Coral reef resilience assessment along the Tanzanian coast: the case of Mafia island, Songosongo and Mnazi bay areas

Measuring the resilience of coral reefs can give insights into the cause and effect of reef degradation, and identify paths for improvement of reef health. This study assessed the resilience of coral reefs of Mafia Island, Songosongo and Mnazi Bay, Tanzania, from October - December 2009. The study analysed size class distribution of hard corals, including coral recruitment and survivorship of the recruits. Hard corals were sampled using a 1 m2 quadrat for small colonies of < 10 cm and a 25 * 1 m belt transects for large colonies of > 10 cm diameters. Hard coral small colonies were further classified as either recruits (< 2.5 cm) or juveniles (2.6 - 10 cm) of colony diameters. Hard coral community structure including abundance and size class distributions of genera were analysed and compared among the study areas. Benthic data were recorded in photoquadrats and analysed using Coral Point Count with excel extension (CPCe) software. Coral recruitment among study areas did not differ significantly (F = 0.095, DF = 66, p = 0.909). Adult colony densities were significantly higher in Songosongo than Mnazi Bay and Mafia (R = 0.172, p < 0.01). However, recruit survivorship was equal among study areas (F = 0.234, DF = 28, p = 0.793). Coral genera abundance based on size class distribution among study areas did not differ significantly (R = 0.091, p = 0.075). Overall results suggest that resilience was highest at Songosongo followed by Mnazi Bay and Mafia Island. I conclude that the resilience of hard coral populations, as measured in size class distributions, depends on a variety of factors including the abundance of reproductive hard corals that enhance recruitment, and survivorship of recruits into juveniles. It is therefore recommended that resilience studies be included in reef monitoring programmes, which will provide useful scientific information for coral reef conservation and management in Tanzania.