Tuesday, June 28, 2016

Sharks vs Dolphin

With the Premiere of Shark vs. Dolphin: Face Off on Shark Week, I thought it would be a good time to write a little bit about the current state of the science.  I've been studying shark-dolphin interactions for about 20 years.  Most of my work has been in Shark Bay, Western Australia (www.sberp.org) where I investigated how often tiger sharks attacked Indo-Pacific bottlenose dolphins and how dolphins changed their behavior to reduce their chances of being attacked.  This led to studies of the influence of tiger sharks on the whole ecosystem.  You can learn more about that at the Shark Bay Ecosystem Research Project webpage.  We put together a video-based lesson for K-12 classrooms that lets students join the work that you can get here. Below are answers to a couple of the most common questions I get.

Do sharks really eat dolphins?  How do we know sharks attack dolphins?

The threat that sharks pose to dolphins is greater than we once thought. Most shark species won’t attack dolphins, but there are some species that do! Although it is very rare, people have seen and filmed dolphins being attacked by sharks.  That means that we have to use other methods to figure out how big a threat sharks are to dolphins.  We can see if particular shark species have dolphins in their stomachs or use chemical markers in the blood or muscle of sharks to see if they have eaten dolphins. 

A scar on the back of a dolphin in Shark Bay
These methods don’t tell us if the sharks attacked a living dolphin or if they ate one that had died already.  By using the scars on the bodies of dolphins that are left behind after an attack, we get an idea of how many dolphins are attacked and, sometimes, what species of shark attacked the dolphin.  In our work in Shark Bay, we found that more than 70% of Indo-Pacific bottlenose dolphins have scars from shark attacks and probably 10% of the population is attacked (but escape) every year!  Around 20% of spotted dolphins in Bimini, Bahamas has shark-inflicted scars and around 30% of common bottlenose dolphins have evidence of shark bites in Sarasota, Florida and Morteon Bay, Australia.  In many locations, shark attacks are very rare.  This could be because populations of large predatory sharks have declined in many parts of the world.

What sharks are a threat to dolphins?

A tiger shark cruises the shallows
Not all sharks are a threat to dolphins and only a few species are probably a regular risk to them.  Within these species, only large individuals will attack dolphins.  Tiger sharks, bull sharks, and white sharks are at the top of the list. Sixgill and sevengill sharks also are likely to take dolphins. Hammerhead sharks, oceanic whitetip sharks, dusky sharks, and mako sharks might occasionally attack and kill dolphins, but it is likely rare and only very large individuals that do so.  The cookie cutter shark is a species that attacks dolphins, but won’t kill them.  Instead it scoops out small bites of blubber and skin, leaving a mark that looks like a cookie cutter made it.

How do dolphins avoid being attacked?

Even for shark species that are major threats to dolphins, it is unlikely that they go too far out of their way to try to eat dolphins.  But, dolphins will make major changes to their behavior in order to reduce the chances that they are attacked!  The success of these anti-predator behaviors is probably one reason that dolphins aren't found in diets of sharks very often. 

A dolphin in the shallows when tiger sharks are not around
The best way for dolphins to avoid an encounter with a shark is to spend their time in the safest places.  This is what we found for dolphins in Shark Bay.  When there are very few large tiger sharks around, many dolphins feed in shallow waters where most of their food (fish) is found.  When the number of tiger sharks increases, these sharks concentrate their foraging in shallow habitats, where there is a lot of prey (other than dolphins).  In response, dolphins shift to feeding more in deep waters that may have less fish but are safer.   In Hawaii, Hawaiian spinner dolphins feed offshore at night and then come into shallow bays with white
Spotted dolphin with two scars
sandy bottoms to rest during the day.  This minimizes the chances that the spinner dolphins run into sharks.  Many dolphins live in large groups, which likely help to increase the chances that predators, like sharks, are detected.  Living in groups can also help reduce the risk to a particular individual because groups can confuse attacking predators.   Some species of dolphins form big groups with other species of dolphins.  These "mixed-species groups" probably help further reduce the risk of predation without greatly increasing competition for food since the species in such groups generally have different favorite foods.

If a dolphin does run into a shark, their best defense is speed and maneuverability.  Dolphins are much more maneuverable than most sharks so they are able to keep out of the way once they see a shark. Dolphins also are faster over the long haul. In Shark Bay, we saw a white shark approach a group of dolphins that didn’t see the shark until the last second.  Even though the shark didn’t attack, the dolphins scattered and leapt away from the shark for a long time before slowing down!  Sometimes, dolphins may group together to “mob” a shark, much like small birds will do to a hawk.  They harass the shark until it swims away.  This behavior has been observed in several locations around the world.

Can a dolphin kill a shark?

They can. Smaller dolphins - like the well-known bottlenose dolphin - will attack and kill small sharks occasionally, but sharks are not a part of their diets.  Accounts of dolphins killing large and dangerous sharks by ramming them in the gills have not been verified. 

Some populations of killer whales (yes, they are dolphins), eat sharks, and can even kill a great white shark.  Off New Zealand, killer whales eat many rays and sharks.  Off the western coast of North America, there are several types of killer whales.  "Resident" whales eat fish like salmon while "transient" killer whales eat marine mammals including seals and sea lions, dolphins, porpoises, and whales.  The "offshore" whales are not as well known and appear to eat a large number of sharks, especially Pacific sleeper sharks which live in fairly deep water. 

False killer whales also have been observed taking dolphins. 

Heithaus lab publications on shark-dolphin interactions:

Connor, R. C. and M. R. Heithaus.  1996.  Approach by great white shark elicits flight response in bottlenose dolphins.  Marine Mammal Science 12: 602-606.

Heithaus, M. R.  2001. Shark attacks on bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia: attack rate, bite scar frequencies, and attack seasonality.  Marine Mammal Science 17: 526-539.

Heithaus, M. R.  2001.  Predator-prey and competitive interactions between sharks (order Selachii) and dolphins (suborder Odontoceti): a review. Journal of Zoology (London) 253:53-68.

Heithaus, M. R. and L. M. Dill. 2002. Food availability and tiger shark predation risk influence bottlenose dolphin habitat use. Ecology 83: 480-491.

Heithaus, M. R. and L. M. Dill.  2006. Does tiger shark predation risk influence foraging habitat use by bottlenose dolphins at multiple spatial scales? Oikos 114:257-264.

Heithaus, M. R., J. J. Kiszka, A. Cadinouche, V. Dulau-Drouot, V. Boucaud, S. Perez-Jorge, and I. Webster. In press. Spatial variation in shark-inflicted injuries to Indo-Pacific bottlenose dolphins (Tursiops aduncus) of the southwestern Indian Ocean. Marine Mammal Science

Selected other publications on shark-dolphin interactions:

Cockcroft, V. G., Cliff, G. and G. J. B. Ross. 1989. Shark predation on Indian Ocean bottlenose dolphins Tursiops truncatus off Natal, South Africa. South African Journal of Zoology 24:305-310.

Corkeron, P. J., R. J. Morris and M. M. Bryden. 1987. Interactions between bottlenose dolphins and sharks in Moreton Bay, Queensland. Aquatic Mammals 13:109-113.

Ford, J. K. B., G. M. Ellis, C. O Matkin, M. H. Wetklo, L. G. Barrett-Lennard, R. E. Withler. 2011. Shark predation and tooth wear in a population of northeastern Pacific killer whales.  Aquatic Biology 11: 213-224.

Kiszka, J., Perrin, W. F., Pusineri, C. and V. Ridoux. 2011. What drives island-associated tropical dolphins to form mixed-species associations in the southwest Indian Ocean? Journal of Mammalogy 92:1105-1111.

Melillo‐Sweeting, K., Turnbull, S. D. and T. L. Guttridge. 2014. Evidence of shark attacks on Atlantic spotted dolphins (Stenella frontalis) off Bimini, The Bahamas. Marine Mammal Science 30:1158-1164.

Norris, K. S. and T. P. Dohl. 1980. Behavior of the Hawaiian spinner dolphin, Stenella longirostris. Fishery Bulletin 77:821-849.

Pyle, P., M. J. Schramm, C. Keiper, and D. Anderson. 1999. Predation on a white shark (Carcharodon carcharias) by a killer whale (Orcinus orca) and a possible case of competitive displacement. Marine Mammal Science 15: 563-568.

Wednesday, June 15, 2016

Turtles, Sharks, and Seagrass of Abaco, Bahamas - Post 1

BRUVs, turtles and seagrass: week 1 in Abaco, Bahamas

By: Laura Thornton

The past week in the beautiful Abaco, Bahamas has been a whirlwind to say the least. Beth Whitman and my fellow members of her intern army, Liberty Boyd and Ryley Parent, began building and deploying BRUV (baited remote underwater video) units for the Global Fin Print Project (https://globalfinprint.org/) on the outer reef. We also ventured out to scout sites for Beth’s study of green turtle foraging behavior and distributions. We checked out a couple of bays for ideal seagrass coverage in order to eventually conduct green turtle, shark (predators) and seagrass (food) abundance surveys. Once we reached our destination of Water Cay (about 20 minutes away), we interns had a crash course on scientific names for local seagrasses and algae and practiced the Braun-Blanquet method for rating abundance.

The next couple days had a very early start to them in order to take advantage of the perfect weather conditions. On the first day e set out with the assistance of Stephen Connett, a turtle researcher, and a volunteer from The Bahamas National Trust to go deploy as many BRUV units as possible (tagging sea turtles after was our reward). After almost 11 hours on the water we deployed twelve BRUV units! After the BRUV deployments, we mustered up the last bit of energy we had for the day to have Stephen take us to a shallow part of Scotland Cay to go find some turtles. We managed to capture, tag and release three green turtles; the largest one being ~26 kg.

The next day was BRUV day round two and we managed 12 more deployments! We learned that chicken wire can slice your hands faster than you can say ouch, and sometimes you have to almost fall off the boat in order to get a good picture (cough cough, Beth). 8:30 PM is a very ideal bedtime in case anyone was curious about the riveting life of a scientist. Overall a great first week of work down here!
A small reef shark investigates a BRUV

Wednesday, May 18, 2016

Sea turtles of Martinique and Guadeloupe

Today we turn over the blog to Caitlyn Webster, who helped PhD student Beth Whitman with her research on sea turtles off the Caribbean islands of Martinique and Guadeloupe!  Coming up soon will be updates on Global FinPrint from Guadeloupe, Martinique and Madagascar!

The turtles of Martinique and Guadeloupe

Caitlyn diving down to survey seagrass
After landing in the tropical paradise of Martinique, Beth Whitman, lab researcher Dr. Jeremy Kiska, and I set out to elucidate the ecological significance of sea turtle grazing on the spread of an invasive seagrass,Halophila stipulacea. We hit the ground running on the first day as we met with local collaborators, including the ever helpful and lovely, Emilie Dumont-Dayot (Coordinater at Rêseau Tortues Marines Martinique), in order to develop an idea of the general seagrass composition within potential field sites. We were quite fortuitous to have boat time, donated by the national park service, for the day to conduct our initial surveys within Grande Anse and to visit the potential sites of Andse du Bourge and Petite Anse. For our surveys we dropped quadrats from the surface, while freediving Beth and I plunged into the pleasant waters within Grand Anse to quantify the seagrass community composition and abundance. We even managed to spot a few turtles when our heads weren’t buried in the seagrass! That evening we returned, a bit sun kissed, and began entering data, running power analyses, and designing our experimental game plan for the next weeks on island. 

In the morning we tested the settings of the DJI Phantom 3 quadcopter/drone over land before releasing it to the elements over an open body of water. France requires fashionable parameters for flight! 

Grand Anse in Martinique.  Beth Whitman (with drone), Dr. Jeremy Kiszka, and Caitlyn Webster are dressed in the required safety gear and the landing pad is roped off!

We launched the drone over Grand Anse for a test flight in hopes of using the aerial vantage point to survey turtle densities and map seagrass cover within the bay. Although the drone took impressive aerial footage, the glare, rippling of the water's surface, and water depth made it difficult to detect turtles while flying high enough to avoid collision with the many boat masts scattered throughout the bay. Admitting this is a fantastic technology, but more applicable to shallow, mast-free environments, we opted for plan B: in-water snorkel surveys of turtle abundance. Over the course of the next week in Martinique we conducted 60 of these surveys along 300-600m long transects, and then 30 more in Guadeloupe! During our snorkel surveys we found that most of the moorings in Grand Anse were broken, and unusable for the many pleasure yachts that inevitably anchored in the seagrass and/or sand throughout the bay. These anchors leave large scars and leaving open areas for the invasive seagrass Halophila stipulacea and dispurse fragments of the seagrass contributing to it’s spread. During our snorkel transects we dodged boats (usually successfully) while recording turtle locations, behaviors and food availability. Some nights we asked ourselves, “Is 7PM too early for bed”? However, our hard work paid off! Surveying until the very morning of our departure, we completed 60 transects throughout Grande Anse. 

Green turtle grazing in a mixed bed of native and invasive seagrass
We also collected focal follow video footage of individual turtles. Armed with GoPros attached to extendable poles and GPSs we filmed foraging green turtles to investigate the turtle behavior and grazing preferences. For the most part turtles proved unperturbed by our presence. As the turtles foraged below, we could often hear the crisp crunching of the naturally occurring seagrass, Syringodium filiforme

Our local collaborator, Emilie, agreed to continue focal follows and seagrass collection (for nutrient analysis at FIU) in our absence after showing us a site she has great interest in, Sainte-Anne. The invasion of H. stipulacea in Sainte Anne is not as as great as in Grand Anse, so it will be interesting to see if our pilot surveys detect a significant difference. Also, keep an eye out for Beth and I in the local Martinique newspaper. After catching enough eyes with our bright orange buoys, clipboards, and zinc-oxide caked faces, Emilie coordinated an interview with a journalist about the research that we were conducting. After twelve days 11, we said “Au revoir!” to Martinique and hopped over to our next field site, Malendure, Guadeloupe.

Beth, Emilie and Caitlyn collecting seagrass samples
In Guadeloupe we continued with our surveys in a much smaller bay than Grand Anse. After researching all day, Beth and I were delighted to wash our gear in the convenient river flowing alongside our temporary home thanks to another local collaborator, Gaëlle Vandersarren , grab a daily baguette down the road, and walk the dogs through the mountainous jungle surrounding our accommodations. We joined forces with an avid marine conservationist, Caroline with Association Evasion Tropicale, to begin a valuable collaboration of integrating years of her data with our own. She also agreed to continue focal follows and seagrass collection in our absence. As our trip was coming to a close, Beth had the terrible misfortune of kicking a long-spined urchin (Diadema sp.) as we were, quite literally, hopping between transects in the shallows. Determined and motivated, Beth continued our surveys sans one fin! Serendipitously, Phil Matich (Heithhaus lab alumni and Post-Doc at Sam Houston State University) came to assist us in the field for a few days in between deploying BRUVs and joining the sperm whale team on board the Saravi III. With a fresh set of particularly tall legs, our team produced an impressive amount of seagrass surveys and collections. Beth and I wrapped up all of the field work and data entry with just enough time, and barely enough endurance, to summit a remarkable volcano, La Grande Soufriѐre, on our last day. Thus concludes our “vacation” at the beach in the French Antilles. Stay tuned for updates about Beth’s next turtle endeavors in the Caribbean at her field site in Abaco!
View from the summit (1,467m) of La Grande Soufriere.

Sunday, April 17, 2016

Guadeloupe Sperm Whales Expedition - 2016

This is the third FIU-SPAWRAC expedition to study sperm whales and other deep-diving whales and dolphins off Guadelupe Island in the eastern Caribbean Sea.  I was able to join the expedition – led by lab researcher Dr. Jeremy Kiska - for 4 days, but the team got started a few days before my arrival and will stay on until the 18th of April.  Below is the account from the days I was there.  For more information on the project, check out our 2014 blog posts, and stay tuned for exciting developments.  We are working on several scientific papers and also have been working with Patrick Greene (SymbioStudios) to produce video projects for teachers to use in K-12 classrooms! We’ll also be sending out tweet updates (@symbiostudios, @mikeheithaus) and we will update the blog once the expedition gets wrapped up!


Day 1 – April 10, 2016

Patrick an I arrived yesterday evening and joined the crew onboad our mothership Saravi III.  So far the trip has been a success!  The team spent the first three days using towed sonar to map the distribution of potential sperm whale prey from nearshore waters to 1500m deep and around a seamount that has been a sperm whale hotspot in the past.  They also managed to encounter a number of sperm whales – including a group being harassed by false killer whales – and Mehdi Bahktiari  (the designer of the whale cameras we are using) deployed the first camera of the year.  Unfortunately, the suction cup worked too well, and the camera has not popped off yet and is missing in action.  The cameras have built-in release mechanisms so it will come off – hopefully before the whale swims far beyond the range of our transmitters we use to pick them up (about 20 miles).

We got underway at 530 and had time to get our gear together while we steamed to the learest gas station (about 2 hours away). Once we had filled up,  we headed straight offshore to 1000m of water to listen for sperm whales.  After the challenges of finding whales during my last trip (2014), I have to say it was very exciting to hear that there were sperm whales in the area the second the hydrophone hit the water.  Because the hydrophone is directional, we knew which way to head and it wasn’t long before we were in the small boat getting ready to deploy a camera!

Putting a camera on a sperm whale isn’t particularly easy.  First, you have to find and stay with the whales, which can be challenging when they regularly dive for more than 25 minutes.  A whale can travel a long way in that amount of time and often the boat is too far away to approach the whales before they dive again.  That is what happened on our first try to get close.  It looked like the same thing would happen on the next surfacing of the whales, but we headed towards them slowly just to try to get an idea of the direction they were heading.  We got lucky.  The whales stayed at the surface for a long time and lab scientist Kirk Gastrich’s slow approach got us close as the whales were taking their last breaths before diving.  We thought we had just missed the group when one last whale popped up for two last breaths close to the boat.  We had one change, and we were able to deploy the camera.

Unfortunately, the deployment didn’t last super long.  After about 15 minutes, the camera was up.  After viewing the video we found out why – a young whale kept bumping into the whale with the camera, several times knocking the camera.  Amazingly, the suction cup held up to three pretty major hits, but on the forth it came off.  Although we didn’t get the data we wanted it was good to see the camera can take so much abuse, and there was a great sequence of the adult whale producing a coda (a train of clicks to a specific rhythm) and the smaller whale answering.  We also saw that our new headlight system will be perfect for getting images from the darkness of the deepsea without disturbing the whales!

Deploying an Exeye camera on a sperm whale

View down the body of a whale wearing the camera

Day 2 – April 11, 2016

Today was one of those simply amazing days!  We got going early in spite of rain showers and headed straight offshore.  We were rewarded with an encounter with Fraser’s dolphins shortly after leaving the harbor at Malendure.  It didn’t last long, which is typical of Fraser’s.  A group split off and came to ride the bow briefly then shot off to continue on their way.   Like yesterday, we heard sperm whales as soon as the hydrophone hit the water and we were off in search of them.  It wasn’t long before we saw the characteristic blows off in the distance!  Unfortunately, the conditions weren’t great at the time and even though we had the tagging boat, Jeremy in the filming boat, and Saravi III we had a hard time staying with a group.  Luckily there were whales all over – individuals and small groups could be seen in the distance, all seeming to head north.  After a couple hours we finally had our chance and got close to a group.  Unfortunately, we weren’t quite close enough and I wasn’t able to get the camera to stick to the whale!  It can actually be hard to get the suction cup to stick because there are only a few parts of the body that aren’t too wrinkly (which keeps the cup from sticking) and sperm whales shed their skin a lot.  If the cup hits a spot where the whale is shedding, the cup can’t stick.

We weren’t thwarted by the miss and stuck with our efforts.  As we went through the day trying to get close to the whales we encountered many other species (7 in total).  Jeremy saw dwarf sperm whales – which are small and shy – and we heard humpback whales while trying to figure out which direction the sperm whales were going.   A group of spotted dolphins came to ride the bow while we were trying to get close to a sperm whale (not helping at all), and we saw a Gervais beaked whale (a deep-diving toothed whale that is not often seen) as well. 

Spotted dolphin

Eventually we got our chance.  The winds had died down and a juvenile whale was lingering at the surface (and showing some interest in the boat).  It was just the opportunity we needed and the camera was deployed…for three minutes.  A look at the video showed that the lip of the cup was on a spot where the whale was sloughing skin.  With about 10 whales spread around the boat we made a quick trip to Saravi III to get ready to deploy again.  On the way a group of rough-toothed dolphins showed up, but we didn’t have time to spend with them and headed back to the whales.

Over the course of the next hour we spent time trying to deploy.  The whales were socializing with each other (especially some juveniles) and were very relaxed so we could take our time, waiting for the perfect opportunity to deploy without disturbing the whales.  My previous three deployments had been on the side of whales’ dorsal fins, so this time I tried for the smooth area on the back between the blowhole and dorsal fin.  A fortuitously timed swell and Kirk’s expert driving gave me the perfect opportunity.  The camera was deployed, and we started tracking the group.  It was amazing to watch the animals for so long without them worried about our presence! 
Sperm whale group

Sperm whale with camera
Whale-cam view of two other sperm whales

It turns out this camera was stuck really well.  In fact, as the sun set it was still on the whale after more than four hours.  With the swell building and multiple boats that need to be towed by Saravi III we had to leave the area far offshore where the whales were to spend time in shallower waters near the coast.  We can’t go into the harbor at dark so we will be on one hour watches tonight as we patrol the shallows listening for the camera to pop off…I get the 2am shift!

Day 3 – April 12, 2016

We started the day very early since we were already offshore when the sun rose.  I have to say that I really enjoyed my 2am watch with the amazing stars and calm seas. Conditions this morning were not ideal, but we headed north to try to listen for the missing camera.  We quickly found sperm whales, and launched both small boats to work with them in hopes of getting biopsy samples. While the small boats tried to find the whales again in the swell and chop, the Saravi III broke off and headed 12 miles north to listen for the VHF signal. Unfortunately, the wind was already picking up and we were unable to get close enough to the whales for a sample.  The rest of the day was spent searching for (and not finding any) sperm whales, dodging nasty rain squalls, and bailing a leaking filming boat. We did have a nice brief encounter with spotted dolphins at least!  Tomorrow we will continue our quest to find the camera – Mehdi thinks he heard it faintly to the north. 

Day 4 – April 13, 2016

It was a nice day to be on the water, with flat calm seas greeting us early in the morning.  But the sperm whales were not cooperating.  The hydrophone produced no sign of sperm whales when Saravi III got out to sea.  It wasn’t a good sign since sound travels so far.  We did run into a large group of pilot whales though.  The group of maybe 100 animals was spread out over a wide area and were resting and traveling very slowly.  We spent a while with them, but the Saravi III and our inflatable tagging boat broke off quickly to go look for sperm whales in an attempt to get one more camera deployed before Patrick and I had to leave and to get Jeremy started with biopsy sampling this year.  Jeremy and I were on the tagging boat and had an amazing encounter with Fraser’s dolphins. We found maybe 400-500 animals in four tight groups leaping to the south, then changing direction before moving south again.  Jeremy had never seen anything like it.  We didn’t linger though – there were sperm whales to find. 

A large group of Fraser's dolphins on the move

The filming boat stayed with the pilot whales for quite a while before they saw a large pilot whale in deeper water come to the surface and violently tail-slap the surface.  Almost immediately, the sleepy pilot whales they had been with woke up and started swimming quickly in the direction of the tail-slaps and deeper water. 

Unfortunately, despite good conditions and a very determined effort we didn’t find any other animals the rest of the day.  Whale watching boats reported a similar experience – no sperm whales to be found on the west coast of Guadeloupe today.  It is interesting that we saw a big group of pilot whales today and no sperm whales.  We had a similar experience in 2014.  When we saw pilot whales, there were no sperm whales and vice versa.  Pilot whales are known to harass sperm whales until they regurgitate – then the pilot whales eat what comes up – so maybe the sperm whales have moved somewhere else temporarily. 

When the day was over it was bittersweet to be packing up.  It was an amazing trip, and I am sure there will be more great work in the next few days before the project wraps up.  After that it is time to analyze the data we have collected, finish the films and classroom videos, and plan the next steps in the research!

Monday, October 19, 2015

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).

Friday, February 13, 2015

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!


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.