Research study shows that Trochus niloticus (semum) population has increased since 2016

A new study released by the Palau International Coral Reef Center (PICRC) and the Bureau of Marine Resources (BMR), shows that the population of trochus in Palau has been increasing, in both size and number. Results of the study show a significant increase in density, population abundance, and size in each of the habitats surveyed from 2016 to 2019. The average size of all trochus counted was 9.34 cm, with 86% of the population greater than the legal harvest size of 3 inches (7.62 cm), and 48% of the population greater than 4 inches (10 cm).  During the last survey in 2016, trochus density at the reef crest was 146 individuals per hectare.  This year’s survey, the number has increased by nearly 20% to 757 individuals per hectare. Overall, trochus populations in Palau, both on the fore reef and reef crest habitats, is an estimated 3,385,995 individuals, where 2,909,980 individuals are of legal harvest size.

The study was conducted in 122 sites located in fore reef and reef crest habitats from Kayangel to Peleliu. This was the second follow up of the same study done by BMR in 2016. The objective of this study was to provide needed information so that BMR, under the Ministry of Natural Resources, Environment and Tourism (MNRET), is better able to make recommendations to the Palau National Congress on the status of the Trochus niloticus (semum) in Palau.

The copy of the full report is available at the PICRC website and can be downloaded at (