IMIS

The field studies were carried out in the inner reef lagoon of the Mombasa Marine National Park (MMNP) and the adjacent Mombasa Marine National Reserve (MMNR). Also the outer reef (only at the park’s side) was investigated for the presence of E. Diadema. During one month (29/9/97-24/10/97), surveys were made at low tide when the water reached a depth maximum 6 m, in the deep channel. Both locations (park and reserve) were subdivided in four different zones along the reef. A minimum of eight circle samples was laid in each zone, with a one-minute swimming distance (approximately 30 m) between them, using mask, snorkel and fins. Circle samples covering an area of 12.6 m^2 were laid. This was done by means of a 2-m line attached to a weight, which functioned respectively as radius and center of the circle. First the depth at the center of each circle sample was measured with a measuring tape and the time of each depth measurement was recorded on a plastic board, like all measurements. Within the circle samples the number of urchins of the species Echinothrix diadema were counted and the substrate cover was estimated. The topographic complexity of the reef was estimated using the rugosity measurement method (McClanahan T.R. & Shafir 1990).