Keeping The Crown-Of-Thorns At Bay

A detailed study of the population dynamics of the Crown-of-Thorns starfish could help local fishermen dependent on the coral reefs of Japan.

AsianScientist (Oct. 15, 2014) – Working with local fishermen, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have conducted an extensive survey of the population dynamics of the Crown-of-Thorns starfish, Acanthaster planci. Their results, published in Marine Biology, could help population control efforts and protect coral reefs.

A. planci is an aggressive predator of hard corals, which are responsible for building coral reefs. While individuals are typically harmless, their population increase poses significant danger because they are capable of consuming large areas of coral colonies. Outbreaks in their population can potentially devastate entire reef habitats.

Due to chronically high populations of A. planci on Japanese coral reefs, the Okinawan local government has been subsidizing population control programs since the early 1970s. Early removal efforts by fishermen were largely ineffective due to the inability to collect on a large enough scale. Sporadic recovery of coral has often been met with more predation from additional outbreaks in A. planci population.

While population control efforts have met with mixed success, collection data on A. planci in the Onna area have been maintained continually, with an improving quality of information. Starting in 1989, the collection area and number of starfish removed were recorded. From 2003, starfish were then sorted into seven different size classes.

Dr. Masako Nakamura displaying juvenile Crown-of-Thorn starfish. Credit: OIST.
Dr. Masako Nakamura displaying juvenile Crown-of-Thorn starfish. Credit: OIST.

The study led by Dr. Masako Nakamura from OIST’s Marine Biophysics Unit not only analyzes the population dynamics of A. planci over time, but also examines the potential factors that might be sustaining such populations off of the Onna village coast. The effects of seawater temperature, number of typhoons and rainfall were all investigated as factors contributing to the sustained population.

While there appeared to be no significant correlation between these factors and the chronically high population rates, the study found that the majority of individuals caught off of the Onna village coast fit within a certain size and age range. This finding suggests that there is a relatively steady number of new recruits coming in from elsewhere as well as locally. The collection data also points to a northern progression in the population densities along the Onna village coastline.

Further research is currently being conducted into ocean current movements in the area and other environmental factors which might be the cause of continuous recruitment. The hope is that this research will provide the foundation for a better understanding of how and where population outbreaks of A. planci occur.

“This research is a first step. Now we are doing more detailed research to help conservation efforts. But we could not have done as much as we have without the help of the local fisherman who know the sea here better than anyone,” Nakamura said.

The article can be found at: Nakamura et al. (2014) Spatial and Temporal Population Dynamics of the Crown-of-Thorns starfish, Acanthaster planci, Over a 24-year Period Along the Central West Coast of Okinawa Island, Japan.

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Source: Okinawa Institute of Science and Technology Graduate University.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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