Lagoon seagrass meadows and green sea turtles

Team members: Al Badush, Elrika D’Souza, Mayuresh Gangal, Nachiket Kelkar, Rohan Arthur, Teresa Alcoverro and Nuria Marba

Project timeline: 2005 - present

Fishers in the seagrass meadows of the Lakshadweep correlate a decline in fish catch with increase in green turtles over the last two decades. Although easy to discount as a disaffected fisher tale, the mechanisms they propose are much harder to dismiss. Apart from breaking nets and driving away fish, fishers are adamant that the turtle overgrazed seagrass are driving a decline in adult fish and hindering recruitment. In 2005, conflict between local lagoon fishers of Agatti Island and green turtles escalated, resulting in the clandestine killing of turtles.

Project objective:

To ascertain the relationship between green turtles, seagrass meadows and lagoon fish populations as hypothesised by local fishers, through both observational and experimental research on turtle herbivory.

To begin, we estimate the magnitude of turtle herbivory and establish that the current level represents a significant risk of overgrazing, and that the herbivory rates and turtle density in the Lakshadweep meadows are among the highest in the world. Our seagrass clipping experiments show that turtles can cause population and biomass declines in seagrass and explore the mechanisms by which herbivory causes species shifts from long-lived, hardy species to short-lived, sediment-tolerant species. In this process, turtles are also affecting the capacity for seagrass recovery.

Additionally, we are tracking fisher perceptions about the turtle problem, and we set out to validate these hypotheses through studies on both turtle herbivory affecting seagrass structural complexity, and the response of fish populations to this change across a gradient of differentially grazed meadows. Our results provide considerable support for the fact that the fishers, for the most part, appear to have gotten it right by associating increasing turtles with declining fish catch. We also show that indirect and unobvious pathways of conflict might be far more serious than direct mechanisms which are generally the focal point of conflict mitigation measures.

Team members: Al Badush, Elrika D’Souza, Mayuresh Gangal, Rohan Arthur, Teresa Alcoverro and Nuria Marba

Timeline: 2005 - present

Shallow lagoons of Lakshadweep were once filled with lush seagrass meadows, intricate ecosystems crucial for nourishing and sheltering various reef and pelagic fishes. Around 2005, a noteworthy concentration of green sea turtles was noted within these meadows. Over two decades of extensive grazing, however, the turtles drove the seagrass meadows to the brink of functional extinction. Presently, green turtle distribution is sparse, and seagrass meadows are largely absent across most islands.

Project objective:

To establish ‘insurance sites’ to safeguard the remaining seagrass patches from complete local extinction

In a targeted intervention, we implemented ‘seagrass insurance sites’ in the meadows of Kalpeni, Agatti and Kavaratti to safeguard remaining seagrass patches from further herbivory. The objective is for these preserved areas to serve as potential sources for recovery when turtle populations decrease. These 'insurance sites’ prevent the complete local extinction of the original meadow's dominant structural species, ensuring their availability for recolonization when feasible. Achieving a delicate equilibrium between seagrass and green turtle conservation proves intricate, necessitating both active and passive measures to sustain both ecosystems and species in robust conditions. Now we have been observing a notable resurgence in seagrass density and visible restoration of seagrass ecosystem functions have been observed through this small-scale initiative. Fish density within the exclosure is three times higher than in the surrounding areas.