Gardeners of the Reef
The coral reefs of Lakshadweep have faced two mass bleaching events in the recent past, the second one occurring as recently as in 2010. As the dead coral structures get covered with algae, the associated marine life disappears, and soon the entire coral reef ‘dies’. Remarkably, nature has corrective methods in place to recover from such losses. The herbivorous fish on the reef help serve this purpose, but only if enough survive to devour the ever-increasing amounts of algae.
It was in 2009 that I first visited Minicoy – the southern-most island in the Lakshadweep archipelago – and dived into the world of myriad patterns, colours and structures, the world of coral reefs. I could barely believe my eyes as I explored the diversity of life that existed on every inch of the surface beneath. The water was so clear that we had a clear view of the corals up to 30 metres below the surface. Damselfishes sought shelter within the branches of the corals; brightly-coloured crabs guarded their homes; spiny sea urchins bored into the coral structures; and feather stars clung to the corals with their arms wide open to filter the current, finding in it all that they need for survival.
Brightly-coloured Anthias swim in the clear waters of Lakshadweep
There was an amazing diversity of fish – the elegantly shaped butterflyfish pecking at the corals, parrotfish clearing the algae off the reef with their massive jaws, clownfish guarding their anemones, and giant groupers lurking in holes and crevices, awaiting prey to steer too close. The creatures here were so exquisitely fashioned that most of them seemed unreal; their beauty too fragile to exist, yet every detail functionally useful. Each time I dived, I gained new insights and understanding into the intricate fabric of life in which every creature in the reef is linked with another and to its surroundings.
A mixed feeding shoal of parrotfish and surgeonfish, foraging in the Kavaratti atoll
When I returned to the Lakshadweep islands in 2014, the reefs surprised me yet again, but this time, not in the way I was expecting. The reefs had transformed into coral graveyards, their skeletons covered with algae – the coral’s competitors for space and light. This was the after-effect of the mass bleaching event of 2010, where within a few short weeks after El Niño, the sea temperatures rose and the corals bleached, with over 80% dying. Coral bleaching is a phenomenon where under stressful conditions, like ocean warming, corals expel their symbiotic zooxanthelae, which carry out photosynthesis to create food. If water temperatures remain constantly high, the corals will eventually die, turning the reef into a barren substrate over which algae quickly proliferates.
In such a scenario, coral reefs depend on herbivorous fishes to keep a check on the growth of the algae. By feeding on the algae, these herbivores keep the reef surface clean for new coral larvae to settle and grow. But not all herbivores feed in equal amounts, because of which they are categorised into feeding guilds such as grazers and scrapers. For example, scrapers such as parrotfishes have powerful jaws, specialised for eating hard, calciferous algae, whereas grazing surgeon-fishes and rabbit-fishes have smaller mouths to consume softer types of algae. Many studies have linked healthy coral populations to healthy herbivore population and emphasized the importance of specific herbivore guilds and species during the times of reef recovery. However, what was happening in Lakshadweep was still unknown.
Were herbivores as critical to this system as they were elsewhere? If so, were they helping the reefs recover?
Blue tangs (Acanthurus leucosternon) on the reefs of Kavaratti. These fish graze on the turf algae as they move from reef to reef
Geared with a range of underwater equipment, I started counting the number of herbivorous fishes and estimating the amount of live coral and algae that existed on these reefs. Most of the reefs were dominated by algae and the coral cover was very low, but there was still a lot of fish. To find out what these fish were doing, I filmed them with cameras in several locations. Interestingly, I discovered that fish in the shallower reefs spent a majority of their time eating algae. In deeper reefs, although the fish were present in large numbers (nearly twice as much), they were not feeding as much. This implied that they were there for a reason other than herbivory. What I suspected, and something which has been documented from other parts of the world is that fish aggregated in huge numbers in areas with a high amount of structure, which acts as a refuge from predators. Other works on coral groupers had also suggested that these larger fish seemed to prefer areas with high structural complexity. Shallow reefs are subjected to severe wave action, especially during the monsoon season, which causes breakage of corals and loss of structure. Deeper reefs, on the other hand, remain relatively sheltered and hence retain more structure. Thus, it seemed that the herbivores were present everywhere, but they were only performing their function in specific areas. While herbivore numbers had often been used as an indicator of the reef’s health, what I was finding out was that numbers may not always translate to functionality. Simply put, what you see is not what you get.
A feather star latched to a dead piece of coral, filtering the current for food particles
What I also noticed is that reefs often differed in the amount of algae present. Some reefs were carpeted with thick mats of algae, while others were covered with finely-chopped, thin algae. What was so different about these locations? Did the composition of herbivores change? To answer these questions, I analysed over 58 hours of underwater footage, noting down the bite rates and which species ate how much in each of the surveyed locations. And the results sure were intriguing! Sites with high amounts of algae had 64% less herbivory than sites which had fewer amounts of algae. Moreover, reefs with more algae had five species of grazers (all surgeonfish) absent. If in due time these reefs lose more species, the algae growth will get out of control, making the entire system even more vulnerable.
A Green sea turtle ascends from a deep dive near the Kadmat lagoon
So where are the missing species? A possible theory is found in the history of the Lakshadweep reefs. The seagrass meadows adjoining the reefs – that act as important recruitment habitats for herbivores and other fish – have vanished dramatically due to overgrazing by Green sea-turtles, resulting in potential recruitment failure. However, all of this requires scrutiny. What we now know for sure is that herbivory is critical and without sufficient numbers of these important marine grazers, even the existing corals would eventually be taken over by algae.
A reef dominated by corals (top) and a reef dominated by algae after a mass bleaching effect (bottom)
LIGHT IN THE DARKNESS
Lakshadweep islands have among the highest human densities in rural India. One would imagine that more people would translate to more fishing, more pollution, and hence rapid reef decline. However, despite the high human fingerprint, the reef fish communities appear to be relatively healthy, even in the wake of the mass coral mortality. a couple of generations ago, fishermen in the archipelago depended heavily on the reef for their sustenance. But since the 1980s, these fishermen have shifted to targeting pelagic tuna. This was part of the government-sponsored fisheries development programme, which turned out to be highly successful, as the fishermen saw great economic benefits in tuna fishery and quickly adopted it. One lucky by-product of this initiative was that it released the reefs from potential overfishing, which has been critical to the resilience observed during the bleaching event.
Fishermen sorting their tuna catch of the day. Tuna is either sold fresh in the local markets or dried and shipped to the mainland.
Image credits: Vardhan Patankar
A Chlorurus strongylocephalus biting off the algae with its massive teeth. This action exposes the invertebrates, such as crabs and shrimps, living in the gaps, which are then consumed by other fishes like wrasses (seen in the pic).
Given the overall fragility of the Lakshadweep reefs, which have suffered from two mass bleaching events in the recent past, it is fundamental that we maintain the inherent spirit of these systems against anticipated climatic and other anthropogenic changes, which will without a doubt have a negative impact on these valuable ecosystems.
This article was originally published in the 2015 April edition of Saevus magazine