Social behavior of green turtles in Western Australia

 Dr. Jordan Thomson

Since 2011 we’ve been using animal-borne video cameras to study the behavior of sea turtles in Shark Bay, Western Australia. Since sea turtles can be hard to observe in the wild for meaningful lengths of time without disrupting their natural behavior, very little is known about how they interact with one another (aside from courtship and mating, when they’re relatively stationary and, well, otherwise occupied). Animal-borne video cameras overcome this challenge by remotely recording the first-hand perspective of a turtle for several hours as it moves freely throughout its habitat and interacts with other turtles. The resulting footage is a treasure trove of insight into their secretive social lives!  To see some of the footage visit the lab YouTube channel.

In 300 hours of video footage recorded from the backs of 93 green turtles, we recorded about one turtle encounter per hour of video – a fairly high encounter rate considering the generally solitary nature of most adult sea turtles. We recorded a huge variety of interactions that ranged from brief one-on-one encounters lasting just a few seconds to behaviorally diverse encounters lasting up to 20 minutes and involving up to seven other turtles.

The main finding of our study was that the nature of turtle encounters depended on what type of habitat it occurred in. Shark Bay is a large (13,000 km²) bay with expansive seagrass meadows, which turtles use for feeding. Hard-bottomed, vertically structured habitats like rock ledges and coral heads are relatively rare and occur mostly in the deeper areas of the bay. Despite their limited availability, structured habitats are really valuable to sea turtles because they use them for a variety of critical non-feeding activities like resting or seeking refuge from predators (Video 1), rubbing their bodies on hard surfaces to “self-clean” by removing epiphytes like algae and parasites (Video 2) or posing to allow fish, like the black and gold fish in the background in this video (Video 3), to clean them in a “mutualistic” interaction where both species benefit (fish get food, turtle gets cleaned to reduce drag while swimming).

Since hard-bottomed habitats are so rare in Shark Bay, and feeding sites (i.e., seagrass beds) are so plentiful, we hypothesized that we might see competition for access to resting/refuge/cleaning sites while we shouldn’t see competition for access to feeding sites. And this is in fact what the video showed. Turtle encounters over seagrass beds tended to be brief, innocuous and typically involved just one other turtle (Video 4). In contrast, turtle encounters in hard-bottomed habitats were much more frequent (Video 5), despite the fact that turtles spent only 5% of their time in these areas, often involved several individuals, and regularly involved physical contests between turtles for access to these spaces (Video 6). This competition could be important to individuals and populations, since less competitive turtles would be displaced from the best resting/refuge/cleaning sites and could experience higher levels of predation or have to spend more energy while swimming (due to drag) as a result.

For more information on this study, check out the article in Marine Ecology Progress Series (http://www.int-res.com/abstracts/meps/v532/p243-256/) or email me for further information (jathoms@fiu.edu).

p.s. if you’ve ever wondered what a Western Australian sunset looks like from the perspective of a sea turtle (haven’t we all?), wonder no more (See the video).

Sea turtles of Abaco, Bahamas

PhD Student Elizabeth Whitman has been working on green turtles of Abaco Bahamas since last summer.   As part of her research she put up large cages (exclosures) that keep turtles from grazing on the seagrass.  By placing exclosures in different areas she hopes to figure out how important turtles are in structuring seagrass beds and whether their importance varies in space and time.  Below is a summary of her brief field trip in November, 2014 to check on her experiment.  Check back later this spring to read about her upcoming work to deploy cameras on turtles on several islands in the Caribbean!

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This past November I revisited my study sites on Abaco to collect data and check on the condition of the exclosure cages.

Linda Gardner performing seagrass shoot counts in one of the full exclosure cages.

At each of the exclosure study sites (Bight of Old Robinson creeks, Snake Creek, and Hill’s Creek) we first visited each plot and marked 10 seagrass shoots that we would collect one week later to measure growth. I was pleasantly surprised at the good condition of the exclosures (they have been out for months now), so over the course of our trip we only needed to perform minor repairs and cleaning. We also surveyed the seagrass and algae, measured seagrass canopy height, and counted shoot density within each plot . 

The location of seagrass shoots marked for growth measurements are marked by the white flag; seagrass and algae surveys were conducted within 50 x 50 cm quadrats at four locations within each plot

Visually, the seagrass appears to be benefiting from the protection from grazing at Hill’s Creek, but I am still working on the analyses and it might take more time to see a big effect if there is one. With the help of local volunteers who will be visiting the cages during my absence, this experiment will run through the summer of 2015 when I will be back for my next round of data collection. By keeping the exclosures in place for one full year, the I will get a more accurate picture of green turtle grazing behavior and the effects of green turtle grazing on the seagrass in the creeks of Abaco.

Back at the lab at Florida International University I am busy, along with a few undergraduate volunteers, prepping the seagrass we collected for nutrient analysis. I am also actively working to secure support for my summer research trip and to expand upon the sea turtle field course and tagging program with Friends of the Environment.

An exclsure cage viewed from above the water.

A small nurse shark resting in the seagrass.