doi:10.1038/nindia.2014.7 Published online 21 January 2014
In the Arabian Sea waters around the Lakshadweep archipelago, coral reefs are recovering after the cataclysmic bleaching event of 1998. But the reef's fish species aren't quite, a recent study has found1 .
Corals are tiny marine animals that secrete a hard exoskeleton made of calcium carbonate. Using tentacles, they feed on small fish and plankton. They also have in-house grocery stores — microscopic algae which live in their bodies, giving them energy and nutrients in return.
In warm, shallow waters with enough light for photosynthesis, corals and their algae thrive to form stunning underwater coral reefs, considered the marine equivalent of tropical rainforests by many. Reefs cover only a tiny fraction of the world's ocean surface but house more than 25% of its marine species. When conditions become unfavourable — such as a rise in ocean water temperature — the coral expel the in-house algae and turn white, a process called bleaching.
The 1998 bleaching event in Lakshadweep wiped out 90% of the coral reefs in the area. These coral reefs have been subject to two major El Niño Southern Oscillation-related coral bleaching events in the last decades (1998 and 2010). A team of marine biologists from the Nature Conservation Foundation has documented how the reefs and its fish communities have been faring after 1998. Deep reefs sheltered from the monsoons have been very stable throughout as opposed to exposed shallow reefs. "Some reefs, which have fast growing coral species, have been able to recover rapidly after the bleaching event. Corals are very sensitive to even one degree rise in temperature, while fish are a lot more tolerant," said Rucha Karkarey, first author of the paper. "This means, a bleaching event has an immediate effect on corals but may take time to manifest in fish."
As a coral reef grows and branches out over years, it creates a home filled with nooks and crannies for different species to take shelter, ambush prey, or to set up nests. After a bleaching event, whether a coral reef recovers the complex structure is crucial for fish and other species to settle in and build populations.
The researchers studied a long living and predatory type of fish called 'groupers'. Lakshadweep fishers told the researchers that they preferred open ocean species like skipjack tuna, and the only fishing inside reefs was relatively non-intrusive, low level artisanal fishing. This made Lakshadweep an ideal place to study the effect of climate change and related bleaching events on a commercially and ecologically important coral reef species, the groupers.
The researchers picked out 60 different reef sites around Lakshadweep with varying degrees of structural complexity and exposure to monsoon. They swam 50 metre long trails underwater, photographing and making notes of fish on waterproof paper. Deep reefs were found to have six times more grouper biomass than shallow sites. In fact, just 10 sites contained about half the groupers found in the Lakshadweep archipelago.
"We found that the rapid recovery of reefs after the bleaching event was not good enough to sustain these long lived species," Karkarey said.
Long-term stability of habitats, and not just structurally complex reefs, is important for long-lived species like groupers. Complex coral reefs house more prey species and provide crevices for groupers to ambush prey. After the 1998 bleaching event, deep reefs recovered rapidly while shallow sites were highly degraded and had a higher proportion of fragile corals, making them unstable.
Michel Kulbicki, Research Director at the Institut de Recherche pour le Développement (IRD) in France, says the study could be extended to the rest of the archipelago. "It is difficult to know if the results can be extrapolated to other regions, as the species pool, fishing level etc. may be quite different; but the results seem logical and I would expect to find something quite similar in other places."
Rohan Arthur, senior author of the paper has been monitoring the Lakshadweep reefs since the 1998 bleaching event. "While corals may be able to recover quite well, the underlying ecological network may be unravelling in subtle ways whose consequences may not be readily apparent," he told Nature India. "This makes the few stable reefs not affected by bleaching all the more important to conserve," Arthur said.