*2.1. Study Area*

The Jaffna Peninsula (JP) and its islands surround an almost enclosed body of shallow water, located on the southern side of the 25–50 km wide Palk Strait. They form a reef and island complex (known as "Sethusamudram") that lies at the south-eastern tip of the Indian mainland, partially enclosed off the Bay of Bengal (BOB) (Figure 1). The islands themselves are often referred to as the "Coral Islands Archipelago" and are believed to have been formed as a result of sea level rise since the late Holocene [8,24,38]. On the SL mainland, fossilised limestone rocks are present in the near-shore area, extending up to 50 m seawards, along the northern coast between Point Pedro and Keerimalai, while sandy to muddy seabed extends westwards from Keerimalai, as well as around the islands of the JP. In contrast, the coastline along the south-eastern shore extending from Point Pedro towards the SL coastline is composed of sandy beaches.

**Figure 1.** Maps showing (**above left**) the location of the Jaffna Peninsula (within the red rectangle) in relation to Sri Lanka and southern India, (**above right**) the relation of the Jaffna Peninsula to the Palk Strait and Palk Bay, and (**below**) the locations of the study sites (blue circles) on the northern side and the island sites (red circles) of the Jaffna Peninsula and its adjacent islands.

Long stretches of the reef, called "paar" by local fishermen and made up mostly of fringing reef, extend along the northern coast and around the islands at depths between 1 and 8 m [24,27]. Typically, there is a reef flat, 15–20 m wide and 1 to 2 m deep, extending seawards from the shore, with a substrate mainly of dead coral rocks and coral gravel [24]. Beyond this, the reef front (including reef crest) and reef slope region are dominated by a mix of live coral and dead coral to a depth of nearly 6 m. The maximum depth of the reef base is no more than8m[24], so no surveys were undertaken beyond this depth.
