The active selection of recent volcanic habitats by juvenile fish following a cyclone could explain the unusual mass-settlement of the Indian Ocean blacktip grouper Epinephelus oceanicus observed on 25 May 2006, few weeks after tropical storm Diwa on the latest lava flows of the Piton de la Fournaise volcano at Reunion Island. Following a cyclonic depression, shallow underwater habitats are devoid of resident fishes, including predators, favouring juvenile fish survival. Otherwise, the recent origin of the volcanic habitats, which vary in age, could facilitate the spatial segregation of adult and juvenile fish on the lava flows, reducing intraspecific competition and adult aggression toward juveniles. Regional characteristics of larval recruitment could also contribute to the infrequency of the described event; the isolated situation of the island would be expected to result in an average density of fish postlarvae 0.25 % to 1 % of that seen on some reefs in the Pacific. The supposed origin of juvenile groupers in the neighbouring island of Mauritius should encourage Reunionese managers to look beyond their own jurisdictions to adopt a collaborative approach to conservation of the entire connected network.