Size frequency distributions of Pocillopora damicornis in coral reefs: responses to anthropogenic impacts

Abstract

Size-frequency distributions of coral colonies reveal important characteristics of composition of coral populations by providing insight into past events and have some predictive power with respect to population development on the reef. The aims of this study were to evaluate the size frequency distribution of Pocillopora damicornis between impacted reef (Polhena, Matara–subjected to anthropogenic activities and influence from Nilwala river discharges) and relatively non-impacted reef (Paraviwella, Tangalle), and to find the size dependent patterns of the partial mortality and bleaching. Data were collected during May and June 2014 using 25 m long line intercept transect. The colony size was determined as its volume using the maximum height, length, width and a “k value” (0.0658) which was estimated from a volumetric method. There was no significant difference in the size frequency distribution of P. damicornis for impacted versus relatively non-impacted reefs (Pearson χ2= 21.55, df= 14, p= 0.088). There were large number of colonies in the lowest size class (<100 cm3) at both sites and the number of colonies in that size class were higher in relatively non impacted reef than in impacted reef indicating that the condition of the reef locality affects to the distribution patterns of corals. There was a significant positive relationship of the size of P. damicornis with the estimated volume of the partial mortality (R2= 0.63, F= 150.82, p= 0.01) and with the estimated volume of bleaching (R2= 0.34, F= 5.23, p= 0.045) revealing that the larger size colonies may not spent much of their energy for repair and maintenance. As P. damicornis is a key reef building species on reefs in Sri Lanka, it is very important to study its population dynamics and this study provide base line data need for coral reef management.