Edited version published in Sanctuary Asia February 2015.
Vardhan Patankar & Elrika D’Souza
Living on the edge: dugongs prefer to feed repeatedly on seagrass meadows that are sparsely distributed
Dugong ordinarily comes into view only briefly, when they part the sea’s shimmer to breath. Though more active at the surface than the most marine mammal, still spends about seventy percent of their lives below the water. What are they doing down there? They roam too widely through rough and remote seas, no wonder, earth wonderful life forms, are still steeped in mystery. Out in the wonderfully clear azure blue seas of the Andaman and Nicobar islands our research has gathered new clews about a crucial part of dugong lives: their feeding habit on seagrass meadows. We found out that some dugongs are feeding repeatedly in single meadow, by making serpentine feeding trails, whereas others abandon the seagrass meadows. What we are not fully able to grasp until recently was what makes them decide to leave the sea grass meadow? And why do they repeatedly graze a single seagrass meadow? Leaving seagrass meadow for few months must be with the aim of giving seagrass time to recover. Repeated grazing in seagrass meadow must be due to low density of dugongs in the islands, which makes their grazing less intense, reducing their need to travel between meadows, resulting in spatially persistent populations. Also we are not clear if they feed regularly or take break. Considering how fat they are, we guessed they could afford to take a break from feeding for a week or so by gulping prodigious quantities of seagrasss and store fat by the ton.
The more time we spent, the more feeding signs we observed. The more we read about other herbivores the more clues we got about the dugong movement. Our quest for the answers started seven years back when we sighted two dugongs while snorkeling around an island in the Ritchie’s archipelago. We called them ‘Alpha’ and ‘Luna’. The individuals we sighted had a unique behavior to us. Most animals in the wild would have preferred to move away from us, but Alpha and Lunar were least bothered, giving us enough time to observe them closely. Every morning we would follow Alpha and Luna for few hours and sometime till dawn. Sometime Alpha and Luna would feed from morning to evening, resting only occasionally. And sometime instead of surfacing every 3-4 minutes for series of breath, they would stay underwater, munching on their favourite seagrass.
Though we have been very lucky, sighting these animals in the wild is extremely rare. At times we would go months without seeing a single individual. If not underwater then only chance of sighting the animal is when it surfaces to take a deep breath. In first few years we sighted only a four individuals in the wild. That’s when we decided to concentrate our efforts on locating distinct serpentine feeding trails of dugongs. So began the search of seagrass meadows. At times we would duck dive in mucky waters without knowing that the area is crocodile frequented. On locating feeding trails we estimated extent of seagrass meadow and took a few measurements on number of feeding trails, size, species present, primary production of seagrass meadow, abundance, shoot density etc. to determine how dugong feeding called herbivory, changes seagrass dynamics. Even then, we were not getting any closer in knowing how dugong herbivory changes seagrass dynamics. That’s when Dr. Rohan Arthur, Dr. Teresa Alcoverro and Dr. Nuria Marba gave us necessary guidance and suggested to set-up an experiment on seagrass meadows. We selected three accessible seagrass meadows located around Neil Island which were consistently used by dugongs. As per protocol explained by the trio, we established a four 1 × 1 m dugong foraging exclosures at each meadow using a mesh of fish line and PVC pipes. At the start of the experiment, we measured shoots density within 20 × 20 cm quadrats inside (control) and outside (treatment) the exclosures. Four months later, we revisited the exclosures and estimated seagrass growth inside and outside the exclosures.
Seagrass exclosures in Neil island When we extrapolated data from the experiments and combined with direct observations we started seeing clearer trends. Though out of forty-four meadows surveyed we observed feedings trails only at eight sites. All eight meadows that were used by dugongs had a distinct characteristics—all being relatively large, un-fragmented, continuous and dominated by the short-lived species Halophila ovalis, Halodule uninervis and Halodule pinifolia. This was interesting finding as in other parts dugongs are known to feed on many more species. Though there was a lot of variation on how much seagrass was consumed by dugongs, we noticed that dugongs on an average consumed approximately 15 % of meadow primary production. The recovery of meadows after a feeding event was also quick, taking a little longer than a week to return to original shoot densities. 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 50 % higher than in meadows that were actively foraged upon.
The data obtained from herbivory exclosures when combined with other observation helped us understand what dugongs are doing down in the islands. Why they are persistently grazing the same seagrass meadows and most importantly how seagrass is coping with feeding of dugongs. With the support of empirical data we think that the proportion of primary production consumed by dugong reduces seagrass meadow level in long-term to levels that are still above levels that trigger meadow abandonment. This ability of seagrasses to cope perhaps explains the long-term site fidelity shown by individual dugongs in these seagrass meadows. Again, we are not certain what is actually going on in the animals large, convoluted brain and what makes them choose one seagrass meadow for repeated grazing. What we know is what we observed. In other parts of the world where meadows are comprises of different seagrass species, and where dugongs are found in higher numbers, they behave very differently than dugongs around Andaman and Nicobar islands. And even in these areas dugong behavior changes over time—from one year to the next, and sometime even during one season to the next based on availability of seagrass. Years of study lie ahead and what we are able to understood about the animal is only a fraction of what they are doing while feeding—either ways findings answers about dugong is as satisfying as it can get. Trying to understand mammal in an environment so different than ours is part of the attraction that keeps us going years after years. Being intelligent marine mammal we think dugongs have a way of turning questions about nature into questions about the nature of knowledge and how we interpret the world.