Population colonization of introduced trochus (Gastropoda) on coral reefs in Samoa

Shellfish have been introduced to countries beyond their native distributions in order to develop new fisheries, but the success of such translocations has been variable. In 2003 and 2006, adult trochus (Rochia nilotica), a herbivorous coral reef gastropod, were translocated from Fiji and Vanuatu to Samoa. This translocation extended their natural range and created a new fishery in Samoa. In 2018, we had the opportunity to assess the population structure of trochus stocks at 28 sites around Samoa's two main islands using underwater visual censuses along transects. This assessment revealed that the distribution of populations showed no correspondence with initial translocation sites. Densities of trochus were spatially variable, and very high (>500 individuals/ha) at some sites. Size-frequency distributions also varied among sites, yet all populations contained some large individuals. There was no evidence of competitive dominance of trochus over native gastropods or negative impacts to coral communities. This study shows that stocked shellfish such as trochus can develop to exploitable population levels within 15 years. Translocations of marine organisms must be considered with great caution. Our study indicates that livelihood benefits of introducing alien shellfish species are likely to be spatially variable. Translocations of the right species could support food webs and provide further food security and livelihood options to coastal fishing communities.