Impact of sea surface temperature anomalies on giant clam population dynamics in Lakshadweep reefs: Inferences from a fourteen years study

Frequent sea surface temperature (SST) anomalies may have global consequences on marine biodiversity conservation through their potential impacts on fragile coral reefs and associated organisms such as the giant clams. However, effects of SST anomalies on giant clam population dynamics is not clearly understood as their slow lifehistory traits require long-term population studies for unravelling these patterns. Here, we have examined the effects of SST anomalies on Tridacna maxima by monitoring their populations in Lakshadweep archipelago across a 14 year period (2004-2017). We monitored T. maxima density annually in three reefs (Agatti, Bangaram-Tinakkara and Kavaratti) using 100mx8m belt transects. The annual patterns of density change rate (lambda(t)) against time since bleaching were modelled and used to develop stochastic population growth models under different scenarios of bleaching incidence. Our study was preceded by one and intercepted by two El Nino events (1998, 2010 and 2016). Mean summer SST > 30 degrees C triggered T. maxima bleaching. Population density showed large inter-annual variations that were synchronous between reefs, showing gradual increase in density during 2004-2010, and decrease during 2010-2017. SST anomalies did not cause immediate clam mortality. However, T. maxima population change rate (lambda) was 0.91 +/- 0.05(SE) immediately after bleaching and increased by 0.02 +/- 0.01(SE) per year, resulting in positive growth only after five years since bleaching. Time-series analysis of SST data (1965-2016) indicated increasing incidence of anomalous temperature (beta= 0.09 +/- 0.04(SE)). Population growth models showed stable density trend after 50 years under past bleaching incidence (20% yr(-1)), but declining density (R= -0.05 +/- 0.02), high risk of quasi-extinction (0.31-0.61 across reefs), and low density (geometric mean: 9-15 individuals ha(-1) across reefs) under the prevailing high bleaching incidence (40% yr(-1)). Thus, climate change induced frequent SST anomalies are likely to compromise the viability of giant clam populations.