Marine Meadows – Following The Feeding Trail Of The Dugong
Following dugongs in the waters around the Andaman and Nicobar islands allowed researchers Vardhan Patankar and Elrika D’Souza to start to decode the mysterious feeding habits of these threatened marine mammals.
Photo: Vardhan Patankar.
Dugongs roam widely through rough and remote seas, no wonder these astonishing life forms are still steeped in mystery. They ordinarily come into view all too briefly, when they part the sea’s shimmer to breathe. Although more active at the surface than most marine mammals, they still spend about 70 per cent of their lives under water. So what are they doing down there? Out in the azure blue seas of the Andaman and Nicobar islands, our research has gathered new clues about a crucial part of dugong lives: their feeding habits in seagrass meadows.
We observed some dugongs feeding repeatedly in a single meadow, while others abandoned meadows after feeding just once. We were curious to know what triggered Dugong dugon’s decision to either leave a seagrass meadow or repeatedly graze it.
ALPHA AND LUNAR
We did have theories. Leaving a seagrass meadow ungrazed for a few months would, for instance, give it time to recover and regrow. Repeated grazing could be on account of the low density of dugongs around these islands, which reduces their grazing impact, reduces travel time between meadows. What is not yet clear is whether these animals feed regularly or take breaks. Considering how large they are, we guessed that they could afford to take a break from feeding for a week or so by eating prodigious quantities of seagrasss and storing the energy they got from it.
But we needed proof to confirm our hypotheses. The more time we spent in the meadows, the more feeding signs we observed. The more we read about other herbivores, the more clues we got about dugong movement. Our quest for these answers had really begun seven years ago when we sighted two dugongs while snorkelling around an island in the Ritchie’s archipelago. We named them Alpha and Lunar. These individuals displayed unique behaviour towards us. Most animals in the wild would have preferred to move away from us, but Alpha and Lunar were unperturbed by our presence, providing us with enough time to observe them closely. Every morning we would follow Alpha and Lunar for a few hours and sometimes even until dusk. Sometimes, they would feed from morning to evening, resting only occasionally. At other times, instead of surfacing every three to four minutes for series of breaths, they would stay underwater, munching on their favourite plants.
Photo: Vardhan Patankar.
We knew we were very lucky; sighting these animals in the wild is extremely rare. At times we would go for months without seeing a single individual. If not underwater then the only chance of spotting the animal is when it surfaces to take a deep breath. In the first few years we saw only four individuals in the wild. That’s when we decided to concentrate our efforts on locating the distinct serpentine feeding trails of dugongs. So began the search for seagrass meadows. At times we would duck dive in mucky waters, not realising the area is saltwater crocodile frequented. That was a scary thought. On locating feeding trails, we would measure a set of variables on the specifics of that seagrass meadow to estimate its primary production and also count the number of feeding trails we saw, to determine how dugong feeding or their role as herbivores influenced seagrass dynamics.
But even then, we were not getting any closer to knowing how dugong herbivory changed seagrass dynamics. Then Dr. Rohan Arthur, Dr. Teresa Alcoverro and Dr. Nuria Marba provided us with the guidance we needed and suggested we set up an experiment on seagrass meadows. We selected three accessible seagrass meadows located around Neil Island, which were consistently used by dugongs. Our protocol involved establishing four 1 x 1 m. dugong foraging exclosures or cages at each meadow using a mesh of fish line and PVC pipes. At the commencement of the experiment, we measured shoot density within 20 x 20 cm. quadrats inside (control) and outside (treatment) the exclosures. Four months later, we revisited these cages and estimated seagrass growth inside and outside them.
Photo: Vardhan Patankar.
When we looked at our results, we found that although we had surveyed 44 meadows, we had observed feeding trails only at eight sites. All eight meadows that were used by dugongs had distinct characteristics – they were relatively large, unfragmented, continuous and dominated by the short-lived species Halophila ovalis, Halodule uninervis and Halodule pinifolia. This was an interesting finding as in other parts of the world, dugongs are known to feed on a greater diversity of seagrass species. Although there was considerable variation in the quantum of seagrass consumed by dugongs, we observed that dugongs on an average consumed approximately 15 per cent of a meadow’s primary production. The recovery of meadows after a feeding event was quick. Within a week original shoot densities would return. Through experimental manipulations, we tried to understand the short-term impacts of dugong herbivory on seagrasses. We found that when herbivory was excluded, the shoot densities were almost fifty percent higher than in meadows that were actively foraged upon.
The data obtained from herbivory cages when combined with other observations helped us understand what dugongs are doing down in the islands, why they were persistently grazing the same seagrass meadows, and more importantly how seagrass was coping with their feeding. The ability of seagrasses to cope with grazing perhaps explains the long-term site fidelity shown by individual dugongs in these meadows.
We still cannot be certain what is actually going on in these animals’ large, convoluted brains, or what makes them choose one seagrass meadow for repeated grazing. What we know is what we have observed. In other parts of the world where meadows are made up of different seagrass species, and where dugongs are found in higher numbers, they behave very differently from the dugongs around the Andaman and Nicobar islands. And even in these areas, dugong behaviour changes over time – from one year to the next, and sometime even during one season to the other. Years of study lie ahead and what we are able to understand about the animal is only a fraction of what they are actually doing while feeding. One way or the other, unravelling secrets on dugong behaviour is as satisfying as it can get. Trying to understand mammals in an environment so different to ours is part of the fascination that keeps us going back year after year. We do not know nearly enough about these remarkable animals yet, but what we do know is that dugongs have a way of turning questions about nature into questions about how we ourselves live in and interpret the world around us.
OF MERMAIDS AND SIRENS
Dugongs Dugong dugon are large marine mammals found in coastal waters from East Africa to Australia including the Red Sea, Indian and Pacific Oceans. They are one of four living species of the order Sirenia, which also includes three species of manatees – the South American manatee Trichechus inunguis, West Indian manatee Trichechus manatus and West African Manatee Trichechus senegalensis. The closest relative of dugongs, the Steller’s sea cow Hydrodamalis gigas, was hunted to extinction in the 18th century less than 30 years after it was discovered by Artic explorers. Though dugongs are similar in appearance and behaviour to manatees, their fluked tail sets them apart. Manatees, have a rounded paddle-like tail. Dugongs spend most of their time alone or in pairs, though sometimes large herds numbering up to 100 individuals are known to gather in one place. Long sought for their meat, oil, skin, bones and teeth, dugongs have been easy targets for coastal fishermen, and their survival is further threatened as they are found in shallow waters where human activity is maximum. Despite being legally protected throughout their range now, the populations are declining, with the IUCN Red List categorising them as vulnerable.
Authors: Vardhan Patankar and Elrika D’Souza First appeared in: Sanctuary Asia, Vol. XXXV No. 1, February 2015.