Management of crown-of-thorns sea star (Acanthaster planci L.) outbreaks: Removal success depends on reef topography and timing within the reproduction cycle
Abstract Removals of crown-of-thorns sea stars (Acanthaster planci L.) are crucial initiatives in limiting the damage to coral reefs during outbreaks, but have often been unable to control the populations. We hypothesized that reef topography and exact timing of removals (before reproduction) determine their success and studied these in reefs along the western shore of Samal Island in the Philippines. An outbreak of A. planci was successfully removed from an isolated reef (surrounding water column >40 m) during 42 dive hours. In August 2009, the removal rate was initially 14 specimens h−1, which sharply decreased to less than 4 h−1 after the third removal. From May 2011 onward, no specimens were observed. All sea stars were found at depths ≤18 m, which confirmed that migration to isolated reefs is very unlikely. Mean gonado-somatic index (GSI) ranged from 6 to 15 in April–May 2010 and March–April 2011. During the rest of the year, mean GSI ranged between 0 and 4. Maximum GSI of 22.0 was found for one female specimen in April 2010. Maturation occurred at a diameter of 13 cm (female) and 16 cm (male). Diameter frequency distributions showed a new cohort (7 ≤ diameter ≤ 10 cm) between November 2009 and February 2010. To accurately compare results of various studies, relationships between underwater diameter, surface diameter, and crown-of-thorns weight, were established. Removals are recommended to be performed in isolated reefs, where migration from adjacent areas is limited, and before reproduction in April (Northern hemisphere), reducing the chance of future outbreaks.