By Bhanu Sridharan
I have lost count of the days. It’s the tide I track now. My eyes automatically scan the tide chart every few hours. How many hours of low tide? How long can I walk freely trying to learn the mangroves? Trying to understand the nature of the trees, the type of soil. Too soon, the tide comes in, water floods the forest and everything is different again. How can I explain what I see, when nothing is constant?
I came to the Andamans to study mangrove forests. My study site, a mangrove patch in the Lohabarak Crocodile Sanctuary, adjoins the beautiful field station of the Andaman and Nicobar Environmental Team. All the literature tells us that mangroves are excellent habitats; a haven of safety and food particularly for young fish that seek to escape the crowded, competitive coral reefs or the dangers of the open sea.These forests act as nurserieswhere juvenile fish may grow and develop before moving onto their adult lives. No amount of reading however prepared me for the actual landscape, for its ever-changing nature.
Armed with notebooks and equipment (that had made airport security eye me suspiciously) I stepped into this weird world, part land part sea. Almost instantly I forgot my study. At first glance, the word that came to me was desolate. Tall dead trees rise against a cloudless sky, beneath lies a vast grey flat land. But then blinking against the sunlight, I look again and I see life. Dead trees are only part of the picture; remnants of a struggle against an environment that is often too harsh. Beyond it stands thickets of green, looking cool in the already hot sun, but strangely uninviting. With my field guide for mangrove flora, I walk around slowly identifying the dead trees by what is left of them, their knee roots. Here once stood Bruguiera gymnorrhiza, tall mangrove trees that grow inland and develop large buttresses and roots that pop out of the ground in little knee shaped bumps. The ‘knees’ helping them breathe in the oxygen poor soil. These trees have now fallen in the onslaught of the tsunami. But others such as the Rhizophora and Avicennia trees, have survived,are even perhaps thriving.
Dense Rhizophora forest with a complex network of stilt roots. Juvenile fish are believed to use these root structures to hide from predators.
To survive in this environment the trees need multiple tricks. The Rhizophora trees are tall and thin. They prop themselves up with an extensive network of stilt roots and strut roots that emerge from the main trunk and help the trees remain stable in loose muddy soil and constant flow of tidal water and wave action. The roots and the bark have lenticels; tiny pores that help them breathe oxygen from the atmosphere. The Avicennia trees have long thin pneumatophores, little snorkels that emerge from the soil to take in oxygen from the air. A good way to remember Avicennia leaves is to taste them, my field guide says. I lick the leaves and taste the salt crystals that the trees expel, away to survive the sea that regularly seeps in. Everything makes sense but the sense of an alien environment does not leave. It takes me awhile to gather courage to walk through the thickets of Rhizophora. To enter the dense undergrowth of roots, to walk barefoot in knee deep muck, because shoes or slippers easily get lost in the marsh.
But slowly the forest starts to become familiar. I realise that the mangroves are not as barren as they first seem. In fact they are teeming with life. The ground is littered with sharp hard stone like things, hundreds of mangrove snails. The snails are clustered around a pile of yellowing Rhizophora leaves, feeding and moving now, but when the tide comes in they will attach themselves to the thick stilt roots or tree trunks staying well above the water, for unlike other marine snails they have a lung and must breathe air.
The quiet is broken by a steady plop, as if someone is tossing pebbles in the water. Here I see strange creatures that are supposed to have gills but move happily on land. Mudskippers,amphibious fish, are perfectly adapted to this land-sea environment. They use their gills to breathe like ordinary fish during the high tideswhen the water comes in, but use the mucous lining on their skin and mouth to store oxygen during low tides when the forest is dry. They move about actively defending territories, finding food and mates, walking or more accurately skipping on land using their pectoral fins.
My eyes adjust more and I realise that every step I take causes a flurry of flight. A thousand tiny fiddler crabs run for cover in the onslaught of my feet.The males have a disproportionately large right claw that they use to communicate and fight for females and territories, tiny holes in the soil.Fights are common and the ground is littered with claws of the defeated crabs.But peace is made and claws grow back, albeit smaller than before. And it seems if fights are common, so are alliances.
|Male and female fiddler crabs||Male blue fiddler crab (Uca sp).|
The larger mangrove snails provide structure; smaller oysters grow on their shells. The Rhizophora tree provides space for all of them, on their roots, leaves and branches. Birds nest on them and butterflies flutter far from the muck, reminding me that the tree is quite like its counterparts in other forests, having simply adapted to the land and sea.
I stop my day dreaming because suddenly the tide has come in. Water is flooding the creeks that were dry just hours ago. Slowly the waterwill spill over, submerging the land of crabs and snails. It is time for the fish. In the precious few hours of high tide, when the forest is flooded, fish of all kinds will enter the mangroves. They come from the nearby channels, the sea and the reef. It takes me a long time to identify them, but wading through the water now I see awhite spotted puffer fish,puffing up in indignation as soon as I touch it; some archer fish shooting jets of water onto the surface to catch insects;a mangrove snapper chasing the archer fish through the dense Rhizophora roots. Shaking off hermit crabs that are nibbling at my toes, I run after a school of juvenile mullets. They dart too quickly now, but they will be stranded in small puddles when the water recedes. The pond herons will then make a meal of them.
Do the fish come here for food or for safety? As the days go by, the picture becomes clearer. The complex network of Rhizophora rootsthat I dread so muchis believed to provide hiding places for fish. But the tide brings all kinds of fish, each with their own agenda and tricks. Not all fish hide and not all fish are successful in hiding. Although I see schools of tiny cardinal fish in these roots, the carnivorous mangrove snappers also seem to hang around here. As the snappers are ambush predators, they may be able to use the roots to their advantage; to creep up on their prey. Not surprisingly, some prey species don’t seem to care too much for these ‘hideouts’. Young mullets for instance swim in the open, darting quickly in tight groups. They probably find safety in numbers. The gobies, fish related to the mudskippers, bury themselves in the soil when disturbed, their mud coloured body giving them the perfect camouflage. The cardinal fish however remain in the roots; risking a hungry snapper. Why would they do that?
The roots are not just able to provide cover to fish, they provide them food. Several species of sesarmid crabs, common in the mangroves sit in the crevices of the Rhizophora roots. Fish don’t just eat fish, they eat the crabs. For the cardinal fish risking an ambush predator may be worth the reward of good crab meal. Juvenile fish have to constantly make this choice. Eat well and risk predators or be safe and go hungry. If they eat well they may grow quickly and become big enough to ward off the predators, but if they take too many risks they may never reach adulthood. In the mangroves, it seems there is always something to eat and be eaten by.
For me however, all this drama is not as fascinating as the limited time for the actors. The tides in the Andamans change every six hours and full high tide often lasts little more than an hour.To eat or be eaten, the fish must make the choice soon. When the water goes out the fish must leave. And before that, I must find out all that I can about the fish and forest. I am distracted however, by the other creatures, my own thoughts and my sense of isolation.
Yet it comes sometimes, signs of people. The isolation is broken by a nearby villager taking firewood, fish traps buried in the sediment, turtles bones left behind, the meat long gone or an empty coke bottle, tossed aside casually, floating in the water. Can people be taken out of this story? I can’t say for I am one of them. But mostly when I look away from data collection, I see the magic of the Rhizophora, the sun pouring through the canopy and the hermit crab making boldly for my feet. Today at least, the day is good.
This article appeared in the January 2013 issue of the Saevus magazine.