, Darjon), some locals (Jeffrey, Tiar) and we catched them using the rodeo technique. This year we recaptured some individuals that we tagged previously in 2009 on Derawan Island. These turtles grew 3-6 cm (curved carapace lenght) or 3-10 kgs. On her first day on Derawan my sister Sabine saw green turtles in all kind of sizes and activities; she saw an female laying eggs, and small hatchlings coming out of their nest and she was my research assistant while cathcing green turtles in the seagrass meadow. Again a lot of pictures, enjoy because this will be the last pictures from the field for a while!
One of the cool things of seagrass meadows is that at first sight it might not seem too rich in animal life. But when working for hours on the seagrass around our cages and sandbags you find cool new animals that you might miss normally. In the last weeks I went out to make the perfect green-turtle-grazes-on-seagrass-picture but every time the visibility was crap or it was too deep or the current was too strong but then during the struggle under water I found 2 of my favourite animals (just in front of my house !): the robust ghostpipefish and a orange frogfish (species still needs identification).
BTW: I can recommend the sandbags to anyone interested in studying the settlement of flora/fauna.
To get an idea of green turtle densities on foraging grounds around the Derawan Archipelago I hopped over the border to Malaysia to visit the islands around Semporna (Mabul, Sipadan, Pom Pom, & Pandanan). Steve Oakley and Celebes Beach resort were so kind to host me on Pom Pom Island. Read the blog post by Steve Oakley on our work here, also if you are looking for a student project on turtles. High densities on the seagrass here! But Derawan turtle densities are still on top. Turtles here are more restricted to a limited time frame for grazing due to a different tide. On Sipadan I found a lot of juvenile turtles that grazed on hairlike Halodule uninervis in only 15cm of water. On the coral reef turtles could be approached so close that I could read the tag numbers and most of the subadult turtles had flippertags all tagged on Sipadan itself. The turtles seem to stay for a long time on the same place. The Green turtles are real fish-spa addicts, so many times you see the green turtle stretching out in a cloud of cleaner wrass and cleaner emperor fish, and on Sipadan I saw a group of 8 turtles fighting (biting!) over taking over the best spa-spot on a big rock.
The last fieldwork of my Phd is finished! While writing this blog post in Dubai on the way back to the Netherlands the dried seagrass samples patiently wait on the seat next of me, together with the other 70kg of equipment where I travel with. The first unofficial results are promising. On the aerial pictures we clearly see different patterns of gaps on the 3 different stations that lie on increasing distance from the beach.
The grazing effect on the seagrass is enormous, which is clear if you remove the cages that kept away the grazing turtles. Seagrass regrows quicker into gaps behind wave barriers. And the effect of waves and grazing seem to differ between the stations, so now it is time to spend some hours analyzing the huge amount of data that we have collected and write the results up.
In these last months we had quite a lot visitors. Research (photo) journalist Hans Wolkers visited a few weeks to write an article about our research here. With his too infected he could not photograph the turtles during his last days so he ended up helping to sort out numerous seagrass samples 🙂 . And we also had family/boy/girlfriends of Iris and Peter and my sister around. it was a lot of fun having him here. Hans, Sara, Sabine, Jelco, Ger thanx a lot for the help!
After crashing my aerial photography plane directly after the first flight, we retrieved the bits and pieces (the plane from the top of a 70m telephone tower & the catapulted-rest by logging a 20m perimeter of bushes), and shipped the plane around the world to get repaired. But 1.5 months later DHL delivered it in Berau and we could fly. And the footage is really cool: here are some first sneak peaks from the air.
High palm trees/ phone tower are not the only obstacles to fly on Derawan.
Obstacle 2: While looking at the flight track suddenly the plane flew 180 degrees the other way than planned, the plane was out of sight so we wondered what the disturbance could have been. The plane corrected itself and safely landed but part of the frame was missing! We repaired it with syrofoam and during our test flight it became clear what did this: Sea eagles! A 1.5m wingspan sea eagle flew 100m above the plane and pulled in his wings in to rocket down on the plane, so after some quick steering I manage to make quick turns and with some shouting the plane escaped from the bird. Every next flight I gave 2 children the task to report any fish eagle and called the plane to the ground until the coast was clear of eagles.
Obstacle 3: Children: The only place to land the airplane is on the “Lapangan” the soccer field right in the middle of the schools. Everytime that I come walking around the corner with the interesting suitcase the teachers don’t manage to keep the children in there classroom and it would be handy to have 8 hands extra to keep the hands of the plane, get the field cleared & stop the kids from running over the plane.
Our second destination in our quest to find the green turtle foraging grounds was Raja Ampat. Some of the tracks of the GPS tagged turtles show foraging greens that travel all the way from Raja Ampat to Kalimantan. We were based at Kri with papua diving and did 2 short expeditions; 1 to Batanta & 1 to Wayag – Sayang. We planned our route using the aerial pictures of Max Ammer and the tips of his friendly staff. During the first trip we saw a dugong feeding at dusk on Enhalus leaves in front of the mangroves. The majority of the seagrass meadows were strips of multispecies (incl Enhalus) meadows in front of mangrove lined islands. Because we were the first foreigners to overnight in the village people did not know what to with us and we ended up sleeping in a empty information house on the jetty. In this area the fisherman are mostly Biak people that migrated here some decades ago. The Biak people explained us that green turtles are still hunted because of the tasty meat, in contrast to dugong meat that they did not like too much. Hence not too many foraging turtles, but a very nice trip. Here is a map of our trip and below it, you can see images of the trip.