We woke up this morning at a delightful 7am in order to go to Culebrita to look for more sea turtles; however, the boat ride turned into quite an adventure! The waves soaked us all in the boat and it was only supposed to get choppier out, so we decided to go to the closer Manglar Bay to net for green turtles again (the same place we went a few days ago). The Manglar area is known for foraging green turtles because of the vast amount of sea grass – the food of choice for these turtles. Green turtles play an important part in the ecosystem because they trim sea grass to allow for younger grass, which is full of nutrients and energy, to grow.
It appeared that the turtles were hungry this morning because as soon as we deployed the net, almost every snorkeling pair was wrestling with their own giant green turtle. We worked to untangle the turtles and take them to waiting boat, while trying to avoid becoming entangled ourselves (our partners kept watch). We ended up catching ten green turtles in around just 30 minutes (the record is 17 turtles in 45 minutes). Three of the turtles weighed more than 50 kg, the maximum our scale could measure!
We recorded information about the turtles in the same way we did previous days, but this time we got to watch Carlos and Matthew put PIT tags in two of the smaller turtles. PIT tags are small, rice grain sized tags that carry a unique identifying code. PIT tags are inserted under the skin either midway on the flipper or closer to the shoulder area (the triceps muscle). There is some debate about which location is best for placing a PIT tag, related to long-term retention. Ideally, when researchers re-capture the turtles, they should be able to detect the PIT tags using a scanner, and thus know the capture history of the animal, and its changes in size and weight. However, PIT tags are more expensive than external flipper tags, so the Culebra project uses PIT tags only on smaller turtles that are likely to be recaptured.
In the afternoon we went into town where everyone bought Culebra souvenirs to bring home (t-shirts, stickers, postcards, etc.). We all got ice cream (passion fruit, coconut, and soursop flavors!) from a street trolley on the main plaza and walked around to all of the shops downtown before coming back to the house to eat leftover burritos for dinner.
After dinner, Wendy gave lectures on the ecological roles of turtles and their sensory physiology. Wendy’s own research focused on sea turtle hearing and behavioral responses to noise, which previously had received little attention from researchers. Earlier research suggested that sea turtles were deaf or had very poor hearing. Wendy’s research focused on measuring electrical impulses from the turtle’s ear to the brain (called auditory evoked potentials) in response to different frequencies of sound. She found that all the species of turtles she tested had similar hearing abilities, and were especially good hearing at low frequencies (possibly even lower frequencies than people!).
We also talked about turtle vision and chemoreception. Loggerhead and green turtles can see in color, particularly green and yellow wavelengths, which correspond to wavelengths that occur where these turtles spend most of their time (near the coast). Leatherbacks, on the other hand, see blues best, which correspond to the predominate wavelengths found in deeper water in the open ocean, where leatherbacks spend much of their time. Turtles sense chemicals, analogous to our sense of smell, with large olfactory glands found in the nasal cavity. They may use this sense to detect prey, avoid predators, and possibly find their way back to their nesting beach. Knowing how sea turtle senses work is important to their conservation: researchers are currently trying to find ways to deter sea turtles from hooks and nets to reduce bycatch while still attracting their target fish. One method recently found to work is attaching green lights to nets, because they illuminate the nets causing fewer turtles to be caught as bycatch.
After lecture, we went out for another night walk on Zoni Beach, the leatherback nesting beach. The moon was so bright that our headlamps weren’t needed at all. We walked down the beach enjoying the stars and the ocean, keeping our eyes out for leatherback turtles. It was a bit cloudy for great star gazing, but we were able to spot the Big Dipper and Scorpio when the clouds moved. Part way through our last walk down the beach, it started to rain so we gave up our search early. We didn’t end up seeing any turtles, but there will be plenty of time to see nesting leatherbacks in a few days when we are in St. Croix!
-Lizzie and Eleanor
Click on this link to check out our last day of netting for green turtles! : medium