First release of hatchery juveniles of the dusky grouper Epinephelus marginatus (Lowe, 1834) (Serranidae: Teleostei) at artificial reefs in the Mediterranean: results from a pilot study

A pilot experiment of stock enhancement based on the release of hatchery-reared juveniles of dusky grouper, Epinephelus marginatus, was carried out at two artificial reefs located 0.9 km offshore the south-western Sicilian coast (Central Mediterranean). The fish assemblages inhabiting the reefs were characterized using underwater visual census (UVC) surveys. Despite some differences in fish species richness, diversity and abundance, both artificial reefs provide suitable feeding resources and shelter opportunities for the settlement of dusky grouper juveniles. A total of 95 dusky grouper juveniles, which were 3 and 4 years old, were released. No stress-related effects on fish swimming or behaviour were observed during the step by step transportation to the sea-bottom at 20 m depth. After release, several groupers showed very low reactivity; freezing and schooling were the most common behaviours. The sighting rate after one month of UVC surveys was 15.7% and 20% at the two artificial reefs. No effect of fish size at release on grouper survival was observed. Some groupers were not recorded on the pyramid of release but elsewhere in the same artificial reef, which demonstrated their ability to move around the reef. The extent of dispersal of the released juveniles largely exceeded the area encompassed by the reefs, which was shown by recaptures of tagged groupers by local fishermen up to 13 km from the release site. This result provides a first insight into the potential of stock enhancement of hatchery reared dusky grouper juveniles for marine ranching and conservation purposes.