Recent survey shows decline in trochus population in Palau

A new study by the Palau International Coral Reef Center (PICRC) in collaboration with the Ministry of Agriculture, Fisheries, and the Environment (MAFE) has found a significant decline in the number of trochus, known locally as Semum, in Palau. The survey was conducted from December 2023 to March 2024 and examined 57 sites from Kayangel to Peleliu, covering the reef crest and fore reef areas.

The trochus is a type of mollusk important to Palauan culture, used for food and in making various products. Over the years, it has been overharvested, leading to population decline. The harvesting season for trochus is opened by the Palau National Congress (Olbiil er a Kelulau) based on assessments of the population’s health.

From 2016 to 2019, there was good news: the trochus population grew from just over 1,000,000 to nearly 4,000,000. Additionally, the mean size of trochus increased significantly, by 38% in fore reef areas and by 72% in reef crest areas. This growth in size meant that more trochus were reaching maturity and legal harvest size, indicating healthy populations. In 2016, about 57% of trochus surveyed were above the maturity size, and this figure rose to 83% in 2019.

However, the recent survey paints a different picture: only 428 were found at the selected sites, with a total estimated population of 1,653,067, a significant decrease from 2019. The Southeast region had the most trochus, while the Southwest had the fewest, consistent with past surveys. While trochus in fore reef areas have grown slightly by 4%, those in the reef crest have decreased by 28%.

Overall, 73% of the trochus surveyed were above maturity size, but there are only about 267 trochus per hectare in the fore reef and 223 in reef crest areas. This is well below the threshold of 600 individuals per hectare recommended for sustainable harvesting, indicating that the current population is not stable enough to support a harvest season.

Since 2016, the trochus harvest season has only been opened once, in 2021, for about three weeks. While it is unclear how much was harvested during this brief period, results from this survey indicate heavy pressure and a slow recovery. Opening the season again could seriously harm the trochus population.

To help the trochus recover, the study suggests several actions: increasing the size limit for harvesting to protect mature individuals so they can reproduce after closure, limiting the number of trochus each person can harvest, and restricting harvesting to certain areas.