Endangered green sea turtles threaten fish population in Lakshadweep seas

At a time when political cauldron is on the boil in Lakshadweep, another problem is brewing in the Union Territory — far away from the land, right in the middle of the sea.

A recent research paper published in Biological Conservation by Mayuresh Gangal et al., shows that overgrazing of seagrass meadows by the endangered green sea turtles in the seas of Lakshadweep is responsible for the decline in many fish species, which are dependent on seagrass for survival.

The 15-year-long research was carried out by researchers from Nature Conservation Foundation, Karnataka, in collaboration with researchers from Ashoka Trust for Research in Ecology and the Environment (ATREE), Karnataka, Lancaster Environment Centre, UK, Global Change Research Group, Spain and Centre d’Estudis Avançats de Blanes, Spain.

“Turtle overgrazing resulted in massive decline in seagrass fish diversity and biomass as well as reduction in sediment stored carbon. Apart from being important conservation flagships, green turtles are strong ecosystem interactors and can potentially cause trophic cascades or functional extinction of seagrass ecosystems,” the authors said.

‘Turtles – Ecosystem Engineers’

Green turtles (Chelonia mydas) are one of the many species of turtles found in Atlantic, Pacific and Indian Oceans. In India, it is largely distributed in Andaman and Nicobar Islands, and Lakshadweep. The International Union for Conservation of Nature (IUCN) lists it as an endangered species.

Green turtles act as ecosystem engineers in their habitat. In tropical waters, they mainly feed on seagrass and algae. In areas where green turtle numbers are lower than they are in Lakshadweep, they act as gardeners of seagrass beds and may even help keep seagrass beds healthy.

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The green turtle population has been declining dramatically in their home ranges in the past century. The systematic harvest of eggs and adults on nesting beaches, incidental catch of turtles in the fishing nets, accidental collisions with boat propellers along with a whole lot of natural threats put the population of green turtles under severe threat.

“Responding to the decline of green turtle populations, a lot of conservation efforts have focused on protecting nesting beaches. One of the most dangerous times in a green turtle’s life is when it makes the journey from nest to sea. Multiple predators, including dogs, crabs, shorebirds, mongooses etc voraciously prey on hatchlings during this short scamper,” said Al Badush, one of the co-authors on the paper.

Of late, shrinking shark populations and overfishing too have contributed to the sustained growth of turtle population.

Seagrass – Nursery sites for fish

The root systems of seagrass form a complex matrix in the sand, which help stabilise coastlines. Tall seagrass leaves also help trap suspended matter in the water column and keep shallow waters clear.

The meadow is also a diverse ecosystem with a huge array of invertebrates and fish. In particular, seagrass meadows make perfect nursery sites for fish that may spend their adult life in neighbouring coral reefs, mangroves or the open ocean. They are a source of food for fish, green sea turtles and dugongs. Notably, dugongs (that are no longer found in Lakshadweep) almost exclusively depend on seagrass.

“As global warming continues, the ability of plants to offset these losses by sequestering carbon becomes increasingly important. While there is considerable emphasis on terrestrial forests, the world’s highest carbon stores are thought to be in the coastal systems that include mangroves, salt marshes and seagrass meadows. Seagrass meadows are important global carbon sinks, being responsible for approximately 10-15% of the annual organic carbon burial in the ocean,” said Elrika D’souza, one of the co-authors of the paper.

Impact on fisheries

Since seagrass acts as a nursery habitat for many fish, its loss is detrimental for their survival. This in turn reduces present as well as future diversity and abundance of fish species which are associated with these nursery habitats. Many coral associated fish also find nursery habitat in seagrass meadows. Thus, decline in seagrass meadow can have far reaching consequences for coral reef associated fish as well.

“Along with acting as nursery seagrass meadows provide habitat for adult fish as well. And decline in this habitat reduces diversity and abundance of adult fish. These fish are important for small scale fishermen who fish them for subsistence. In addition, these seagrass meadows provide habitat for bait fish which are important for commercial tuna fishermen. Decline in seagrass habitat, leads to decline in bait fish and subsequently affects commercial tuna fisheries as well” said Rohan Arthur, one of the co-authors.

Is seagrass restoration possible?

While this is the case, how can we handle the problem of overgrazing in the sea? Is it possible to control the population of green sea turtles? Can seagrass be restored?

“While protecting turtles is essential, we also need to think carefully about shark conservation, adequate protection of seagrass and related ecosystems, and the livelihood needs of dependent human communities,” the authors say.

It has been observed that once the turtles overgraze the meadow and there is very little seagrass left, they tend to leave the meadow in search of new foraging grounds. From an ecological view, it may be said that the problem of turtles in the Lakshadweep meadows is a self-limiting process – give it enough time and it will take care of itself.

“What we can do is to assist the process of recovery once green turtles have moved away – this seems like the most pragmatic solution to the problem,” Gangal added.

The principle behind assisted recovery is to maximise the natural ability of seagrass meadows to recover on their own. With the help of exclosures, we can protect some remnant patches of seagrass as insurance sites. These small locations help ensure that the seagrass species from the original meadow are present in the lagoon to kick-start recovery, he said.

“Alongside this, we need to minimise other local disturbances that may reduce the ability of meadows to grow and spread from these patches. These include disturbances like anchoring, constructing structures in the lagoon, like jetties, cabins, houses etc. Along with that we have to make sure no effluents are entering the water. Such long term assisted natural recovery is critical to bring back seagrass meadows with all of its species richness. We need more imaginative conservation models to make this work,” said Gangal.