Timing within the reproduction cycle modulates the efficiency of village-based crown-of-thorns starfish removal

- A subsampling method was proposed to document COTS spawning patterns.
- Estimating GSI from three random arms provides errors < 1, a cost-effective strategy.
- Hand-removal practices can in certain cases induce synchronized spawning in COTS
- Framing removals within COTS reproductive cycle may increase their efficiency.

In Pacific countries, outbreaks of the crown-of-thorns starfish Acanthaster planci (COTS) raise increasing conservation concerns in areas where people heavily depend upon coral reefs for their livelihoods. Small-scale cleanups are often used in an attempt to regulate COTS densities, but their efficiency is challenged by knowledge gaps and the difficulty for the communities to frame their decisions in an ecological perspective. Here, we investigated COTS reproductive seasonality and its potential impact on removals. The spawning period of COTS was documented in two islands of the Vanuatu archipelago using the gonado-somatic index (GSI). Peak spawning occurred one month after the 28 °C threshold was exceeded and was delayed by 2-3 weeks with increasing latitude, theoretically allowing cleanups to be framed within the biological cycle. We demonstrated that subsampling of two/three arms per starfish provide GSI errors < 1, a cost-effective strategy to document local spawning patterns. This approach was used during a major cleanup effort that took place during the spawning season, where fishermen removed COTS from a 3.8 km fringing reef. The density drastically dropped from 518.8 to 66.7 ind.ha− 1 after participants extirpated > 4 tons (13,000 starfish). At the end of the removals, GSI was reduced by half when compared to control areas, suggesting that traditional practices may under certain conditions trigger synchronized spawning. More effectively controlling COTS outbreaks will require teaching good ecological practices to avoid potential side-effects, a challenge in countries where a lack of knowledge on COTS does not really foster environmental concerns among local communities.