Persistent and expanding population outbreaks of the corallivorous starfish Acanthaster planci in the Northwestern Indian Ocean: Are They really a consequence of unsustainable starfish predator removal through overfishing in coral reefs, or a response to a changing environment?

Abstract

Population outbreaks of the starfish Acanthaster planci have been persisting for at least the past 25 yr on coral reefs in the Gulf of Oman, in the northwestern Indian Ocean. A survey conducted in 2001 showed that the A. planci population on the Dimaniyat Is. was as abundant (around 5 individuals (ind.)/transect; equivalent to 100 ind./ha) as that recorded during an outbreak in the early 1980s. Local authorities are controlling starfish populations by culling relatively large adult individuals. These outbreaks cause considerable damage to coral communities, as observed specimens were adult individuals of about 60 cm in total diameter (no juveniles were observed). The situation has persisted for over 2 decades, and has now spread to coral reefs in the Arabian Gulf. Although A. planci population outbreaks were associated in the past with overfishing of starfish predators in coral reef areas, in the present study, we found no connection between this theory and starfish outbreaks, as stomach contents of carnivorous fish specimens likely to prey on this starfish species, caught on coral reefs on the Gulf of Oman, and sold at local fish markets (in Barka, Muscat, and Sur), showed no presence of A. planci in their diets. Therefore, the reason for A. planci population outbreaks could not have been due to overfishing of predator species, but is most likely to have been caused by the frequent input of nutrients, due to frequent upwelling events in the northwestern Indian Ocean, leading to planktonic blooms which thus enhance A. planci recruitment.